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PLoS One. 2012;7(11):e50140.
RUNX3, EGR1 and SOX9B form a regulatory cascade
required to modulate BMP-signaling during cranial
cartilage development in zebrafsh.
Dalcq J, Pasque V, Ghaye A, Larbuisson A, Motte P, Martial JA, Muller M.
Abstract
The cartilaginous elements forming the pharyngeal arches of the zebrafsh derive from cranial
neural crest cells. Their proper diferentiation and patterning are regulated by reciprocal
interactions between neural crest cells and surrounding endodermal, ectodermal and meso-
dermal tissues. In this study, we show that the endodermal factors Runx3 and Sox9b form a
regulatory cascade with Egr1 resulting in transcriptional repression of the fsta gene, encoding
a BMP antagonist, in pharyngeal endoderm. Using a transgenic line expressing a dominant nega-
tive BMP receptor or a specifc BMP inhibitor (dorsomorphin), we show that BMP signaling
is indeed required around 30 hpf in the neural crest cells to allow cell diferentiation and
proper pharyngeal cartilage formation. Runx3, Egr1, Sox9b and BMP signaling are required
for expression of runx2b, one of the key regulator of cranial cartilage maturation and bone
formation. Finally, we show that egr1 depletion leads to increased expression of fsta and
inhibition of BMP signaling in the pharyngeal region. In conclusion, we show that the successive
induction of the transcription factors Runx3, Egr1 and Sox9b constitutes a regulatory cascade
that controls expression of Follistatin A in pharyngeal endoderm, the latter modulating BMP
signaling in developing cranial cartilage in zebrafsh.
Runx3, Egr1 and Sox9b form a regulatory cascade required to modulate Bmp-signaling
during cranial cartilage development in zebrafsh.
Signaling model in wild-type embryos (A) and in embryos lacking the endodermal regulatory cascade
(B). (A) In wild-type embryos, pharyngeal endoderm expresses a regulatory cascade composed of three
transcription factors, Runx3, Egr1 and Sox9b, which down-regulates fsta expression that codes for a
Bmp antagonist. This down-regulation of fsta enables Bmp ligands to bind to their heterodimeric recep-
tor (BmpRI and BmpRII) and induce runx2b expression in cranial neural crest cells (cNCC). (B) Embryos
lacking any member of the Runx3-Egr1-Sox9b cascade display an over-expression of fsta, which its
coding protein is secreted from the endoderm. Antagonist Fsta binds to Bmp ligands and inhibit them
to bind to their receptor, with the consequence that no Bmp-signaling occurs towards the cNCC and no
runx2b expression.