The protein kinase MLTK Is a key regulator for chondrogenesis.

Sox9 acts together with Sox5 or Sox6 as a master regulator for chondrocyte differentiation; however, how these transcription factors functionally interact and collaborate to regulate chondrogenesis remains unclear. Here we show that the protein kinase MLTK plays an essential role in the onset of chondrogenesis through triggering the induction of Sox6 by Sox9. Knockdown of MLTK in Xenopus embryos results in drastic loss of craniofacial cartilages without defects in neural crest formation. We also find that Sox6 is specifically induced during craniofacial chondrogenesis and this induction is inhibited by MLTK knockdown. Remarkably, Sox6-knockdown embryos display essentially the same phenotype as the MLTK-knockdown embryos; the drastic loss of cartilages and the marked down-regulation of genes involved in chondrogenesis. Microarray analysis reveals a remarkable similarity between Sox6-depleted and MLTK-depleted embryos in their gene expression pattern. Moreover, we find that ectopic expression of MLTK can induce Sox6 expression in a Sox9-dependent manner. These results identify a novel, key regulator for chondrogenesis.

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Suzuki T, Kusakabe M, Nakayama K, Nishida E