Wnt and BMP signaling cooperate with Hox in the control of Six2 expression in limb tendon precursor.

The number and shape of limb tendons vary along the proximodistal axis, and the autopod contains more tendons than the zeugopod. The transcription factor Six2 is expressed in the developing tendons, and its expression can be traced back to a group of limb mesenchymal cells that are thought to be tendon precursor cells. We tried to elucidate the mechanism controlling position-specific tendon pattern formation using Six2 as a tendon marker. Six2 expression was always found in cells between the limb cartilage and ectoderm. Administration of BMP-2 or BMP antagonist Noggin to the limb bud, respectively repressed or facilitated Six2 expression. Removal of the ectoderm or administration of the Wnt antagonist sFRP-2 abolished Six2 expression and ectopic Wnt expression induced ectopic Six2 expression. Taken together, Six2 expression is induced in the cells located at the point where cartilage-derived Noggin and ectoderm-derived Wnt signals meet. Misexpression of the autopod-specific Hoxa-13 or Hoxd-13 induced ectopic expression of Six2 in the zeugopodal mesenchymal cells of the chick limb bud. Six2 expression in the dorsal autopodal mesenchyme was not detected in Hoxa-13(-/-);HoxD(del/del) mice, indicating that autopod-specific Hox is required for the regulation of Six2 expression. Misexpression of Wnt in the autopod induced ectopic Six2 expression in the autopod. On the other hand, Wnt misexpression alone never induced Six2 expression in the zeugopod, yet co-misexpression of Hoxa-13 and Wnt in the zeugopod enhanced ectopic Six2 expression. Our results indicate that autopodal Hox genes regulate Six2 expression in the autopodal tendon precursor in cooperation with the factors from cartilage and ectoderm.