Transcriptional changes in chick wing bud polarization induced by retinoic acid

BACKGROUND: Retinoic acid is implicated in the induction of the gene encoding Sonic hedgehog (Shh) that specifies antero-posterior positional values and promotes growth of the developing limb bud. However, because retinoic acid is involved in limb initiation, it has been difficult to determine if it could have additional roles in antero-posterior patterning. To investigate this, we implanted retinoic acid-soaked beads to the anterior margin of the chick wing bud and performed microarray analyses prior to onset of Shh expression.

RESULTS: Retinoic acid upregulates expression of Hoxd11-13 that encode transcription factors implicated in inducing Shh transcription and that are involved in digit development. In our assay, retinoic acid induces Shh transcription and consequently a new pattern of digits at a much later stage than anticipated. Retinoic acid represses many anteriorly-expressed genes including Bmp4, Lhx9, Msx2 and Alx4. We provide evidence that retinoic acid influences transcription via induction of dHAND and inhibition of Gli3 to establish a new antero-posterior pre-pattern. We show that transient exposure to retinoic acid can suppress distal development and expedite cells to transcriptionally respond to Shh.

CONCLUSIONS: Our findings reveal how retinoic acid and Shh signalling could co-operate in antero-posterior patterning of the limb. This article is protected by copyright. All rights reserved.