CHICKEN TRANSCRIPTION FACTOR AP-2 - CLONING, EXPRESSION AND ITS ROLE IN OUTGROWTH OF FACIAL PROMINENCES AND LIMB BUDS

Embryonic facial development in chick embryos involves a sequential ac tivation of genes that control differential growth and patterning of t he beak. In the present study we isolate one such gene, the transcript ion factor, AP-2, that is known to be expressed in the face of mouse e mbryos. The protein sequence of chick AP-2 alpha is 94% homologous to human and mouse AP-2. Wholemount in situ hybridization with a probe fo r chick AP-2 identifies expression from primitive streak stages up to stage 28. The most striking expression patterns in the head are during neural crest cell migration when AP-2 transcripts follow closely the tracts previously mapped for neural crest cells. Later, expression in the facial mesenchyme is strongest in the frontonasal mass and lateral nasal prominences and is do unregulated in the maxillary and mandibul ar prominences. Once limb buds are visible, high expression is seen in the distal mesenchyme but not in the apical ectodermal ridge. The exp ression patterns of AP-2 in stage 20 embryos suggested that the gene m ay be important in ''budding out'' of facial prominences and limb buds . We implanted beads soaked in retinoic acid in the right nasal pit of stage 20 embryos resulting in a specific inhibition of outgrowth of t he frontonasal mass and lateral nasal prominences. AP-2 expression was completely down-regulated in the lateral nasal within 8 hr of bead ap plication. In addition, the normal up regulation of AP-2 in the fronto nasal mass did not occur following retinoic-acid treatment. There was an increase in programmed cell death around the right nasal pit that a ccompanied the down-regulation of AP-2. Prominences whose morphogenesi s were not affected by retinoic acid did not have altered expression p atterns. We removed the apical ectodermal ridge in stage 20 limb buds and found that AP-2 expression was partially do unregulated 4 hr follo wing ridge removal and completely downregulated 8 hr following strippi ng. Application of an FGF-4 soaked bead to the apex of the limb bud ma intained AP-2 expression. Thus AP-2 is involved in outgrowth and could be regulated by factors such as FGFs that are present in the ectoderm of both the face and limb. (C) 1997 Academic Press.