BMPs negatively regulate structure and function of the limb apical ectodermal ridge

The apical ectodermal ridge (AER), a transient specialized epithelium at th e distal limb tip, is essential for vertebrate embryonic limb outgrowth alo ng the proximodistal axis. Among all the molecules expressed in the AER, on ly the Fibroblast Growth Factors (FGFs) have been shown to substitute for i ts function in limb outgrowth. After specification of the skeletal progenit ors is complete, the AER regresses, having fulfilled its function. However, the cellular processes underlying AER regression remain largely unclear, a nd the molecular ones, totally unknown. Members of the Bone Morphogenetic Protein (BMP) family are expressed in the AER throughout its life and in the mesenchyme, Our studies using misexpres sion of Noggin, a BMP inhibitor, reveal an unsuspected role for BMPs in the negative regulation of Fgf expression and AER function. We find that BMPs limit limb outgrowth by promoting AER regression, as BMP inhibition results in persistence of the AER, prolonged Fgf expression and excess soft-tissue growth. In addition, the Noggin misexpression studies uncover an earlier r ole for BMPs in repression of AER function. Noggin overexpression results i n extension of the AER anteriorly and loss of AER asymmetry. We show that o verall the AER becomes taller, and its anterior half becomes more similar t o a normal posterior AER, In addition, Fgf4 transcripts, which are usually restricted to the posterior half of the AER, are now also expressed anterio rly. Moreover, ectopicFgf4 expression is induced independently of Sonic Hed gehog, contrary to current models of Fgf4 regulation in the limb. Our studi es also provide insight into the activity of the hypothesized apical ectode rmal maintenance factor (AEMF), which is thought to maintain the tall shape of the posterior part of the AER, Our work shows that the AER is negativel y regulated by BMP.