A BMP-Shh negative-feedback loop restricts Shhexpression during limb development

Normal patterning of tissues and organs requires the tight restriction of signaling molecules to well-defined organizing centers. In the limb bud, one of the main signaling centers is the zone of polarizing activity (ZPA) that controls growth and patterning through the production of sonic hedgehog (SHH). The appropriate temporal and spatial expression of Shh is crucial for normal limb bud patterning, because modifications, even if subtle, have important phenotypic consequences. However, although there is a lot of information about the factors that activate and maintain Shhexpression, much less is known about the mechanisms that restrict its expression to the ZPA. In this study, we show that BMP activity negatively regulates Shh transcription and that a BMP-Shhnegative-feedback loop serves to confine Shh expression. BMP-dependent downregulation of Shh is achieved by interfering with the FGF and Wnt signaling activities that maintain Shh expression. We also show that FGF induction of Shh requires protein synthesis and is mediated by the ERK1/2 MAPK transduction pathway. BMP gene expression in the posterior limb bud mesoderm is positively regulated by FGF signaling and finely regulated by an auto-regulatory loop. Our study emphasizes the intricacy of the crosstalk between the major signaling pathways in the posterior limb bud.Acknowledgements: We thank Kim Cooper, John Fallon, Cheryll Tickle, Miguel Torres and Mat Towers for critically reading the manuscript. We appreciate the excellent technical assistance of Marisa Junco and Dolores Delgado, Iñigo Casafont and members of the Piero Crespo and Ros laboratories for support. Supported by grant BFU2008-00397 from the Spanish Ministry of Education and Science. R.S. is a FPU fellow from the Spanish Ministry of Science and Innovation