Genetic evidence that FGFs have an instructive role in limb proximal–distal patterning

Abstract

Half a century ago, the apical ectodermal ridge (AER) at the distal tip of the tetrapod limb bud was shown to produce signals necessary for development along the proximal–distal (P–D) axis, but how these signals influence limb patterning is still much debated1,2. Fibroblast growth factor (FGF) gene family members are key AER-derived signals3,4, with Fgf4, Fgf8, Fgf9 and Fgf17 expressed specifically in the mouse AER5. Here we demonstrate that mouse limbs lacking Fgf4, Fgf9 and Fgf17 have normal skeletal pattern, indicating that Fgf8 is sufficient among AER-FGFs to sustain normal limb formation. Inactivation of Fgf8 alone causes a mild skeletal phenotype6,7; however, when we also removed different combinations of the other AER-FGF genes, we obtained unexpected skeletal phenotypes of increasing severity, reflecting the contribution that each FGF can make to the total AER-FGF signal. Analysis of the compound mutant limb buds revealed that, in addition to sustaining cell survival, AER-FGFs regulate P–D-patterning gene expression during early limb bud development, providing genetic evidence that AER-FGFs function to specify a distal domain and challenging the long-standing hypothesis that AER-FGF signalling is permissive rather than instructive for limb patterning. We discuss how a two-signal model for P–D patterning can be integrated with the concept of early specification to explain the genetic data presented here.

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Figure 1: Fgf8 is sufficient for normal limb development.
Figure 2: Effects of inactivating AER-FGF genes on skeletal development.
Figure 3: Effects of inactivating AER-FGF genes on cell survival, limb bud size and Meis1 expression.
Figure 4: AER-FGF mutant phenotypes can be explained by a two-signal dynamic specification model for limb proximal–distal patterning.

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Acknowledgements

We thank D. Ornitz for providing mice carrying the Fgf9 and Fgf17 null alleles before publication, and D. Lakeland for help with the statistical analysis. We are grateful to P. Ghatpande, A. Nemati and E. Yu for technical assistance. We also thank M. Barna, X. Sun, R. Metzger and our laboratory colleagues for comments on the manuscript. F.M. was supported by a fellowship from the American Heart Association and a National Research Service Award (NIH). This work was supported by a grant from the National Institutes of Health (R01 HD34380) to G.R.M.

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Correspondence to Gail R. Martin.

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The file contains Supplementary Table 1, Supplementary Figures 1-2 with Legends and additional references. (PDF 2541 kb)

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Mariani, F., Ahn, C. & Martin, G. Genetic evidence that FGFs have an instructive role in limb proximal–distal patterning. Nature 453, 401–405 (2008). https://doi.org/10.1038/nature06876

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