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Shh and Gli3 are dispensable for limb skeleton formation but regulate digit number and identity

Abstract

Most current models propose Sonic hedgehog (Shh) as the primary determinant of anteroposterior development of amniote limbs1. Shh protein is said to be required to direct the formation of skeletal elements and to specify digit identity through dose-dependent activation of target gene expression. However, the identity of genes targeted by Shh, and the regulatory mechanisms controlling their expression, remain poorly understood. Gli3 (the gene implicated in human Greig cephalopolysyndactyly syndrome) is proposed to negatively regulate Shh by restricting its expression and influence to the posterior mesoderm2,3,4. Here we report genetic analyses in mice showing that Shh and Gli3 are dispensable for formation of limb skeletal elements: Shh-/- Gli3-/- limbs are distally complete and polydactylous, but completely lack wild-type digit identities. We show that the effects of Shh signalling on skeletal patterning and ridge maintenance are necessarily mediated through Gli3. We propose that the function of Shh and Gli3 in limb skeletal patterning is limited to refining autopodial morphology, imposing pentadactyl constraint on the limb's polydactyl potential, and organizing digit identity specification, by regulating the relative balance of Gli3 transcriptional activator and repressor activities.

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Figure 1: Gli3 regulates digit number and identity.
Figure 2: Shh signalling regulates Gli3 processing in the mouse limb.
Figure 3: Regulation of putative SHH target genes in the limb. E10.5 wild-type, Shh-/-, Gli3-/-, Shh-/- Gli3-/- and Shh-/- Gli3+/- forelimb buds were examined for expression of Gli1, Ptch1, Hoxd12, dHand and Bmp2.
Figure 4: Shh maintains ridge function by relieving an antagonistic regulatory cascade.

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Acknowledgements

We thank Y.-F. Wang, R. Mernaugh and J. Lancman for technical assistance; R. Harland and B. Vogelstein for reagents; and S. Carroll and members of the Fallon laboratory for critical comments on the manuscript. This work was supported by grants from the National Institutes of Health to C.C. and J.F.F.

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Correspondence to John F. Fallon or Chin Chiang.

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Litingtung, Y., Dahn, R., Li, Y. et al. Shh and Gli3 are dispensable for limb skeleton formation but regulate digit number and identity. Nature 418, 979–983 (2002). https://doi.org/10.1038/nature01033

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