Abstract
Vertebrate limb outgrowth is driven by a positive feedback loop that involves Sonic hedgehog (Shh) and Gremlin1 (Grem1) in the posterior limb bud mesenchyme and Fibroblast growth factors (Fgfs) in the overlying epithelium. Proper spatio-temporal control of these signaling activities is required to avoid limb malformations such as polydactyly. Here we show that, in Tbx2-deficient hindlimbs, Shh/Fgf4 signaling is prolonged, resulting in increased limb bud size and duplication of digit 4. In turn, limb-specific Tbx2 overexpression leads to premature termination of this signaling loop with smaller limbs and reduced digit number as phenotypic manifestation. We show that Tbx2 directly represses Grem1 in distal regions of the posterior limb mesenchyme allowing Bone morphogenetic protein (Bmp) signaling to abrogate Fgf4/9/17 expression in the overlying epithelium. Since Tbx2 itself is a target of Bmp signaling, our data identify a growth-inhibiting positive feedback loop (Bmp/Tbx2/Grem1). We propose that proliferative expansion of Tbx2-expressing cells mediates self-termination of limb bud outgrowth due to their refractoriness to Grem1 induction.
Publication types
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Research Support, Non-U.S. Gov't
MeSH terms
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Animals
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Bone Morphogenetic Proteins / metabolism
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Cytokines
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Epithelium / metabolism
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Fibroblast Growth Factors / genetics*
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Fibroblast Growth Factors / metabolism
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Gene Expression Regulation, Developmental
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Hedgehog Proteins / genetics*
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Hedgehog Proteins / metabolism
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Intercellular Signaling Peptides and Proteins / genetics*
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Intercellular Signaling Peptides and Proteins / metabolism
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Limb Buds / growth & development*
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Limb Buds / metabolism
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Mesoderm / growth & development
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Mesoderm / metabolism
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Mice
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Phenotype
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Signal Transduction
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T-Box Domain Proteins / genetics*
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T-Box Domain Proteins / metabolism
Substances
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Bone Morphogenetic Proteins
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Cktsf1b1 protein, mouse
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Cytokines
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Hedgehog Proteins
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Intercellular Signaling Peptides and Proteins
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Shh protein, mouse
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T-Box Domain Protein 2
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T-Box Domain Proteins
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Fibroblast Growth Factors
Grants and funding
Work in the laboratory of VMC was supported by the European Community's Seventh Framework Programme contract (“CardioGeNet” 223463). Work in the laboratory of AK was supported by funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for the grant KI 728/5-1 DFG and for the Cluster of Excellence REBIRTH (from Regenerative Biology to Reconstructive Therapy) at Medizinische Hochschule Hannover. HFF was supported by an EMBO long-term fellowship. Publication charges were supported by the Deutsche Forschungsgemeinschaft DFG in the framework of the program “Open Access Publishing” at Medizinische Hochschule Hannover. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.