Tgfβ2 3 are coexpressed with their extracellular regulator Ltbp1 in the early limb bud and modulate mesodermal outgrowth and BMP signaling in chicken embryos

Background: Transforming growth factor beta proteins (Tgf beta s) are secreted cytokines with well-defined functions in the differentiation of the musculoskeletal system of the developing limb. Here we have studied in chicken embryos, whether these cytokines are implicated in the development of the embryonic limb bud at stages preceding tissue differentiation. Results: Immunohistochemical detection of phosphorylated Smad2 and Smad3 indicates that signaling by this pathway is active in the undifferentiated mesoderm and AER. Gene expression analysis shows that transcripts of tgf beta 2 and tgf beta 3 but not tgf beta 1 are abundant in the growing undifferentiated limb mesoderm. Transcripts of tgf beta 2 are also found in the AER, which is the signaling center responsible for limb outgrowth. Furthermore, we show that Latent Tgf beta Binding protein 1 (LTBP1), which is a key extracellular modulator of Tgf beta ligand bioavailability, is coexpressed with Tgf beta s in the early limb bud. Administration of exogenous Tgf beta s to limb buds growing in explant cultures provides evidence of these cytokines playing a role in the regulation of mesodermal limb proliferation. In addition, analysis of gene regulation in these experiments revealed that Tgf beta signaling has no effect on the expression of master genes of musculoskeletal tissue differentiation but negatively regulates the expression of the BMP-antagonist Gremlin. Conclusion: We propose the occurrence of an interplay between Tgf beta and BMP signaling functionally associated with the regulation of early limb outgrowth by modulating limb mesenchymal cell proliferation.