Background: Transforming growth factor beta proteins (Tgfbetas) 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 tgfbeta2 and tgfbeta3 but not tgfbeta1 are abundant in the growing undifferentiated limb mesoderm. Transcripts of tgfbeta2 are also found in the AER, which is the signaling center responsible for limb outgrowth. Furthermore, we show that Latent Tgfbeta Binding protein 1 (LTBP1), which is a key extracellular modulator of Tgfbeta ligand bioavailability, is coexpressed with Tgfbetas in the early limb bud. Administration of exogenous Tgfbetas 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 Tgfbeta 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 Tgfbeta and BMP signaling functionally associated with the regulation of early limb outgrowth by modulating limb mesenchymal cell proliferation.