The molecular programme underlying tendon development has not been fully identified. Interactions with components of the musculoskeletal system are important for limb tendon formation. Limb tendons initiate their development independently of muscles; however, muscles are required for further tendon differentiation. We show that both FGF/ERK MAPK and TGFβ/SMAD2/3 signalling pathways are required and sufficient for SCX expression in chick undifferentiated limb cells, whereas the FGF/ERK MAPK pathway inhibits Scx expression in mouse undifferentiated limb mesodermal cells. During differentiation, muscle contraction is required to maintain SCX, TNMD and THBS2 expression in chick limbs. The activities of FGF/ERK MAPK and TGFβ/SMAD2/3 signalling pathways are decreased in tendons under immobilisation conditions. Application of FGF4 or TGFβ2 ligands prevents SCX downregulation in immobilised limbs. TGFβ2 but not FGF4 prevent TNMD and THBS2 downregulation under immobilisation conditions. We did not identify any intracellular crosstalk between both signalling pathways in their positive effect on SCX expression. Independently of each other, both FGF and TGFβ promote tendon commitment of limb mesodermal cells and act downstream of mechanical forces to regulate tendon differentiation during chick limb development.
Author contributions
D.D. designed the experiments. E.H., M.-A.B., J.E.d.L., B.C. and C.M. performed experiments. E.H. and D.D. analysed the data and D.D. wrote the manuscript. All of the authors have read and approved the final manuscript.
Funding
This work was supported by the Fondation pour la Recherche Médicale (FRM) [DEQ20140329500]; the Agence Nationale de la Recherche (ANR) [ANR-12-BSV1-0038]; the Association Française contre les Myopathies (AFM) [16752/16826]; Institut National de la Santé et de la Recherche Médicale (INSERM); the Centre National de la Recherche Scientifique (CNRS); and the Université Pierre et Marie Curie (UPMC).
