Loss of Smad4 in the scleraxis cell lineage results in postnatal joint contracture.

Growth of the musculoskeletal system requires precise coordination between bone, muscle, and tendon during development. Insufficient elongation of the muscle-tendon unit relative to bone growth results in joint contracture, a condition characterized by reduction or complete loss of joint range of motion. Here we establish a novel murine model of joint contracture by targeting Smad4 for deletion in the tendon cell lineage using Scleraxis-Cre (ScxCre). Smad4 mutants develop a joint contracture shortly after birth. The contracture is stochastic in direction and increases in severity with age. Smad4 mutant tendons exhibited a stable reduction in cellularity and a progressive reduction in extracellular matrix volume. Collagen fibril diameters were reduced in the Smad4 mutants, suggesting a role for Smad4 signaling in the regulation of matrix accumulation. Although ScxCre also has sporadic activity in both cartilage and muscle, we demonstrate an essential role for Smad4 loss in tendons for the development of joint contractures. Disrupting the canonical TGFβ-pathway in Smad2;3 mutants did not result in joint contractures. Conversely, disrupting the BMP pathway by targeting BMP receptors (Alk3/Alk6) recapitulated many features of the Smad4 contracture phenotype, suggesting that joint contracture in Smad4 mutants is caused by disruption of BMP signaling. Overall, these results establish a model of murine postnatal joint contracture and a role for BMP signaling in tendon elongation and extracellular matrix accumulation.