Receptor Tyrosine Kinase Regulated Growth and Remodeling of Tendon

Abstract

Tendon connects skeletal muscle to bone and transmits force generated from muscular contraction to produce locomotion. Despite the importance of tendon to the overall function of the musculoskeletal system, relatively little is known about the cellular and molecular mechanisms that control its growth, maintenance and repair. This lack of knowledge represents a significant barrier to improved management of tendon injuries, which remains difficult with poor outcomes. The objective of this dissertation was to determine the role of key signaling pathways activated during tendon growth and injury, and to investigate the biological mechanisms behind these responses. Since a central feature of tendon disorders is abnormal fibroblast morphology and a grossly disrupted appearance of the tendon extracellular matrix (ECM), my overall working hypothesis is molecular programs that regulate the interaction between tendon fibroblasts and their local ECM environment during tendon growth play an important role in the healing response of injured tendons. Herein, initial studies identified epithelial-to-mesenchymal transition and matrix metalloproteinases as important sets of genes upregulated in the proliferative and remodeling phases of tendon regeneration. A large number of these genes are known to be regulated by a family of receptor tyrosine kinases known as the platelet-derived growth factor receptors (PDGFRs), yet the role of PDGFR signaling in tendon growth and remodeling has remained largely unexplored. Using hindlimb synergist ablation to overload tendons and induce tendon growth in mice, we showed that tendon fibroblasts express both PDGFRs, PDGFRα and PDGFRβ, and the inhibition of PDGFR signaling suppressed the normal growth of tendon tissue after mechanical overload due to defects in fibroblast proliferation and migration. Membrane type-1 matrix metalloproteinase (MT1-MMP) was also identified as an essential proteinase for fibroblast migration through tendon ECM. Using a combination of small molecule inhibitors and cell-surface biotinylation experiments, we showed that MT1-MMP translation was regulated by PI3K/Akt within tendon fibroblasts, while ERK1/2 controlled post-translational trafficking of MT1-MMP. Taken together, these findings demonstrate that PDGFR signaling is necessary for postnatal tendon growth and remodeling, and that MT1-MMP is a critical mediator of tendon fibroblast migration and a potential target for the treatment of tendon injuries and diseases.