Exploring Mechanobiology Within the Extracellular Microenvironment of the Impinged Achilles Tendon Insertion.

다국어 초록 (Multilingual Abstract)

The composition and structure of the extracellular matrix (ECM) is crucial to the primary mechanical function of tendon, with fibrillar collagen aligned longitudinally to transmit tensile force generated by muscle to bone. A multitude of tendons, however, experience bony impingement due to normal anatomical positioning, which produces a multiaxial, spatially heterogeneous mechanical strain environment with significantly elevated transverse compressive strain. In impinged regions, tendon exhibits a distinct fibrocartilage phenotype discernible by unaligned fibrillar collagen matrix enriched with glycosaminoglycans (GAGs) via aggrecan, a large, GAG-rich proteoglycan. Appreciated as a normal adaptation to mechanical impingement, these same features are clinically recognized as hallmarks of tendinopathy, a common degenerative tendon disease that frequently emerges with chronic impingement. These phenotypic landmarks, however, insufficiently capture the biochemical spectrum of aggrecan in fibrocartilage, which contains a landscape of aggrecan fragments due to constitutive aggrecanase catabolism. This biochemical profile is distinct from that of tensile tendon, suggesting aggrecan turnover may govern ECM mechanics and may itself be regulated by mechanical environment. To test this concept, the overall objective of this thesis was to determine if mechanical impingement modulates the biochemical profile of aggrecan and ECM mechanical properties in association with fibrocartilage formation. In the absence of suitable experimental models, we developed and characterized a novel murine hind limb explant model for studying the mechanobiology of fibrocartilage formation at the Achilles tendon insertion in situ through controlled prescription of impingement via passive ankle dorsiflexion. We also designed a rigid boot to restrict the ankle of mice to a dorsiflexed position to interrogate fibrocartilage mechanobiology at the impinged Achilles tendon insertion in vivo. We performed immunofluorescence to capture biochemical change in aggrecan composition secondary to impingement and atomic force microscopy (AFM) for spatially-guided nanomechanical characterization of the ECM. Our results indicated that impingement elicits distinct biochemical profiles in aggrecan composition and significantly augments ECM indentation modulus in situ. These findings are significant because they elucidate the molecular foundations of mechanobiology in tendon fibrocartilage, which will ultimately enhance our understanding of, and ability to diagnose and treat, impingement-associated tendinopathies.