Regulators of ECM Structure Enable Functional Adaptation to Tensile Loading in Tendon Explants

Extracellular matrix (ECM) remodeling is essential for adaptation to changing mechanical demands, yet the mechanisms linking altered strain to functional outcomes remain poorly understood. This study aimed to define molecular and cellular programs driving the adaptation of tendon to increased (exercise) and decreased (disuse) strain. Male murine flexor tendon explants were cultured in incubator-housed tensile bioreactors and subjected to step changes in cyclic strain. After acclimation at 1% cyclic strain, exercise tendons experienced a step increase to 5% strain, while disuse tendons underwent stress deprivation. Increased strain produced significant mechanical adaptations, including increased elastic modulus and failure stress. Multiscale analyses of matrix organization, tissue composition, protein synthesis, signaling factors, and proteolytic activity revealed the mechanisms underlying these adaptations. Exercise-induced functional improvements were linked to an anabolic remodeling program characterized by TGF-{beta} and IL-6 signaling, small leucine-rich proteoglycan expression, MMP suppression, and enhanced collagen alignment. These findings indicate that regulators of matrix organization and turnover, beyond synthesis alone, are critical for functional adaptation. In contrast, mechanical unloading reduced collagen synthesis and alignment and promoted an MMP-dominant, catabolic phenotype favoring matrix breakdown. This study provides a comprehensive characterization of ECM remodeling, linking defined mechanical perturbations to molecular regulation and emergent structure-function relationships. These findings identify targetable mediators of adaptive remodeling and establish a framework for future studies of maladaptive ECM changes in aging, injury, and disease.