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Background: Exercise is widely recognized for its health benefits, including the release of bioactive proteins into the bloodstream, which exert systemic effects on various organs. Previous research has demonstrated that certain types of exercise can promote tendon healing, however, the specific exercise modalities that yield the most beneficial effects, as well as the underlying mechanisms, remain poorly understood. This thesis project aimed to address these knowledge gaps by utilizing an in vitro cell loading model to simulate exercise and investigate how different types and intensities of mechanical loading on myoblasts (muscle cells) influence the secretion (i.e. production and release) of bioactive proteins that may enhance tendon healing.

Aim: This thesis comprised four studies. The first study aimed to determine whether the secretome, derived from statically or dynamically loaded myoblasts, has a greater impact on tendon wound healing. This was assessed through measuring key processes, such as tenocyte (tendon cell) migration, proliferation, healing phenotype, and collagen production. The second and third studies sought to identify the optimal intensity of static loading that induces the secretion of proteins with potential roles in tendon healing. The fourth study employed a three-dimensional (3D) tendon formation model to elucidate how factors secreted by mechanically loaded myoblasts influence tendon cell phenotype, extracellular matrix (ECM) protein production, tendon structure, and the underlying molecular mechanisms.

Results: The Paper I demonstrated that secretory factors from statically loaded myoblasts significantly enhanced tenocyte migration, increased the type I/III collagen ratio and induced a myofibroblast-like phenotype in tenocytes compared with both dynamically loaded myoblasts and unloaded controls. These results suggest that molecules secreted from statically loaded myoblasts play a crucial role in tendon healing. In the Papers II and III, RNA sequencing and proteomic analyses, followed by validation experiments, identified insulin-like growth factor 1 (IGF-1) and neuroblastoma suppressor of tumorigenicity 1 (NBL1) as key factors secreted from myoblasts subjected to low-intensity (2%) static loading as compared with mild (5%) and high (10%) intensity loading and unloaded control. IGF-1 was found to enhance tenocyte proliferation, while NBL1 promoted tenocyte migration. The Paper IV revealed that secretome derived from myoblasts under 2% static loading increased the expression of key ECM proteins in tenocytes, including type I and III collagen, while also upregulated the expression of tenocyte-specific markers using an in vitro 3D tendon formation model.

Conclusion: This thesis work showed that the secretome derived from myoblasts subjected to low-intensity static loading improved tendon healing related parameters in tenocytes. This presents a potential novel strategy to support tendon healing during the critical immobilization phase following tendon injury. By stimulating the secretion of bioactive proteins into the circulation through targeted muscle loading—without directly subjecting the injured tendon to mechanical stress—this approach presents a promising method for promoting tendon healing. Furthermore, IGF-1 and NBL1 may serve as potential therapeutic targets for enhancing tendon healing.