Proteomics Identifies Differences in Fibrotic Potential of Exosomes from Human Tendon and Muscle Fibroblasts

Fibroblasts are the powerhouses responsible for the production and assembly of extracellular matrix. Their activity needs to be tightly controlled especially within the musculoskeletal system, where changes to extracellular matrix composition affect force transmission and mechanical loading that are required for effective movement of the body. Extracellular vesicles, including exosomes, are a mode of cell-cell communication within and between tissues, which has been largely characterised in cancer. However, it is unclear what the role of healthy fibroblast-derived extracellular vesicles is during tissue homeostasis. Here, we performed proteomic analysis of exosomes derived from primary human muscle and tendon cells to identify the potential functions of healthy fibroblast-derived exosomes. Mass spectrometry-based proteomics revealed comprehensive profiles for exosomes released from healthy human fibroblasts from different tissues. We found that fibroblast-derived exosomes were more similar than exosomes from differentiating myoblasts, but there were significant differences between tendon fibroblasts and muscle fibroblasts exosomes. Exosomes from tendon fibroblasts contain higher levels of proteins that support extracellular matrix synthesis, including TGFβ1, and muscle fibroblast exosomes contain a higher abundance of MMP2. Our data demonstrates a marked heterogeneity among healthy fibroblast-derived exosomes, indicating shared tasks between exosomes of skeletal muscle myoblasts and fibroblast, whereas tendon fibroblast exosomes has a marked fibrotic potential in human tendon tissue. These findings suggest an important role for exosomes in tissue homeostasis of both tendon and skeletal muscle in humans.