Scleraxis-lineage cells are required for tendon homeostasis and their depletion induces an accelerated extracellular matrix aging phenotype

Aged tendons have disrupted homeostasis, increased injury risk, and impaired healing capacity. Understanding mechanisms of homeostatic disruption is crucial for developing therapeutics to retain tendon health through the lifespan. Here, we developed a novel model of accelerated tendon extracellular matrix (ECM) aging via depletion of Scleraxis-lineage (Scx ^Lin ) cells in young mice (DTR). DTR recapitulates many aspects of tendon aging including comparable declines in cellularity, alterations in ECM structure, organization, and composition. Single cell RNA-sequencing demonstrated a conserved decline in tenocytes associated with ECM biosynthesis in aged and DTR tendons, identifying the requirement for Scx ^Lin cells during homeostasis. However, the remaining cells in aged and DTR tendons demonstrate functional divergence. Aged tenocytes become pro-inflammatory and lose proteostasis. In contrast, DTR tenocytes demonstrate enhanced remodeling capacity. Collectively, this study defines DTR a novel model of accelerated tendon ECM aging and identifies novel biological intervention points to maintain tendon function through the lifespan.