Loss of Cochlin drives impairments in tendon structure and function

Aging tendons undergo disruptions in homeostasis, increased susceptibility to injury, and reduced capacity for healing. Exploring the mechanisms behind this disruption in homeostasis is essential for developing therapeutics aimed at maintaining tendon health through the lifespan. We have previously identified that the extracellular matrix protein, Cochlin, which is highly expressed in healthy flexor tendon, is consistently lost during both natural aging and upon depletion of Scleraxis-lineage cells in young animals, which recapitulates many aging-associated homeostatic disruptions. Therefore, we examined the effects of Cochlin on tendon maturation and hypothesized that loss of Cochlin would disrupt normal tendon maturation and recapitulate phenotypes associated with disrupted adult tendon homeostasis, including alterations in collagen fibril organization, and impaired tendon mechanics. By 3-months of age, Cochlin flexor tendons exhibited altered collagen structure, with these changes persisting through at least 9-months. In addition, Cochlin tendons demonstrated significant declines in structural and material properties at 6-months, and structural properties at 9-months. While Cochlin did not drastically change the overall tendon proteome, consistent decreases in proteins associated with RNA metabolism, extracellular matrix production and the cytoskeleton were observed in Cochlin ^-/-. Interestingly, disrupted tendon maturation via Cochlin did not impair the tendon healing process. Taken together, these data define a critical role for Cochlin in facilitating physiological tendon maturation.