In tendons, differing physiological requirements lead to distinct patterns of MMP-1 degradation

Collagen fibrils from high-stress, energy-storing tendons critical to locomotion are smaller in diameter with increased intermolecular crosslinking compared to fibrils from low-stress, positional tendons. This results in distinct loading mechanics thought to limit fatigue damage in energy-storing tendons. However, there appears to be a functional trade-off with energy-storing tendons also having reduced remodeling ability. Energy-storing tendons have lower collagen turnover and increased injury rates compared to positional tendons. In a recent study, a causative factor for this lower collagen turnover was suggested: resistance to degradation by MMP-1. To validate the prior study's results obtained from single fibrils, the current study undertook population level assessment of fibril degradation by MMP-1. Predictive degradation models were created to assess fibril diameter distribution changes. Positional and energy-storing tendon sections were incubated for 24 h with buffer or MMP-1, imaged with scanning electron microscopy, and analysed with a custom pipeline for piece-wise fibril measurement. Enzyme treated sections showed evidence of degradation with reduced fibril diameter, decreased alignment, increased curvature, and decreased D-band length. Energy-storing tendon fibrils were more resistant to enzymolysis, with only the large diameter fibril subpopulation affected by MMP-1 (15% diameter reduction compared to control), while the entire population of positional tendon fibrils decreased in diameter (41%). Comparison to model predictions confirmed a linear relationship of degradation with fibril size. Larger fibrils experienced greater diameter decreases combined with increased longitudinal diameter variation and D-band decreases. Crosslinking is thought to be responsible for both fibril type and size findings, the latter suggesting higher density crosslinking in the fibril core.