MMP-9 selectively cleaves non-D-banded material on collagen fibrils with discrete plasticity damage in mechanically-overloaded tendon

The mechanical properties of tendon are due to the properties and arrangement of its collagen fibril content. Collagen fibrils are highly-organized supermolecular structures with a periodic banding pattern (D-band) indicative of the geometry of molecular organization. Following mechanical overload of whole tendon, collagen fibrils may plastically deform at discrete sites along their length, forming kinks, and acquiring a fuzzy, non-D-banded, outer layer (shell). Termed discrete plasticity, such non-uniform damage to collagen fibrils suggests localized cellular response at the fibril level during subsequent repair/replacement. Matrix metallo-proteinases (MMPs) are enzymes which act upon the extracellular matrix, facilitating cell mobility and playing important roles in wound healing. A sub-group within this family are the gelatinases, MMP-2 and MMP-9, which selectively cleave denatured collagen molecules. Of these two, MMP-9 is specifically upregulated during the initial stages of tendon repair. This suggests a singular function in damage debridement. Using atomic force microscopy (AFM), a novel fibril-level enzymatic assay was employed to assess enzymatic removal of material by trypsin and MMP-9 from individual fibrils which were: (i) untreated, (ii) partially heat denatured, (iii) or displaying discrete plasticity damaged after repeated mechanical overload. Both enzymes removed material from heat denatured and discrete plasticity-damaged fibrils; however, only MMP-9 demonstrated the selective removal of non-D-banded material, with greater removal from more damaged fibrils. The selectivity of MMP-9, coupled with documented upregulation, suggests a likely mechanism for the in vivo debridement of individual collagen fibrils, following tendon overload injury, and prior to deposition of new collagen.