Collagen III Deficiency Following Injury in Female Murine Tendons Alters Matrix Composition, Structure, Organization...

Tendons withstand large forces due to an aligned, dense collagen matrix. However, their low cellularity and relative inability to recruit reparative cells post-injury, coupled with a susceptibility to excessive scarring results in loss of tendon structure and function. Type III collagen (COL3) plays a key role in regulating matrix architecture and limiting scar formation following cutaneous injury; however, its role in tendon remains unclear. We examined the impact of reduced COL3 using an established murine Col3a1 knockdown model. Uninjured tendons in Col3a1^+/- mice had a broader distribution of fibrils compared to Col3a1^+/+ mice. Fibrils in injured tendons of Col3a1^+/- mice were larger than those in Col3a1^+/+ mice at 3-weeks post injury but were smaller than their littermates at 6-weeks. Injured Col3a1^+/- tendons had enhanced fiber alignment at 1- and 6-weeks post-injury and an increase in ɑSMA⁺ myofibroblasts at 3-weeks post-injury. Differential expression of matrix components, as well as markers of cells, cell-ECM interaction, and inflammation were discovered. Anti-inflammatory macrophages were decreased in Col3a1^+/- tendons 1-week following injury, with no differences between genotypes later in healing. Pro-inflammatory macrophages remained unchanged between genotypes early in healing, but were increased in Col3a1^+/+ tendons compared to Col3a1^+/- tendons at 3-weeks post-injury. Finally, altered quasistatic mechanical properties was noted in COL3-deficient injured tendons. Our data suggests COL3 plays a complex role in regulating cell phenotype, activity, and fate, as well as collagen matrix architecture in the tendon injury microenvironment, which impacts tendon structure-function post-injury.