Fascicular Elastin Impacts Tendon-Specific Ramp-to-Failure and Fatigue Mechanics

Fascicular elastic fibers have recently been shown to play a significant role in tendon mechanics despite relatively low abundance, leading to increased linear modulus in ramp to failure mechanical testing with elastin knockdown. Despite elastin providing fatigue and recoil properties in a variety of tissues such as vasculature and lungs, its role in tendon fatigue mechanics is largely unknown. Therefore, this study validated and leveraged a novel murine model of local elastin knockout in the limbs (Prx1Cre+; Elnfl/fl) to study the impacts of elastin on tendon stress relaxation, ramp to failure, and fatigue mechanics for functionally distinct Achilles tendons (ATs) and tibialis anterior tendons (TBs). Elastin knockout was confirmed via gene expression analysis, biochemical protein quantification, immunofluorescence confocal imaging, and analysis of 3D two-photon image stacks. Consistent with previous results of elastin reduction or disruption in ramp to failure mechanical testing, Prx1Cre+; Elnfl/fl ATs exhibited increased linear modulus; in contrast, TBs exhibited decreased linear modulus, demonstrating tendon-specific effects. In agreement with previous results, mechanical changes corresponded to alterations in dynamic alignment of collagen fibers, suggesting elastin can mediate collagen fiber orientation and recruitment. Furthermore, elastin knockdown led to increased strain during fatigue testing in ATs but increased early hysteresis and decreased fatigue-life in both tendon types. Damage metrics showed opposite trends for collagen denaturation by tendon type, but consistent results for kinking after fatigue loading. Results suggest different mechanisms underly each type of damage and that the impact of elastic fiber knockdown is tendon-dependent.