Mechanical properties of tendon are highly influenced by structural protein composition and microscopic sub-structures. Within the largest subunit of tendon, the fascicle, the role of the elastin protein remains understudied despite impressive extensibility and fatigue resistance of its resulting elastic fibers. While previous work catalogued contributions of fascicular elastin across tendon type and species, the anticipated effects of elastin in fatigue and healing have not yet been explored. While previous knockout mouse models showed that disruption of elastic fibers led to altered mechanical properties (e.g., increased linear modulus), these models depended on heterozygous elastin deficiency or indirect knockout of proteins related to elastic fibers. Thus, opportunities for in vivo studies were limited, since remaining elastin could possess inherent bioactivity even with elastic fiber disruption. To this end, a novel murine model of local elastin knockout Prx1Cre⁺;​Eln^fl/fl was developed.

Tendons from Prx1Cre⁺;​Eln^fl/fl mice were examined and found to share similarities with previous models, but with increased sensitivity to previously unknown elastin contributions. Results differed by tendon type, with linear modulus increases in energy-storing tendons (i.e., Achilles or AT) and decreases in positional tendons (i.e., tibialis anterior or TB). Next, fatigue-to-failure testing was performed, where Prx1Cre⁺;​Eln^fl/fl tendons had decreased fatigue life and increased energy loss in early cycles. Elastin knockout resulted in increased AT strain accumulation throughout testing and variable effects on damage accumulation by tendon type, as measured by collagen hybridizing peptide (CHP) assay and collagen fiber kinking (with development of custom image processing software).

A subsequent study applied 50% fatigue failure to functionally distinct tendons followed by subsequent stress relaxation (SR) and ramp-to-failure testing. Mechanical properties were observed to be reduced after cycling, though no differences in damage accumulation by genotype were measured, suggesting that evidence of damage likely emerges in the second half (i.e., 50-100%) of fatigue testing. While differences in ramp properties between genotypes were no longer apparent for sub-failure fatigue tendons, SR properties were altered in Prx1Cre⁺;​Eln^fl/fl tendons. Moreover, elastin was shown to modulate dynamic fiber engagement during AT cycling, which persisted post-rupture as quantified by the spread of fiber orientations. These results suggested that elastin may rely on different underlying mechanisms to impact mechanics depending on tendon type.

Lastly, the role of elastin in post-rupture healing was investigated in Prx1Cre⁺;​Eln^fl/fl mice through unilateral partial AT transection and treatment injections (tropoelastin, saline, or no injections). Notably, we found non-injected Prx1Cre⁺;​Eln^fl/fl mice exhibited worse functional (i.e., gait) outcomes than all other groups, as measured by high-speed video and a custom automated DeepLabCut/Python pipeline. Moreover, non-injected Prx1Cre⁺;​Eln^fl/fl tendons alone exhibited significant tendon lengthening and contralateral imbalance between multiple mechanical properties. While injection of any kind modulated some morphological and mechanical properties, saline injections produced more unintended effects (e.g., tendon thickening) than tropoelastin.

Overall, this study has shown that the presence of elastin impacts tendon health at multiple levels, including improved mechanics, fatigue resistance, and recovery following injury. This work provides multiple novel insights that are impactful on their own but also lay the foundation for future research to optimize elastin-informed treatment strategies to limit tendon degeneration and increase healing following injury