Functional Role of Tenomodulin in the distinct stages of Achilles Tendon repair

The Achilles tendon (AT) is the thickest, largest, and strongest tendon in the human body. The main function of the AT is to connect the triceps surae muscle, composed of gastrocnemius-soleus muscle complex to the heel bone. The AT transmits the energy generated by the contraction of the calf muscles to induce plantar flexion allowing skeletal movements such as walking, running, or jumping. The AT can withstand forces 12 times greater than the bodyweight during running, which together with the natural poor blood supply in the AT predispose the tendon to injuries. One possible outcome after tendon injury is the formation of ectopic bone in the tendinous tissue, which is known as Heterotopic Ossification (HO). To date, neither effective nor successful treatments have been put in practice in the clinic. “Tenomodulin (Tnmd), the best-known mature marker for tendon lineage cells, has important effects in tendon tissue aging and function. We have previously reported that loss of Tnmd leads to inferior early tendon repair characterized by fibrovascular scaring and therefore hypothesized that its absence will persistently cause deficient repair during later stages. In this study, Tnmd knockout (Tnmd-/-) and wild-type (WT) animals were subjected to complete AT surgical transection followed by end-to-end suture. Lineage tracing analyses revealed the formation of a fibrotic scar tissue, characterized by reduction in tendon-lineage cells (marked by ScleraxisGFP-expression), and the significant increase in the presence of alpha smooth muscle actin myofibroblasts in Tnmd-/- tendon scars”. At the proliferative stage, no significant differences were detected between both genotypes regarding innervation, vasculature formation, collagen fibril orientation and maturation. “However, significantly more pro-inflammatory M1 macrophages and larger collagen II cartilaginous template were detected in the injured tendons of Tnmd-deficient mice. Furthermore, qRT-PCR analysis showed a clear dysregulation of classical tendon-related genes and collagen fibril regulators in mutant tendons, which might be associated to a delayed activation of tenogenesis in the scar”. These results go in line with our TEM data, where significantly thicker collagen fibrils were measured in non-injured and injured AT of Tnmd-/- tendons. “At the remodeling stage, histological scoring revealed lower repair quality in the injured Tnmd-deficient tendons, which was coupled with higher HO quantified by micro-CT. Tendon biomechanical properties were compromised in both groups upon injury, however we identified an abnormal stiffening on non-injured Tnmd-/- tendons, which possessed higher static and dynamic E-moduli. Pathologically thicker and abnormally shaped collagen fibrils were observed by TEM in Tnmd-/- tendons and this together with augmented HO resulted in diminished running capacity of Tnmd-/- mice. These novel findings demonstrate that Tnmd plays a protecting role against trauma-induced endochondral HO and can inspire the generation of novel therapeutics to accelerate repair