Changes in Collagen With Aging Maintain Molecular Stability After Overload: Evidence From an In Vitro Tendon Model

Abstract

Soft tissue injuries are poorly understood at the molecular level. Previous work using differential scanning calorimetry (DSC) has shown that tendon collagen becomes less thermally stable with rupture. However, most soft tissue injuries do not result in complete tissue rupture but in damaging fiber overextension. Covalent crosslinking, which increases with animal maturity and age, plays an important role in collagenous fiber mechanics. It is also a determinant of tissue strength and is hypothesized to inhibit the loss of thermal stability of collagen due to mechanical damage. Controlled overextension without rupture was investigated to determine if overextension was sufficient to reduce the thermal stability of collagen in the bovine tail tendon (BTT) model and to examine the effects of aging on the phenomenon. Baseline data from DSC and hydrothermal isometric tension (HIT) techniques were compared between two groups: steers aged 24-30 months (young group), and skeletally mature bulls and oxen aged greater than five years (old group). Covalent crosslinks were quantified by ion exchange chromatography. Overextension resulted in reduced collagen thermal stability in the BTT model. The Young specimens, showing detectably lower tissue thermomechanical competence, lost more thermal stability with overextension than did the old specimens. The effect on old specimens, while smaller, was detectable. Multiple overextension cycles increased the loss of stability in the young group. Compositional differences in covalent crosslinking corresponded with tissue thermomechanical competence and therefore inversely with the loss of thermal stability. HIT testing gave thermal denaturation temperatures similar to those measured with DSC. The thermal stability of collagen was reduced by overextension of the tendon--without tissue rupture--and this effect was amplified by increased cycles of overextension. Increased tissue thermomechanical competence with aging seemed to mitigate the loss of collagen stability due to mechanical overextension. Surprisingly, the higher tissue thermomechanical competence did not directly correlate with the concentration of endogenous enzymatically derived covalent crosslinking on a mole per mole of collagen basis. It did, however, correlate with the percentage of mature and thermally stable crosslinks. Compositional changes in fibrous collagens that occur with aging affect fibrous collagen mechanics and partially determine the nature of mechanical damage at the intermolecular level. As techniques develop and improve, this new information may lead to important future studies concerning improved detection, prediction, and modeling of mechanical damage at much finer levels of tissue hierarchy than currently possible.

Leather quality of beefalo‐Nellore cattle in different production systems

“…Loss of mechanical tension appears to be the major factor underlying decreased collagen synthesis in aged skin, as collapsed fibroblasts produce low levels of collagen and high levels of collagen-degrading enzymes (Varani et al 2006). However, a 30 to 36 month difference in age has been shown to result in leather of different quality, with lower quality with increased level of maturity (Willett et al 2010). Results on transversal force and longitudinal force from the tensile test of leather were similar among treatments and close to the values of 171.6 N and 122.9 N, respectively, reported by Villarroel et al (2004) for 236 days old sheep.…”

“…Leather weight after standardization (i.e. Increased level of maturity also has been responsible for differences in properties of collagen between steers aged 24-30 months and bulls aged 5 years (Willett et al 2010). However, numerical values were greater for cattle in the intensive than the extensive system likely due to physiological differences between young and old animals.…”

Torn human rotator cuff tendons have reduced collagen thermal properties on differential scanning calorimetry

“…Lee et al23 proposed that the T peak is affected by intrahelical cross‐links formed between two polypeptide chains in the same helix, rather than interhelical cross‐links. Willett et al24 have demonstrated that the thermal stability of collagen is sensitive to mechanical forces, as stability is reduced simply by overextension rather than rupture of tendons.…”

Macrophage-like U937 cells recognize collagen fibrils with strain-induced discrete plasticity damage

“…For this first study of macrophage response to overload‐damaged collagen fibrils, we combined two established models. Overload‐damaged collagen fibrils were produced using a well‐characterized bovine tail tendon damage model . Cellular response to the damaged fibrils was assessed using a similarly well‐characterized U937 macrophage‐like cell model …”