Dysregulated assembly of elastic fibers in fibulin-5 knockout mice results in a tendon-specific increase in elastic modulus

The alteration of the structure and macroscopic mechanical response of porcine patellar tendon by elastase digestion

Background: The treatment of patellar tendon injury has always been an unsolved problem, and mechanical characterization is very important for its repair and reconstruction. Elastin is a contributor to mechanics, but it is not clear how it affects the elasticity, viscoelastic properties, and structure of patellar tendon. Methods: The patellar tendons from six fresh adult experimental pigs were used in this study and they were made into 77 samples. The patellar tendon was specifically degraded by elastase, and the regional mechanical response and structural changes were investigated by: (1) Based on the previous study of elastase treatment conditions, the biochemical quantification of collagen, glycosaminoglycan and total protein was carried out; (2) The patellar tendon was divided into the proximal, central, and distal regions, and then the axial tensile test and stress relaxation test were performed before and after phosphate-buffered saline (PBS) or elastase treatment; (3) The dynamic constitutive model was established by the obtained mechanical data; (4) The structural relationship between elastin and collagen fibers was analyzed by two-photon microscopy and histology. Results: There was no statistical difference in mechanics between patellar tendon regions. Compared with those before elastase treatment, the low tensile modulus decreased by 75%-80%, the high tensile modulus decreased by 38%-47%, and the transition strain was prolonged after treatment. For viscoelastic behavior, the stress relaxation increased, the initial slope increased by 55%, the saturation slope increased by 44%, and the transition time increased by 25% after enzyme treatment. Elastin degradation made the collagen fibers of patellar tendon become disordered and looser, and the fiber wavelength increased significantly. Conclusion: The results of this study show that elastin plays an important role in the mechanical properties and fiber structure stability of patellar tendon, which supplements the structure-function relationship information of patellar tendon. The established constitutive model is of great significance to the prediction, repair and replacement of patellar tendon injury. In addition, human patellar tendon has a higher elastin content, so the results of this study can provide supporting information on the natural properties of tendon elastin degradation and guide the development of artificial patellar tendon biomaterials.

Contributions of collagen and elastin to elastic behaviours of tendon fascicle

Tendon exhibits the capacity to be stretched and to return to its original length without suffering structural damage in vivo, a capacity known as elastic recoil. Collagen fibres are aligned longitudinally and elastin fibres mostly run parallel to collagen fibres in tendon. However, their interactions and contributions to tendon elastic behaviours are not well understood. The present study examined functional roles of collagen and elastin in tendon elastic behaviours using a variety of mechanical tests. We prepared three types of fascicle specimens from mouse tail tendon: fascicles freshly isolated, those digested with elastase in PBS to selectively remove elastin, and those incubated in PBS without elastase. A quasi-static tensile test demonstrated that elastase-treated fascicles had higher tangent moduli and strength compared to fresh and PBS fascicles. Cyclic stretching tests showed that fresh and PBS fascicles could withstand cyclic strain at both small and large amplitudes, but elastase-treated fascicles could only behave elastically to a limited degree. Fibre-sliding analysis revealed that fresh fascicles could be elongated both through stretching of collagen fibers and through movement of the fibres. However, elastase-treated fascicles could be stretched only via fibre stretching. This evidence suggests that normal tendons can be extended through both fibre stretching and fibre sliding, whereas tendons without elastin can only extend as much as collagen fibers can withstand. Accordingly, collagen fibres mainly contribute to tendon elastic behaviours by furnishing rigidity and elasticity, whereas elastin provides tendon viscoelasticity and also enables sliding of collagen fibres during elastic behaviours. STATEMENT OF SIGNIFICANCE: The present study revealed distinct mechanical functions of collagen and elastin fibres in elastic behaviours of mouse tail tendon fascicle using a variety of mechanical tests at both microscopic and macroscopic levels. It was demonstrated that collagen mainly governs tendon fascicle rigidity and elasticity, but only possesses limited extensibility, whereas elastin contributes to viscoelasticity and collagen fibre sliding, enabling elastic recoil behaviour against relatively large deformation. By their interactions, tendon can be elongated without suffering major structural damage and withstand a large magnitude of tensile force in response to mechanical loading. Such information should be particularly useful in designing collagen-based biomaterials such as artificial tendons, in that previous studies have merely considered collagen without incorporation of elastin.

Papillary and reticular fibroblasts generate distinct microenvironments that differentially impact angiogenesis

Papillary and reticular dermis show distinct extracellular matrix (ECM) and vascularization corresponding to their specific functions. These characteristics are associated with gene expression patterns of fibroblasts freshly isolated from their native microenvironment. In order to assess the relevance of these fibroblast subpopulations in a tissue engineering context, we investigated their contribution to matrix production and vascularization using cell sheet culture conditions. We first performed RNA-seq differential expression analysis to determine whether several rounds of cell amplification and high-density culture affected their gene expression profile. Bioinformatics analysis revealed that expression of angiogenesis-related and matrisome gene signatures were maintained, resulting in papillary and reticular ECMs that differ in composition and structure. The impact of secreted or ECM-associated factors was then assessed using two independent 3D angiogenesis assays: -1/ a fibrin hydrogel-based assay allowing investigation of diffusible secreted factors, -2/ a scaffold-free cell-sheet based assay for investigation of fibroblast-produced microenvironment. These analyses revealed that papillary fibroblasts secrete highly angiogenic factors and produce a microenvironment characterised by ECM remodelling capacity and dense and branched microvascular network, whereas reticular fibroblasts produced more structural core components of the ECM associated with less branched and larger vessels. These features mimick the characteristics of both the ECM and the vasculature of dermis subcompartments. In addition to showing that skin fibroblast populations differentially regulate angiogenesis via both secreted and ECM factors, our work emphasizes the importance of papillary and reticular fibroblasts for engineering and modelling dermis microenvironment and vascularization. STATEMENT OF SIGNIFICANCE: Recent advances have brought to the forefront the central role of microenvironment and vascularization in tissue engineering for regenerative medicine and microtissue modelling. We have investigated the role of papillary and reticular fibroblast subpopulations using scaffold-free cell sheet culture. This approach provides differentiated cells conditions allowing the production of their own microenvironment. Analysis of gene expression profiles and characterisation of the matrix produced revealed strong and specific angiogenic properties that we functionally characterized using 3D angiogenesis models targeting the respective role of either secreted or matrix-bound factors. This study demonstrates the importance of cell-generated extracellular matrix and questions the importance of cell source and the relevance of hydrogels for developing physio-pathologically relevant tissue engineered substitutes.

Fascicular elastin within tendon contributes to the magnitude and modulus gradient of the elastic stress response across tendon type and species

Elastin, the main component of elastic fibers, has been demonstrated to significantly influence tendon mechanics using both elastin degradation studies and elastinopathic mouse models. However, it remains unclear how prior results differ between species and functionally distinct tendons and, in particular, how results translate to human tendon. Differences in function between fascicular and interfascicular elastin are also yet to be fully elucidated. Therefore, this study evaluated the quantity, structure, and mechanical contribution of elastin in functionally distinct tendons across species. Tendons with an energy-storing function had slightly more elastin content than tendons with a positional function, and human tendon had at least twice the elastin content of other species. While distinctions in the organization of elastic fibers between fascicles and the interfascicular matrix were observed, differences in structural arrangement of the elastin network between species and tendon type were limited. Mechanical testing paired with enzyme-induced elastin degradation was used to evaluate the contribution of elastin to tendon mechanics. Across all tendons, elastin degradation affected the elastic stress response by decreasing stress values while increasing the modulus gradient of the stress-strain curve. Only the contributions of elastin to viscoelastic properties varied between tendon type and species, with human tendon and energy-storing tendon being more affected. These data suggest that fascicular elastic fibers contribute to the tensile mechanical response of tendon, likely by regulating collagen engagement under load. Results add to prior findings and provide evidence for a more mechanistic understanding of the role of elastic fibers in tendon. STATEMENT OF SIGNIFICANCE: Elastin has previously been shown to influence the mechanical properties of tendon, and degraded or abnormal elastin networks caused by aging or disease may contribute to pain and an increased risk of injury. However, prior work has not fully determined how elastin contributes differently to tendons with varying functional demands, as well as within distinct regions of tendon. This study determined the effects of elastin degradation on the tensile elastic and viscoelastic responses of tendons with varying functional demands, hierarchical structures, and elastin content. Moreover, volumetric imaging and protein quantification were used to thoroughly characterize the elastin network in each distinct tendon. The results presented herein can inform tendon-specific strategies to maintain or restore native properties in elastin-degraded tissue.

Method development and characterization of chick embryo tendon mechanical properties

Tendons are involved in multiple disorders and injuries, ranging from birth deformities to tendinopathies to acute ruptures. The ability to characterize embryonic tendon mechanical properties will enable elucidation of mechanisms responsible for functional tendon formation. In turn, an understanding of tendon development could inform approaches for adult and embryonic tendon tissue engineering and regenerative medicine. The chick embryo is a scientifically relevant model that we have been using to study Achilles (calcaneal) tendon development. Chick embryo calcaneal tendons are challenging to mechanically test due to small size and delicate nature, and difficulty distinguishing embryonic tendons from muscle and fibrocartilage using the naked eye. Here, we developed and implemented a "marking protocol" to identify and isolate calcaneal tendons at different stages of chick embryonic development. Mechanical testing of tendons isolated using the marking protocol revealed trends in mechanical property development that were not observed with tendons isolated by naked eye (eyeballing). Marked tendons exhibited non-linear increases in tensile modulus and ultimate tensile strength, whereas eyeballed tendons exhibited linear increases in the same properties, reflecting a need for the marking protocol. Furthermore, the tensile mechanical properties characterized for marked tendons are consistent with previously reported trends in cell length-scale mechanical properties measured using atomic force microscopy. This report establishes new methodology to enable tensile testing of chick embryo tendons and provides new information about embryonic tendon mechanical property development.

Coordinate roles for collagen VI and biglycan in regulating tendon collagen fibril structure and function

Tendon is a vital musculoskeletal tissue that is prone to degeneration. Proper tendon maintenance requires complex interactions between extracellular matrix components that remain poorly understood. Collagen VI and biglycan are two matrix molecules that localize pericellularly within tendon and are critical regulators of tissue properties. While evidence suggests that collagen VI and biglycan interact within the tendon matrix, the relationship between the two molecules and its impact on tendon function remains unknown. We sought to elucidate potential coordinate roles of collagen VI and biglycan within tendon by defining tendon properties in knockout models of collagen VI, biglycan, or both molecules. We first demonstrated co-expression and co-localization of collagen VI and biglycan within the healing tendon, providing further evidence of cooperation between the two molecules during nascent tendon matrix formation. Deficiency in collagen VI and/or biglycan led to significant reductions in collagen fibril size and tendon mechanical properties. However, collagen VI-null tendons displayed larger reductions in fibril size and mechanics than seen in biglycan-null tendons. Interestingly, knockout of both molecules resulted in similar properties to collagen VI knockout alone. These results indicate distinct and non-additive roles for collagen VI and biglycan within tendon. This work provides better understanding of regulatory interactions between two critical tendon matrix molecules.

Elastic energy storage across speeds during steady-state hopping of desert kangaroo rats (Dipodomys deserti)

Small bipedal hoppers, including kangaroo rats, are thought to not benefit from substantial elastic energy storage and return during hopping. However, recent species-specific material properties research suggests that, despite relative thickness, the ankle extensor tendons of these small hoppers are considerably more compliant than had been assumed. With faster locomotor speeds demanding higher forces, a lower tendon stiffness suggests greater tendon deformation and thus a greater potential for elastic energy storage and return with increasing speed. Using the elastic modulus values specific to kangaroo rat tendons, we sought to determine how much elastic energy is stored and returned during hopping across a range of speeds. In vivo techniques were used to record tendon force in the ankle extensors during steady-speed hopping. Our data support the hypothesis that the ankle extensor tendons of kangaroo rats store and return elastic energy in relation to hopping speed, storing more at faster speeds. Despite storing comparatively less elastic energy than larger hoppers, this relationship between speed and energy storage offer novel evidence of a functionally similar energy storage mechanism, operating irrespective of body size or tendon thickness, across the distal muscle-tendon units of both small and large bipedal hoppers.

Tendon Extracellular Matrix Assembly, Maintenance and Dysregulation Throughout Life

In his Lissner Award medal lecture in 2000, Stephen Cowin asked the question: "How is a tissue built?" It is not a new question, but it remains as relevant today as it did when it was asked 20 years ago. In fact, research on the organization and development of tissue structure has been a primary focus of tendon and ligament research for over two centuries. The tendon extracellular matrix (ECM) is critical to overall tissue function; it gives the tissue its unique mechanical properties, exhibiting complex non-linear responses, viscoelasticity and flow mechanisms, excellent energy storage and fatigue resistance. This matrix also creates a unique microenvironment for resident cells, allowing cells to maintain their phenotype and translate mechanical and chemical signals into biological responses. Importantly, this architecture is constantly remodeled by local cell populations in response to changing biochemical (systemic and local disease or injury) and mechanical (exercise, disuse, and overuse) stimuli. Here, we review the current understanding of matrix remodeling throughout life, focusing on formation and assembly during the postnatal period, maintenance and homeostasis during adulthood, and changes to homeostasis in natural aging. We also discuss advances in model systems and novel tools for studying collagen and non-collagenous matrix remodeling throughout life, and finally conclude by identifying key questions that have yet to be answered.