The Connection of Composition, Structure, and Dynamic Processes to Tendon Mechanics: Structure-Function Relationships in Collagen V Deficient Tendons

Tendons are able to withstand the broad range of stresses and strains via their finely tuned composition and structure. In addition, tendons undergo a coordinated set of dynamic responses, specifically collagen uncrimping, re-alignment, sliding and deformation, within the matrix. To date, a complete understanding of the hierarchical structure-function relationships in tendon is lacking. Therefore, the overall goal of this thesis was to measure tendon structure and function in a mouse supraspinatus model of altered structure, and to analyze links between mechanical properties, dynamic processes and composition/structure using a series of statistical analyses. In the studies presented here, we used novel and established methods to measure the multi-scale composition, structure and mechanical function of mouse supraspinatus tendons from wild type, collagen V heterozygous and collagen V null mice. Overall, we found that the experimental groups were mechanically inferior to the wild type group, with larger changes in both macroscale function and the dynamic responses (re-alignment, crimp, deformation, sliding). In addition, while fibril morphology was altered at both locations, the insertion site also exhibited alterations in cell and fiber morphology as well as extracellular matrix composition. Finally, using a novel regression approach, we found that the contribution of composition and structure as well as the contribution of dynamic processes to determining macroscale mechanical function was highly dependent on location and that the dynamic processes were significant mediators of the relationship between composition/structure and mechanical properties. Overall, we conclude that although collagen V is a quantitatively minor component in mature tendon/ligament, it is a major regulator of composition and structure during development which ultimately leads to mechanical function. Furthermore, we conclude that the dynamic responses to load are crucial factors in ultimately determining regionally-dependent mechanical function. This information will help to guide clinicians in developing preventative techniques and appropriate rehabilitation strategies, as well as help to define the appropriate and important parameters on which to base tissue engineering efforts for tendon augmentation or replacement. Finally, this work presents a strong foundation on which to develop future experimental and modeling efforts in order to fully understand the complex structure-function relationships present in tendon

Senescence contributes to death resistance of aged tenocytes in a model of secondary joint damage

NIH/National Institutes of Health; The American Association of University Wome

Earlier proteoglycan turnover promotes higher efficiency matrix remodeling in MRL/MpJ tendons

While most mammalian tissue regeneration is limited, the Murphy Roths Large (MRL/MpJ) mouse has been identified to regenerate several tissues, including tendon. Recent studies have indicated that this regenerative response is innate to the tendon tissue and not reliant on a systemic inflammatory response. Therefore, we hypothesized that MRL/MpJ mice may also exhibit a more robust homeostatic regulation of tendon structure in response to mechanical loading. To assess this, MRL/MpJ and C57BL/6J Flexor digitorum longus tendon explants were subjected to stress-deprived conditions in vitro for up to 14 days. Explant tendon health (metabolism, biosynthesis, and composition), matrix metalloproteinase activity, gene expression, and tendon biomechanics were assessed periodically. We found a more robust response to a loss of mechanical stimulus in the MRL/MpJ tendon explants, exhibiting an increase in collagen production and MMP activity consistent with previous in vivo studies. This greater collagen turnover was preceded by an early expression of small leucine rich proteoglycans and proteoglycan-degrading MMP-3, promoting efficient regulation and organization of newly synthesized collagen and allowing for more efficient overall turnover in MRL/MpJ tendons. Therefore, mechanisms of MRL/MpJ matrix homeostasis may be fundamentally different from that of B6 tendons and may indicate better recovery from mechanical microdamage in MRL/MpJ tendons. We demonstrate here the utility of the MRL/MpJ model in elucidating mechanisms of efficient matrix turnover and its potential to shed light on new targets for more effective treatments for degenerative matrix changes brought about by injury, disease, or aging. This article is protected by copyright. All rights reserved.NIH/National Institutes of Healt

Differential effect of acute compressive injury on young and aged tendons

5R00AG063896-04 REVISED - NIH/National Institute on Agin

In situ AFM-based nanoscale rheology reveals regional non-uniformity in viscoporoelastic mechanical behavior of the murine periodontal ligament

The periodontal ligament (PDL) is a critical player in the maintenance of tooth health, acting as the primary stabilizer of tooth position. Recent studies have identified two unique regions within the PDL, the ‘dense collar’ region and the ‘furcation’ region, which exhibit distinct structural and compositional differences. However, specific functional differences between these regions have yet to be investigated. We adapted an AFM-based nanoscale rheology method to regionally assess mechanical properties and poroelasticity in the mouse PDL while minimizing the disruption of the 3-dimensional native boundary conditions, and then explored tissue mechanical function in four different regions within the dense collar as well as in the furcation region. We found significant differences between the collar and furcation regions, with the collar acting as a stabilizing ligamentous structure and the furcation acting as both a compressive cushion for vertical forces and a conduit for nutrient transport. While this finding supports our hypothesis, based on previous studies investigating structural and compositional differences, we also found surprising inhomogeneity within the collar region itself. This inhomogeneity supports previous findings of a tilting movement in the buccal direction of mandibular molar teeth and the structural adaptation to prevent lingual movement. Future work will aim to understand how different regions of the PDL change functionally during biological or mechanical perturbations, such as orthodontic tooth movement, development, or aging, with the ultimate goal of better understanding the mechanobiology of the PDL function in health and disease.Accepted manuscrip

Tendon exhibits complex poroelastic behavior at the nanoscale as revealed by high-frequency AFM-based rheology

Published in final edited form as: J Biomech. 2017 March 21; 54: 11–18. doi:10.1016/j.jbiomech.2017.01.029.Tendons transmit load from muscle to bone by utilizing their unique static and viscoelastic tensile properties. These properties are highly dependent on the composition and structure of the tissue matrix, including the collagen I hierarchy, proteoglycans, and water. While the role of matrix constituents in the tensile response has been studied, their role in compression, particularly in matrix pressurization via regulation of fluid flow, is not well understood. Injured or diseased tendons and tendon regions that naturally experience compression are known to have alterations in glycosaminoglycan content, which could modulate fluid flow and ultimately mechanical function. While recent theoretical studies have predicted tendon mechanics using poroelastic theory, no experimental data have directly demonstrated such behavior. In this study, we use high-bandwidth AFM-based rheology to determine the dynamic response of tendons to compressive loading at the nanoscale and to determine the presence of poroelastic behavior. Tendons are found to have significant characteristic dynamic relaxation behavior occurring at both low and high frequencies. Classic poroelastic behavior is observed, although we hypothesize that the full dynamic response is caused by a combination of flow-dependent poroelasticity as well as flow-independent viscoelasticity. Tendons also demonstrate regional dependence in their dynamic response, particularly near the junction of tendon and bone, suggesting that the structural and compositional heterogeneity in tendon may be responsible for regional poroelastic behavior. Overall, these experiments provide the foundation for understanding fluid-flow-dependent poroelastic mechanics of tendon, and the methodology is valuable for assessing changes in tendon matrix compressive behavior at the nanoscale.Accepted manuscrip

Bioinspired materials and tissue engineering approaches applied to the regeneration of musculoskeletal tissues

The musculoskeletal tissues have a prime role in the biomechanical support and metabolic activities of the human body. As musculoskeletal tissues are highly prone to injuries, conditions afflicting these tissues have a great impact on the quality of life of patients worldwide. Tissue engineering approaches hold the promise to develop bioengineered substitutes aiming at the regeneration of failing and injured tissue and organs. To effectively address the tissue-specific structural and biochemical features of musculoskeletal tissues, different biomaterials and techniques have been employed envisioning biomimetic solutions. Herein, the unique composition, structure, and function of the musculoskeletal tissues, namely bone, cartilage, and tendon, as well as state-of-the-art technologies to develop bioinspired strategies for tissue regeneration will be overviewed. Finally, this chapter will also discuss the unmet challenges and future perspectives in the field.FCT Project MagTT PTDC/CTM-CTM/29930/2017 (POCI-01- 0145-FEDER-29930) for A.I.G postdoc grant, the FCT Project PTDC/NAN-MAT/30595/2017 (POCI-01-0145-FEDER-30595) for P.S.B. postdoc grant, and for the assistant researcher contract (RL1) of M.T.R from the project “Accelerating tissue engineering and personalized medicine discoveries by the integration of key enabling nanotechnologies, marine-derived biomaterials and stem cells” supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). Authors acknowledge the financial support from the European Union Framework Programme for Research and Innovation HORIZON 2020, under the TEAMING Grant agreement No. 739572—The Discoveries CTR and the European Research Council 2017-CoG MagTendon (No. 772817