Advanced glycation end-products diminish tendon collagen fiber sliding

The instructive extracellular matrix of the lung: basic composition and alterations in chronic lung disease

The pulmonary extracellular matrix (ECM) determines the tissue architecture of the lung, and provides mechanical stability and elastic recoil, which are essential for physiological lung function. Biochemical and biomechanical signals initiated by the ECM direct cellular function and differentiation, and thus play a decisive role in lung development, tissue remodelling processes and maintenance of adult homeostasis. Recent proteomic studies have demonstrated that at least 150 different ECM proteins, glycosaminoglycans and modifying enzymes are expressed in the lung, and these assemble into intricate composite biomaterials. These highly insoluble assemblies of interacting ECM proteins and their glycan modifications can act as a solid phase-binding interface for hundreds of secreted proteins, which creates an information-rich signalling template for cell function and differentiation. Dynamic changes within the ECM that occur upon injury or with ageing are associated with several chronic lung diseases. In this review, we summarise the available data about the structure and function of the pulmonary ECM, and highlight changes that occur in idiopathic pulmonary fibrosis (IPF), pulmonary arterial hypertension (PAH), chronic obstructive pulmonary disease (COPD), asthma and lung cancer. We discuss potential mechanisms of ECM remodelling and modification, which we believe are relevant for future diagnosis and treatment of chronic lung disease.

Experimental evaluation of multiscale tendon mechanics

Tendon's primary function is a mechanical link between muscle and bone. The hierarchical structure of tendon and specific compositional constituents are believed to be critical for proper mechanical function. With increased appreciation for tendon importance and the development of various technological advances, this review paper summarizes recent experimental approaches that have been used to study multiscale tendon mechanics, includes an overview of studies that have evaluated the role of specific tissue constituents, and also proposes challenges/opportunities facing tendon study. Tendon has been demonstrated to have specific structural characteristics (e.g., multi-level hierarchy, crimp pattern, helix) and complex mechanical properties (e.g., non-linearity, anisotropy, viscoelasticity). Physical mechanisms including uncrimping, fiber sliding, and collagen reorganization have been shown to govern tendon mechanical responses under both static and dynamic loading. Several tendon constituents with relatively small quantities have been suggested to play a role in its mechanics, although some results are conflicting. Further research should be performed to understand the interplay and communication of tendon mechanical properties across levels of the hierarchical structure, and further show how each of these components contribute to tendon mechanics. The studies summarized and discussed in this review have helped elucidate important aspects of multiscale tendon mechanics, which is a prerequisite for analyzing stress/strain transfer between multiple scales and identifying key principles of mechanotransduction. This information could further facilitate interpreting the functional diversity of tendons from different species, different locations, and even different developmental stages, and then better understand and identify fundamental concepts related to tendon degeneration, disease, and healing. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1353-1365, 2017.

Methylglyoxal in Metabolic Disorders: Facts, Myths, and Promises

Glucose and fructose metabolism originates the highly reactive byproduct methylglyoxal (MG), which is a strong precursor of advanced glycation end products (AGE). The MG has been implicated in classical diabetic complications such as retinopathy, nephropathy, and neuropathy, but has also been recently associated with cardiovascular diseases and central nervous system disorders such as cerebrovascular diseases and dementia. Recent studies even suggested its involvement in insulin resistance and beta-cell dysfunction, contributing to the early development of type 2 diabetes and creating a vicious circle between glycation and hyperglycemia. Despite several drugs and natural compounds have been identified in the last years in order to scavenge MG and inhibit AGE formation, we are still far from having an effective strategy to prevent MG-induced mechanisms. This review summarizes the endogenous and exogenous sources of MG, also addressing the current controversy about the importance of exogenous MG sources. The mechanisms by which MG changes cell behavior and its involvement in type 2 diabetes development and complications and the pathophysiological implication are also summarized. Particular emphasis will be given to pathophysiological relevance of studies using higher MG doses, which may have produced biased results. Finally, we also overview the current knowledge about detoxification strategies, including modulation of endogenous enzymatic systems and exogenous compounds able to inhibit MG effects on biological systems.

Tendinopathy in diabetes mellitus patients-Epidemiology, pathogenesis, and management

Chronic tendinopathy is a frequent and disabling musculo-skeletal problem affecting the athletic and general populations. The affected tendon is presented with local tenderness, swelling, and pain which restrict the activity of the individual. Tendon degeneration reduces the mechanical strength and predisposes it to rupture. The pathogenic mechanisms of chronic tendinopathy are not fully understood and several major non-mutually exclusive hypotheses including activation of the hypoxia-apoptosis-pro-inflammatory cytokines cascade, neurovascular ingrowth, increased production of neuromediators, and erroneous stem cell differentiation have been proposed. Many intrinsic and extrinsic risk/causative factors can predispose to the development of tendinopathy. Among them, diabetes mellitus is an important risk/causative factor. This review aims to appraise the current literature on the epidemiology and pathology of tendinopathy in diabetic patients. Systematic reviews were done to summarize the literature on (a) the association between diabetes mellitus and tendinopathy/tendon tears, (b) the pathological changes in tendon under diabetic or hyperglycemic conditions, and (c) the effects of diabetes mellitus or hyperglycemia on the outcomes of tendon healing. The potential mechanisms of diabetes mellitus in causing and exacerbating tendinopathy with reference to the major non-mutually exclusive hypotheses of the pathogenic mechanisms of chronic tendinopathy as reported in the literature are also discussed. Potential strategies for the management of tendinopathy in diabetic patients are presented.

Changes in Cartilage and Tendon Composition of Patients With Type I Diabetes Mellitus: Identification by Quantitative Sodium Magnetic Resonance Imaging at 7 T

OBJECTIVE

The aim of this study was to investigate possible biochemical alterations in tendons and cartilage caused by type 1 diabetes mellitus (DM1), using quantitative in vivo 7 T sodium magnetic resonance (MR) imaging.

MATERIALS AND METHODS

The institutional review board approved this prospective study, and written informed consent was obtained. Eight DM1 patients with no history of knee trauma and 9 healthy age- and weight-matched volunteers were examined at 7 T using dedicated knee coils.All participants underwent morphological and sodium MR imaging. Region-of-interest analysis was performed manually for the non-weight-bearing area of the femoral condyle cartilage and for the patella tendon. Two readers read the image data sets independently, twice, for intrareader and interreader agreement. Normalized mean sodium signal intensity (NMSI) values were compared between patients and volunteers for each reader using analysis of variance.

RESULTS

On morphological images, cartilage in the non-weight-bearing area and the patellar tendon was intact in all patients. On sodium MR imaging, bivariate analysis of variance showed significantly lower mean NMSI values in the cartilage (P = 0.008) and significantly higher values in the tendons (P = 0.025) of patients compared with those of volunteers.

CONCLUSION

Our study showed significantly different NMSI values between DM1 patients and matched volunteers. Differences observed in the cartilage and tendon might be associated with a DM1-related alteration of biochemical composition that occurs before it can be visualized on morphological MR sequences.

Effect of aging and exercise on the tendon

Here, we review the literature on how tendons respond and adapt to ageing and exercise. With respect to aging, there are considerable changes early in life, but this seems to be maturation rather than aging per se. In vitro data indicate that aging is associated with a decreased potential for cell proliferation and a reduction in the number of stem/progenitor-like cells. Further, there is persuasive evidence that turnover in the core of the tendon after maturity is very slow or absent. Tendon fibril diameter, collagen content, and whole tendon size appear to be largely unchanged with aging, while glycation-derived cross-links increase substantially. Mechanically, aging appears to be associated with a reduction in modulus and strength. With respect to exercise, tendon cells respond by producing growth factors, and there is some support for a loading-induced increase in tendon collagen synthesis in humans, which likely reflects synthesis at the very periphery of the tendon rather than the core. Average collagen fibril diameter is largely unaffected by exercise, while there can be some hypertrophy of the whole tendon. In addition, it seems that resistance training can yield increased stiffness and modulus of the tendon and may reduce the amount of glycation. Exercise thereby tends to counteract the effects of aging.

Increased AGE-RAGE ratio in idiopathic pulmonary fibrosis

BACKGROUND

The abnormal epithelial-mesenchymal restorative capacity in idiopathic pulmonary fibrosis (IPF) has been recently associated with an accelerated aging process as a key point for the altered wound healing. The advanced glycation end-products (AGEs) are the consequence of non-enzymatic reactions between lipid and protein with several oxidants in the aging process. The receptor for AGEs (RAGEs) has been implicated in the lung fibrotic process and the alveolar homeostasis. However, this AGE-RAGE aging pathway has been under-explored in IPF.

METHODS

Lung samples from 16 IPF and 9 control patients were obtained through surgical lung biopsy. Differences in AGEs and RAGE expression between both groups were evaluated by RT-PCR, Western blot and immunohistochemistry. The effect of AGEs on cell viability of primary lung fibrotic fibroblasts and alveolar epithelial cells was assessed. Cell transformation of fibrotic fibroblasts cultured into glycated matrices was evaluated in different experimental conditions.

RESULTS

Our study demonstrates an increase of AGEs together with a decrease of RAGEs in IPF lungs, compared with control samples. Two specific AGEs involved in aging, pentosidine and Nε-Carboxymethyl lysine, were significantly increased in IPF samples. The immunohistochemistry identified higher staining of AGEs related to extracellular matrix (ECM) proteins and the apical surface of the alveolar epithelial cells (AECs) surrounding fibroblast foci in fibrotic lungs. On the other hand, RAGE location was present at the cell membrane of AECs in control lungs, while it was almost missing in pulmonary fibrotic tissue. In addition, in vitro cultures showed that the effect of AGEs on cell viability was different for AECs and fibrotic fibroblasts. AGEs decreased cell viability in AECs, even at low concentration, while fibroblast viability was less affected. Furthermore, fibroblast to myofibroblast transformation could be enhanced by ECM glycation.

CONCLUSIONS

All of these findings suggest a possible role of the increased ratio AGEs-RAGEs in IPF, which could be a relevant accelerating aging tissue reaction in the abnormal wound healing of the lung fibrotic process.

Advanced glycation end-products: Mechanics of aged collagen from molecule to tissue

Concurrent with a progressive loss of regenerative capacity, connective tissue aging is characterized by a progressive accumulation of Advanced Glycation End-products (AGEs). Besides being part of the typical aging process, type II diabetics are particularly affected by AGE accumulation due to abnormally high levels of systemic glucose that increases the glycation rate of long-lived proteins such as collagen. Although AGEs are associated with a wide range of clinical disorders, the mechanisms by which AGEs contribute to connective tissue disease in aging and diabetes are still poorly understood. The present study harnesses advanced multiscale imaging techniques to characterize a widely employed in vitro model of ribose induced collagen aging and further benchmarks these data against experiments on native human tissues from donors of different age. These efforts yield unprecedented insight into the mechanical changes in collagen tissues across hierarchical scales from molecular, to fiber, to tissue-levels. We observed a linear increase in molecular spacing (from 1.45nm to 1.5nm) and a decrease in the D-period length (from 67.5nm to 67.1nm) in aged tissues, both using the ribose model of in vitro glycation and in native human probes. Multiscale mechanical analysis of in vitro glycated tendons strongly suggests that AGEs reduce tissue viscoelasticity by severely limiting fiber-fiber and fibril-fibril sliding. This study lays an important foundation for interpreting the functional and biological effects of AGEs in collagen connective tissues, by exploiting experimental models of AGEs crosslinking and benchmarking them for the first time against endogenous AGEs in native tissue.

Factors affecting the range of motion of the ankle and first metatarsophalangeal joints in patients undergoing hemodialysis who walk daily

[Purpose] Increased plantar pressure during walking is a risk factor for foot ulcers because of reduced range of motion at the ankle and first metatarsophalangeal joints. However, the range of motion in patients undergoing hemodialysis has not yet been determined. A cross-sectional study was performed to investigate the factors affecting the range of motion of the ankle and first metatarsophalangeal joints in patients undergoing hemodialysis who walk daily. [Subjects and Methods] Seventy feet of 35 patients receiving hemodialysis therapy were examined. Measurements included the passive range of motion of plantar flexion and dorsiflexion of the ankle joint, and flexion and extension of the first metatarsophalangeal joint. [Results] Hemodialysis duration was not associated with ankle and first metatarsophalangeal joint range of motion in patients undergoing hemodialysis. Diabetes duration was significantly associated with limited ankle joint mobility. Finally, blood hemoglobin levels, body mass index, and age were associated with first metatarsophalangeal joint range of motion. [Conclusion] The present study identified age, diabetes, and decreased physical activity, but not hemodialysis duration, to be risk factors for limited joint mobility of the ankle and first metatarsophalangeal joints in patients undergoing hemodialysis.

Ultrastructural Changes Associated with Reversible Stiffening in Catch Connective Tissue of Sea Cucumbers

The dermis of sea cucumbers is a catch connective tissue or a mutable collagenous tissue that shows rapid, large and reversible stiffness changes in response to stimulation. The main component of the dermis is the extracellular material composed of collagen fibrils embedded in a hydrogel of proteoglycans. The stiffness of the extracellular material determines that of the dermis. The dermis has three mechanical states: soft (Sa), standard (Sb) and stiff (Sc). We studied the ultrastructural changes associated with the stiffness changes. Transverse sections of collagen fibrils in the dermis showed irregular perimeters with electron-dense protrusions or arms that cross-bridged between fibrils. The number of cross-bridges increased in stiffer dermis. The distance between the fibrils was shorter in Sc than that in other states, which was in accord with the previous report that water exuded from the tissue in the transition Sb→Sc. The ultrastructure of collagen fibrils that had been isolated from the dermis was also studied. Fibrils aggregated by tensilin, which causes the transition Sa→Sb possibly through an increase in cohesive forces between fibrils, had larger diameter than those dispersed by softenin, which antagonizes the effect of tensilin. No cross-bridges were found in isolated collagen fibrils. From the present ultrastructural study we propose that three different mechanisms work together to increase the dermal stiffness. 1.Tensilin makes collagen fibrils stronger and stiffer in Sa→Sb through an increase in cohesive forces between subfibrils that constituted fibrils; 2. Cross-bridging by arms caused the fibrils to be a continuous network of bundles both in Sa→Sb and in Sb→Sc; 3. The matrix embedding the fibril network became stiffer in Sb→Sc, which was produced by bonding associated with water exudation.

Hormones and tendinopathies: the current evidence

BACKGROUND

Tendinopathies negatively affect the quality of life of millions of people, but we still do not know the factors involved in the development of tendon conditions.

SOURCES OF DATA

Published articles in English in PubMed and Google Scholar up to June 2015 about hormonal influence on tendinopathies onset. One hundred and two papers were included following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines.

AREAS OF AGREEMENT

In vitro and in vivo, tenocytes showed changes in their morphology and in their functional properties according to hormonal imbalances.

AREAS OF CONTROVERSY

Genetic pattern, sex, age and comorbidities can influence the hormonal effect on tendons.

GROWING POINTS

The increasing prevalence of metabolic disorders prompts to investigate the possible connection between metabolic problems and musculoskeletal diseases.

AREAS TIMELY FOR DEVELOPING RESEARCH

The influence of hormones on tendon structure and metabolism needs to be further investigated. If found to be significant, multidisciplinary preventive and therapeutic strategies should then be developed.

Modelling approaches for evaluating multiscale tendon mechanics

Tendon exhibits anisotropic, inhomogeneous and viscoelastic mechanical properties that are determined by its complicated hierarchical structure and varying amounts/organization of different tissue constituents. Although extensive research has been conducted to use modelling approaches to interpret tendon structure-function relationships in combination with experimental data, many issues remain unclear (i.e. the role of minor components such as decorin, aggrecan and elastin), and the integration of mechanical analysis across different length scales has not been well applied to explore stress or strain transfer from macro- to microscale. This review outlines mathematical and computational models that have been used to understand tendon mechanics at different scales of the hierarchical organization. Model representations at the molecular, fibril and tissue levels are discussed, including formulations that follow phenomenological and microstructural approaches (which include evaluations of crimp, helical structure and the interaction between collagen fibrils and proteoglycans). Multiscale modelling approaches incorporating tendon features are suggested to be an advantageous methodology to understand further the physiological mechanical response of tendon and corresponding adaptation of properties owing to unique in vivo loading environments.

A preliminary technical study on sodium dodecyl sulfate-induced changes of the nano-structural and macro-mechanical properties in human iliotibial tract specimens

INTRODUCTION

Acellular scaffolds are frequently used for the surgical repair of ligaments and tendons. Even though data on the macro-mechanical properties related to the acellularization process exist, corresponding data on the nano-structural properties are still lacking. Such data would help identify target proteins of the formed extracellular matrix that are chemically altered by the acellularization. In this study we examined the altered structure by comparing molecular properties of collagens from native and acellular iliotibial tract samples to the macroscopic stress-strain behavior of tract samples.

MATERIAL AND METHODS

Matched pairs of five human iliotibial tract samples were obtained from five donors (mean age 28.2±4.7 years). One of each pair was acellularized using 1vol% sodium dodecyl sulfate (SDS) for 7 days. (13)C magic angle spinning nuclear magnetic resonance spectroscopy ((13)C CP MAS NMR) was utilized to compare the collagen overall secondary structure and internal dynamics of collagen-typical amino acid proteins. The resulting data was compared to age-matched stress-strain data of tract samples obtained in an uniaxial tensile setup and histologically.

RESULTS

Typical and nearly identical collagen (13)C CP MAS NMR spectra were found in the tract samples before and after acellularization with SDS. The characteristic collagen backbone remained intact in the native and acellular samples. Collagen molecular composition was largely unaltered in both conditions. Furthermore, a similar dynamic behavior was found for the amino acids Hyp γ, Pro α/Hyp α, Ala α, Gly α and Ala β. These minute alterations in the collagens' molecular properties related to acellularization with SDS were in line with the similarly minute changes in the macro-mechanical tensile behavior, such as the elastic modulus and ultimate stress. Histology showed intact type I collagens, minute amounts of elastins before and after acellularization and evidence for acellularization-induced reductions of proteoglycans.

DISCUSSION

Nano-structural properties of collagens are minutely affected by SDS treatment for acellularization, indicated by the molecular composition and dynamics. The lacking acellularization-related changes in the molecular structure properties of collagens in iliotibial tract samples are in line with the small alterations in their macro-mechanical tensile behavior. Though the given setup approaches soft tissue mechanics from both scaling extremes of mechanical testing, further structural analyzes are needed in a larger sample size to substantiate these findings.

How High Glucose Levels Affect Tendon Homeostasis

Among the many factors playing a role in tendon disease, unregulated biochemical reactions between glucose and the collagen extracellular matrix are coming increasingly into focus. We have shown that formation of advanced glycation end-products that cross-link the collagen extracellular matrix can drastically affect cellular level mechanical properties of the matrix, and in turn affect cell-level biomechanical stimuli during physiological loading of the tissue. We suggest that these may adversely affect tendon cell response to matrix damage, as well as the quality of the consequent repair. If such mechanical feedback loops are altered, the ability of tendon cells to maintain tissue in a functional, healthy state may be compromised. Although key foundational elements of biochemical, biomechanical, and biological understanding are now in place, the full extent of how these aspects interact, including the precise mechanisms by which advanced glycation end-products pathologically disrupt connective tissue homeostasis and damage repair, are only beginning to be adequately appreciated.

Rehabilitation of Tendon Problems in Patients with Diabetes Mellitus

Exercise is crucial in the management of diabetes mellitus and its associated complications. However, individuals with diabetes have a heightened risk of musculoskeletal problems, including tendon pathologies. Diabetes has a significant impact on the function of tendons due to the accumulation of advanced glycation end-products in the load-bearing collagen. In addition, tendon vascularity and healing may be reduced due to diabetes-induced changes in the peripheral vascular system, and impaired synthesis of collagen and glycosaminoglycan. The current chapter presents an evidence-based discussion of considerations for the rehabilitation of tendon problems in people with diabetes. The following conditions are discussed in detail - calcific tendinopathy, tenosynovitis, tendon rupture, and non-calcifying tendinopathy. Common diabetes-related findings are presented, along with their potential impact on tendinopathy management and suggested modifications to standard tendinopathy treatment protocols. A holistic approach should be used to optimize musculotendinous function, including a comprehensive exercise prescription addressing strength, flexibility, and aerobic fitness.

Capturing relevant extracellular matrices for investigating cell migration

Much progress in understanding cell migration has been determined by using classic two-dimensional (2D) tissue culture platforms. However, increasingly, it is appreciated that certain properties of cell migration in vivo are not represented by strictly 2D assays. There is much interest in creating relevant three-dimensional (3D) culture environments and engineered platforms to better represent features of the extracellular matrix and stromal microenvironment that are not captured in 2D platforms. Important to this goal is a solid understanding of the features of the extracellular matrix—composition, stiffness, topography, and alignment—in different tissues and disease states and the development of means to capture these features

Human Achilles tendon glycation and function in diabetes

Diabetic patients have an increased risk of foot ulcers, and glycation of collagen may increase tissue stiffness. We hypothesized that the level of glycemic control (glycation) may affect Achilles tendon stiffness, which can influence gait pattern. We therefore investigated the relationship between collagen glycation, Achilles tendon stiffness parameters, and plantar pressure in poorly (n = 22) and well (n = 22) controlled diabetic patients, including healthy age-matched (45-70 yr) controls (n = 11). There were no differences in any of the outcome parameters (collagen cross-linking or tendon stiffness) between patients with well-controlled and poorly controlled diabetes. The overall effect of diabetes was explored by collapsing the diabetes groups (DB) compared with the controls. Skin collagen cross-linking lysylpyridinoline, hydroxylysylpyridinoline (136%, 80%, P < 0.01) and pentosidine concentrations (55%, P < 0.05) were markedly greater in DB. Furthermore, Achilles tendon material stiffness was higher in DB (54%, P < 0.01). Notably, DB also demonstrated higher forefoot/rearfoot peak-plantar-pressure ratio (33%, P < 0.01). Overall, Achilles tendon material stiffness and skin connective tissue cross-linking were greater in diabetic patients compared with controls. The higher foot pressure indicates that material stiffness of tendon and other tissue (e.g., skin and joint capsule) may influence foot gait. The difference in foot pressure distribution may contribute to the development of foot ulcers in diabetic patients.

Relationship Between Skin Intrinsic Fluorescence--an Indicator of Advanced Glycation End Products-and Upper Extremity Impairments in Individuals With Diabetes Mellitus

BACKGROUND

Accumulation of advanced glycation end products (AGEs) is thought to contribute to limited joint mobility in people with diabetes mellitus (DM), but the relationships among AGEs, shoulder structural changes, movement, and disability are not understood.

OBJECTIVE

The purpose of this study was to determine the differences and relationships among skin intrinsic fluorescence (SIF), a proxy measure of AGEs, biceps and supraspinatus tendon thickness, upper extremity movement, and disability in groups with and without DM.

DESIGN

This was a cross-sectional, case-control study.

METHODS

Fifty-two individuals participated: 26 with type 2 DM and 26 controls matched for sex, age, and body mass index. The main outcome measures were: SIF; biceps and supraspinatus tendon thickness; 3-dimensional peak shoulder motion; and Disability of the Arm, Shoulder and Hand (DASH) questionnaire scores.

RESULTS

Mean SIF measurements were 19% higher in the DM group compared with the control group (P<.05). Biceps tendons (mean and 95% confidence interval [CI]) (4.7 mm [4.4, 5.0] versus 3.2 mm [2.9, 3.5]) and supraspinatus tendons (6.4 mm [5.9, 6.8] versus 4.9 mm [4.4, 5.3]) were thicker and peak humerothoracic elevation (139° [135°, 146°] versus 150° [146°, 155°]) and glenohumeral external rotation (35° [26°, 46°] versus 51° [41°, 58°]) were reduced in the DM group compared with the control group (P<.05). In the DM group, SIF was correlated to biceps tendon thickness, DASH score, and shoulder motion (r=.44-.51, P<.05). The SIF score and shoulder strength explained 64% of the DASH scores (P<.01).

LIMITATIONS

Because this was a cross-sectional study design, a cause-effect relationship could not be established.

CONCLUSIONS

Accumulation of AGEs in the connective tissues of individuals with DM appears to be associated with increased tendon thickness and decreased shoulder joint mobility and upper extremity function. Physical therapists should be aware of these possible metabolic effects on structure, movement, and disability when treating people with diabetes.

Molecular and intermolecular effects in collagen fibril mechanics: a multiscale analytical model compared with atomistic and experimental studies

Both atomistic and experimental studies reveal the dependence of collagen fibril mechanics on biochemical and biophysical features such as, for instance, cross-link density, water content and protein sequence. In order to move toward a multiscale structural description of biological tissues, a novel analytical model for collagen fibril mechanics is herein presented. The model is based on a multiscale approach that incorporates and couples: thermal fluctuations in collagen molecules; the uncoiling of collagen triple helix; the stretching of molecular backbone; the straightening of the telopeptide in which covalent cross-links form; slip-pulse mechanisms due to the rupture of intermolecular weak bonds; molecular interstrand delamination due to the rupture of intramolecular weak bonds; the rupture of covalent bonds within molecular strands. The effectiveness of the proposed approach is verified by comparison with available atomistic results and experimental data, highlighting the importance of cross-link density in tuning collagen fibril mechanics. The typical three-region shape and hysteresis behavior of fibril constitutive response, as well as the transition from a yielding-like to a brittle-like behavior, are recovered with a special insight on the underlying nanoscale mechanisms. The model is based on parameters with a clear biophysical and biochemical meaning, resulting in a promising tool for analyzing the effect of pathological or pharmacological-induced histochemical alterations on the functional mechanical response of collagenous tissues.

Shear loads induce cellular damage in tendon fascicles

Tendon is vital to musculoskeletal function, transferring loads from muscle to bone for joint motion and stability. It is an anisotropic, highly organized, fibrous structure containing primarily type I collagen in addition to tenocytes and other extracellular matrix components contributing to maintenance and function. Tendon is generally loaded via normal stress in a longitudinal direction. However, certain situations, including fiber breakage, enzymatic remodeling, or tendon pathology may introduce various degrees of other loading modalities, such as shear-lag at the fiber level, potentially affecting cellular response and subsequent function. Fascicles from rat tail tendon were dissected and placed in one of three paired groups: intact, single laceration, or double laceration. Each pair had a mechanically tested and control specimen. Single laceration fascicles contained one transverse laceration to mimic a partial tear. Double laceration fascicles had overlapping, longitudinally separated lacerations on opposite sides to cause intra-fascicular shear transfer to be the primary mechanism of loading. Elastic properties of the fascicle, e.g. peak load, steady state load, and stiffness, decreased from intact to single laceration to double laceration groups. Surprisingly, 45% of the intact strength was maintained when shear was the primary internal load transfer mechanism. Cellular viability decreased after mechanical testing in both laceration groups; cell death appeared primarily in a longitudinal plane where high shear load transfer occurred. This cell death extended far from the injury site and may further compromise an already damaged tendon via enzymatic factors and subsequent remodeling associated with cell necrosis.

The adverse effects of diabetes on osteoarthritis: update on clinical evidence and molecular mechanisms

Projected increases in the prevalence of both diabetes mellitus (DM) and osteoarthritis (OA) ensure their common co-existence. In an era of increasing attention to personalized medicine, understanding the influence of common comorbidities such as DM should result in improved care of patients with OA. In this narrative review, we summarize the literature addressing the interactions between DM and OA spanning the years from 1962 to 2014. We separated studies depending on whether they investigated clinical populations, animal models, or cells and tissues. The clinical literature addressing the influence of DM on OA and its therapeutic outcomes suggests that DM may augment the development and severity of OA and that DM increases risks associated with joint replacement surgery. The few high quality studies using animal models also support an adverse effect of DM on OA. We review strengths and weaknesses of the common rodent models of DM. The heterogeneous literature derived from studies of articular cells and tissues also supports the existence of biochemical and biomechanical changes in articular tissues in DM, and begins to characterize molecular mechanisms activated in diabetic-like environs which may contribute to OA. Increasing evidence from the clinic and the laboratory supports an adverse effect of DM on the development, severity, and therapeutic outcomes for OA. To understand the mechanisms through which DM contributes to OA, further studies are clearly necessary. Future studies of DM-influenced mechanisms may shed light on general mechanisms of OA pathogenesis and result in more specific and effective therapies for all OA patients.

Shear load transfer in high and low stress tendons

BACKGROUND

Tendon is an integral part of joint movement and stability, as it functions to transmit load from muscle to bone. It has an anisotropic, fibrous hierarchical structure that is generally loaded in the direction of its fibers/fascicles. Internal load distributions are altered when joint motion rotates an insertion site or when local damage disrupts fibers/fascicles, potentially causing inter-fiber (or inter-fascicular) shear. Tendons with different microstructures (helical versus linear) may redistribute loads differently.

METHOD OF APPROACH

This study explored how shear redistributes axial loads in rat tail tendon (low stress tendons with linear microstructure) and porcine flexor tendon (high stress with helical microstructure) by creating lacerations on opposite sides of the tendon, ranging from about 20% to 60% of the tendon width, to create various magnitudes of shear. Differences in fascicular orientation were quantified using polarized light microscopy.

RESULTS AND CONCLUSIONS

Unexpectedly, both tendon types maintained about 20% of pre-laceration stress values after overlapping cuts of 60% of tendon width (no intact fibers end to end) suggesting that shear stress transfer can contribute more to overall tendon strength and stiffness than previously reported. All structural parameters for both tendon types decreased linearly with increasing laceration depth. The tail tendon had a more rapid decline in post-laceration elastic stress and modulus parameters as well as a more linear and less tightly packed fascicular structure, suggesting that positional tendons may be less well suited to redistribute loads via a shear mechanism.

Achilles tendons in people with type 2 diabetes show mildly compromised structure: an ultrasound tissue characterisation study

BACKGROUND

Musculotendinous overuse injuries are prevalent in people with type 2 diabetes. Non-enzymatic glycosylation of collagen resulting in tendon stiffening may play a role. In this case-control study we determined whether patients with diabetes had poorer ultrasonographic structure in their Achilles tendons compared to age-matched controls.

METHODS

People with type 1 diabetes or type 2 diabetes, and age-matched controls, had computerised ultrasound tissue characterisation of both Achilles tendons. In contiguous ultrasonographic images of the tendon, echopatterns were quantified and categorised into four echo-types. Tendon abnormality was quantified as sum of echo-types III+IV. Furthermore, skin autofluorescence (AF) of the forearm (AF-value) was gathered.

RESULTS

Twenty four type 2 diabetes patients, 24 controls, 24 type 1 diabetes patients and 20 controls were included. AF-value was higher in type 1 diabetes (1.55±0.17) than in their controls (1.39±0.18, p<0.001) and in type 2 diabetes (2.28±0.38) compared to their controls (1.84±0.32, p<0.001) Achilles tendons of type 2 diabetes patients contained more echo-types III+IV (14.1±7.9%) than matched controls (8.0±5.4%, p<0.001). There was a trend towards a difference in echo-types III+IV between type 1 diabetes patients (9.5±5.3%) and their controls (6.5±3.7%, p=0.055). In a stepwise linear regression analysis, body mass index (BMI) was moderately associated with tendon abnormality in patients with diabetes and controls (β=0.393, p<0.001).

CONCLUSIONS

Type 2, and possibly type 1, diabetes patients showed poorer ultrasonographic Achilles tendon structure that may be a risk factor for tendinopathy. Although markers for accumulation of advanced glycation end products were elevated in both diabetes populations, only BMI was associated with these abnormalities.

TRIAL REGISTRATION NUMBER

NTR2209.

Advanced glycation end-products reduce collagen molecular sliding to affect collagen fibril damage mechanisms but not stiffness

Advanced glycation end-products (AGE) contribute to age-related connective tissue damage and functional deficit. The documented association between AGE formation on collagens and the correlated progressive stiffening of tissues has widely been presumed causative, despite the lack of mechanistic understanding. The present study investigates precisely how AGEs affect mechanical function of the collagen fibril--the supramolecular functional load-bearing unit within most tissues. We employed synchrotron small-angle X-ray scattering (SAXS) and carefully controlled mechanical testing after introducing AGEs in explants of rat-tail tendon using the metabolite methylglyoxal (MGO). Mass spectrometry and collagen fluorescence verified substantial formation of AGEs by the treatment. Associated mechanical changes of the tissue (increased stiffness and failure strength, decreased stress relaxation) were consistent with reports from the literature. SAXS analysis revealed clear changes in molecular deformation within MGO treated fibrils. Underlying the associated increase in tissue strength, we infer from the data that MGO modified collagen fibrils supported higher loads to failure by maintaining an intact quarter-staggered conformation to nearly twice the level of fibril strain in controls. This apparent increase in fibril failure resistance was characterized by reduced side-by-side sliding of collagen molecules within fibrils, reflecting lateral molecular interconnectivity by AGEs. Surprisingly, no change in maximum fibril modulus (2.5 GPa) accompanied the changes in fibril failure behavior, strongly contradicting the widespread assumption that tissue stiffening in ageing and diabetes is directly related to AGE increased fibril stiffness. We conclude that AGEs can alter physiologically relevant failure behavior of collagen fibrils, but that tissue level changes in stiffness likely occur at higher levels of tissue architecture.

In situ fibril stretch and sliding is location-dependent in mouse supraspinatus tendons

Tendons are able to transmit high loads efficiently due to their finely optimized hierarchical collagen structure. Two mechanisms by which tendons respond to load are collagen fibril sliding and deformation (stretch). Although many studies have demonstrated that regional variations in tendon structure, composition, and organization contribute to the full tendon׳s mechanical response, the location-dependent response to loading at the fibril level has not been investigated. In addition, the instantaneous response of fibrils to loading, which is clinically relevant for repetitive stretch or fatigue injuries, has also not been studied. Therefore, the purpose of this study was to quantify the instantaneous response of collagen fibrils throughout a mechanical loading protocol, both in the insertion site and in the midsubstance of the mouse supraspinatus tendon. Utilizing a novel atomic force microscopy-based imaging technique, tendons at various strain levels were directly visualized and analyzed for changes in fibril d-period with increasing tendon strain. At the insertion site, d-period significantly increased from 0% to 1% tendon strain, increased again from 3% to 5% strain, and decreased after 5% strain. At the midsubstance, d-period increased from 0% to 1% strain and then decreased after 7% strain. In addition, fibril d-period heterogeneity (fibril sliding) was present, primarily at 3% strain with a large majority occurring in the tendon midsubstance. This study builds upon previous work by adding information on the instantaneous and regional-dependent fibrillar response to mechanical loading and presents data proposing that collagen fibril sliding and stretch are directly related to tissue organization and function.

Incorporating plasticity of the interfibrillar matrix in shear lag models is necessary to replicate the multiscale mechanics of tendon fascicles

Despite current knowledge of tendon structure, the fundamental deformation mechanisms underlying tendon mechanics and failure are unknown. We recently showed that a shear lag model, which explicitly assumed plastic interfibrillar load transfer between discontinuous fibrils, could explain the multiscale fascicle mechanics, suggesting that fascicle yielding is due to plastic deformation of the interfibrillar matrix. However, it is unclear whether alternative physical mechanisms, such as elastic interfibrillar deformation or fibril yielding, also contribute to fascicle mechanical behavior. The objective of the current work was to determine if plasticity of the interfibrillar matrix is uniquely capable of explaining the multiscale mechanics of tendon fascicles including the tissue post-yield behavior. This was examined by comparing the predictions of a continuous fibril model and three separate shear lag models incorporating an elastic, plastic, or elastoplastic interfibrillar matrix with multiscale experimental data. The predicted effects of fibril yielding on each of these models were also considered. The results demonstrated that neither the continuous fibril model nor the elastic shear lag model can successfully predict the experimental data, even if fibril yielding is included. Only the plastic or elastoplastic shear lag models were capable of reproducing the multiscale tendon fascicle mechanics. Differences between these two models were small, although the elastoplastic model did improve the fit of the experimental data at low applied tissue strains. These findings suggest that while interfibrillar elasticity contributes to the initial stress response, plastic deformation of the interfibrillar matrix is responsible for tendon fascicle post-yield behavior. This information sheds light on the physical processes underlying tendon failure, which is essential to improve our understanding of tissue pathology and guide the development of successful repair.

Influence of cross-link structure, density and mechanical properties in the mesoscale deformation mechanisms of collagen fibrils

Collagen is a ubiquitous protein with remarkable mechanical properties. It is highly elastic, shows large fracture strength and enables substantial energy dissipation during deformation. Most of the connective tissue in humans consists of collagen fibrils composed of a staggered array of tropocollagen molecules, which are connected by intermolecular cross-links. In this study, we report a three-dimensional coarse-grained model of collagen and analyze the influence of enzymatic cross-links on the mechanics of collagen fibrils. Two representatives immature and mature cross-links are implemented in the mesoscale model using a bottom-up approach. By varying the number, type and mechanical properties of cross-links in the fibrils and performing tensile test on the models, we systematically investigate the deformation mechanisms of cross-linked collagen fibrils. We find that cross-linked fibrils exhibit a three phase behavior, which agrees closer with experimental results than what was obtained using previous models. The fibril mechanical response is characterized by: (i) an initial elastic deformation corresponding to the collagen molecule uncoiling, (ii) a linear regime dominated by molecule sliding and (iii) the second stiffer elastic regime related to the stretching of the backbone of the tropocollagen molecules until the fibril ruptures. Our results suggest that both cross-link density and type dictate the stiffness of large deformation regime by increasing the number of interconnected molecules while cross-links mechanical properties determine the failure strain and strength of the fibril. These findings reveal that cross-links play an essential role in creating an interconnected fibrillar material of tunable toughness and strength.

The extent of ligament injury and its influence on pelvic stability following type II anteroposterior compression pelvic injuries--A computer study to gain insight into open book trauma

Surgical stabilization of the pelvis following type II anteroposterior compression pelvic injuries (APCII) is based on the assumption that the anterior sacroiliac, sacrospinous, and sacrotuberous ligaments disrupt simultaneously. Recent data on the ligaments contradict this concept. We aimed at determining the mechanisms of ligament failure in APCII computationally. In an individual osteoligamentous computer model of the pelvis, ligament load, and strain were observed for the two-leg stance, APCII with 100-mm symphyseal widening and for two-leg stance with APCII-related ligament failure, and validated with body donors. The anterior sacroiliac and sacrotuberous ligaments had the greatest load with 80% and 17% of the total load, respectively. APCII causes partial failure of the anterior sacroiliac ligament and the pelvis to become horizontally instable. The other ligaments remained intact. The sacrospinous ligament was negligibly loaded but stabilized the pelvis vertically. The interosseous sacroiliac and sacrotuberous ligaments are likely responsible for reducing the symphysis and might serve as an indicator of vertical stability. The sacrospinous ligament appears to be of minor significance in APCII but plays an important role in vertical stabilization. Further research is necessary to determine the influence of alterations in ligament and bone material properties.

DTAF dye concentrations commonly used to measure microscale deformations in biological tissues alter tissue mechanics

Identification of the deformation mechanisms and specific components underlying the mechanical function of biological tissues requires mechanical testing at multiple levels within the tissue hierarchical structure. Dichlorotriazinylaminofluorescein (DTAF) is a fluorescent dye that is used to visualize microscale deformations of the extracellular matrix in soft collagenous tissues. However, the DTAF concentrations commonly employed in previous multiscale experiments (≥2000 µg/ml) may alter tissue mechanics. The objective of this study was to determine whether DTAF affects tendon fascicle mechanics and if a concentration threshold exists below which any observed effects are negligible. This information is valuable for guiding the continued use of this fluorescent dye in future experiments and for interpreting the results of previous work. Incremental strain testing demonstrated that high DTAF concentrations (≥100 µg/ml) increase the quasi-static modulus and yield strength of rat tail tendon fascicles while reducing their viscoelastic behavior. Subsequent multiscale testing and modeling suggests that these effects are due to a stiffening of the collagen fibrils and strengthening of the interfibrillar matrix. Despite these changes in tissue behavior, the fundamental deformation mechanisms underlying fascicle mechanics appear to remain intact, which suggests that conclusions from previous multiscale investigations of strain transfer are still valid. The effects of lower DTAF concentrations (≤10 µg/ml) on tendon mechanics were substantially smaller and potentially negligible; nevertheless, no concentration was found that did not at least slightly alter the tissue behavior. Therefore, future studies should either reduce DTAF concentrations as much as possible or use other dyes/techniques for measuring microscale deformations.

Interfibrillar shear stress is the loading mechanism of collagen fibrils in tendon

Despite the critical role tendons play in transmitting loads throughout the musculoskeletal system, little is known about the microstructural mechanisms underlying their mechanical function. Of particular interest is whether collagen fibrils in tendon fascicles bear load independently or if load is transferred between fibrils through interfibrillar shear forces. We conducted multiscale experimental testing and developed a microstructural shear lag model to explicitly test whether interfibrillar shear load transfer is indeed the fibrillar loading mechanism in tendon. Experimental correlations between fascicle macroscale mechanics and microscale interfibrillar sliding suggest that fibrils are discontinuous and share load. Moreover, for the first time, we demonstrate that a shear lag model can replicate the fascicle macroscale mechanics as well as predict the microscale fibrillar deformations. Since interfibrillar shear stress is the fundamental loading mechanism assumed in the model, this result provides strong evidence that load is transferred between fibrils in tendon and possibly other aligned collagenous tissues. Conclusively establishing this fibrillar loading mechanism and identifying the involved structural components should help develop repair strategies for tissue degeneration and guide the design of tissue engineered replacements.

Dose- and time-dependent effects of genipin crosslinking on cell viability and tissue mechanics - toward clinical application for tendon repair

The crosslinking agent genipin is increasingly invoked for the mechanical augmentation of collagen tissues and implants, and has previously been demonstrated to arrest mechanical damage accumulation in various tissues. This study established an in vitro dose-response baseline for the effects of genipin treatment on tendon cells and their matrix, with a view to in vivo application to the repair of partial tendon tears. Regression models based on a broad range of experimental data were used to delineate the range of concentrations that are likely to achieve functionally effective crosslinking, and predict the corresponding degree of cell loss and diminished metabolic activity that can be expected. On these data, it was concluded that rapid mechanical augmentation of tissue properties can only be achieved by accepting some degree of cytotoxicity, yet that post-treatment cell survival may be adequate to eventually repopulate and stabilize the tissue. On this basis, development of delivery strategies and subsequent in vivo study seems warranted.

Nanoscale changes in collagen are reflected in physical and mechanical properties of bone at the microscale in diabetic rats

Diabetes detrimentally affects the musculoskeletal system by stiffening the collagen matrix due to increased advanced glycation end products (AGEs). In this study, tibiae and tendon from Zucker diabetic Sprague-Dawley (ZDSD) rats were compared to Sprague-Dawley derived controls (CD) using Atomic Force Microscopy. ZDSD and CD tibiae were compared using Raman Spectroscopy and Reference Point Indentation (RPI). ZDSD bone had a significantly different distribution of collagen D-spacing than CD (p=0.015; ZDSD n=294 fibrils; CD n=274 fibrils) which was more variable and shifted to higher values. This shift between ZDSD and CD D-spacing distribution was more pronounced in tendon (p<0.001; ZDSD n=350; CD n=371). Raman revealed significant increases in measures of bone matrix mineralization in ZDSD (PO4(3-) ν1/Amide I p=0.008; PO4(3-) ν1/CH2 wag p=0.047; n=5 per group) despite lower bone mineral density (aBMD) and ash fraction indicating diabetes may preferentially reduce the Raman signature of collagen. Decreased indentation distance increase (p=0.010) and creep indentation distance (p=0.040) measured by RPI (n=9 per group) in ZDSD rats suggest a matrix more resistant to indentation under the high stresses associated with RPI at this length scale. There were significant correlations between Raman and RPI measurements in the ZDSD population (n=18 locations) but not the CD population (n=16 locations) indicating that while RPI is relatively unaffected by biological noise, it is sensitive to disease-induced compositional changes. In conclusion, diabetes in the ZDSD rat causes changes to the nanoscale morphology of collagen that result in compositional and mechanical effects in bone at the microscale.

Multiscale analysis of morphology and mechanics in tail tendon from the ZDSD rat model of type 2 diabetes

Type 2 diabetes (T2D) impacts multiple organ systems including the circulatory, renal, nervous and musculoskeletal systems. In collagen-based tissues, one mechanism that may be responsible for detrimental mechanical impacts of T2D is the formation of advanced glycation end products (AGEs) leading to increased collagen stiffness and decreased toughness, resulting in brittle tissue behavior. The purpose of this study was to investigate tendon mechanical properties from normal and diabetic rats at two distinct length scales, testing the hypothesis that increased stiffness and strength and decreased toughness at the fiber level would be associated with alterations in nanoscale morphology and mechanics. Individual fascicles from female Zucker diabetic Sprague-Dawley (ZDSD) rats had no differences in fascicle-level mechanical properties but had increased material-level strength and stiffness versus control rats (CD). At the nanoscale, collagen fibril D-spacing was shifted towards higher spacing values in diabetic ZDSD fibrils. The distribution of nanoscale modulus values was also shifted to higher values. Material-level strength and stiffness from whole fiber tests were increased in ZDSD tails. Correlations between nanoscale and microscale properties indicate a direct positive relationship between the two length scales, most notably in the relationship between nanoscale and microscale modulus. These findings indicate that diabetes-induced changes in material strength and modulus were driven by alterations at the nanoscale.

Glycation cross-linking induced mechanical-enzymatic cleavage of microscale tendon fibers

Recent molecular modeling data using collagen peptides predicted that mechanical force transmitted through intermolecular cross-links resulted in collagen triple helix unwinding. These simulations further predicted that this unwinding, referred to as triple helical microunfolding, occurred at forces well below canonical collagen damage mechanisms. Based in large part on these data, we hypothesized that mechanical loading of glycation cross-linked tendon microfibers would result in accelerated collagenolytic enzyme damage. This hypothesis is in stark contrast to reports in literature that indicated that individually mechanical loading or cross-linking each retards enzymatic degradation of collagen substrates. Using our Collagen Enzyme Mechano-Kinetic Automated Testing (CEMKAT) System we mechanically loaded collagen-rich tendon microfibers that had been chemically cross-linked with sugar and tested for degrading enzyme susceptibility. Our results indicated that cross-linked fibers were >5 times more resistant to enzymatic degradation while unloaded but became highly susceptible to enzyme cleavage when they were stretched by an applied mechanical deformation.