Age-related changes in the physical properties, cross-linking, and glycation of collagen from mouse tail tendon

Nails in older adults

As the world's population of adults greater than 60 years old continues to increase, it is important to manage nail disorders that may impact their daily lives. Nail disorders may have significant impact on quality of life due to decreased functionality, extreme pain, or social embarrassment. In this review, we discuss nail disorders affecting older patients, including physiologic, traumatic, drug-induced, infectious, environmental, inflammatory, and neoplastic conditions. Diagnosis of these conditions involves a detailed history, physical examination of all 20 nails, and depending on the condition, a nail clipping or biopsy and/or diagnostic imaging. Nails grow even more slowly in older adults compared to younger individuals, and therefore it is important for accurate diagnosis, and avoidance of inappropriate management and delay of treatment. Increased awareness of nail pathologies may help recognition and management of nail conditions in older adults.

Patellar tendon biomechanical and morphologic properties and their relationship to serum clinical variables in persons with prediabetes and type 2 diabetes

Tendon biomechanical properties and fibril organization are altered in patients with diabetes compared to healthy individuals, yet few biomarkers have been associated with in vivo tendon properties. We investigated the relationships between in vivo imaging-based tendon properties, serum variables, and patient characteristics across healthy controls (n = 14, age: 45 ± 5 years, body mass index [BMI]: 24 ± 1, hemoglobin A1c [HbA1c]: 5.3 ± 0.1%), prediabetes (n = 14, age: 54 ± 5 years, BMI: 29 ± 2; HbA1c: 5.7 ± 0.1), and type 2 diabetes (n = 13, age: 55 ± 3 years, BMI: 33 ± 2, HbA1c: 6.7 ± 0.3). We used ultrasound speckle-tracking and measurements from magnetic resonance imaging (MRI) to estimate the patellar tendon in vivo tangent modulus. Analysis of plasma c-peptide, interleukin-1β (IL-1β), IL-6, IL-8, tumor necrosis factor-α (TNF-α), adiponectin, leptin, insulin-like growth factor 1 (IGF-1), and C-reactive protein (CRP) was completed. We built regression models incorporating statistically significant covariates and indicators for the clinically defined groups. We found that tendon cross-sectional area normalized to body weight (BWN CSA) and modulus were lower in patients with type 2 diabetes than in healthy controls (p < 0.05). Our regression analysis revealed that a model that included BMI, leptin, high-density lipoprotein (HDL), low-density lipoprotein (LDL), age, and group explained ~70% of the variability in BWN CSA (R2 = 0.70, p < 0.001). For modulus, including the main effects LDL, groups, HbA1c, age, BMI, cholesterol, IGF-1, c-peptide, leptin, and IL-6, accounted for ~54% of the variability in modulus (R2 = 0.54, p < 0.05). While BWN CSA and modulus were lower in those with diabetes, group was a poor predicter of tendon properties when considering the selected covariates. These data highlight the multifactorial nature of tendon changes with diabetes and suggest that blood variables could be reliable predictors of tendon properties.

Mechanical Models of Collagen Networks for Understanding Changes in the Failure Properties of Aging Skin

Skin undergoes mechanical alterations due to changes in the composition and structure of the collagenous dermis with aging. Previous studies have conflicting findings, with both increased and decreased stiffness reported for aging skin. The underlying structure-function relationships that drive age-related changes are complex and difficult to study individually. One potential contributor to these variations is the accumulation of nonenzymatic crosslinks within collagen fibers, which affect dermal collagen remodeling and mechanical properties. Specifically, these crosslinks make individual fibers stiffer in their plastic loading region and lead to increased fragmentation of the collagenous network. To better understand the influence of these changes, we investigated the impact of nonenzymatic crosslink changes on the dermal microstructure using discrete fiber networks representative of the dermal microstructure. Our findings suggest that stiffening the plastic region of collagen's mechanical response has minimal effects on network-level stiffness and failure stresses. Conversely, simulating fragmentation through a loss of connectivity substantially reduces network stiffness and failure stress, while increasing stretch ratios at failure.

Interfacial Tissue Regeneration with Bone

PURPOSE OF REVIEW

Interfacial tissue exists throughout the body at cartilage-to-bone (osteochondral interface) and tendon-to-bone (enthesis) interfaces. Healing of interfacial tissues is a current challenge in regenerative approaches because the interface plays a critical role in stabilizing and distributing the mechanical stress between soft tissues (e.g., cartilage and tendon) and bone. The purpose of this review is to identify new directions in the field of interfacial tissue development and physiology that can guide future regenerative strategies for improving post-injury healing.

RECENT FINDINGS

Cues from interfacial tissue development may guide regeneration including biological cues such as cell phenotype and growth factor signaling; structural cues such as extracellular matrix (ECM) deposition, ECM, and cell alignment; and mechanical cues such as compression, tension, shear, and the stiffness of the cellular microenvironment. In this review, we explore new discoveries in the field of interfacial biology related to ECM remodeling, cellular metabolism, and fate. Based on emergent findings across multiple disciplines, we lay out a framework for future innovations in the design of engineered strategies for interface regeneration. Many of the key mechanisms essential for interfacial tissue development and adaptation have high potential for improving outcomes in the clinic.

Depth-Dependent Strain Model (1D) for Anisotropic Fibrils in Articular Cartilage

The mechanical response of articular cartilage (AC) under compression is anisotropic and depth-dependent. AC is osmotically active, and its intrinsic osmotic swelling pressure is balanced by its collagen fibril network. This mechanism requires the collagen fibers to be under a state of tensile pre-strain. A simple mathematical model is used to explain the depth-dependent strain calculations observed in articular cartilage under 1D axial compression (perpendicular to the articular surface). The collagen fibers are under pre-strain, influenced by proteoglycan concentration (fixed charged density, FCD) and collagen stiffness against swelling stress. The stiffness is introduced in our model as an anisotropic modulus that varies with fibril orientation through tissue depth. The collagen fibers are stiffer to stretching parallel to their length than perpendicular to it; when combined with depth-varying FCD, the model successfully predicts how tissue strains decrease with depth during compression. In summary, this model highlights that the mechanical properties of cartilage depend not only on proteoglycan concentration but also on the intrinsic properties of the pre-strained collagen network. These properties are essential for the proper functioning of articular cartilage.

Advanced glycation end-products as mediators of the aberrant crosslinking of extracellular matrix in scarred liver tissue

The extracellular matrix of cirrhotic liver tissue is highly crosslinked. Here we show that advanced glycation end-products (AGEs) mediate crosslinking in liver extracellular matrix and that high levels of crosslinking are a hallmark of cirrhosis. We used liquid chromatography-tandem mass spectrometry to quantify the degree of crosslinking of the matrix of decellularized cirrhotic liver samples from patients and from two mouse models of liver fibrosis and show that the structure, biomechanics and degree of AGE-mediated crosslinking of the matrices can be recapitulated in collagen matrix crosslinked by AGEs in vitro. Analyses via cryo-electron microscopy and optical tweezers revealed that crosslinked collagen fibrils form thick bundles with reduced stress relaxation rates; moreover, they resist remodelling by macrophages, leading to reductions in their levels of adhesion-associated proteins, altering HDAC3 expression and the organization of their cytoskeleton, and promoting a type II immune response of macrophages. We also show that rosmarinic acid inhibited AGE-mediated crosslinking and alleviated the progression of fibrosis in mice. Our findings support the development of therapeutics targeting crosslinked extracellular matrix in scarred liver tissue.

Effect of Aging on Tendon Biology, Biomechanics and Implications for Treatment Approaches

Tendon aging is associated with an increasing prevalence of tendon injuries and/or chronic tendon diseases, such as tendinopathy, which affects approximately 25% of the adult population. Aged tendons are often characterized by a reduction in the number and functionality of tendon stem/progenitor cells (TSPCs), fragmented or disorganized collagen bundles, and an increased deposition of glycosaminoglycans (GAGs), leading to pain, inflammation, and impaired mobility. Although the exact pathology is unknown, overuse and microtrauma from aging are thought to be major causative factors. Due to the hypovascular and hypocellular nature of the tendon microenvironment, healing of aged tendons and related injuries is difficult using current pain/inflammation and surgical management techniques. Therefore, there is a need for novel therapies, specifically cellular therapy such as cell rejuvenation, due to the decreased regenerative capacity during aging. To augment the therapeutic strategies for treating tendon-aging-associated diseases and injuries, a comprehensive understanding of tendon aging pathology is needed. This review summarizes age-related tendon changes, including cell behaviors, extracellular matrix (ECM) composition, biomechanical properties and healing capacity. Additionally, the impact of conventional treatments (diet, exercise, and surgery) is discussed, and recent advanced strategies (cell rejuvenation) are highlighted to address aged tendon healing. This review underscores the molecular and cellular linkages between aged tendon biomechanical properties and the healing response, and provides an overview of current and novel strategies for treating aged tendons. Understanding the underlying rationale for future basic and translational studies of tendon aging is crucial to the development of advanced therapeutics for tendon regeneration.

Mechanotransduction through hemidesmosomes during aging and longevity

Hemidesmosomes are structural protein complexes localized at the interface of tissues with high mechanical demand and shear forces. Beyond tissue anchoring, hemidesmosomes have emerged as force-modulating structures important for translating mechanical cues into biochemical and transcriptional adaptation (i.e. mechanotransduction) across tissues. Here, we discuss the recent insights into the roles of hemidesmosomes in age-related tissue regeneration and aging in C. elegans, mice and humans. We highlight the emerging concept of preserved dynamic mechanoregulation of hemidesmosomes in tissue maintenance and healthy aging.

Strategic outline of interventions targeting extracellular matrix for promoting healthy longevity

The extracellular matrix (ECM), composed of interlinked proteins outside of cells, is an important component of the human body that helps maintain tissue architecture and cellular homeostasis. As people age, the ECM undergoes changes that can lead to age-related morbidity and mortality. Despite its importance, ECM aging remains understudied in the field of geroscience. In this review, we discuss the core concepts of ECM integrity, outline the age-related challenges and subsequent pathologies and diseases, summarize diagnostic methods detecting a faulty ECM, and provide strategies targeting ECM homeostasis. To conceptualize this, we built a technology research tree to hierarchically visualize possible research sequences for studying ECM aging. This strategic framework will hopefully facilitate the development of future research on interventions to restore ECM integrity, which could potentially lead to the development of new drugs or therapeutic interventions promoting health during aging.

Quantitative nanohistology of aging dermal collagen

The skin is the largest organ in the body and is essential for protecting us from environmental stressors such as UV radiation, pollution, and pathogens. As we age, our skin undergoes complex changes that can affect its function, appearance, and health. These changes result from intrinsic (chronological) and extrinsic (environmental) factors that can cause damage to the skin's cells and extracellular matrix. As higher-resolution microscopical techniques, such as Atomic Force Microscopy (AFM), are being deployed to support histology, it is possible to explore the biophysical properties of the dermal scaffold's constituents, such as the collagen network. In this study, we demonstrate the use of our AFM-based quantitative nanohistology, performed directly on unfixed cryosections of 30 donors (female, Caucasian), to differentiate between dermal collagen from different age groups and anatomical sites. The initial 420 (10 × 10 μm²) Atomic Force Microscopy images were segmented into 42,000 (1 × 1 μm²) images before being classified according to four pre-defined empirical collagen structural biomarkers to quantify the structural heterogeneity of the dermal collagen. These markers include interfibrillar gap formation, undefined collagen structure, and registered or unregistered dense collagen fibrillar network with evident D-banding. The structural analysis was also complemented by extensive nanoindentation (∼1,000 curves) performed on individual fibrils from each section, yielding 30,000 indentation curves for this study. Principal Component Analysis was used to reduce the complexity of high-dimensional datasets. The % prevalence of the empirical collagen structural biomarkers between the papillary and reticular dermis for each section proves determinant in differentiating between the donors as a function of their age or the anatomical site (cheek or breast). A case of abnormal biological aging validated our markers and nanohistology approach. This case also highlighted the difference between chronological and biological aging regarding dermal collagen phenotyping. However, quantifying the impact of chronic and pathological conditions on the structure and function of collagen at the sub-micron level remains challenging and lengthy. By employing tools such as the Atomic Force Microscope as presented here, it is possible to start evaluating the complexity of the dermal matrix at the nanoscale and start identifying relevant collagen morphology which could be used toward histopathology standards.

Studying the Geroprotective Properties of YAP/TAZ Signaling Inhibitors on Drosophila melanogaster Model

The transcriptional coactivators Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are the main downstream effectors of the evolutionarily conserved Hippo signaling pathway. YAP/TAZ are implicated in the transcriptional regulation of target genes that are involved in a wide range of key biological processes affecting tissue homeostasis and play dual roles in the aging process, depending on the cellular and tissue context. The aim of the present study was to investigate whether pharmacological inhibitors of Yap/Taz increase the lifespan of Drosophila melanogaster. Real-time qRT-PCR was performed to measure the changes in the expression of Yki (Yorkie, the Drosophila homolog of YAP/TAZ) target genes. We have revealed a lifespan-increasing effect of YAP/TAZ inhibitors that was mostly associated with decreased expression levels of the wg and E2f1 genes. However, further analysis is required to understand the link between the YAP/TAZ pathway and aging.

Impact of elevated serum advanced glycation end products and exercise on intact and injured murine tendons

OVERVIEW

Delayed tendon healing is a significant clinical challenge for those with diabetes. We explored the role of advanced glycation end-products (AGEs), a protein modification present at elevated levels in serum of individuals with diabetes, on injured and intact tendons using a mouse model. Cell proliferation following tissue injury is a vital component of healing. Based on our previous work demonstrating that AGEs limit cell proliferation, we proposed that AGEs are responsible for the delayed healing process commonly observed in diabetic patients. Further, in pursuit of interventional strategies, we suggested that moderate treadmill exercise may support a healing environment in the presence of AGEs as exercise has been shown to stimulate cell proliferation in tendon tissue.

MATERIALS AND METHODS

Mice began receiving daily intraperitoneal injections of bovine serum albumin (BSA)-Control or AGE-BSA injections (200μg/ml) at 16-weeks of age. A tendon injury was created in the central third of both patellar tendons. Animals assigned to an exercise group began a moderate treadmill protocol one week following injury. The intact Achilles tendon and soleus muscle were also evaluated to assess the effect of BSA and AGE-BSA on un-injured muscle and tendon.

RESULTS

We demonstrate that our injection dosing and schedule lead to an increase in serum AGEs. Our findings imply that AGEs indeed modulate gene expression following a patellar tendon injury and have modest effects on gene expression in intact muscle and tendon.

CONCLUSIONS

While additional biomechanical analysis is warranted, these data suggest that elevated serum AGEs in persons with diabetes may impact tendon health.

Age-related changes in mechanical properties of semitendinosus tendon used for anterior cruciate ligament reconstruction

Background

Hamstring tendons are a popular choice for autografts in anterior cruciate ligament (ACL) reconstruction. However, there is increasing evidence that hamstring tendon autografts carry a high risk of revision and residual instability in young patients. To elucidate the reasons for the inferior outcome of the reconstructed ACL with hamstring tendon autografts in young patients, we investigated the Young’s modulus and the extent of cyclic loading-induced slackening of the semitendinosus tendon used for ACL reconstruction across a broad range of ages.

Methods

Twenty-six male patients (aged 17–53 years), who were scheduled for ACL reconstruction surgery using the semitendinosus tendon autograft, participated in this study. The distal portion of the harvested semitendinosus tendon, which was not used to construct the autograft, was used for cyclic tensile testing to calculate the Young’s modulus and the extent of slackening (i.e., increase in slack length).

Results

Spearman correlation analysis revealed that the Young’s modulus of the semitendinosus tendon was positively correlated with the patient’s age (ρ = 0.559, P = 0.003). In contrast, the extent of tendon slackening did not correlate with the patient’s age.

Conclusions

We demonstrated that the Young’s modulus of the semitendinosus tendon increases with age, indicating that the semitendinosus tendon used for ACL reconstruction is compliant in young patients.

Aging and matrix viscoelasticity affect multiscale tendon properties and tendon derived cell behavior

Aging is the largest risk factor for Achilles tendon associated disorders and rupture. Although Achilles tendon macroscale elastic properties are suggested to decline with aging, less is known about the effect of maturity and aging on multiscale viscoelastic properties and their effect on tendon cell behavior. Here, we show dose dependent changes in native multiscale tendon mechanical and structural properties and uncover several nanoindentation properties predicted by tensile mechanics and echogenicity. Alginate hydrogel systems designed to mimic juvenile tendon microscale mechanics revealed that stiffness and viscoelasticity affected Achilles tendon cell aspect ratio and proliferation during aging. This knowledge provides further evidence for the negative impact of maturity and aging on tendon and begins to elucidate how viscoelasticity can control tendon derived cell morphology and expansion. STATEMENT OF SIGNIFICANCE: Aging is the largest risk factor for Achilles tendon associated disorders and rupture. Although Achilles tendon macroscale elastic properties are suggested to decline with aging, less is known about the effect of maturity and aging on multiscale viscoelastic properties and their effect on tendon cell behavior. Here, we show dose dependent changes in native multiscale tendon mechanical and structural properties and uncover several nanoindentation properties predicted by tensile mechanics and echogenicity. Alginate hydrogel systems designed to mimic juvenile tendon microscale mechanics revealed that stiffness and viscoelasticity affected Achilles tendon cell spreading and proliferation during aging. This knowledge provides further evidence for the negative impact of maturity and aging on tendon and begins to elucidate how viscoelasticity can control tendon derived cell morphology and expansion.

Regulators of collagen crosslinking in developing and adult tendons

Tendons are collagen-rich musculoskeletal tissues that possess the mechanical strength needed to transfer forces between muscles and bones. The mechanical development and function of tendons are impacted by collagen crosslinks. However, there is a limited understanding of how collagen crosslinking is regulated in tendon during development and aging. Therefore, the objective of the present review was to highlight potential regulators of enzymatic and non-enzymatic collagen crosslinking and how they impact tendon function. The main collagen crosslinking enzymes include lysyl oxidase (LOX) and the lysyl oxidase-like isoforms (LOXL), whereas non-enzymatic crosslinking is mainly mediated by the formation of advanced glycation end products (AGEs). Regulators of the LOX and LOXL enzymes may include mechanical stimuli, mechanotransducive cell signaling pathways, sex hormones, transforming growth factor (TGF)β family, hypoxia, and interactions with intracellular or extracellular proteins. AGE accumulation in tendon is due to diabetic conditions and aging, and can be mediated by diet and mechanical stimuli. The formation of these enzymatic and non-enzymatic collagen crosslinks plays a major role in tendon biomechanics and in the mechanisms of force transfer. A more complete understanding of how enzymatic and non-enzymatic collagen crosslinking is regulated in tendon will better inform tissue engineering and regenerative therapies aimed at restoring the mechanical function of damaged tendons.

An integrated approach to investigate age-related modifications of morphological, mechanical and structural properties of type I collagen

The main propose of this study is to characterize the impact of chronological aging on mechanical, structural, biochemical, and morphological properties of type I collagen. We have developed an original approach combining a stress-strain measurement device with a portable Raman spectrometer to enable simultaneous measurement of Raman spectra during stress vs strain responses of young adult, adult and old rat tail tendon fascicles (RTTFs). Our data showed an increase in all mechanical properties such as Young's modulus, yield strength, and ultimate tensile strength with aging. At the molecular level, Raman data revealed that the most relevant frequency shift was observed at 938 cm^-1 in Old RTTFs, which is assigned to the C-C. This suggested a long axis deformation of the peptide chains in Old RTTFs during tensile stress. In addition, the intensity of the band at 872 cm^-1, corresponding to hydroxyproline decreased for young adult RTTFs and increased for the adult ones, while it remained unchanged for Old RTTFs during tensile stress. The amide III band (1242 and 1265 cm^-1) as well as the band ratios I₁₆₃₁/ I₁₆₆₃ and I₁₆₄₅ / I₁₆₆₃ responses to tensile stress were depending on mechanical phases (toe, elastic and plastic). The quantification of advanced glycation end-products by LC-MS/MS and spectrofluorometry showed an increase in their content with aging. This suggested that the accumulation of such products was correlated to the alterations observed in the mechanical and molecular properties of RTTFs. Analysis of the morphological properties of RTTFs by SHG combined with CT-FIRE software revealed an increase in length and straightness of collagen fibers, whereas their width and wavy fraction decreased. Our integrated study model could be useful to provide additional translational information to monitor progression of diseases related to collagen remodeling in musculoskeletal disorders. STATEMENT OF SIGNIFICANCE: Type I collagen is the major component of the extracellular matrix. Its architectural and structural organization plays an important role in the mechanical properties of many tissues at the physiological and pathological levels. The objective of this work is to develop an integrated approach to bring a new insight on the impact of chronological aging on the structural organization and mechanical properties of type I collagen. We combined a portable Raman spectrometer with a mechanical tensile testing device in order to monitor in real time the changes in the Raman fingerprint of type I collagen fibers during the mechanical stress. Raman spectroscopy allowed the identification of the type I collagen bonds that were affected by mechanical stress in a differential manner with aging.

The role of cell-matrix interactions in connective tissue mechanics

Living tissue is able to withstand large stresses in everyday life, yet it also actively adapts to dynamic loads. This remarkable mechanical behaviour emerges from the interplay between living cells and their non-living extracellular environment. Here we review recent insights into the biophysical mechanisms involved in the reciprocal interplay between cells and the extracellular matrix and how this interplay determines tissue mechanics, with a focus on connective tissues. We first describe the roles of the main macromolecular components of the extracellular matrix in regards to tissue mechanics. We then proceed to highlight the main routes via which cells sense and respond to their biochemical and mechanical extracellular environment. Next we introduce the three main routes via which cells can modify their extracellular environment: exertion of contractile forces, secretion and deposition of matrix components, and matrix degradation. Finally we discuss how recent insights in the mechanobiology of cell-matrix interactions are furthering our understanding of the pathophysiology of connective tissue diseases and cancer, and facilitating the design of novel strategies for tissue engineering.

Collagen hydroxylysine glycosylation: non-conventional substrates for atypical glycosyltransferase enzymes

Collagen is a major constituent of the extracellular matrix (ECM) that confers fundamental mechanical properties to tissues. To allow proper folding in triple-helices and organization in quaternary super-structures, collagen molecules require essential post-translational modifications (PTMs), including hydroxylation of proline and lysine residues, and subsequent attachment of glycan moieties (galactose and glucose) to specific hydroxylysine residues on procollagen alpha chains. The resulting galactosyl-hydroxylysine (Gal-Hyl) and less abundant glucosyl-galactosyl-hydroxylysine (Glc-Gal-Hyl) are amongst the simplest glycosylation patterns found in nature and are essential for collagen and ECM homeostasis. These collagen PTMs depend on the activity of specialized glycosyltransferase enzymes. Although their biochemical reactions have been widely studied, several key biological questions about the possible functions of these essential PTMs are still missing. In addition, the lack of three-dimensional structures of collagen glycosyltransferase enzymes hinders our understanding of the catalytic mechanisms producing this modification, as well as the impact of genetic mutations causing severe connective tissue pathologies. In this mini-review, we summarize the current knowledge on the biochemical features of the enzymes involved in the production of collagen glycosylations and the current state-of-the-art methods for the identification and characterization of this important PTM.

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.

The human Descemet's membrane and lens capsule: Protein composition and biomechanical properties

The Descemet's membrane (DM) and the lens capsule (LC) are two ocular basement membranes (BMs) that are essential in maintaining stability and structure of the cornea and lens. In this study, we investigated the proteomes and biomechanical properties of these two materials to uncover common and unique properties. We also screened for possible protein changes during diabetes. LC-MS/MS was used to determine the proteomes of both BMs. Biomechanical measurements were conducted by atomic force microscopy (AFM) in force spectroscopy mode, and complemented with immunofluorescence microscopy. Proteome analysis showed that all six existing collagen IV chains represent 70% of all LC-protein, and are thus the dominant components of the LC. The DM on the other hand is predominantly composed of a single protein, TGF-induced protein, which accounted for around 50% of all DM-protein. Four collagen IV-family members in DM accounted for only 10% of the DM protein. Unlike the retinal vascular BMs, the LC and DM do not undergo significant changes in their protein compositions during diabetes. Nanomechanical measurements showed that the endothelial/epithelial sides of both BMs are stiffer than their respective stromal/anterior-chamber sides, and both endothelial and stromal sides of the DM were stiffer than the epithelial and anterior-chamber sides of the LC. Long-term diabetes did not change the stiffness of the DM and LC. In summary, our analyses show that the protein composition and biomechanical properties of the DM and LC are different, i.e., the LC is softer than DM despite a significantly higher concentration of collagen IV family members. This finding is unexpected, as collagen IV members are presumed to be responsible for BM stiffness. Diabetes had no significant effect on the protein composition and the biomechanical properties of both the DM and LC.

COLLAGEN: STRUCTURE, METABOLISM, PRODUCTION AND INDUSTRIAL APPLICATION

This review presents the current scientific literature data about structure, properties, and functions of collagen, which is known as one of the most abundant human and animal proteins. The building of collagen molecule from the primary structure to submolecular formations, the main stages of its synthesis and biodegradation are briefly described. The information about collagen diversity, its features and metabolic ways in various tissues, including skin, tendons, bones, etc. is presented. The problems of pathologies caused by collagen synthesis and breakdown disorders as well as age-related changes in collagen properties and their causes are discussed. A comparative analysis of the advantages and disadvantages of collagen and its derivatives obtaining from various sources (animals, marine, and recombinant) is given. The most productive methods for collagen extraction from various tissues are shown. The concept of collagen hydrolysis conditions influence on the physicochemical properties and biological activity of the obtained products is described. The applications of collagen and its products in various fields of industrial activity, such as pharmaceutical, cosmetic industry and medicine, are discussed. Further prospective directions of fundamental and applied investigations in this area of research are outlined.

Mechanical stretching changes crosslinking and glycation levels in the collagen of mouse tail tendon

Collagen I is a major tendon protein whose polypeptide chains are linked by covalent crosslinks. It is unknown how the crosslinking contributes to the mechanical properties of tendon or whether crosslinking changes in response to stretching or relaxation. Since their discovery, imine bonds within collagen have been recognized as being important in both crosslink formation and collagen structure. They are often described as acidic or thermally labile, but no evidence is available from direct measurements of crosslink levels whether these bonds contribute to the mechanical properties of collagen. Here, we used MS to analyze these imine bonds after reduction with sodium borohydride while under tension and found that their levels are altered in stretched tendon. We studied the changes in crosslink bonding in tail tendon from 11-week-old C57Bl/6 mice at 4% physical strain, at 10% strain, and at breaking point. The crosslinks hydroxy-lysino-norleucine (HLNL), dihydroxy-lysino-norleucine (DHLNL), and lysino-norleucine (LNL) in-creased or decreased depending on the specific crosslink and amount of mechanical strain. We also noted a decrease in glycated lysine residues in collagen, indicating that the imine formed between circulating glucose and lysine is also stress labile. We also carried out mechanical testing, including cyclic testing at 4% strain, stress relaxation tests, and stress-strain profiles taken at breaking point, both with and without sodium borohydride reduction. The results from both the MS studies and mechanical testing provide insights into the chemical changes during tendon stretching and directly link these chemical changes to functional collagen properties.

Enzymatic and non-enzymatic crosslinks found in collagen and elastin and their chemical synthesis

This review summarized the enzymatic and non-enzymatic crosslinks found in collagen and elastin and their organic synthesis.