Putative role of lysyl hydroxylation and pyridinoline cross-linking during adolescence in the occurrence of osteoarthritis at old age

Glycosylation Modulates the Structure and Functions of Collagen: A Review

Collagens are fundamental constituents of the extracellular matrix and are the most abundant proteins in mammals. Collagens belong to the family of fibrous or fiber-forming proteins that self-assemble into fibrils that define their mechanical properties and biological functions. Up to now, 28 members of the collagen superfamily have been recognized. Collagen biosynthesis occurs in the endoplasmic reticulum, where specific post-translational modification-glycosylation-is also carried out. The glycosylation of collagens is very specific and adds β-d-galactopyranose and β-d-Glcp-(1→2)-d-Galp disaccharide through β-O-linkage to hydroxylysine. Several glycosyltransferases, namely COLGALT1, COLGALT2, LH3, and PGGHG glucosidase, were associated the with glycosylation of collagens, and recently, the crystal structure of LH3 has been solved. Although not fully understood, it is clear that the glycosylation of collagens influences collagen secretion and the alignment of collagen fibrils. A growing body of evidence also associates the glycosylation of collagen with its functions and various human diseases. Recent progress in understanding collagen glycosylation allows for the exploitation of its therapeutic potential and the discovery of new agents. This review will discuss the relevant contributions to understanding the glycosylation of collagens. Then, glycosyltransferases involved in collagen glycosylation, their structure, and catalytic mechanism will be surveyed. Furthermore, the involvement of glycosylation in collagen functions and collagen glycosylation-related diseases will be discussed.

PLOD2, a key factor for MRL MSC metabolism and chondroprotective properties

BACKGROUND

Initially discovered for its ability to regenerate ear holes, the Murphy Roth Large (MRL) mouse has been the subject of multiple research studies aimed at evaluating its ability to regenerate other body tissues and at deciphering the mechanisms underlying it. These enhanced abilities to regenerate, retained during adulthood, protect the MRL mouse from degenerative diseases such as osteoarthritis (OA). Here, we hypothesized that mesenchymal stromal/stem cells (MSC) derived from the regenerative MRL mouse could be involved in their regenerative potential through the release of pro-regenerative mediators.

METHOD

To address this hypothesis, we compared the secretome of MRL and BL6 MSC and identified several candidate molecules expressed at significantly higher levels by MRL MSC than by BL6 MSC. We selected one candidate, Plod2, and performed functional in vitro assays to evaluate its role on MRL MSC properties including metabolic profile, migration, and chondroprotective effects. To assess its contribution to MRL protection against OA, we used an experimental model for osteoarthritis induced by collagenase (CiOA).

RESULTS

Among the candidate molecules highly expressed by MRL MSC, we focused our attention on procollagen-lysine,2-oxoglutarate 5-dioxygenase 2 (PLOD2). Plod2 silencing induced a decrease in the glycolytic function of MRL MSC, resulting in the alteration of their migratory and chondroprotective abilities in vitro. In vivo, we showed that Plod2 silencing in MRL MSC significantly impaired their capacity to protect mouse from developing OA.

CONCLUSION

Our results demonstrate that the chondroprotective and therapeutic properties of MRL MSC in the CiOA experimental model are in part mediated by PLOD2.

Cross-Communication Between Knee Osteoarthritis and Fibrosis: Molecular Pathways and Key Molecules

Knee fibrosis is characterized by the presence of excessive connective tissue due to dysregulated fibroblast activation following local or systemic tissue damage. Knee fibrosis constitutes a major clinical problem in orthopaedics due to the severe limitation in the knee range of motion that leads to compromised function and patient disability. Knee osteoarthritis is an extremely common orthopedic condition that is associated with patient disability and major costs to the health-care systems worldwide. Although knee fibrosis and osteoarthritis (OA) have traditionally been perceived as two separate pathologic entities, recent research has shown common ground between the pathophysiologic processes that lead to the development of these two conditions. The purpose of this review was to identify the pathophysiologic pathways as well as key molecules that are implicated in the development of both knee OA and knee fibrosis in order to understand the relationship between the two diagnoses and potentially identify novel therapeutic targets.

Nutrient supply and nucleus pulposus cell function: effects of the transport properties of the cartilage endplate and potential implications for intradiscal biologic therapy

OBJECTIVE

Intradiscal biologic therapy is a promising strategy for managing intervertebral disc degeneration. However, these therapies require a rich nutrient supply, which may be limited by the transport properties of the cartilage endplate (CEP). This study investigated how fluctuations in CEP transport properties impact nutrient diffusion and disc cell survival and function.

DESIGN

Human CEP tissues harvested from six fresh cadaveric lumbar spines (38-66 years old) were placed at the open sides of diffusion chambers. Bovine nucleus pulposus (NP) cells cultured inside the chambers were nourished exclusively by nutrients diffusing through the CEP tissues. After 72 h in culture, depth-dependent NP cell viability and gene expression were measured, and related to CEP transport properties and biochemical composition determined using fluorescence recovery after photobleaching and Fourier transform infrared (FTIR) spectroscopy.

RESULTS

Solute diffusivity varied nearly 4-fold amongst the CEPs studied, and chambers with the least permeable CEPs appeared to have lower aggrecan, collagen-2, and matrix metalloproteinase-2 gene expression, as well as a significantly shorter viable distance from the CEP/nutrient interface. Increasing chamber cell density shortened the viable distance; however, this effect was lost for low-diffusivity CEPs, which suggests that these CEPs may not provide enough nutrient diffusion to satisfy cell demands. Solute diffusivity in the CEP was associated with biochemical composition: low-diffusivity CEPs had greater amounts of collagen and aggrecan, more mineral, and lower cross-link maturity.

CONCLUSIONS

CEP transport properties dramatically affect NP cell survival/function. Degeneration-related CEP matrix changes could hinder the success of biologic therapies that require increased nutrient supply.

Meniscus, articular cartilage and nucleus pulposus: a comparative review of cartilage-like tissues in anatomy, development and function

The degradation of cartilage in the human body is impacted by aging, disease, genetic predisposition and continued insults resulting from daily activity. The burden of cartilage defects (osteoarthritis, rheumatoid arthritis, intervertebral disc damage, knee replacement surgeries, etc.) is daunting in light of substantial economic and social stresses. This review strives to broaden the scope of regenerative medicine and tissue engineering approaches used for cartilage repair by comparing and contrasting the anatomical and functional nature of the meniscus, articular cartilage (AC) and nucleus pulposus (NP). Many review papers have provided detailed evaluations of these cartilages and cartilage-like tissues individually but none have comprehensively examined the parallels and inconsistencies in signaling, genetic expression and extracellular matrix composition between tissues. For the first time, this review outlines the importance of understanding these three tissues as unique entities, providing a comparative analysis of anatomy, ultrastructure, biochemistry and function for each tissue. This novel approach highlights the similarities and differences between tissues, progressing research toward an understanding of what defines each tissue as distinctive. The goal of this paper is to provide researchers with the fundamental knowledge to correctly engineer the meniscus, AC and NP without inadvertently developing the wrong tissue function or biochemistry.

Molecular insights into prolyl and lysyl hydroxylation of fibrillar collagens in health and disease

Collagen is a macromolecule that has versatile roles in physiology, ranging from structural support to mediating cell signaling. Formation of mature collagen fibrils out of procollagen α-chains requires a variety of enzymes and chaperones in a complex process spanning both intracellular and extracellular post-translational modifications. These processes include modifications of amino acids, folding of procollagen α-chains into a triple-helical configuration and subsequent stabilization, facilitation of transportation out of the cell, cleavage of propeptides, aggregation, cross-link formation, and finally the formation of mature fibrils. Disruption of any of the proteins involved in these biosynthesis steps potentially result in a variety of connective tissue diseases because of a destabilized extracellular matrix. In this review, we give a revised overview of the enzymes and chaperones currently known to be relevant to the conversion of lysine and proline into hydroxyproline and hydroxylysine, respectively, and the O-glycosylation of hydroxylysine and give insights into the consequences when these steps are disrupted.

Matrix and cell phenotype differences in Dupuytren's disease

Background

Dupuytren’s disease is a fibroproliferative disease of the hand and fingers, which usually manifests as two different phenotypes within the same patient. The disease first causes a nodule in the palm of the hand, while later, a cord develops, causing contracture of the fingers.

Results

We set out to characterize the two phenotypes by comparing matched cord and nodule tissue from ten Dupuytren’s patients. We found that nodule tissue contained more proliferating cells, CD68-positive macrophages and α-smooth muscle actin (α-SMA)-positive myofibroblastic cells. qPCR analysis showed an increased expression of COL1A1, COL1A2, COL5A1, and COL6A1 in nodule tissue compared to cord tissue. Immunohistochemistry showed less deposition of collagen type I in nodules, although they contained more fibronectin, collagen type V, and procollagen 1. Lower collagen levels in nodule were confirmed by HPLC measurements of the Hyp/Pro ratio. PCOLCE2, an activator of BMP1, the main enzyme cleaving the C-terminal pro-peptide from procollagen, was also reduced in nodule. Cord tissue not only contained more collagen I, but also higher levels of hydroxylysylpyridinoline and lysylpyridinoline residues per triple helix, indicating more crosslinks.

Conclusions

Our results clearly show that in Dupuytren’s disease, the nodule is the active disease unit, although it does not have the highest collagen protein levels. The difference in collagen type I deposition compared to mRNA levels and procollagen 1 levels may be connected to a decrease in procollagen processing.

Electronic supplementary material

The online version of this article (doi:10.1186/s13069-016-0046-0) contains supplementary material, which is available to authorized users.

Unravelling osteoarthritis-related synovial fibrosis: a step closer to solving joint stiffness

Synovial fibrosis is often found in OA, contributing heavily to joint pain and joint stiffness, the main symptoms of OA. At this moment the underlying mechanism of OA-related synovial fibrosis is not known and there is no cure available. In this review we discuss factors that have been reported to be involved in synovial fibrosis. The aim of the study was to gain insight into how these factors contribute to the fibrotic process and to determine the best targets for therapy in synovial fibrosis. In this regard, the following factors are discussed: TGF-β, connective tissue growth factor, procollagen-lysine, 2-oxoglutarate 5-dioxygenase 2, tissue inhibitor of metalloproteinase 1, A disintegrin and metalloproteinase domain 12, urotensin-II, prostaglandin F2α and hyaluronan.

Gene expression analysis of murine and human osteoarthritis synovium reveals elevation of transforming growth factor β-responsive genes in osteoarthritis-related fibrosis

OBJECTIVE

Synovial fibrosis is a major contributor to joint stiffness in osteoarthritis (OA). Transforming growth factor β (TGFβ), which is elevated in OA, plays a key role in the onset and persistence of synovial fibrosis. However, blocking of TGFβ in OA as a therapeutic intervention for fibrosis is not an option since TGFβ is crucial for cartilage maintenance and repair. Therefore, we undertook the present study to seek targets downstream of TGFβ for preventing OA-related fibrosis without interfering with joint homeostasis.

METHODS

Experiments were performed to determine whether genes involved in extracellular matrix turnover were responsive to TGFβ and were elevated in OA-related fibrosis. We analyzed gene expression in TGFβ-stimulated human OA synovial fibroblasts and in the synovium of mice with TGFβ-induced fibrosis, mice with experimental OA, and humans with end-stage OA. Gene expression was determined by microarray, low-density array, or quantitative polymerase chain reaction analysis.

RESULTS

We observed an increase in expression of procollagen genes and genes encoding collagen crosslinking enzymes under all of the OA-related fibrotic conditions investigated. Comparison of gene expression in TGFβ-stimulated human OA synovial fibroblasts, synovium from mice with experimental OA, and synovium from humans with end-stage OA revealed that the genes PLOD2, LOX, COL1A1, COL5A1, and TIMP1 were up-regulated in all of these conditions. Additionally, we confirmed that these genes were up-regulated by TGFβ in vivo in mice with TGFβ-induced synovial fibrosis.

CONCLUSION

Most of the up-regulated genes identified in this study would be poor targets for therapy development, due to their crucial functions in the joint. However, the highly up-regulated gene PLOD2, responsible for the formation of collagen crosslinks that make collagen less susceptible to enzymatic degradation, is an attractive and promising target for interference in OA-related synovial fibrosis.

The effect of collagen crosslinking on the biphasic poroviscoelastic cartilage properties determined from a semi-automated microindentation protocol for stress relaxation

Given the important role of the collagenous structure in cartilage mechanics, there is considerable interest in the relationship between collagen crosslinking and the mechanical behavior of the cartilage matrix. While crosslink-induced alterations to the elastic modulus of cartilage have been described, changes to time-dependent behavior have not yet been determined. The objective of the study was to quantify changes to cartilage material properties, including viscoelastic coefficients, with crosslinking via indentation. To accomplish this, a semi-autonomous microindentation stress relaxation protocol was first developed, validated and then applied to cartilage specimens before and after crosslinking. The change in mechanical properties with crosslinking was analyzed both in the unloading portions of the test via the Oliver-Pharr method and in the holding portion with an inverse iterative finite element model that represented cartilage as a biphasic poroviscoelastic material. Although both techniques suggested a similar increase in equilibrium modulus in the crosslinked specimens as compared to the controls, distinct differences in the control specimens were apparent, suggesting that the two different techniques may be capturing different aspects of the material behavior. No differences in time-dependent properties were observed between the crosslinked and the control specimens. These results give further insight into the effects of crosslinking in cartilage mechanical behavior. Additionally, the microindentation stress relaxation protocol may enable increased automation for high-throughput testing.

Temporal changes in collagen cross-links in spontaneous articular cartilage repair

OBJECTIVE: Little is known about how the biochemical properties of collagen change during tissue regeneration following cartilage damage. In the current study, temporal changes in cartilage repair tissue biochemistry were assessed in a rabbit osteochondral defect. DESIGN: Bilateral full thickness 3mm osteochondral trochlear groove defects were created in 54 adult male skeletally mature New Zealand white rabbits and tissue repair monitored over 16 weeks. Collagen content, cross-links, lysyl hydroxylation, gene expression, histological grading, and FTIR analyses were performed at 2, 4, 6, 8, 12, and 16 weeks. RESULTS: Defect fill occurred at ~4 weeks post-injury, however, histological grading showed that the repair tissue never became normal, primarily due to the presence of fibrocartilage. Gene expression levels of Col1a1 and Col1a2 were higher in the defect compared to adjacent regions. Collagen content in the repair tissue reached the level of normal cartilage at 6 weeks, but it took 12 weeks for the extent of lysine hydroxylation to return to normal. Divalent immature cross-links markedly increased in the early stages of repair. Though the levels gradually diminished thereafter, they never returned to the normal levels. The mature cross-link, pyridinoline, gradually increased with time and nearly reached normal levels by week 16. Infrared imaging data of protein content paralleled the biochemical data. However, collagen maturity, a parameter previously shown to reflect collagen cross-link ratios in bone, did not correlate with the biochemical determination of cross-links in the repair tissue.. CONCLUSION: Collagen biochemical data could provide markers for clinical monitoring in a healing defect.

Immunohistochemical distribution of advanced glycation end products (AGEs) in human osteoarthritic cartilage

Advanced glycation end products (AGEs) may be associated with osteoarthritis (OA), because the accumulation of AGEs in articular cartilage are among the most striking age-related changes. AGEs modify the tissue protein structure and function and stimulate the cellular responses mediated by a specific receptor for AGEs (RAGE). This study investigated the localization of AGEs in degenerated cartilage using newly identified epitope-specific antibodies to determine the linkage between the distribution of AGEs and the development and progression of OA. Osteochondral specimens of the tibial plateau from OA patients were immunostained by specific antibodies against N(ε)-(carboxymethyl)lysine (CML), N(ε)-(carboxyethyl)lysine (CEL), pentosidine, GA-pyridine, and RAGE. The immunohistochemical distribution of these epitopes was evaluated during cartilage degeneration. The immunoreactivity (IR) of AGEs and RAGE was stronger in cells rather than in the extracellular matrix. Higher IR of cellular CML and CEL was observed in both mild and severe OA cartilage in comparison to macroscopically intact cartilage. There was a strong association between GA-pyridine and RAGE in the pattern of increasing IR with the OA grade. These IR patterns of AGEs varying with cartilage degeneration indicate that AGE modified proteins are associated with cartilage degeneration. The coincidental up-regulation of GA-pyridine and RAGE suggests that GA-pyridine is the most significant AGE for cartilage degeneration via the RAGE pathway.

Regulation of osteoarthritis by omega-3 (n-3) polyunsaturated fatty acids in a naturally occurring model of disease

OBJECTIVE

To examine effects of high omega-3 (n-3) polyunsaturated fatty acid (PUFA) diets on development of osteoarthritis (OA) in a spontaneous guinea pig model, and to further characterise pathogenesis in this model. Modern diets low in n-3 PUFAs have been linked with increases in inflammatory disorders, possibly including OA. However, n-3 is also thought to increases bone density, which is a possible contributing factor in OA. Therefore we aim to determine the net influence of n-3 in disease development.

METHOD

OA-prone Dunkin-Hartley (DH) Guinea pigs were compared with OA-resistant Bristol Strain-2s (BS2) each fed a standard or an n-3 diet from 10 to 30 weeks (10/group). We examined cartilage and subchondral bone pathology by histology, and biochemistry, including collagen cross-links, matrix metalloproteinases (MMPs), alkaline phosphatase, glycosaminoglycan (GAG), and denatured type II collagen.

RESULTS

Dietary n-3 reduced disease in OA-prone animals. Most cartilage parameters were modified by n-3 diet towards those seen in the non-pathological BS2 strain - significantly active MMP-2, lysyl-pyridinoline and total collagen cross-links - the only exception being pro MMP-9 which was lower in the BS2, yet increased with n-3. GAG content was higher and denatured type II lower in the n-3 group. Subchondral bone parameters in the DH n-3 group also changed towards those seen in the non-pathological strain, significantly calcium:phosphate ratios and epiphyseal bone density.

CONCLUSION

Dietary n-3 PUFA reduced OA in the prone strain, and most disease markers were modified towards those of the non-OA strain, though not all significantly so. Omega-3 did not increase markers of pathology in either strain.

Skin and urine pentosidine weakly correlate with joint damage in a cohort of patients with early signs of osteoarthritis (CHECK)

OBJECTIVES

Age-related changes in articular cartilage are likely to play a role in the aetiology of osteoarthritis (OA). One of the major age-related changes in cartilage is the accumulation of advanced-glycation-endproducts (AGEs). Since, cartilage tissue is not readily available from patients for studying AGE levels, alternative approaches such as analyzing skin and urine are needed to study the role of cartilage AGE levels in OA.

METHODS

Paired human skin and cartilage samples were obtained post mortem. Paired skin and urine samples were obtained from the CHECK cohort (early OA patients). Pentosidine levels were measured by high-performance liquid chromatography (HPLC). As marker of cumulative cartilage damage X-rays of both knees and hips were scored. Urinary CTXII (uCTXII) levels were measured, to assess current cartilage breakdown.

RESULTS

Cartilage and skin pentosidine correlate well (R=0.473, P=0.05). Skin pentosidine was higher in mild (summed (Kellgren & Lawrence K&L) over four large joints ≥4) compared to no (summed K&L≤3) radiographic OA (P=0.007). Urinary pentosidine was not different between these two groups. Skin pentosidine levels were not related to cartilage breakdown (highest vs lowest tertile of uCTXII). Urinary pentosidine, however, was higher in the highest compared to the lowest uCTXII tertile (P=0.009). Multiple regression analysis showed age to be the only predictor of the summed K&L score and age, creatinine clearance and urinary pentosidine as predictors of uCTXII.

CONCLUSION

The higher skin and urinary pentosidine levels in those with mild compared to no radiographic joint damage and low vs high cartilage breakdown respectively suggest that AGEs may contribute to disease susceptibility and/or progression. However, relations are weak and cannot be used as surrogate markers of severity of OA.

Biomarkers in the development of novel disease-modifying therapies for osteoarthritis

Identification and utilization of biomarkers is vitally important for the successful development of disease-modifying osteoarthritis drugs. Biochemical and imaging platforms hold great promise to deliver such biomarkers. Studies indicate a marked increase in biochemical products arising from the breakdown and biosynthesis of collagen, extracellular matrix and bone in osteoarthritis. These molecules have been associated with disease severity and may also have prognostic value as indicators of disease progression. However, issues including biological variability and lack of tissue specificity currently hinder the utility of these molecular markers in drug development. Imaging technologies hold great potential for sensitive and accurate measurement of disease-related structural damage. Drawbacks, including expense, need for validation and limited accessibility also limit the utility of these technologies. In this article, the potential value and challenges in developing and utilizing biomarkers in disease-modifying osteoarthritis drug development will be discussed.

Decreased collagen organization and content are associated with reduced strength of demineralized and intact bone in the SAMP6 mouse

To examine the link between bone material properties and skeletal fragility, we analyzed the mechanical, histological, biochemical, and spectroscopic properties of bones from a murine model of skeletal fragility (SAMP6). Intact bones from SAMP6 mice are weak and brittle compared with SAMR1 controls, a defect attributed to reduced strength of the bone matrix. The matrix weakness is attributed primarily to poorer organization of collagen fibers and reduced collagen content.

INTRODUCTION

The contribution of age-related changes in tissue material properties to skeletal fragility is poorly understood. We previously reported that bones from SAMP6 mice are weak and brittle versus age-matched controls. Our present objectives were to use the SAMP6 mouse to assess bone material properties in a model of skeletal fragility and to relate defects in the mechanical properties of bone to the properties of demineralized bone and to the structure and organization of collagen and mineral.

MATERIALS AND METHODS

Femora from 4- and 12-month-old SAMR1 (control) and SAMP6 mice were analyzed using bending and torsional mechanical testing of intact bones, tensile testing of demineralized bone, quantitative histology (including collagen fiber orientation), collagen cross-links biochemistry, and Raman spectroscopic analysis of mineral and collagen.

RESULTS

Intact bones from SAMP6 mice have normal elastic properties but inferior failure properties, with 60% lower fracture energy versus SAMR1 controls. The strength defect in SAMP6 bones was associated with a 23% reduction in demineralized bone strength, which in turn was associated with poorer collagen fiber organization, lower collagen content, and higher hydroxylysine levels. However, SAMP6 have normal levels of collagen cross-links and normal apatite mineral structure.

CONCLUSIONS

Bones from SAMP6 osteoporotic mice are weak and brittle because of a defect in the strength of the bone matrix. This defect is attributed primarily to poorer organization of collagen fibers and reduced collagen content. These findings highlight the role of the collagen component of the bone matrix in influencing skeletal fragility.

Musculoskeletal responses of 2-year-old Thoroughbred horses to early training. Conclusions

This paper summarises and presents in context the main findings of an extensive series of studies of early training lasting 13 weeks in which the tissue responses of 2-year-old Thoroughbred horses were assessed using a combination of methods. Negligible clinical injury was detected and thus the study fulfilled the intention of investigating adaptive change rather than injury. Cancellous and cortical bone, some digital tendons, and articular cartilage responded to early training exercise to a greater or lesser degree. Clinical examination and ancillary diagnostic aids currently in veterinary clinical use are not sufficient to detect early abnormalities in metacarpo-phalangeal joint (MCPJ) cartilage found in both trained and untrained horses. Future work should centre on detection of such changes, on the precise registration of training workload, and on the manipulation of the responses of musculoskeletal tissues by careful investigation of the effects of introducing conditioning exercise at a young age.

AGEing and osteoarthritis: a different perspective

PURPOSE OF REVIEW

Across the world, osteoarthritis is the most commonly occurring musculoskeletal disease of the elderly, affecting more than 25% of the population older than 60 years of age. By far the single greatest risk factor for the development of osteoarthritis is age, but a mechanism to explain this relation has not yet been identified. If such a mechanism is identified, this potentially also provides a novel target for osteoarthritis therapy. The identification of new therapeutic targets is of utmost importance, because a disease-modifying treatment for osteoarthritis is not available and, because of the graying of the population, the number of patients with osteoarthritis will continue to increase, which will pose an enormous social and economic burden on society.

RECENT FINDINGS

Advanced glycation end products accumulate in human articular cartilage with increasing age, and affect biomechanical, biochemical, and cellular characteristics of the tissue. As an illustration, accumulation of advanced glycation end products increase cartilage stiffness and brittleness while decreasing the synthesis and degradation of cartilage matrix constituents. Articular cartilage becomes more prone to damage, and thus osteoarthritis, at elevated concentrations of advanced glycation end products.

SUMMARY

The reviewed literature demonstrates that the age-related accumulation of advanced glycation end products in articular cartilage may provide a molecular mechanism capable of (at least in part) explaining the age-related increase in the incidence of osteoarthritis. This conclusion paves the way for new strategies to prevent or treat osteoarthritis via inhibition and/or reversal of this process.

Advanced glycation endproducts and osteoarthritis

Non-enzymatic glycation of proteins, such as collagen, results in the formation of advanced glycation endproducts (AGE). Advanced glycation endproducts result in pathologic stiffening of cartilage and extracellular matrix and accumulate with age. Pentosidine, an AGE, is present in serum, synovial fluid, and articular cartilage from patients with osteoarthritis (OA). However, AGE levels are not always increased, and may be decreased locally, in association with osteoarthritic pathology. The finding of pentosidine in articular cartilage of individuals with OA may not be specific for that disease, independent of chronologic age. Advanced glycation endproduct modification of normal articular cartilage increases its stiffness, increases chondrocyte-mediated proteoglycan degradation, reduces its susceptibility to matrix metalloproteinase-mediated degradation, and decreases proteoglycan synthesis by chondrocytes. These observations parallel findings in osteoarthritic cartilage, which suggests that AGE modification could contribute to the pathogenesis of OA. However, a causative link between AGEs and OA has not yet been established.