ELISA for the core protein of the cartilage large aggregating proteoglycan, aggrecan: comparison with the concentrations of immunogenic keratan sulphate in synovial fluid, serum and urine

A novel graphene oxide-based fluorescence method for detection of urine glycosaminoglycans

Glycosaminoglycans (GAGs) serve as a biomarker for mucopolysaccharidoses disease. In this study, a novel fluorometric method was developed to measure total GAGs in urine. Graphene oxide (GO) and rhodamine B (RhB), a cationic fluorescent dye, were employed in the development of the method. RhB attaches to the GO surface via electrostatic attraction, leading to the quenching of its fluorescence upon the establishment of the RhB-GO complex. However, the presence of GAGs prompts a resurgence of intense fluorescence. The linear range of the method is between 5.00 and 70.00 mg/L. The total GAG levels of urine samples analyzed using the method agree with the results of the biochemistry analysis laboratory (65.85 and 79.18 mg/L; 73.30 ± 1.76 and 72.21 ± 2.21). The method is simple, accurate, and sensitive and may be used for both first-step diagnosis of the mucopolysaccharidoses and detection of individual GAGs for studies of GAG-related research and other biological applications.

Detection of Glycosaminoglycans in Biological Specimens

Proteoglycans (PGs) are macromolecules formed by a protein backbone to which one or more glycosaminoglycan (GAG) side chains are covalently attached. Most PGs are present in connective tissues, cell surfaces, and intracellular compartments. The major biological function of PGs derives from the GAG component of the molecule, which is involved in cell growth and proliferation, embryogenesis, maintenance of tissue hydration, and interactions of the cells via receptors. PGs are categorized into four groups based on their cellular and subcellular localization, including cell surfaces and extracellular, intracellular, and pericellular locations. GAGs are a crucial component of PGs involved in various physiological and pathological processes. GAGs also serve as biomarkers of metabolic diseases such as mucopolysaccharidoses and mucolipidoses. Detection of specific GAGs in various biological fluids helps manage various genetic metabolic disorders before it causes irreversible damage to the patient (Amendum et al., Diagnostics (Basel) 11(9):1563, 2021). There are several methods for detecting GAGs; this chapter focuses on measuring GAGs using enzyme-linked immunosorbent assay, liquid chromatographic tandem mass spectrometry, and automated high-throughput mass spectrometry.

Fluorescence sensing of heparin and heparin-like glycosaminoglycans by stabilizing intramolecular charge transfer state of dansyl acid-labeled AG73 peptides with glutathione-capped gold nanoclusters

Sensors that can specifically and accurately detect glycosaminoglycans are rare. Here, a dual-mode platform for fluorescence intensity and lifetime sensing of plasma heparin and fluorescence imaging of heparan sulfate proteoglycan-expressed cancer cells was developed by stabilizing the intramolecular charge transfer (ICT) state of dansyl acid-labeling AG73 (DA-AG73) peptide with glutathione-capped gold nanoclusters (GSH-AuNCs). DA-AG73 peptides, including an electron-donor dimethylamino group and an electron-withdrawing sulfonamide moiety in the labeled DA molecules, emitted weak fluorescence due to the formation of the twisted ICT excited state. The complexation of heparin with DA-AG73 peptides followed by interacting with the GSH-AuNCs could restrict the rotation of the dimethylamino groups of the labeled DA molecules, triggering the transition from their twisted ICT state to ICT excited state. As a result, the fluorescence intensity and lifetime of the labeled DA molecules in DA-AG73 peptides were gradually enhanced with increasing the heparin concentration. The proposed platform provided excellent selectivity toward heparin and heparan sulfate and exhibited two linear calibration curves for quantifying 20-800 nM and 20-1000 nM heparin in the fluorescence intensity and lifetime modes, respectively. The proposed platform was practically applied for the fluorescence intensity and lifetime determination of plasma heparin and for the selective imaging of heparan sulfate proteoglycan-expressed cells.

Higher aggrecan 1-F21 epitope concentration in synovial fluid early after anterior cruciate ligament injury is associated with worse knee cartilage quality assessed by gadolinium enhanced magnetic resonance imaging 20 years later

Background

To investigate if cartilage related biomarkers in synovial fluid are associated with knee cartilage status 20 years after an anterior cruciate ligament (ACL) injury.

Methods

We studied 25 patients with a complete ACL rupture without subsequent ACL reconstruction or radiographic knee OA. All had a delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC) 20 years after the ACL injury, using the T1 transverse relaxation time in the presence of gadolinium (T1Gd) which estimates the concentration of glycosaminoglycans in hyaline cartilage. Synovial fluid samples were aspirated acutely (between 0 and 18 days) and during 1 to 5 follow up visits between 0.5 and 7.5 years after injury. We quantified synovial fluid concentrations of aggrecan (epitopes 1-F21 and ARGS), cartilage oligomeric matrix protein, matrix metalloproteinase-3 and tissue inhibitor of metalloproteinase-1 by immunoassays, and sulfated glycosaminoglycans by Alcian blue precipitation. Western blot was used for qualitative analyses of aggrecan fragments in synovial fluid and cartilage samples.

Results

Western blot indicated that the 1-F21 epitope was located within the chondroitin sulfate 2 region of aggrecan. Linear regression analyses (adjusted for age, sex, body mass index and time between injury and sampling) showed that acute higher synovial fluid 1-F21-aggrecan concentrations were associated with shorter T1Gd values 20 years after injury, i.e. inferior cartilage quality (standardized effects between − 0.67 and − 1.0). No other statistically significant association was found between molecular biomarkers and T1Gd values.

Conclusion

Higher acute synovial fluid 1-F21-aggrecan concentrations in ACL injured patients, who managed to cope without ACL reconstruction and were without radiographic knee OA, were associated with inferior knee cartilage quality assessed by dGEMRIC 20 years after injury.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-020-03819-9.

Glycosaminoglycans analysis in blood and urine of patients with mucopolysaccharidosis

To explore the correlation between glycosaminoglycan (GAG) levels and mucopolysaccharidosis (MPS) type, we have evaluated the GAG levels in blood of MPS II, III, IVA, and IVB and urine of MPS IVA, IVB, and VI by tandem mass spectrometry. Dermatan sulfate (DS), heparan sulfate (HS), keratan sulfate (KS; mono-sulfated KS, di-sulfated KS), and the ratio of di-sulfated KS in total KS were measured. Patients with untreated MPS II had higher levels of DS and HS in blood while untreated MPS III had higher levels of HS in blood than age-matched controls. Untreated MPS IVA had higher levels of KS in blood and urine than age-matched controls. The ratio of blood di-sulfated KS/total KS in untreated MPS IVA was constant and higher than that in controls for children up to 10 years of age. The ratio of urine di-sulfated KS/total KS in untreated MPS IVA was also higher than that in age-matched controls, but the ratio in untreated MPS IVB was lower than controls. ERT reduced blood DS and HS in MPS II, and urine KS in MPS IVA patients, although GAGs levels remained higher than the observed in age-matched controls. ERT did not change blood KS levels in MPS IVA. MPS VI under ERT still had an elevation of urine DS level compared to age-matched controls. There was a positive correlation between blood and urine KS in untreated MPS IVA patients but not in MPS IVA patients treated with ERT. Blood and urine KS levels were secondarily elevated in MPS II and VI, respectively. Overall, measurement of GAG levels in blood and urine is useful for diagnosis of MPS, while urine KS is not a useful biomarker for monitoring therapeutic efficacy in MPS IVA.

Clinical presentation and diagnosis of mucopolysaccharidoses

Mucopolysaccharidoses (MPS) are estimated to affect1 in 25,000 live births although specific rates vary between the ethnic origin and country. MPS are a group of lysosomal storage disorders, which cause the buildup of GAG(s) due to insufficient or absent GAG-degrading enzymes. With seven types of MPS disorders and eleven subtypes, the MPS family presents unique challenges for early clinical diagnosis due to the molecular and clinical heterogeneity between groups and patients. Novel methods of early identification, particularly newborn screening through mass spectrometry, can change the flow of diagnosis, allowing enzyme and GAG quantification before the presentation of clinical symptoms improving outcomes. Genetic testing of patients and their families can also be conducted preemptively. This testing enables families to make informed decisions about family planning, leading to prenatal diagnosis. In this review, we discuss the clinical symptoms of each MPS type as they initially appear in patients, biochemical and molecular diagnostic methods, and the future of newborn screening for this group of disorders.

Surfen-Assembled Graphene Oxide for Fluorescence Turn-On Detection of Sulfated Glycosaminoglycans in Biological Matrix

Sulfated glycosaminoglycans (GAGs) not only serve as a biomarker for mucopolysaccharidoses disease but also participate in various biological processes, such as blood clot medication (heparin) and signal transduction (heparan sulfate). However, few fluorescent sensors, such as 1,9-dimethylmethylene blue, have been developed for the detection of sulfated GAGs in the real world. Herein, we fabricated a surfen/few-layer graphene oxide (FLGO) nanocomplex for sensing sulfated GAGs in biological fluids. Surfen molecules are self-assembled onto the surface of FLGO through electrostatic attraction, and their fluorescence was then quenched by the creation of the FLGO-surfen complex (static quenching) and partially combined with the energy transfer from surfen to FLGO (dynamic quenching). The presence of sulfated GAGs resulted in the fluorescence recovery through the formation of the surfen-GAGs complex, which exhibits weak binding to FLGO and keeps surfen molecules away from the FLGO surface. Because FLGO efficiently reduced the fluorescence background from surfen and competed with sulfated GAGs for binding to surfen, surfen-assembled FLGO exhibited higher sensitivity and better selectivity for sulfated GAGs than surfen. The strategy mentioned above was exemplified by the analysis of heparin in human plasma and sulfated GAGs in an artificial cerebrospinal fluid; the limits of detection at a signal-to-noise ratio of 3 for heparin, dermatan sulfate, and heparin sulfate were determined to be 30, 30, and 60 ng/mL, respectively.

Concentrations of synovial fluid biomarkers and the prediction of knee osteoarthritis 16 years after anterior cruciate ligament injury

OBJECTIVE

To describe the longitudinal patterns of release, and investigate the association between a set of synovial fluid biomarkers at the acute and chronic stage and the development of radiographic knee osteoarthritis (OA) after an anterior cruciate ligament (ACL) injury.

DESIGN

Synovial fluid was aspirated from the acutely ACL-injured knee within the first 2weeks (acute samples), and yearly (chronic samples) up to 7.5 years after injury in 88 subjects (60% men). Non-injured subjects (n = 12) were used as reference group. Aggrecan, cartilage oligomeric matrix protein (COMP), matrix metalloproteinase (MMP)-3 and tissue inhibitor of metalloproteinase (TIMP)-1 in synovial fluid were quantified by immunoassays. The presence of radiographic tibiofemoral (TF) or patellofemoral (PF) OA [Kellgren and Lawrence (K&L) ≥2] was examined with weight-bearing knee radiography 16 years after the ACL injury.

RESULTS

The average acute and chronic SF concentrations of COMP and aggrecan were elevated in comparison with the reference group (P < 0.001). The levels of COMP and aggrecan clearly decreased approximately half a year after the ACL injury, and returned to reference values during the 7.5 years of follow-up. Using logistic regression analysis neither acute nor chronic concentrations of the four biomarkers were associated with the development of radiographic knee OA at the 16 year follow-up.

CONCLUSION

Increased synovial fluid concentrations of aggrecan and COMP was related to knee injury, but acute and chronic synovial fluid concentrations of aggrecan, COMP, MMP-3 and TIMP-1 failed to predict knee OA 16 years after ACL injury.

Glycosaminoglycans detection methods: Applications of mass spectrometry

Glycosaminoglycans (GAGs) are long blocks of negatively charged polysaccharides. They are one of the major components of the extracellular matrix and play multiple roles in different tissues and organs. The accumulation of undegraded GAGs causes mucopolysaccharidoses (MPS). GAGs are associated with other pathological conditions such as osteoarthritis, inflammation, diabetes mellitus, spinal cord injury, and cancer. The need for further understanding of GAG functions and mechanisms of action boosted the development of qualitative and quantitative (alcian blue, toluidine blue, paper and thin layer chromatography, gas chromatography, high pressure liquid chromatography, capillary electrophoresis, 1,9-dimethylmethylene blue, enzyme linked-immunosorbent assay, mass spectrometry) techniques. The availability of quantitative techniques has facilitated translational research on GAGs into the medical field for: 1) diagnosis, monitoring, and screening for MPS; 2) analysis of GAG synthetic and degradation pathways; and 3) determination of physiological and pathological roles of GAGs. This review provides a history of development of GAG assays and insights about the use of tandem mass spectrometry and its applications for GAG analysis.

An ARGS-aggrecan assay for analysis in blood and synovial fluid

OBJECTIVE

To validate a modified ligand-binding assay for the detection of aggrecanase generated aggrecan fragments with the ARGS neoepitope in synovial fluid (SF) and blood, and to verify the identity of aggrecan fragments found in blood.

DESIGN

An enzyme-linked immunosorbent assay (ELISA) on the Meso Scale Discovery (MSD) platform for detection of ARGS-aggrecan was validated, using a standard made from recombinant human aggrecan. Matched samples of SF, serum, plasma, and urine were obtained from 36 subjects at different time points after knee injury, and analysed for ARGS-aggrecan content. Aggrecan was purified from serum and plasma pools and analysed by Western blot.

RESULTS

The limits of quantification for the ARGS-aggrecan assay was between 0.2 and 0.025 pmol ARGS/ml, and the sensitivity of the assay was improved two-fold compared to when using a standard purified from human donors. The ARGS concentrations were highest in SF (mean, range; 3.02, 0.36-30.22 pmol/ml), 20 times lower in the blood samples (0.14, 0.055-0.28 pmol/ml serum and 0.13, 0.053-0.28 pmol/ml plasma), and 80 times lower in urine (0.036, below detection - 0.087 pmol/ml). Serum-ARGS and plasma-ARGS concentrations were similar, and correlated (r(S) = 0.773, P < 0.001). SF concentration correlated with serum concentrations (r(S) = 0.420, P = 0.011). In blood, we identified 129-138 kDa aggrecan fragments containing the ARGS neoepitope.

CONCLUSIONS

This novel ARGS-aggrecan assay is highly sensitive and suited for analysis of SF and blood samples. Both SF and blood contains ARGS-aggrecan, and ARGS concentrations in SF and serum are correlated.

Binding of chondroitin 4-sulfate to cathepsin S regulates its enzymatic activity

Human cysteine cathepsin S (catS) participates in distinct physiological and pathophysiological cellular processes and is considered as a valuable therapeutic target in autoimmune diseases, cancer, atherosclerosis, and asthma. We evaluated the capacity of negatively charged glycosaminoglycans (heparin, heparan sulfate, chondroitin 4/6-sulfates, dermatan sulfate, and hyaluronic acid) to modulate the activity of catS. Chondroitin 4-sulfate (C4-S) impaired the collagenolytic activity (type IV collagen) and inhibited the peptidase activity (Z-Phe-Arg-AMC) of catS at pH 5.5, obeying a mixed-type mechanism (estimated Ki = 16.5 ± 6 μM). Addition of NaCl restored catS activity, supporting the idea that electrostatic interactions are primarly involved. Furthermore, C4-S delayed in a dose-dependent manner the maturation of procatS at pH 4.0 by interfering with the intermolecular processing pathway. Binding of C4-S to catS was demonstrated by gel-filtration chromatography, and its affinity was measured by surface plasmon resonance (equilibrium dissociation constant Kd = 210 ± 40 nM). Moreover, C4-S induced subtle conformational changes in mature catS as observed by intrinsic fluorescence spectroscopy analysis. Molecular docking predicted three specific binding sites on catS for C4-S that are different from those found in the crystal structure of the cathepsin K-C4-S complex. Overall, these results describe a novel glycosaminoglycan-mediated mechanism of catS inhibition and suggest that C4-S may modulate the collagenase activity of catS in vivo.

Human aggrecanase generated synovial fluid fragment levels are elevated directly after knee injuries due to proteolysis both in the inter globular and chondroitin sulfate domains

OBJECTIVE

To examine different aggrecanase generated fragments in synovial fluid (SF) from patients with acute and chronic knee injuries and from knee healthy subjects.

METHODS

We prepared SF-D1 samples from acute (n=35) and chronic (n=35) knee injury patients and knee healthy subjects (n=10). Aggrecan fragments were analyzed in the SF-D1 samples by quantitative (G1, ARGS, KEEE and G3 antibodies) and non-quantitative (GRGT and AGEG antibodies) Western blot.

RESULTS

ARGS-SELE, ARGS-chondroitin sulfate (CS)1, GRGT-, GLGS- and AGEG-G3 fragments were the main ARGS and G3 fragments in injured and reference samples. In the acute injury samples the concentrations of these fragments were increased compared to the reference, and the level of the ARGS-SELE remained elevated for at least 2 years after the joint injury. Both SF ARGS fragments and aggrecanase generated G3 fragments had high sensitivity and specificity as biomarkers in distinguishing injured from healthy knee joints, although the ARGS fragments had higher area under the receiver operating characteristic curve (AUC) values for injuries (74-86%) than the G3 fragments (AUC values 63-68%).

CONCLUSION

Our results suggest that during the acute phase after knee injury there is an increased aggrecanase activity against both the interglobular domain (IGD) and the CS2 cleavage sites of joint cartilage aggrecan. This increase in SF aggrecanolytic fragments is present for several years after the injury. SF ARGS fragments are better biomarkers than the aggrecanase generated G3-fragments in distinguishing injured from healthy knee joints.

Development of a novel clinical biomarker assay to detect and quantify aggrecanase-generated aggrecan fragments in human synovial fluid, serum and urine

OBJECTIVE

Proteolytic degradation of aggrecan in articular cartilage is a hallmark feature of osteoarthritis (OA). The present study was aimed at developing a sensitive enzyme linked immunosorbent assay (ELISA) for the detection of aggrecanase-cleaved fragments of aggrecan in human serum and urine to facilitate the clinical development of aggrecanase inhibitors for OA.

METHODS

The BC3 monoclonal antibody that detects the ARGS neoepitope sequence in aggrecanase-cleaved aggrecan was engineered and optimized using complementarity determining region (CDR)-saturation mutagenesis to improve its binding affinity to the neoepitope. A sandwich ELISA (BC3-C2 ELISA) was developed using the optimized alpha-ARGS antibody (BC3-C2) as capture antibody and a commercially available antibody directed against the hyaluronic-acid binding region (HABR) of aggrecan as detection antibody. Aggrecanase-cleaved fragments of aggrecan present in in vitro digests, human cartilage explant culture supernatants and in human synovial fluid, serum and urine were detected and quantified using this ELISA.

RESULTS

The optimized antibody had a 4-log improvement in affinity for the ARGS containing peptide compared to the parental BC3 antibody, while maintaining the ability to not cross-react with a spanning peptide. The BC3-C2 ELISA demonstrated the ability to detect aggrecanase-cleaved aggrecan fragments in the native state, without the need for deglycosylation. This ELISA was able to measure aggrecanase-generated ARGS containing aggrecan fragments in human articular cartilage (HAC) explant cultures in the basal state (without cytokine stimulation). Treatment with an aggrecanase inhibitor resulted in a dose-dependent inhibition of ARGS neoepitope released into the culture supernatant. The ELISA assay also enabled the detection of ARGS containing fragments in human synovial fluid, serum and urine, suggesting its potential utility as a biomarker of aggrecanase activity.

CONCLUSIONS

We have developed a novel ELISA using an optimized ARGS antibody and have demonstrated for the first time, an ELISA-based measurement of aggrecan degradation products in human serum and urine. This assay has the potential to serve as a mechanistic drug activity biomarker in the clinic and is expected to significantly impact/accelerate the clinical development of aggrecanase inhibitors and other disease modifying drugs for OA.

An immunoaffinity liquid chromatography-tandem mass spectrometry assay for detection of endogenous aggrecan fragments in biological fluids: Use as a biomarker for aggrecanase activity and cartilage degradation

The degradation of articular cartilage by aggrecanases (ADAMTS-4 and ADAMTS-5) plays a significant role in the pathology of osteoarthritis (OA). To monitor aggrecanase activity in OA, we have developed a sensitive, accurate, and versatile assay for detection of two specific cleavage sites on aggrecan. The assay uses an immunoaffinity-based liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to detect cleavage at the (374)ARGS site and the (1820)AGEG site. The dynamic range of the assay is more than three orders of magnitude, with interassay precision less than 15%. It has been successfully applied to various biological fluids and species, including rat, bovine, dog, and human. The assay has been analytically qualified for use in human urine and synovial fluid (SF). The limits of detection (LODs) for ARGS in urine and SF are 2.5 and 10 pg/ml, respectively, whereas the LOD for AGEG is 20 pg/ml in SF. Analysis of these biomarkers from OA subjects and normal healthy volunteers revealed a significant elevation of both markers in OA. Similarly, in a rat model of cartilage degradation, both ARGS and AGEG were elevated, demonstrating the utility of these biomarkers for translational research. These data suggest that the ARGS and AGEG biomarkers developed have potential as measures of aggrecanase activity in OA and may contribute to our understanding of OA pathology.

Synovial fluid level of aggrecan ARGS fragments is a more sensitive marker of joint disease than glycosaminoglycan or aggrecan levels: a cross-sectional study

INTRODUCTION

Aggrecanase cleavage at the 392Glu-393Ala bond in the interglobular domain (IGD) of aggrecan, releasing N-terminal 393ARGS fragments, is an early key event in arthritis and joint injuries. Here, we use a quantitative immunoassay of aggrecan ARGS neoepitope fragments in human synovial fluid to determine if this cleavage-site specific method better identifies joint pathology than previously available less specific aggrecan assays.

METHODS

Synovial fluid (SF) from 26 people with healthy knees (reference) and 269 patients were analyzed in a cross-sectional study. Patient groups were acute inflammatory arthritis, acute knee injury, chronic knee injury and knee osteoarthritis (OA). Aggrecan ARGS fragments were assayed by ELISA using the monoclonal antibody OA-1. Total aggrecan content was analyzed by an ELISA using the monoclonal antibody 1-F21, and sulfated glycosaminoglycan by Alcian blue precipitation.

RESULTS

Aggrecan ARGS fragment concentrations in all groups differed from the reference group (P < 0.001). The acute inflammatory arthritis group had the highest median level, 177-fold greater than that of the reference group. Median levels (in pmol ARGS/ml SF) were: reference 0.5, acute inflammatory arthritis 88.5, acute knee injury 53.9, chronic knee injury 0.5 and OA 4.6. In contrast, aggrecan and sulfated glycosaminoglycan concentrations varied much less between groups, and only acute inflammatory arthritis and acute knee injury were found to have a two-fold increase in median levels compared to the reference.

CONCLUSIONS

Levels of aggrecan ARGS fragments in human synovial fluid are increased in human arthritis, OA and after knee injury, likely reflecting an enhanced cleavage at the 392Glu-393Ala bond in the IGD by aggrecanase. An assay that specifically quantified these fragments better distinguished samples from joints with pathology than assays monitoring aggrecan or glycosaminoglycan concentrations. The newly developed ARGS fragment assay can be used to monitor aggrecanase activity in human joint disease and experimental models.

Western blot quantification of aggrecan fragments in human synovial fluid indicates differences in fragment patterns between joint diseases

OBJECTIVE

To develop a Western blot method for quantification of multiple aggrecan fragments in human synovial fluids (SFs).

METHOD

SF aggrecan fragments were prepared from knee healthy (reference), knee injury and arthritis subjects by CsCl gradient centrifugations collecting D1 fractions. Samples were analyzed by Western blot, using antibodies against the N-terminal epitope ARGS and the G3 domain, and fragments were quantified using a digital luminescence image analyzer.

RESULTS

The method had a coefficients of variation of 10-30%, and a high correlation (r(S)=0.86) with a corresponding enzyme-linked immunosorbent assay (ELISA). The SFs from reference, knee injured and arthritic subjects contained two major ARGS fragments, ARGS-SELE and ARGS-CS1, and three major G3 fragments (GRGT-G3, GLGS-G3 and AGEG-G3). Compared to the reference, the acute arthritis and acute joint injury groups had a 30-fold elevated concentration of ARGS fragments, and both groups had a higher proportion of the aggrecan in joint fluid as ARGS fragments compared to the other groups. The reference and chronic injury groups had an excess of ARGS-CS1 fragments over ARGS-SELE fragments, while subjects with acute arthritis or osteoarthritis had a more even distribution between these fragments.

CONCLUSIONS

We have developed a novel Western blot quantification method for quantification of SF aggrecan fragments which can differentiate fragments of different sizes sharing the same epitope. The anti-ARGS and anti-G3 quantitative Western blots provided information important for a better understanding of the proteolytic pathways in aggrecan breakdown, information that discriminates between different joint diseases, and may aid in identification of new biomarkers.

Application of biomarkers in the clinical development of new drugs for chondroprotection in destructive joint diseases: a review

Emerging evidence supports the concept that biochemical markers are clinically useful non-invasive diagnostic tools for the monitoring of changes in cartilage turnover in patients with destructive joint diseases such as osteoarthritis (OA) and rheumatoid arthritis (RA). Epidemiological studies demonstrated that measurements of different degradation products of proteins in the extracellular matrix of hyaline cartilage in urine or serum samples are (1) increased in OA or RA patients compared with healthy individuals, (2) correlate with disease activity, and (3) are predictive for the rate of changes in radiographic measures of cartilage loss. The present review provides an updated list of available biomarkers and summarize the research data arguing for their clinical utility. In addition, it addresses the question whether or not the monitoring of biomarkers during different treatment modalities could be a useful approach to characterize the chondro-protective effects of approved and candidate drugs. Finally, it briefly reviews the in vitro/ex vivo experimental settings - isolated chondrocyte cultures and articular cartilage explants - that can assist in the verification of novel markers, but also studies assessing direct effects of drug candidates on chondrocytes. Collectively, biomarkers may acquire a function as established efficacy parameters in the clinical development of novel chondro-protective agents.

Can biochemical markers serve as surrogates for imaging in knee osteoarthritis?

OBJECTIVE

Osteoarthritis (OA) is a complex heterogeneous joint disease affecting more than 35 million people worldwide. The current gold standard diagnostic investigation is the plain radiograph, which lacks sensitivity. Biochemical markers have the potential to act as adjunct markers for imaging in the assessment of knee OA. We undertook this study to determine the association between individual biochemical markers and radiographic features, and to establish whether the association is strengthened when selected biochemical markers are combined into a single factor (a theoretical marker).

METHODS

Twenty serum and urinary biochemical markers were analyzed in 119 patients with predominantly tibiofemoral knee OA. Pearson's correlation was performed, and corresponding coefficients of determination (R(2)) were calculated to determine the association between biochemical markers and a range of imaging features from radiographs and dual x-ray absorptiometry of the knee. Biochemical markers demonstrating a significant association (P < 0.05) with a specific imaging feature were combined by principal components analysis (PCA). Pearson's correlation was repeated to establish whether the combined panel of biochemical markers showed a stronger association with imaging than the best single marker.

RESULTS

Fourteen biochemical markers showed significant associations with one or more imaging features. By combining specific panels of biochemical markers to form factors, the association of markers with imaging features (R(2)) increased from 11.9% to 22.7% for the Kellgren/Lawrence (K/L) score, from 5.9% to 9.2% for joint space width (JSW), from 6.6% to 10.8% for sclerosis, from 13.5% to 22.6% for osteophytes, and from 12.0% to 14.2% for bone mineral density (BMD). Biochemical markers identifying patients with osteophytes overlapped with those correlated with a high K/L score, while markers of subchondral BMD formed a completely separate group. Biochemical markers of JSW included markers associated with both osteophytes and BMD.

CONCLUSION

The PCA results suggest that biochemical marker combinations may be more sensitive than individual biochemical markers for reflecting structural damage in patients with knee OA. The differences in biochemical marker profiles associated with osteophytes compared with those associated with subchondral BMD raise the possibility that these 2 processes, commonly seen in bone in knee OA, have underlying biologic differences.

Development and characterization of a highly specific and sensitive sandwich ELISA for detection of aggrecanase-generated aggrecan fragments

OBJECTIVE

To develop an enzyme linked immunosorbent assay (ELISA) to quantify the levels of specific aggrecan fragments generated by aggrecanase-mediated cleavage at the 373Glu-374 Ala bond within the aggrecan interglobular domain.

METHODS

The ELISA employs a commercially available monoclonal antibody to capture aggrecan fragments containing keratan sulfate (KS). Aggrecan fragments generated by cleavage at the Glu-Ala bond were then detected using a monoclonal neoepitope antibody (mAb OA-1) that specifically recognizes the N-terminal sequence 'ARGSVIL'.

RESULTS

The mAb OA-1 antibody was highly specific for the immunizing neoepitope peptide since neither peptides spanning the cleavage site nor mutated peptides were detected. Aggrecan fragments generated by ADAMTS-4 digested human aggrecan monomers and from IL-1-stimulated human cartilage explants were quantified by the ELISA, and we observed increased sensitivity of the ELISA compared to mAb OA-1 Western analysis. We also observed that the basal, as well as IL-1-stimulated production of ARGS aggrecan fragments from human articular cartilage explants was blocked by a selective aggrecanase inhibitor, consistent with generation of the ARGS neoepitope in human articular cartilage being mediated by aggrecanase. Using purified human aggrecan digested by ADAMTS-4 as standard to quantify ARGS aggrecan fragments in human synovial fluids, we determined that the calculated amount of ARGSVIL-aggrecan fragments by ELISA measurement is in agreement with the published levels of these fragments, supporting its potential utility as a biomarker assay for osteoarthritis.

CONCLUSION

We have developed an assay that detects and quantifies specific aggrecan fragments generated by aggrecanase-mediated cleavage. Because aggrecanase mediates degradation of human articular aggrecan in joint disease, the KS/mAb OA-1 ELISA may serve as a biomarker assay for evaluation of preclinical and clinical samples.

Glycosaminoglycans Modulate Activation, Activity, and Stability of Tripeptidyl-peptidase I in Vitro and in Vivo

Tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal exopeptidase that sequentially removes tripeptides from the N termini of polypeptides and shows a minor endoprotease activity. Mutations in TPP I lead to classic late-infantile neuronal ceroid lipofuscinosis, a neurodegenerative lysosomal storage disease. TPP I proenzyme is converted in lysosomes into a mature enzyme with the assistance of another protease and is able to autoactivate in acidic pH in vitro via a unimolecular mechanism. Because autoactivation in vitro at the pH values reported for lysosomes generated inactive enzyme, we intended to determine whether physiologically relevant factors can modify this process to also make it plausible in vivo. Here, we report that high ionic strength and glycosaminoglycans (GAGs) increase yields (ionic strength) or yields and rates (GAGs) of activation, enhance degradation of liberated TPP I prosegment fragments, and switch effective autoactivation of TPP I proenzyme toward less acidic pH values (up to pH 6.0). Although ionic strength and GAGs also inhibited TPP I activity in vitro and in living cells, the degree of inhibition (from 20 to 60%) appears to be of rather limited functional significance. Importantly, binding to GAGs improved thermal stability of TPP I and protected the enzyme against alkaline pH-induced denaturation in vitro (t((1/2)) of mature enzyme at pH 7.4 increased by approximately 8-fold in the presence of heparin) and in vivo ( approximately 2-fold higher loss of TPP I in cells deficient in GAGs than in control cells after bafilomycin A1 treatment). These findings elucidate a potent physiologically relevant mechanism of TPP I regulation by GAGs and suggest that generation of the active enzyme via autoactivation can be accomplished not only in vitro but in vivo as well.

The release of crosslinked peptides from type II collagen into human synovial fluid is increased soon after joint injury and in osteoarthritis

OBJECTIVE

To determine concentrations of crosslinked C-telopeptide fragments of type II collagen (CTX-II) in synovial fluid (SF) from patients with joint injury, osteoarthritis (OA), or other knee arthritides.

METHODS

Two study groups were used: a cross-sectional group, which included healthy-knee volunteers (reference group [REF]) and patients with pseudogout (PPA), an anterior cruciate ligament tear with or without a meniscus tear (INJ), or primary knee OA (POA); and a longitudinal group, which included patients with arthroscopic cartilage changes or septic arthritis. CTX-II was quantified by competition enzyme-linked immunosorbent assay (ELISA) based on a monoclonal antibody that recognized the C-terminus of the peptide EKGPDP as a proteolytic neoepitope. Aggrecan fragments, matrix metalloproteinases 1 and 3, and tissue inhibitor of metalloproteinases 1 were determined by ELISAs.

RESULTS

Concentrations of CTX-II in SF were higher in patients with PPA, INJ, and POA than in the REF group (P < 0.001). After joint injury, mean levels of CTX-II in SF were increased above REF levels at all time intervals (P < 0.001), and were highest within hours after trauma. In those in the longitudinal study group with joint cartilage damage, variation coefficients for CTX-II were 81% (between patients) and 64% (within patient), monitored over 1 year. In a patient with septic arthritis, SF CTX-II increased at the onset of symptoms, and peaked 30-fold higher than the baseline. Concentrations of all biomarkers decreased with successful treatment.

CONCLUSION

This is the first report to describe the release into SF of soluble molecular fragments specific for the degradation of mature, crosslinked, type II collagen (CII) in human OA and joint injury. The results provide strong evidence that the integrity of the CII network of cartilage is compromised soon after joint injury and in arthritis. This early degradation of CII may represent an important treatment target.

Immunoblotting assays for keratan sulfate

The detection of microquantities of glycosaminoglycans (GAGs) in biological samples has been hampered by the lack of sensitive methods. In this paper we describe the modification and development of three sensitive assays capable of detecting nanogram quantities of GAGs in biological samples. The first assay detects total GAGs. It is a modified Alcian blue dye precipitation assay in which the dye binds to the negatively charged GAGs in CsCl-fractionated extracts from chicken tendons. This assay compares favorably with the widely used uronic acid assay in terms of its sensitivity and ability to detect all classes of GAGs, including keratan sulfate (KS). Two other assays, dot-blotting and immunoblotting, detect KS in complex mixtures and can be easily adapted for the detection of other GAGs. Both take advantage of binding of carboxyl and sulfate groups of GAGs to trivalent neodymium. In dot-blotting, samples were directly blotted onto nitrocellulose membrane soaked in Nd(2)(SO(4))(3) buffer, and KS was detected with the monoclonal anti-KS 5-D-4 antibody and an avidin-biotin complex detection system. In immunoblotting, the samples were first separated in 28% polyacrylamide gels, transferred onto a Nd(2)(SO(4))(3)-soaked nitrocellulose membrane using a phosphate buffer system, and stained and developed using the same protocol as in dot-blotting. Whereas dot-blotting allows the use of very low quantities of samples because of its high sensitivity (lower detection limit was 5 ng), immunoblotting provides more specificity.

Evaluation of potential organ culture media for eye banking using human donor corneas

AIM

To evaluate the ability of different commercially available cell culture solutions to preserve human donor corneas during 3 weeks of "closed system" organ culture at physiological temperature. This screening was performed in an attempt to establish a rational basis for the development of a serum-free organ culture medium for eye banking.

METHODS

72 normal human donor corneas were organ cultured for 21 days at 31 degrees C in eight different test media (nine corneas in each group). The basic culture solutions included: minimal essential medium (MEM), MEM with stabilised L-glutamine, M199, DIF-1000, SFM, F99, and F99 with ascorbic acid, insulin, bFGF, transferrin, selenium, and lipids (termed F99-Sr). All media were supplemented with 2% fetal calf serum (FCS), except for MEM, which was also studied at 8% FCS. The evaluation parameters included: (1) the endothelial cell loss as evaluated using trypan blue staining; (2) the ability of keratocytes and endothelial cells to incorporate tritiated uridine into RNA as evaluated using autoradiography and digital image analysis; (3) the leakage of immunogenic keratan sulphate as assessed using ELISA; and (4) changes in storage medium pH, glucose, and lactate content.

RESULTS

SFM induced the lowest endothelial cell loss of 14% (SD 2%) and the highest RNA synthesis rates of all test solutions supplemented with 2% FCS. Corneas stored in SFM also showed the least leakage of keratan sulphate and the highest glucose consumption and lactate production. In five media (MEM with 2% FCS, MEM with stabilised L-glutamine, M199, F99, and F99-Sr), comparable and intermediate potentials for organ culture were observed with endothelial cell loss of 16-19%. By contrast, 29% (4%) of the endothelium was lost after storage in DIF-1000. Interestingly, the use of 8% FCS (in MEM) had a marked protective effect on the endothelium, which showed the highest RNA synthetic activity combined with a cell loss of only 11% (4%), compared with 19% (6%) at 2% FCS (p<0.05).

CONCLUSION

Among the present test solutions, SFM appears to be the most prominent candidate for a new corneal organ culture medium and should be further tested and possibly refined to effectively substitute serum addition.

Changes in joint cartilage aggrecan after knee injury and in osteoarthritis

OBJECTIVE

To determine the concentrations of aggrecan fragments in synovial fluid from patients with knee joint injury, osteoarthritis (OA), or acute pyrophosphate arthritis (PPA; pseudogout), and to test their relative reactivity with the 846 epitope, a putative marker of cartilage aggrecan synthesis.

METHODS

Samples of knee joint fluid from 385 patients and 9 healthy-knee volunteers were obtained in a cross-sectional study. Study groups were acute PPA/ pseudogout (n = 60), anterior cruciate ligament (ACL) rupture (n = 159), meniscus lesion (n = 129), and primary knee OA (n = 37). The 846 epitope on aggrecan was assayed by competitive solution-phase radioimmunoassay. Aggrecan fragments were assayed by enzyme-linked immunosorbent assay using a monoclonal antibody (1-F21). Cartilage oligomeric matrix protein (COMP), C-propeptide of type II collagen (CPII), bone sialoprotein, matrix metalloproteinases 1 and 3, and tissue inhibitor of metalloproteinases 1 were previously quantified by immunoassays.

RESULTS

Reactivity of the 846 epitope was increased in all study groups compared with the reference group, and was highest in patients with primary OA. The median levels (in microg fetal aggrecan equivalents/ml) of the epitope were 0.28 (range 0.24-0.47) in the reference group, 0.48 (range 0.26-1.32) in PPA/pseudogout, 0.61 (range 0.12-2.87) in ACL rupture, 0.53 (range 0.22-3.02) in meniscus lesion, and 0.68 (range 0.31-4.31) in primary OA. The 846 epitope reactivity per microg aggrecan fragments in the joint fluid was higher in late-stage OA than in early-stage OA. Epitope 846 reactivity correlated positively with several markers of matrix turnover, particularly with COMP (r(s) = 0.421) and CPII (r(s) = 0.307).

CONCLUSION

The observed differences in 846 epitope reactivity in synovial fluid, and its concentration in relation to aggrecan and other markers of matrix turnover, were consistent with marked ongoing changes in aggrecan turnover after joint injury and in the development of OA. OA is thus a disease characterized by dynamic changes in tissue macromolecule turnover, which is reflected by measurable changes in aggrecan epitopes in the synovial fluid.

Keratan sulfate as a potential biomarker of loading of the intervertebral disc

STUDY DESIGN

A review of the literature.

OBJECTIVE

To investigate the potential of serum levels of keratan sulfate as a biomarker of the effects of loading of the spine.

SUMMARY OF BACKGROUND DATA

Exposure to mechanical loading of the spine causes changes in metabolism of intervertebral discs, eventually leading to accelerated disc degeneration. This process is characterized by the degradation of proteoglycans, which is reflected by an increase in the blood level of proteoglycan components. The serum level of keratan sulfate, an epitope present on these proteoglycan components, has been suggested as a marker of changes in metabolism of cartilaginous tissues.

METHODS

A review of the literature on serum keratan sulfate levels in relation to degenerative changes in cartilaginous tissue.

RESULTS

In a number of studies keratan sulfate in serum was reported to be related to degeneration of articular cartilage in patients with osteoarthritis. In addition, massive and rapid degradation of intervertebral discs was determined to result in a large rise in serum keratan sulfate levels. Whether degenerative changes of intervertebral discs induced by mechanical stress also cause a detectable increase in serum keratan sulfate should be subjected to further investigation.

CONCLUSION

Quantification of keratan sulfate in serum offers a promising measure for the early effects of mechanical loading of the spine, but research is needed for validation.

Viability of human corneal keratocytes during organ culture

The viability of human corneal keratocytes was assessed during four weeks of 'closed system' organ culture at 31 degrees C. After 28 days of culturing, the entire keratocyte population was still alive and viable because all cells incorporated uridine; a parameter for RNA-synthesis. During the first 14 days, mitoses were found in the anterior half of the stroma (0.23% mitoses per 48 h), while only few keratocytes were able to divide at day 28 (0.01% mitoses per 48 h). Metabolic parameters revealed a progressing acidosis in the medium with oxygen and glucose depletion. Immunological measurements of keratan sulphate proteoglycan suggested that approximately 1% of the total content was lost during the period. In conclusion, our current organ culture technique can maintain a viable keratocyte population for four weeks; a viable stroma can be grafted within this period.

Demonstration of immunogenic keratan sulphate in commercial chondroitin 6-sulphate from shark cartilage. Implications for ELISA assays

The prototype monoclonal keratan sulphate (KS) antibody 5D4 that is widely used for detection of KS in tissues and biological fluids reacts strongly with commercial low grade shark cartilage chondroitin 6-sulphate. Characterization of the immunogenic material by chondroitinase ABC digestion, ELISA inhibition studies, immunoblotting and HPLC analyses confirmed the presence of substantial amounts of KS, probably as a large proteoglycan (> 120 kDa). Commercial and heterogenic glycosaminoglycan preparations therefore must be used with great caution in immunological analyses. On the other hand the shark cartilage chondroitin 6-sulphate is an easy accessible source of immunogenic KS that can be used as a reference standard and as coating antigen in KS-ELISAs. The concentration of immunogenic KS in synovial fluid measured with an ELISA based solely on reagents of shark cartilage chondroitin 6-sulphate correlated well (r = 0.90) with the concentrations obtained with a traditional KS-ELISA that uses purified aggrecan as standard and coating antigen, and KS in both serum and synovial fluid could be measured with sufficient linearity.