quantitative determination of collagen cross-links.

The primary functional role of collagen is as a supporting tissue and it is now established that the aggregated forms of the collagen monomers are stabilised to provide mechanical strength by a series of intermolecular cross-links. In order to understand the mechanical properties of collagen, it is necessary to identify and quantitatively determine the concentration of the cross-links during their changes with maturation, ageing and disease. These cross-links are formed by oxidative deamination of the epsilon-amino group of the single lysine or hydroxylysine in the amino and carboxy telopeptides of collagen by lysyl oxidase, the aldehyde formed reacting with a specific lysine or hydroxylysine in the triple helix. The divalent Schiff base and keto-amine bonds so formed link the molecules head to tail and spontaneously convert during maturation to trivalent cross-links, a histidine derivative and cyclic pyridinolines and pyrroles, respectively. These latter bonds are believed to be transverse inter-fibrillar cross-links, and are tissue rather than species specific. We describe the determination of these cross-links in detail.Elastin is also stabilised by cross-linking based on oxidative deamination of most of its lysine residues to yield tetravalent cross-links, desmosine and iso-desmosine, the determination of which is also described.A second cross-linking pathway occurs during ageing (and to a greater extent in diabetes mellitus) involving reaction with tissue glucose. The initial product glucitol-lysine can be determined as furosine and pyridosine, and determination of advanced glycation end-products believed to be cross-links, such as pentosidine, are also described.

Maturation of tissue engineered cartilage implanted in injured and osteoarthritic human knees.

The regeneration of damaged organs requires that engineered tissues mature when implanted at sites of injury or disease. We have used new analytic techniques to determine the extent of tissue regeneration after treatment of knee injury patients with a novel cartilage tissue engineering therapy and the effect of pre-existing osteoarthritis on the regeneration process. We treated 23 patients, with a mean age of 35.6 years, presenting with knee articular cartilage defects 1.5 cm2 to 11.25 cm2 (mean, 5.0 cm2) in area. Nine of the patients had X-ray evidence of osteoarthritis. Chondrocytes were isolated from healthy cartilage removed at arthroscopy. The cells were cultured for 14 days, seeded onto esterified hyaluronic acid scaffolds (Hyalograft C), and grown for a further 14 days before implantation. A second-look biopsy was taken from each patient after 6 to 30 months (mean, 16 months). After standard histological analysis, uncut tissue was further analyzed using a newly developed biochemical protocol involving digestion with trypsin and specific, quantitative assays for type II collagen, type I collagen, and proteoglycan, as well as mature and immature collagen crosslinks. Cartilage regeneration was observed as early as 11 months after implantation and in 10 out of 23 patients. Tissue regeneration was found even when implants were placed in joints that had already progressed to osteoarthrosis. Cartilage injuries can be effectively repaired using tissue engineering, and osteoarthritis does not inhibit the regeneration process.

The role of the alpha2 chain in the stabilization of the collagen type I heterotrimer: a study of the type I homotrimer in oim mouse tissues.

We have previously reported that the fragility of skin, tendon and bone from the oim mouse is related to a significant reduction in the intermolecular cross-linking. The oim mutation is unlikely to affect the efficacy of the lysyl oxidase, suggesting that the defect is in the molecule and fibre. We have therefore investigated the integrity of both the oim collagen molecules and the fibre by differential scanning calorimetry. The denaturation temperature of the oim molecule in solution and the fibre from tail tendon were found to be higher than the wild-type by 2.6deg.C and 1.9deg.C, respectively. With the loss of the alpha2 chain, the hydroxyproline content of the homotrimer is higher than the heterotrimer, which may account for the increase. There is a small decrease in the enthalpy of the oim fibres but it is not significant, suggesting that the amount of disorder of the triple-helical molecules and of the fibres is small and involves only a small part of the total bond energy holding the helical structure together. The difference in denaturation temperature of the skin collagen molecules (t(m)) and fibres (t(d)) is significantly lower for the oim tissues, 19.9deg.C against 23.1deg.C, indicating reduced molecular interactions and hence packing of the molecules in the fibre. Computation of the volume fraction of the water revealed that the interaxial separation of the oim fibres was indeed greater, increasing from 19.6A to 21.0A. This difference of 1.4A, equivalent to a C-C bond, would certainly decrease the ability of the telopeptide aldehyde to interact with the epsilon -amino group from an adjacent molecule and form a cross-link. We suggest, therefore, that the reduction of the cross-linking is due to increased water content of the fibre rather than a distortion of the molecular structure. The higher hydrophobicity of the alpha2 chain appears to play a role in the stabilisation of heterotrimeric type I collagen, possibly by increasing the hydrophobic interactions between the heterotrimeric molecules, thereby reducing the water content and increasing the binding of the molecules in the fibre.

Quantitative outcome measures of cartilage repair in patients treated by tissue engineering.

Reliable and reproducible outcome measures are essential to assess the efficacy of competing and novel tissue-engineering techniques. The aim of this study was to compare traditional histological analyses with newly developed quantitative biochemical outcome measures for the repair of articular cartilage. The production of a new anti-peptide antibody and the development and validation of a novel method for the extraction and immunoassay of type I collagen are described. The assay was used, in conjunction with existing assays for type II collagen and proteoglycans, to measure levels of the matrix components in repair tissue biopsies obtained from patients treated with the new tissue-engineering therapy Hyalograft C. Frozen sections cut from the same biopsies were stained for proteoglycans, using safranin O, and immunohistochemical analysis was used to assess type I and II collagen staining. Although there was general agreement between the extent of staining and the amounts of the three matrix components, there was a large degree of overlap in biochemical content between biopsies classified histologically on the basis of low, moderate, or abundant staining. The results demonstrate that histological grading of matrix protein abundance to classify repair cartilage as hyaline or fibrocartilagenous is often misleading. In addition, we demonstrate for the first time the ability to measure collagen cross-links in repair tissue biopsies and show that it can be used as a surrogate marker for tissue maturity. Our new range of biochemical techniques provides a standardized method to assess the quality of both engineered cartilage produced in vitro and repair tissue biopsies obtained after in vivo implantation.

Effects of in vitro preculture on in vivo development of human engineered cartilage in an ectopic model.

We investigated whether, and under which conditions (i.e., cell-seeding density, medium supplements), in vitro preculture enhances in vivo development of human engineered cartilage in an ectopic nude mouse model. Monolayer-expanded adult human articular chondrocytes (AHACs) were seeded into Hyalograft C disks at 1.3 x 10(7) cells/cm3 (low density) or 7.6 x 10(7) cells/cm3 (high density). Constructs were directly implanted subcutaneously in nude mice for up to 8 weeks or precultured for 2 weeks before implantation. Preculture medium contained either transforming growth factor-beta1 (TGF-beta1, 1 ng/mL), fibroblast growth factor-2, and platelet-derived growth factor (proliferating medium) or TGF-beta1 (10 ng/mL) and insulin (differentiating medium). Both in vitro and after in vivo implantation, constructs derived by cell seeding at high versus low density and precultured in differentiating versus proliferating medium generated more cartilaginous tissues containing higher amounts of glycosaminoglycan and collagen type II and lower amounts of collagen type I, and with higher equilibrium moduli. As compared with direct implantation of freshly seeded scaffolds, preculture of AHAC-Hyalograft C constructs in differentiating medium, but not in proliferating medium, supported enhanced in vivo development of engineered cartilage. The effect of preculture was more pronounced when constructs were seeded at low density as compared with high density. This study indicates that preculture of human engineered cartilage in differentiating medium has the potential to provide grafts with higher equilibrium moduli and enhanced in vivo developmental capacity than freshly seeded scaffolds. These findings need to be validated in an orthotopic model system.

Phenotypic expression of osteoblast collagen in osteoarthritic bone: production of type I homotrimer.

The metabolism and total amount of the collagen of subchondral bone are increased several fold in osteoathritic femurs compared with controls. We now report for the first time that the quality of the collagen is modified by the formation of type I homotrimer. The homotrimer fibre has been reported to possess a reduced mechanical strength and mineralisation in bone. The presence of the latter therefore accounts for narrower disorganised collagen fibres and decreased mineralisation, and a reduction in mechanical stability of the osteoarthritic femoral head. These changes in the subchondral bone are likely to be of considerable importance in the pathogenesis of osteoarthritis.

Quantitative analysis of repair tissue biopsies following chondrocyte implantation.

Outcome measures for cartilage repair techniques include clinical assessment of functional status, magnetic resonance imaging, mechanical indentation in situ and second-look biopsies, which are used for detailed ex vivo histological and immunohistochemical assessment. Biopsy analysis is considered an important outcome measure, despite being highly invasive, since it provides a visual record of the spatial organization of matrix proteins and cells. We propose that the value of second-look biopsies would be significantly enhanced if accurate quantification of cartilage matrix molecules could also be obtained. The goal of our work has been to develop a combined method for histological and biochemical analysis of a single biopsy. We have developed a method of cutting frozen sections of cartilage and recovering the uncut tissue for subsequent biochemical analysis. We have also developed a range of miniaturized assays that can be performed after cartilage digestion with trypsin. In this way we are now able to analyse biopsies with a wet weight as low as 5 mg using both histological and biochemical methods, so obtaining the maximum amount of information from the minimum volume of tissue. This new approach will allow a more accurate assessment of the quality of cartilage repair tissue than histological analysis alone.

Biochemical and mechanical properties of subchondral bone in osteoarthritis.

The subchondral bone has long been known to thicken in osteoarthritis. However, recent evidence has demonstrated that the turnover of the bone is increased several fold, and further suggests that the thickening occurs prior to degradation of the articular cartilage, indicating that it plays a role in the pathogenesis of osteoarthritis. The mechanical and biochemical properties of the subchondral bone are therefore of particular interest in any attempt to determine the nature of the factors initiating osteoarthritis. We have shown that the subchondral bone collagen of the femoral head possessed a 20-fold increase in turnover, as assessed by procollagen rate of synthesis and metalloproteinase degradation, and a 25% decrease in mineralisation. This increased metabolism and high lysyl hydroxylation leads to narrower and weaker fibres. Additionally the phenotypic expression of the osteoblasts is modified to produce increasing proportions of type I homotrimer in addition to the normal type I heterotrimer, which further reduces the mechanical strength of the bone. Overall, the narrow immature collagen fibres, the reduction in pyrrole cross-linking, decreased mineralisation, and increased amounts of type I homotrimer, all contribute to a weakening of the mechanical properties of the subchondral bone.

Three-dimensional cartilage tissue engineering using adult stem cells from osteoarthritis patients.

OBJECTIVE: To determine whether it is possible to engineer 3-dimensional hyaline cartilage using mesenchymal stem cells derived from the bone marrow (BMSCs) of patients with osteoarthritis (OA). METHODS: Expanded BMSCs derived from patients with hip OA were seeded onto polyglycolic acid scaffolds and differentiated using transforming growth factor beta3 in the presence or absence of parathyroid hormone-related protein (PTHrP) to regulate hypertrophy. Micromass pellet cultures were established using the same cells for comparison. At the end of culture, the constructs or pellets were processed for messenger RNA (mRNA) analysis by quantitative real-time reverse transcription-polymerase chain reaction. Matrix proteins were analyzed using specific assays. RESULTS: Cartilage constructs engineered from BMSCs were at least 5 times the weight of equivalent pellet cultures. Histologic, mRNA, and biochemical analyses of the constructs showed extensive synthesis of proteoglycan and type II collagen but only low levels of type I collagen. The protein content was almost identical to that of cartilage engineered from bovine nasal chondrocytes. Analysis of type X collagen mRNA revealed a high level of mRNA in chondrogenic constructs compared with that in undifferentiated BMSCs, indicating an increased risk of hypertrophy in the tissue-engineered cells. However, the inclusion of PTHrP at a dose of 1 microM or 10 microM during the culture period resulted in significant suppression of type X collagen mRNA expression and a significant decrease in alkaline phosphatase activity, without any loss of the cartilage-specific matrix proteins. CONCLUSION: Three-dimensional hyaline cartilage can be engineered using BMSCs from patients with OA. This method could thus be used for the repair of cartilage lesions.

Biochemical markers of the mechanical quality of engineered hyaline cartilage.

The aim of this study was to determine whether or not biochemical markers can be used as surrogate measures for the mechanical quality of tissue engineered cartilage. The biochemical composition of tissue engineered cartilage constructs were altered by varying either (i) the initial cell seeding density of the scaffold (seeding density protocol) or (ii) the length of time the engineered tissue was cultured (culture period protocol). The aggregate or Young's moduli of the constructs were measured (by confined or unconfined compression respectively), and compared with the composition of the extracellular matrix by quantitative measurement of the glycosaminoglycan (GAG), hydroxyproline, collagen I and collagen II and collagen cross-links. The aggregate modulus correlated positively with both GAG and collagen II content, but not with collagen I content. Young's modulus correlated positively with GAG, collagen II and collagen I content, and the ratio of mature to immature cross-links. There was no significant correlation of Young's Modulus with total collagen measured as hydroxyproline content. These results suggested that hydroxyproline determination may be an unreliable indicator of mechanical quality of tissue engineered cartilage, and that a measure of collagen II and GAG content is required to predict the biomechanical quality of tissue engineered cartilage.

Cruciate ligament laxity and femoral intercondylar notch narrowing in early-stage knee osteoarthritis.

OBJECTIVE: The influence of the cruciate ligaments in spontaneous osteoarthritis (OA) is not understood, although ligament rupture is known to cause secondary OA. Additionally, femoral notch narrowing at the anterior cruciate ligament (ACL) insertion site is associated with disease severity, but it is unknown whether ligament deterioration precedes or follows osteophyte formation. We examined cruciate ligament mechanics and metabolism and the intercondylar notch width in OA-prone Dunkin-Hartley (DH) guinea pigs at ages up to and including the age at OA onset (24 weeks), and compared the data with those in age-matched controls (Bristol strain 2 [BS2] guinea pigs). METHODS: Guinea pigs were assessed at 3, 6, 9, 12, 16, 20, 24, and 36 weeks of age. ACLs were mechanically tested, and the intercondylar notch width index (NWI) was determined. Cruciate ligament metabolism was determined by measuring the following markers of collagen turnover: matrix metalloproteinase 2 (MMP-2), tissue inhibitor of metalloproteinases 2, C-terminal type I procollagen propeptide (PICP), and the immature collagen-derived crosslink dihydroxylysinonorleucine (DHLNL). RESULTS: DH guinea pigs had significantly laxer ACLs than did BS2 guinea pigs, at 12, 16, and 24 weeks. We observed elevated levels of pro and active MMP-2, PICP, and DHLNL in the cruciate ligaments of DH animals at most ages, compared with BS2 guinea pigs. The NWI in DH animals was significantly lower than that in BS2 guinea pigs at 24 and 36 weeks. CONCLUSION: In DH guinea pigs, laxer ACLs, which are associated with increased collagen turnover, may cause joint instability and predispose these animals to the early onset of OA. Decreased intercondylar notch width in the DH animals indicates that bone remodeling at the ACL insertion site is a response to elevated ACL laxity.