Cartilage type IX collagen is cross-linked by hydroxypyridinium residues

Application of Alginate Hydrogels for Next-Generation Articular Cartilage Regeneration

The articular cartilage has insufficient intrinsic healing abilities, and articular cartilage injuries often progress to osteoarthritis. Alginate-based scaffolds are attractive biomaterials for cartilage repair and regeneration, allowing for the delivery of cells and therapeutic drugs and gene sequences. In light of the heterogeneity of findings reporting the benefits of using alginate for cartilage regeneration, a better understanding of alginate-based systems is needed in order to improve the approaches aiming to enhance cartilage regeneration with this compound. This review provides an in-depth evaluation of the literature, focusing on the manipulation of alginate as a tool to support the processes involved in cartilage healing in order to demonstrate how such a material, used as a direct compound or combined with cell and gene therapy and with scaffold-guided gene transfer procedures, may assist cartilage regeneration in an optimal manner for future applications in patients.

Aspartic acid racemization reveals a high turnover state in knee compared with hip osteoarthritic cartilage

OBJECTIVE

We investigated tissue turnover in healthy and osteoarthritic cartilage. We challenge long held views that osteoarthritis (OA) is dominated by a similar turnover process in all joints and present evidence that hip and knee cartilage respond very differently to OA.

METHODS

d- and l-Aspartate (Asp) were quantified for whole cartilage, collagen and non-collagenous components of cartilage obtained at the time of joint replacement. We computed the Asp racemization ratio (Asp-RR = d/d + l Asp), reflecting the proportion of old to total protein, for each component.

RESULTS

Compared with hip OA, knee OA collagen fibrils (P < 0.0001), collagen (P = 0.007), and non-collagenous proteins (P = 0.0003) had significantly lower age-adjusted mean Asp-RRs consistent with elevated protein synthesis in knee OA. Knee OA collagen had a mean hydroxyproline/proline (H/P) ratio of 1.2 consistent with the presence of type III collagen whereas hip OA collagen had a mean H/P ratio of 0.99 consistent with type II collagen. Based on Asp-RR, the relative age was significantly different in knee and hip OA (P < 0.0005); on average OA knees were estimated to be 30 yrs 'younger', and OA hips 10 yrs 'older' than non-OA.

CONCLUSIONS

The metabolic response to OA was strikingly different by joint site. Knee OA cartilage evinced an anabolic response that appeared to be absent in hip OA cartilage. These results challenge the long held view that OA cartilage is capable of only minimal repair and that collagen loss is irreversible.

The development of a mature collagen network in cartilage from human bone marrow stem cells in Transwell culture

Damaged hyaline cartilage shows a limited capacity for innate repair. Potential sources of cells to augment the clinical repair of cartilage defects include autologous chondrocytes and mesenchymal stem cells. We have reported that culture of human bone marrow mesenchymal stem cells with specific growth and differentiation factors as shallow multilayers on Transwell permeable membranes provided ideal conditions for chondrogenesis. Rigid translucent cartilaginous disks formed and expressed cartilage-specific structural proteins aggrecan and type II collagen. We report here the analysis of the collagen network assembled in these cartilage constructs and identify key features of the network as it became mature during 28 days of culture. The type II collagen was co-polymerized with types XI and IX collagens in a fibrillar network stabilized by hydroxylysyl pyridinoline cross-links as in epiphyseal and hyaline cartilages. Tandem ion-trap mass-spectrometry identified 3-hydroxylation of Proline 986 and Proline 944 of the α1(II) chains, a post-translational feature of human epiphyseal cartilage type II collagen. The formation of a type II collagen based hydroxy-lysyl pyridinoline cross-linked network typical of cartilage in 28 days shows that the Transwell system not only produces, secretes and assembles cartilage collagens, but also provides all the extracellular mechanisms to modify and generate covalent cross-links that determine a robust collagen network. This organized assembly explains the stiff, flexible nature of the cartilage constructs developed from hMSCs in this culture system.

Effect of exercise on articular cartilage

This review primarily focuses on how the macromolecular composition and architecture of articular cartilage and its unique biomechanical properties play a pivotal role in the ability of articular cartilage to withstand mechanical loads several magnitudes higher than the weight of the individual. Current findings on short-term and long-term effects of exercise on human articular cartilage are reviewed, and the importance of appropriate exercises for individuals with normal and diseased or aberrated cartilage is discussed.

Type IX collagen neo-deposition in degenerative discs of surgical patients whether genotyped plus or minus for COL9 risk alleles

STUDY DESIGN

Immunohistochemical analysis of type IX collagen in disc tissue from spinal fusion patients.

OBJECTIVE

To determine if collagen IX can be detected in adult disc tissue removed at spinal fusion surgery from patients either with or without degeneration-associated tryptophan single nucleotide polymorphisms (SNPs) and whether the distribution is associated either with severity of degeneration or incidence of a collagen IX SNP genotype.

SUMMARY OF BACKGROUND DATA

Genetic factors are strongly associated with risk of development and/or progression of disc degeneration. Two SNPs that introduce tryptophan polymorphisms in COL9A2 and COL9A3 are independently linked to an increased risk of lumbar disc disease. Although tryptophan variants are associated with accelerated degeneration, it is not known if collagen IX can be detected in adult disc tissue.

METHODS

We selected age-matched disc samples from five clinical groups: fracture with Trp(-) (six cases), herniation (six cases), degeneration (five cases), spondylolisthesis with Trp(-) (eight cases), and spondylolisthesis/herniation/fracture with Trp(+) (six cases of Trp3 allele and one case of Trp2 allele). Using hematoxylin and eosin staining and immunohistochemical staining (collagens IX and IIA), 78 sections from 32 patients were analyzed. Selected disc tissues were assayed biochemically for collagen IX.

RESULTS

Focal deposition of collagen IX was observed in regions of adult human disc tissue from spines showing degenerative changes in patients whether or not they were positive for a tryptophan SNP. However, in nondegenerative control disc tissue from fracture cases, little or no collagen IX was detected. The latter finding was confirmed by direct biochemical analyses for collagen IX in pooled samples of normal adult human annulus fibrosus or nucleus pulposus.

CONCLUSION

During growth and maturation of the disc, collagen IX is presumably removed completely during matrix remodeling so that the protein is absent from normal adult annulus and nucleus but can reappear at sites of degeneration presumably as part of a repair response to mechanical injury.

Micro- and nanomechanical analysis of articular cartilage by indentation-type atomic force microscopy: validation with a gel-microfiber composite

As documented previously, articular cartilage exhibits a scale-dependent dynamic stiffness when probed by indentation-type atomic force microscopy (IT-AFM). In this study, a micrometer-size spherical tip revealed an unimodal stiffness distribution (which we refer to as microstiffness), whereas probing articular cartilage with a nanometer-size pyramidal tip resulted in a bimodal nanostiffness distribution. We concluded that indentation of the cartilage's soft proteoglycan (PG) gel gave rise to the lower nanostiffness peak, whereas deformation of its collagen fibrils yielded the higher nanostiffness peak. To test our hypothesis, we produced a gel-microfiber composite consisting of a chondroitin sulfate-containing agarose gel and a fibrillar poly(ethylene glycol)-terephthalate/poly(butylene)-terephthalate block copolymer. In striking analogy to articular cartilage, the microstiffness distribution of the synthetic composite was unimodal, whereas its nanostiffness exhibited a bimodal distribution. Also, similar to the case with cartilage, addition of the negatively charged chondroitin sulfate rendered the gel-microfiber composite's water content responsive to salt. When the ionic strength of the surrounding buffer solution increased from 0.15 to 2 M NaCl, the cartilage's microstiffness increased by 21%, whereas that of the synthetic biomaterial went up by 31%. When the nanostiffness was measured after the ionic strength was raised by the same amount, the cartilage's lower peak increased by 28%, whereas that of the synthetic biomaterial went up by 34%. Of interest, the higher peak values remained unchanged for both materials. Taken together, these results demonstrate that the nanoscale lower peak is a measure of the soft PG gel, and the nanoscale higher peak measures collagen fibril stiffness. In contrast, the micrometer-scale measurements fail to resolve separate stiffness values for the PG and collagen fibril moieties. Therefore, we propose to use nanostiffness as a new biomarker to analyze structure-function relationships in normal, diseased, and engineered cartilage.

Differences in chain usage and cross-linking specificities of cartilage type V/XI collagen isoforms with age and tissue

Collagen type V/XI is a minor but essential component of collagen fibrils in vertebrates. We here report on age- and tissue-related variations in isoform usage in cartilages. With maturation of articular cartilage, the alpha1(V) chain progressively replaced the alpha2(XI) chain. A mix of the molecular isoforms, alpha1(XI)alpha1(V)alpha3(XI) and alpha1(XI)alpha2(XI)alpha3(XI), best explained this finding. A prominence of alpha1(V) chains is therefore characteristic and a potential biomarker of mature mammalian articular cartilage. Analysis of cross-linked peptides showed that the alpha1(V) chains were primarily cross-linked to alpha1(XI) chains in the tissue and hence an integral component of the V/XI polymer. From nucleus pulposus of the intervertebral disc (in which the bulk collagen monomer is type II as in articular cartilage), type V/XI collagen consisted of a mix of five genetically distinct chains, alpha1(XI), alpha2(XI), alpha3(XI), alpha1(V), and alpha2(V). These presumably were derived from several different molecular isoforms, including alpha1(XI)alpha2(XI)alpha3(XI), (alpha1(XI))(2)alpha2(V), and others. Meniscal fibrocartilage shows yet another V/XI phenotype. The findings support and extend the concept that the clade B subfamily of COL5 and COL11 gene products should be considered members of the same collagen subfamily, from which, in combination with clade A gene products (COL2A1 or COL5A2), a range of molecular isoforms has evolved into tissue-dependent usage. We propose an evolving role for collagen V/XI isoforms as an adaptable polymeric template of fibril macro-architecture.

Review: Collagen markers in early arthritic diseases

In arthritic diseases e.g. osteoarthritis (OA) and rheumatoid arthritis (RA), the stability of the collagen type II (CII) fibers, a major component of articular cartilage, is compromised with extensive proteolytic breakdown leading to cartilage erosion and joint deterioration. A clinical need for molecular markers that give instantaneous measure of rate of joint deterioration has developed, as other measurements e.g. arthroscopy, and joint space narrowing are insensitive to small changes in disease status over short periods of time. Owing to its exclusive presence in cartilaginous tissues, markers of CII synthesis and degradation have been extensively studied. Assays that measure these markers in biological fluids e.g. synovial fluid (SF), serum, and urine have been developed and applied to detect early disease onset, monitor disease progression, and response to anti-arthritic drugs. CII synthesis markers include the procollagen type II C-propeptide (PIICP) and the procollagen type IIA N-propeptide (PIIANP). CII degradation markers include CII C-telopeptide (CII-X), CII neoepitope (TIINE), helix II, C2C, CNBr 9.7, Coll 2-1, and Coll 2-1 NO(2). Most of these markers differentiate between early stages of OA, RA and reference controls. The best correlations with structural changes occur when measurements are made in SF while serum measurement frequently did not correlate with structural changes. Although the selection of an optimal marker or a set of markers is still problematic, few markers are of considerable utility in early detection and monitoring of arthritic diseases. The current challenge is to improve the discriminatory power of these markers so they can be used to guide therapeutic decisions.

Cryopreservation and in vitro expansion of chondroprogenitor cells isolated from the superficial zone of articular cartilage

Understanding the proliferation mechanisms of chondroprogenitor cells and their influence on cell differentiation is crucial in order to develop large-scale expansion processes for tissue engineering applications. Proliferation control mechanisms were mainly attributed to substrate limitation and cell-cell contact inhibition. The limiting substrates were found to be components of the FCS, with an optimal proliferation rate achieved in the presence of 40% FCS. In addition, the medium supply rate was found to be essential in reducing substrate limitation. In terms of FCS, 10 microL FCS cm(-2) h(-1) was the threshold feed rate required to prevent substrate limitation. Above this rate, maximum cell densities of 5.3 x 10(5) cells/cm2 were achieved, representing a 53-fold expansion. To reduce the need for high supply rates, the effect of specific growth factors was also investigated. Cell densities of 3.3 x 10(5) cells/cm2 were achieved in batch cultures using 40% FCS and 1 ng/mL TGF-beta1. Chondroprogenitor cells were expanded in this medium up to three passages without compromising their ability to differentiate and produce cartilage-like matrix in pellet cultures. In addition to substrate limitation, cell-cell contact, even at very sparse subconfluent densities, appeared capable of exerting some degree of growth inhibition. The cells exhibited deceleratory growth kinetics, characterized by a decrease of specific growth rates over time.

Cellular invasion of the chicken corneal stroma during development: regulation by multiple matrix metalloproteases and the lens

Avian corneal development requires cellular invasion into the acellular matrix of the primary stroma. Previous results show that this invasion is preceded by the removal of the fibril-associated type IX collagen, which possibly stabilizes matrices through interfibrillar cross-bridges secured by covalent crosslinks. In the present study, we provide evidence for the expression of three matrix metalloproteinases (MMPs) in early corneas, two of which act cooperatively to selectively remove type IX collagen in situ. In organ cultures, MMP inhibitors (either TIMP-2 or a synthetic inhibitor) resulted in arrested development, in which collagen IX persisted, and the stroma remained compact and acellular. We also show that blocking covalent crosslinking of collagen allows for cellular invasion to occur, even when the removal of type IX collagen is prevented. Thus, one factor regulating corneal invasion is the physical structure of the matrix, which can be modified by either selective proteolysis or reducing interfibrillar cross-bridges. We also detected another level of regulation of cellular invasion involving inhibition by the underlying lens. This block, which seems to influence invasive behavior independently of matrix modification, is a transient event that is released in ovo just before invasion proceeds.

Covalent cross-linking of the NC1 domain of collagen type IX to collagen type II in cartilage

From a study to understand the mechanism of covalent interaction between collagen types II and IX, we present experimental evidence for a previously unrecognized molecular site of cross-linking. The location relative to previously defined cross-linking sites predicts a specific manner of interaction and folding of collagen IX on the surface of nascent collagen II fibrils. The initial evidence came from Western blot analysis of type IX collagen extracted by pepsin from fetal human cartilage, which showed a molecular species that had properties indicating an adduct between the alpha1(II) chain and the C-terminal domain (COL1) of type IX collagen. A similar component was isolated from bovine cartilage in sufficient quantity to confirm this identity by N-terminal sequence analysis. Using an antibody that recognized the putative cross-linking sequence at the C terminus of the alpha1(IX) chain, cross-linked peptides were isolated by immunoaffinity chromatography from proteolytic digests of human cartilage collagen. They were characterized by immunochemistry, N-terminal sequence analysis, and mass spectrometry. The results establish a link between a lysine near the C terminus (in the NC1 domain) of alpha1(IX) and the known cross-linking lysine at residue 930 of the alpha1(II) triple helix. This cross-link is speculated to form early in the process of interaction between collagen IX molecules and collagen II polymers. A model of molecular folding and further cross-linking is predicted that can spatially accommodate the formation of all six known cross-linking interactions to the collagen IX molecule on a fibril surface. Of particular biological significance, this model can accommodate potential interfibrillar as well as intrafibrillar links between the collagen IX molecules themselves, so providing a mechanism whereby collagen IX could stabilize a collagen fibril network.

Intervertebral disc collagen. Usage of the short form of the alpha1(IX) chain in bovine nucleus pulposus

Nucleus pulposus, the central zone of the intervertebral disc, is gel-like and has a similar collagen phenotype to that of hyaline cartilage. Amino-terminal protein sequence analysis of the alpha1(IX)COL3 domain purified from bovine nucleus pulposus gave a different sequence to that of the long alpha1(IX) transcript expressed in hyaline cartilage and matched the predicted sequence of short alpha1(IX). The findings indicate that the matrix of bovine nucleus pulposus contains only the short form of alpha1(IX) that lacks the NC4 domain. The sequence encoded by exon 7, predicted from human COL9A1, is absent from both short and long forms of alpha1(IX) from bovine nucleus pulposus and articular cartilage. A structural analysis of the cross-linking sites occupied in type IX collagen from nucleus pulposus showed that usage of the short alpha1(IX) transcript in disc tissue had no apparent effect on cross-linking behavior. As in cartilage, type IX collagen of nucleus pulposus was heavily cross-linked to type II collagen and to other molecules of type IX collagen with a similar site occupancy.

Bioreactors mediate the effectiveness of tissue engineering scaffolds

We hypothesized that the mechanically active environment present in rotating bioreactors mediates the effectiveness of three-dimensional (3D) scaffolds for cartilage tissue engineering. Cartilaginous constructs were engineered by using bovine calf chondrocytes in conjunction with two scaffold materials (SM) (benzylated hyaluronan and polyglycolic acid); three scaffold structures (SS) (sponge, non-woven mesh, and composite woven/non-woven mesh); and two culture systems (CS) (a bioreactor system and petri dishes). Construct size, composition [cells, glycosaminoglycans (GAG), total collagen, and type-specific collagen mRNA expression and protein levels], and mechanical function (compressive modulus) were assessed, and individual and interactive effects of model system parameters (SM, SS, CS, SM*CS and SS*CS) were demonstrated. The CS affected cell seeding (higher yields of more spatially uniform cells were obtained in bioreactor-grown than dish-grown 3-day constructs) and subsequently affected chondrogenesis (higher cell numbers, wet weights, wet weight GAG fractions, and collagen type II levels were obtained in bioreactor-grown than dish-grown 1-month constructs). In bioreactors, mesh-based scaffolds yielded 1-month constructs with lower type I collagen levels and four-fold higher compressive moduli than corresponding sponge-based scaffolds. The data imply that interactions between bioreactors and 3D tissue engineering scaffolds can be utilized to improve the structure, function, and molecular properties of in vitro-generated cartilage.

Growth factors for sequential cellular de- and re-differentiation in tissue engineering

A model system for the in vitro generation of cartilaginous constructs was used to study a tissue engineering paradigm whereby sequentially applied growth factors promoted chondrocytes to first de-differentiate into a proliferative state and then re-differentiate and undergo chondrogenesis. Early cultivation in medium with supplemental TGF-beta1/FGF-2 doubled cell fractions in 2-week constructs compared to unsupplemented controls. Subsequent culture with supplemental IGF-I yielded large 4-week constructs with high fractions of cartilaginous extracellular matrix (ECM) and high compressive moduli, whereas prolonged culture with supplemental FGF-2 yielded small 4-week constructs with low ECM fractions and moduli. Sequential supplementation with TGF-beta1/FGF-2 and then IGF-I yielded 4-week constructs with type-specific mRNA expression and protein levels that were high for type II and negligible for type I collagen, in contrast to other growth factor regimens studied. The data demonstrate that structural, functional, and molecular properties of engineered cartilage can be modulated by sequential application of growth factors.

Stimulation of matrix formation in rabbit chondrocyte cultures by ascorbate. 1. Effect of ascorbate analogs and beta-aminopropionitrile

The most consistent effects of 0.2 mM L-ascorbate on monolayer cultures of rabbit articular chondrocytes were a diversion of incorporated radiosulfate into a pericellular matrix and enhancement of cell proliferation. Only with certain batches of fetal bovine serum (FBS) was there a cell-for-cell increase of proteoglycan synthesis. These actions increased as the cell inoculum rose from 0.5 to 2 x 10(5) cells/T25 flask. Maximal effects of ascorbate and D-isoascorbate were found over a range of 0.05-0.2 mM. L-Dehydroascorbic acid was less effective than either, and no stimulatory action was exerted by L-cysteine, glutathione, dithiothreitol, methylene blue, or phenazine methosulfate. Ascorbate increased the hypro:pro ratio of newly synthesized proteins. beta-Aminopropionitrile (1 mM) reduced the proportion of [3H]hydroxyproline and [35S]O4-proteoglycans in the ascorbate-supplemented matrix 31 and 7%, respectively. In corresponding electronmicrographs, the number of pericellular filaments was reduced. We conclude: (a) Ascorbate has a general anabolic effect on chondrocytes in culture and enhances matrix assembly through mechanisms other than its redox function; (b) deposition of proteoglycans in the matrix is not simply the result of mechanical entrapment by allysine- or hydroxyallysine-derived cross-linking of collagen; and (c) contradictory reports on the subject result from variations in the serum employed, inoculum density, and concentration of ascorbate.

Identification of an enhancer sequence within the first intron required for cartilage-specific transcription of the alpha2(XI) collagen gene

Type XI collagen, a heterotrimer composed of alpha1(XI), alpha2(XI) and alpha3(XI), is primarily synthesized by chondrocytes in cartilage and is also present in some other tissues. Type XI collagen plays a critical role in collagen fibril formation and skeletal morphogenesis. We investigated a tissue-specific transcriptional enhancer in the first intron of the alpha2(XI) collagen gene (Col11a2). Transient transfection assays using reporter gene constructs revealed that a 60-base pair (bp) segment within intron 1 increased promoter activity of Col11a2 in rat chondrosarcoma cells but not in either BalB/3T3 cells or undifferentiated ATDC5 cells, suggesting that it contained cell type-specific enhancer activity. In transgenic mice, this 60-bp fragment was also able to target beta-galactosidase expression to cartilage including the limbs and axial skeleton, with similar localization specificity as the full-length intron 1 fragment. Competition experiments in gel shift assays using mutated oligonucleotides showed that recombinant Sox9 bound to a 7-bp sequence, CTCAAAG, within the 60-bp segment. Anti-Sox9 antibodies supershifted the complex of the 60-bp segment with recombinant Sox9 or with rat chondrosarcoma cell extracts, confirming the binding of Sox9 to the enhancer. Moreover, a site-specific mutation within the 7-bp segment resulted in essentially complete loss of the enhancer activity in chondrosarcoma cells and transgenic mice. These results suggest that the 7-bp sequence within intron 1 plays a critical role in the cartilage-specific enhancer activity of Col11a2 through Sox9-mediated transcriptional activation.

Collagen in tissue-engineered cartilage: types, structure, and crosslinks

The function of articular cartilage as a weight-bearing tissue depends on the specific arrangement of collagen types II and IX into a three-dimensional organized collagen network that can balance the swelling pressure of the proteoglycan/water gel. To determine whether cartilage engineered in vitro contains a functional collagen network, chondrocyte-polymer constructs were cultured for up to 6 weeks and analyzed with respect to the composition and ultrastructure of collagen by using biochemical and immunochemical methods and scanning electron microscopy. Total collagen content and the concentration of pyridinium crosslinks were significantly (57% and 70%, respectively) lower in tissue-engineered cartilage that in bovine calf articular cartilage. However, the fractions of collagen types II, IX, and X and the collagen network organization, density, and fibril diameter in engineered cartilage were not significantly different from those in natural articular cartilage. The implications of these findings for the field of tissue engineering are that differentiated chondrocytes are capable of forming a complex structure of collagen matrix in vitro, producing a tissue similar to natural articular cartilage on an ultrastructural scale.

Ageing and zonal variation in post-translational modification of collagen in normal human articular cartilage. The age-related increase in non-enzymatic glycation affects biomechanical properties of cartilage

A biomechanical failure of the collagen network is postulated in many hypotheses of the development of osteoarthritis with advancing age. Here we investigate the accumulation of non-enzymatic glycation (NEG) products in healthy human articular cartilage, its relation to tissue remodelling and its role in tissue stiffening. Pentosidine levels were low up to age 20 years, and increased linearly after this age. This indicates extensive tissue remodelling at young age, and slow turnover of collagen after maturity has been reached. The slow remodelling is supported by the finding that enzymatic modifications of collagen (hydroxylysine, hydroxylysylpyridinoline, and lysylpyridinoline) were not related to age. The high remodelling is supported by levels of the crosslink lysylpyridinoline (LP) as a function of distance from the articular surface. LP was highest at the surface in mature cartilage (>20 years), whereas in young cartilage (<10 years) the opposite was seen; highest levels were close to the bone. LP levels in cartilage sections at age 14 years are high at the surface and close to the bone, but they are low in the middle region. This indicates that maturation of cartilage in the second decade of life starts in the upper half of the tissue, and occurs last in the tissue close to the bone. The effect of NEG products on instantaneous deformation of cartilage was investigated as a functional of topographical variations in pentosidine levels in vivo and in relation to in vitro induced NEG. Consistently, higher pentosidine levels were associated with a stiffer collagen network. A stiffer and more crosslinked collagen network may become more brittle and more prone to fatigue.

Immunolocalization of type IX collagen in normal and spontaneously osteoarthritic canine tibial cartilage and isolated chondrons

OBJECTIVE

The pericellular localization of type IX collagen in avian and mammalian hyaline cartilages remains controversial, while its distribution during osteoarthritic degeneration is poorly understood. This study aimed to compare and contrast the immunohistochemical distribution of type IX collagen in normal mature and spontaneously osteoarthritic canine tibial cartilage.

DESIGN

Thick vibratome sectioning techniques were evaluated and compared with isolated chondrons using a range of streptavidin-linked probes in combination with light, confocal and transmission electron microscopy.

RESULTS

In normal intact samples, type IX collagen was concentrated in the pericellular microenvironment, while a weaker extracellular reaction around each chondron separated the territorial matrix from the unstained interterritorial matrix. Further differentiation was evident in isolated chondrons where the fibrous pericellular capsule stained more intensely than the tail and interconnecting segments between columnated chondrons. Two regions of type IX reactivity were identified in osteoarthritic tissue: an intensely stained superficial reactive region below the eroding margins, and normal deep layer cartilage where pericellular staining persists. The superficial reactive region was characterized by chondron swelling and chondrocyte cluster formation, a loss of pericellular type IX staining, and a significant increase in matrix staining between clusters. Disintegration and loss of fibrillar collagens was evident in both the swollen microenvironment and adjacent territorial matrices.

CONCLUSIONS

The results suggest that changes in type IX distribution, expansion of the pericellular microenvironment and chondrocyte proliferation represent key elements in the chondron remodeling and chondrocyte cluster formation associated with osteoarthritic degeneration.

Collagen type IX from human cartilage: a structural profile of intermolecular cross-linking sites

Type IX collagen, a quantitatively minor collagenous component of cartilage, is known to be associated with and covalently cross-linked to type II collagen fibrils in chick and bovine cartilage. Type IX collagen molecules have also been shown to form covalent cross-links with each other in bovine cartilage. In the present study we demonstrate by structural analysis and location of cross-linking sites that, in human cartilage, type IX collagen is covalently cross-linked to type II collagen and to other molecules of type IX collagen. We also present evidence that, if the proteoglycan form of type IX collagen is present in human cartilage, it can only be a minor component of the matrix, similar to findings with bovine cartilage.

Structural analysis of cross-linking domains in cartilage type XI collagen. Insights on polymeric assembly

The collagen framework of hyaline cartilage is based on copolymers of types II, IX, and XI collagens. Previous studies have established specific covalent interactions between types II and IX collagens. The present study examined cross-linking sites in type XI collagen to define better the full heteropolymeric assembly. Pepsinsolubilized type XI collagen was purified from fetal bovine cartilage. The cross-linking amino acids in the preparation were primarily divalent, borohydride-reducible structures; pyridinoline residues were essentially absent. Individual alpha 1(XI), alpha 2(XI), and alpha 3(XI) chains were resolved by high performance liquid chromatography. Telopeptides still attached by cross-links to helical sites were released by periodate oxidation and identified by microsequencing. Analysis of cross-linked peptides isolated from trypsin digest of each alpha-chain identified the attachment helical sites for the telopeptides. A high degree of interchain specificity was evident in the cross-linking between N-telopeptides and the COOH terminus of the triple-helix, consistent with a head-to-tail interaction of molecules staggered by 4D (D = 67 nm) periods. In addition, alpha 1(II) C-telopeptide was linked to the amino-terminal site of the alpha 1(XI) triple helix. In summary, the results show that type XI collagen molecules are primarily cross-linked to each other in cartilage, implying that a homopolymer is initially formed. Links to type II collagen are also indicated, consistent with an eventual cofibrillar assembly. Analysis of cartilage extracts showed that all three chains, alpha 1(XI), alpha 2(XI), and alpha 3(XI), had at least in part retained their N-propeptides in cartilage matrix and that the alpha 3 (XI) chain was the IIB splicing variant product of the COL2A1 gene. Of particular note was the finding that the N-telopeptide cross-linking site in both alpha 1(XI) and alpha 2(XI) is located amino-terminal to the putative N-propeptidase cleavage site. This structural feature provides a potential mechanism for the proteolytic depolymerization of type XI collagen by proteases that can cleave between the cross-link and the triple helix (e.g. stromelysin).

Analysis of transcriptional isoforms of collagen types IX, II, and I in the developing avian cornea by competitive polymerase chain reaction

The genes for the alpha 1(IX), alpha 1(II), and alpha 2(I) collagen chains can give rise to different isoforms of mRNA, generated by alternative promotor usage [for alpha 1(IX) and alpha 2(I)] or alternative splicing [for alpha 1(II)]. In this study, we employed competitive reverse transcriptase PCR to quantitate the amounts of transcriptional isoforms for these genes in the embryonic avian cornea from its inception (about 3 1/2 days of development) to 11 days. In order to compare values at different time points, the results were normalized to those obtained for the "housekeeping" enzyme, glycerol-3-phosphate dehydrogenase (G3PDH). These values were compared to those obtained from other tissues (anterior optic cup and cartilage) that synthesize different combinations of the collagen isoforms. We found that, in the cornea, transcripts from the upstream promotor of alpha 1(IX) collagen (termed "long IX") were predominant at stage 18-20 (about 3 1/2 days), but then fell rapidly, and remained at a low level. By 5 days (just before stromal swelling) the major mRNA isoform of alpha 1(IX) was from the downstream promoter (termed "short IX"). The relative amount of transcript for the short form of type IX collagen rose to a peak at about 6 days of development, and then declined. Throughout this period, the predominant transcriptional isoform of the collagen type II gene was IIA (i.e., containing the alternatively spliced exon 2). This indicates that the molecules of type II collagen that are assembled into heterotypic fibrils with type I collagen possess, at least transiently, an amino-terminal globular domain similar to that found in collagen types I, III, and V. For type I, the "bone/tendon" mRNA isoform of the alpha 2(I) collagen gene was predominant; transcripts from the downstream promotor were at basal levels. In other tissues expressing collagen types IX and II, long IX was expressed predominantly with the IIA form in the anterior optic cup at stage 22/23; in 14 1/2 day cartilage, long IX was expressed predominantly along with the IIB form of alpha 1(II). The downstream transcript of the alpha 2(I) gene (Icart) was found at high levels only in cartilage.

Abnormality of type IX collagen in a patient with diastrophic dysplasia

There is growing evidence that a spectrum of chondrodysplasias are caused by mutations in the gene coding for type II collagen. The basic molecular defect in diastrophic dysplasia has not been defined, but it appears not to be in collagen type II. Cartilage contains other tissue-specific collagens, types IX, X, and XI, but no mutations have yet been found in their genes in clinical disease. Type IX collagen is hypothesized to play a role in the regulation of type II collagen fibril organization and structure in cartilage extracellular matrix. In this study, we have examined iliac crest growth cartilage from a patient with diastrophic dysplasia. Although collagen fibrils were markedly increased in diameter on transmission electron microscopy, type II collagen appeared to be normal biochemically. Type XI collagen was also normal. However, type IX collagen appeared abnormal on sodium dodecyl sulfate polyacrylamide gel electrophoresis with a pronounced excess of the COL1 domain of the molecule in pepsin extracts. The findings point to an abnormality in structure or metabolism of type IX collagen in diastrophic dysplasia.

Quantification and immunolocalisation of porcine articular and growth plate cartilage collagens

The collagens of growth plate and articular cartilage from 5–6 month old commercial pigs were characterised. Growth plate cartilage was found to contain less total collagen than articular cartilage as a proportion of the dry weight. Collagen types I, II, VI, IX and XI are present in both growth plate and articular cartilage whereas type X is found exclusively in growth plate cartilage. Types III and V collagen could not be detected in either cartilage. Type I collagen makes up at least 10% of the collagenous component of both cartilages. There are significant differences in the ratios of the quantifiable collagen types between growth plate and articular cartilage. Collagen types I, II, and XI were less readily extracted from growth plate than from articular cartilage following pepsin treatment, although growth plate cartilage contains less of the mature collagen cross-links, hydroxylysyl-pyridinoline and lysyl-pyridinoline. Both cartilages contain significant amounts of the divalent reducible collagen cross-links, hydroxylysyl-ketonorleucine and dehydro-hydroxylysinonorleucine. Immunofluorescent localisation indicated that type I collagen is located predominantly at the surface of articular cartilage but is distributed throughout the matrix in growth plate. Types II and XI are located in the matrix of both cartilages whereas type IX is predominantly pericellular in the calcifying region of articular cartilage and the hypertrophic region of the growth plate. Collagen type VI is located primarily as a diffuse area at the articular surface.

Preliminary observations of chondral abrasion in a canine model

Articular cartilage repair was followed for one year in skeletally mature dogs after destabilisation by anterior cruciate ligament transection of the stifle joint (CT), abrasion of the inferior medial condyle (ABR) to bleeding bone, or anterior cruciate transection followed by chondral abrasion (CT/ABR). ABR animals formed repair cartilage at the abrasion site (ABR and CT/ABR) at six months as determined by arthroscopy and at necropsy. CT and CT/ABR animals had an additional cartilage ulcer on the superior aspect of the medial condyle. The abraded site extended in CT/ABR condyles. Repair cartilage (ABR and CT/ABR) contained reduced amounts of proteoglycan as seen by histological loss of safranin O staining and reduced uronic acid content. Fibrocartilage was suggested by histological appearance, hypocellularity, and a higher hydroxyproline content. In contrast with ABR animals, the repair cartilage in the CT/ABR animals contained near normal amounts of hydroxyproline. Collagen profiles of abrasion site repair cartilage in ABR animals had more types I and V collagens, similar amounts of type VI collagen, and decreased amounts of types II, IX, and XI collagens than CT/ABR animals. The results of this study are consistent with abrasion chondroplasty leading to a repair cartilage. Despite extended ulcers, repair cartilage from the destabilised joint (CT/ABR) animals was more hyaline-like in its hydroxyproline content and collagen composition than repair cartilage from the stable joint (ABR animals). In these models additional measures appear to be needed as the defects induced by abrasion chondroplasty did not form a functional hyaline cartilage.

The collagens of articular cartilage

Articular cartilage contains at least five genetically distinct types of collagen. Types II, IX, and XI are cartilage-specific and are cross-linked together in a copolymeric network that forms the extracellular framework of the tissue. Fibrils of type II collagen provide the basic architecture. Type XI, a quantitatively minor fibril-forming collagen, is probably copolymerized with type II collagen in the matrix. Type IX collagen accounts for approximately 1% of the collagenous protein in adult articular cartilage and its molecules exist in the tissue covalently linked to the surface of type II collagen fibrils. Its suspected functions include regulating fibril diameters and mediating fibril-fibril and fibril-proteoglycan interactions. Stromelysin, a matrix metalloproteinase, was recently shown to degrade type IX collagen. This action may cause the collagen network swelling seen in articular cartilage in early experimental osteoarthritis, (OA). Collagen type X is restricted to the underlying calcified zone of articular cartilage, a zone that exhibits active remodeling in joints with OA. Degradation products of the various cartilage collagens show promise as molecular markers of joint disease.

Identification and partial characterization of two type XII-like collagen molecules

We have identified two distinct collagenous macromolecules in extracts of fetal bovine skin. Each of the molecules appears to contain three identical alpha-chains with short triple-helical domains of approximately 25 kD, and nontriple-helical domains of approximately 190 kD. Consistent with these observations, extracted molecules contain a relatively short triple-helical domain (75 nm) and a large globular domain comprised of three similar arms. Despite these similarities, the purified collagenase-resistant domains are distinguished by a number of criteria. The globular domains can be chromatographically separated on the basis of charge distribution. Peptide profiles generated by V8 protease digestion are dissimilar. These molecules are immunologically unique and have distinct distributions in tissue. Finally, rotary shadow analysis of purified domains identifies size and conformation differences. Structurally, the molecules are very similar to type XII collagen, but differ in tissue distribution, since both these molecules are present in cartilage, while type XII is reported to be absent from that tissue.

Degradation of type IX collagen by matrix metalloproteinase 3 (stromelysin) from human rheumatoid synovial cells

The degradation of type IX collagen, a minor collagen in cartilage, was examined by treatment with three different types of matrix metalloproteinases (MMPs) purified from the culture medium of rheumatoid synovial cells. Neither MMP-1 (collagenase) nor MMP-2 (so-called 'gelatinase') could digest type IX collagen, but MMP-3 (stromelysin) readily degraded it into smaller fragments. This suggests that MMP-3 may be responsible for the pathological degradation and/or normal turnover of type IX collagen.

Age-related changes in the synthesis of matrix macromolecules by bovine articular cartilage

Calf and mature cow articular cartilage was labeled in vitro with [35S]SO4 and [3H]glycine and kinetics of incorporation of both isotopes by cartilage fragments was determined by scintillation spectroscopy. The cartilage fragments were then extracted in sequence with 4M GuHCl (Guanidium chloride) and pepsin. The pepsin digest was adjusted to 1.3 M NaCl and pepsin-solubilized collagen salted out. The 4M GuHCl extract, collagen and pepsin-resistent residue were then freeze-dried. The 4M GuHCl extract was further fractionated by DEAE (Diethylaminoethyl) 52 ion exchange chromatography to obtain protein and PG (Proteoglycan) fractions. The protein fraction was also characterised by SDS-PAGE and PG fraction by Sepharose C1-2B chromatography under associative conditions in the presence and absence of an exogenous HA (Hyaluronic acid). The GAG (Glycosaminoglycan) side chains of the PG samples were analysed by Sephadex G-200 column chromatography and their composition determined by paper chromatography after chondroitinase ABC digestion. Linear incorporation of both isotopes was observed from 1 to 18 hours of incubation and roughly equal amounts of [35S]SO4 counts were found on per cell bases in both cartilages although less [3H]glycine was incorporated by cow chondrocytes. It was also found that calf chondrocytes synthesize much greater proportion of the collagen whereas the cow cells synthesize PGs of smaller hydrodynamic sizes, bearing shorter GAG side chains that are enriched in KS (Keratan sulfate) and Ch-6S (Chondroitin-6 sulfate isomer). A failure of cow 35S-PGs monomers to interact with an exogenous HA in the presence of other extracted components was also demonstrated. The relevance of these findings for the mechanism of cartilage damage in aging and osteoarthritis is discussed.

Covalent interactions of type IX collagen in cartilage

The cross-linking of type IX collagen in fetal bovine cartilage was investigated. The main cross-link was dihydroxy-lysinonorleucine (borohydride-reduced) with a lesser amount of the mature cross-link, pyridinoline. Dihydroxylysinonorleucine was present in all three chains of the COL2 domain of the type IX molecule, but only two of them contained pyridinoline even in mature cartilage. Amino acid sequence analysis of individual tryptic peptides that contained 3H-labeled cross-links showed that they had derived from sites of covalent interaction between type IX collagen and the telopeptide sequences of type II collagen. One two-chained peptide was a helical sequence of alpha 2 (IX) COL2 linked to an alpha 1 (II) N-telopeptide. A second peptide was a different helical sequence from another type IX chain linked to an alpha 1(II) c-telopeptide. This latter helical sequence was also the principal site of pyridinoline cross-linking in type IX collagen.

The carboxypropeptide trimer of type II collagen is a prominent component of immature cartilages and intervertebral-disc tissue

Immature bovine cartilages and intervertebral-disc tissue all revealed a prominent protein, not present in the adult tissues, in non-denaturing extracts made with chondroitin ABC lyase (EC 4.2.2.4), Streptomyces hyaluronidase (EC 4.2.2.1) or 1 M NaCl. The protein ran on SDS-polyacrylamide electrophoresis, before disulphide reduction, as a close doublet of bands of apparent molecular weight 110,000 and 105,000. After reduction, they dissociated respectively into two protein bands at 37,000 and 35,000, indicating that the initial molecules were disulphide-bonded trimers. Amino-terminal sequence analysis established the identity of both proteins (Mr 110,000 and Mr 105,000) as forms of the carboxypropeptide of type II collagen. The larger molecule appeared to be the trimer of intact alpha 1(II) carboxypropeptides and the smaller, a version composed of chains that were ten residues shorter at their amino-terminal ends. The material appears to be identical to chondrocalcin, a protein previously found to be enriched in fetal growth plate and named on the basis that it may play a role in cartilage calcification. The present findings, however, indicate that the protein is equally abundant in all type II collagen-synthesizing young cartilages, including nucleus pulposus of the intervertebral disc and other cartilages that never calcify.

Type VI collagen of the intervertebral disc. Biochemical and electron-microscopic characterization of the native protein

The collagen framework of the intervertebral disc contains two major fibril-forming collagens, types I and II. Smaller amounts of other types of collagen are also present. On examination of the nature and distribution of these minor collagens within bovine disc tissue, type VI collagen was found to be unusually abundant. It accounted for about 20% of the total collagen in calf nucleus pulposus, and about 5% in the annulus fibrosus. It was discovered by serially digesting disc tissue with chondroitin ABC lyase and Streptomyces hyaluronidase that native covalent polymers of type VI collagen could be extracted. Electron micrographs of this material prepared by rotary shadowing revealed the characteristic dimensions of tetramers and double tetramers of type VI molecules, with their central rods and terminal globular domains. Molecular-sieve column chromatography on agarose under non-reducing non-denaturing conditions gave a series of protein peaks with molecular sizes equivalent to the tetramer, double tetramer and higher multimers. On SDS/polyacrylamide-gel electrophoresis after disulphide cleavage, these fractions of type VI collagen all showed a main band at Mr 140,000 and four lesser bands between Mr 180,000 and 240,000. On electrophoresis without disulphide cleavage in agarose/2.4% polyacrylamide only dimeric (six chains) and tetrameric (12 chains) forms of type VI molecules were present. The ability to extract all the type VI collagen of the tissue in 4 M-guanidinium chloride, and absence of aldehyde-mediated cross-linking residues on direct analysis, showed that, in contrast with most matrix collagens, type VI collagen does not function as a covalently cross-linked structural polymer.

Collagen type IX: evidence for covalent linkages to type II collagen in cartilage

A major site of pyridinoline cross-linking in bovine type IX collagen was traced to a tryptic peptide derived from one of the molecule's HMW chains. This peptide gave two amino acid sequences (in 2/1 ratio) consistent with it being a three-chained structure. The major sequence matched exactly that of the C-telopeptide of type II collagen from the same tissue. A second HMW chain that contained pyridinoline cross-links also gave two amino-terminal sequences, one from its own amino terminus, the other matching exactly the N-telopeptide cross-linking sequence of type II collagen. We conclude that type IX collagen molecules are covalently cross-linked in cartilage to molecules of type II collagen, probably at fibril surfaces.

Selective decrease of type I collagen synthesis in Fraser mice skin

Quantification and biosynthesis of type I and type III collagens were determined in skin of control and Fraser mice (CatFraser mutation), which exhibit a genetically determined cataract. Skin organ cultures were labelled with [3H]proline. Pepsin-solubilized collagens were studied using three different approaches: (a) differential salt precipitation at neutral pH, followed by SDS-polyacrylamide gel electrophoresis; (b) differential salt precipitation at acid pH followed by SDS-polyacrylamide gel electrophoresis. (c) CNBr peptide analysis. These methods gave consistent and reproducible results, indicating a selective decrease of type I collagen in Fraser mouse skin as compared to control mouse skin. Metabolic labelling of skin organ cultures showed a decreased specific radioactivity of hydroxy[3H]proline in type I collagen of Fraser mouse skin. The concordant results of these experiments suggest a genetically determined alteration of interstitial collagen metabolism in the Fraser mutation apparently specifically concerning the expression of type I collagen gene(s).