Age-related changes in the structure of proteoglycan link proteins present in normal human articular cartilage

Articular cartilage changes in maturing athletes: new targets for joint rejuvenation

Context:

Articular cartilage has a unique functional architecture capable of providing a lifetime of pain-free joint motion. This tissue, however, undergoes substantial age-related physiologic, mechanical, biochemical, and functional changes that reduce its ability to overcome the effects of mechanical stress and injury. Many factors affect joint function in the maturing athlete—from chondrocyte survival and metabolism to structural composition and genetic/epigenetic factors governing cartilage and synovium. An evaluation of age-related changes for joint homeostasis and risk for osteoarthritis is important to the development of new strategies to rejuvenate aging joints.

Objective:

This review summarizes the current literature on the biochemical, cellular, and physiologic changes occurring in aging articular cartilage.

Data Sources:

PubMed (1969-2013) and published books in sports health, cartilage biology, and aging.

Study Selection:

Keywords included aging, athlete, articular cartilage, epigenetics, and functional performance with age.

Study Design:

Systematic review.

Level of Evidence:

Level 3.

Data Extraction:

To be included, research questions addressed the effect of age-related changes on performance, articular cartilage biology, molecular mechanism, and morphology.

Results:

The mature athlete faces challenges in maintaining cartilage health and joint function due to age-related changes to articular cartilage biology, morphology, and physiology. These changes include chondrocyte loss and a decline in metabolic response, alterations to matrix and synovial tissue composition, and dysregulation of reparative responses.

Conclusion:

Although physical decline has been regarded as a normal part of aging, many individuals maintain overall fitness and enjoy targeted improvement to their athletic capacity throughout life. Healthy articular cartilage and joints are needed to maintain athletic performance and general activities. Genetic and potentially reversible epigenetic factors influence cartilage physiology and its response to mechanical and injurious stimuli. Improved understandings of the physical and molecular changes to articular cartilage with aging are important to develop successful strategies for joint rejuvenation.

Effect of synthetic link N peptide on the expression of type I and type II collagens in human intervertebral disc cells

Intervertebral disc (IVD) degeneration is associated with proteolytic degradation of proteoglycan aggregates present within the extracellular matrix of the disc. Link N peptide (DHLSDNYTLDHDRAIH) is the N-terminal peptide of link protein, which stabilizes the proteoglycan aggregates. It is generated in vivo by proteolytic degradation during tissue turnover. It has been previously shown that this peptide can stimulate the synthesis of collagens by articular cartilage and bovine IVD cells in vitro. Being a synthetic peptide, Link N has considerable financial benefits for clinical use over recombinant growth factors because it is extremely cheap to produce. The purpose of the present study was to determine the effect of Link N on the expression of types I and II collagen and investigate the cellular mechanisms of Link N signal transduction in human IVD cells. The present results suggest that Link N stimulates the expression of types I and II collagen in human IVD cells. More specifically, Link N stimulated the expression of type I in nucleus pulposus (NP) cells, but not in annulus fibrosus cells. As Link N also decreased the phosphorylation of p38 in NP cells only, results suggest that p38 is a mediator of the effect of Link N on type I collagen expression. p38 is a member of the mitogen-activated protein kinase family highlighted by three major cascades: p38, c-Jun amino-terminal kinase, and extracellular signal-regulated kinase pathways. Link N showed no effect on the latter two pathways, suggesting a specific effect of Link N on the p38 cascade. On the other hand, Link N stimulated the expression of type II collagen in both NP and annulus fibrosus, suggesting that other mechanisms are implicated in the control of type II collagen expression in disc cells, without excluding p38 for the NP. In conclusion, the present study showed that Link N can modulate the expression of collagen in human IVD cells.

Cartilage in normal and osteoarthritis conditions

The preservation of articular cartilage depends on keeping the cartilage architecture intact. Cartilage strength and function depend on both the properties of the tissue and on their structural parameters. The main structural macromolecules are collagen and proteoglycans (aggrecan). During life, cartilage matrix turnover is mediated by a multitude of complex autocrine and paracrine anabolic and catabolic factors. These act on the chondrocytes and can lead to repair, remodeling or catabolic processes like those that occur in osteoarthritis. Osteoarthritis is characterized by degradation and loss of articular cartilage, subchondral bone remodeling, and, at the clinical stage of the disease, inflammation of the synovial membrane. The alterations in osteoarthritic cartilage are numerous and involve morphologic and metabolic changes in chondrocytes, as well as biochemical and structural alterations in the extracellular matrix macromolecules.

Cartilage proteoglycans

The structure of the protein core of the high molecular weight aggregating proteoglycan from pig laryngeal cartilage has been investigated. Mild trypsin digestion of proteoglycan aggregates released a large (Mr approximately equal to 150K) protein-rich fragment that contained the hyaluronate-binding region (Mr 66K). Rotary-shadowing electron microscopy of this preparation showed it to contain 'double globe' structures, similar to those seen with intact proteoglycans. Interaction studies and immunochemical evidence showed that one of the globular domains was the binding region. The second globular domain did not interact with hyaluronate or share any major antigenic determinants with the binding region and its function remains unknown. Further evidence from rotary shadowing also suggested that the protein core contained a third globular domain at the C-terminal end. The complete protein core sequence thus contains long folded globular protein regions, in addition to the extended regions bearing glycosaminoglycan chains. Studies of proteoglycan turnover in explants of pig articular cartilage showed that proteoglycan fragments were continuously released into the medium during culture. These included large non-aggregating proteoglycan fragments, free binding region and also link protein. Proteoglycans retained within the cartilage matrix remained intact and able to aggregate. Only in the presence of interleukin 1 was there evidence of more extensive proteolytic digestion. The results suggest normal turnover to be a conservative mechanism involving the selective cleavage of proteoglycan close to the hyaluronate-binding region. This releases the major glycosaminoglycan-bearing domain and enables it to diffuse out of the matrix. The site of the initial cleavage appears to be in the region of the N-terminal globular domains.

Link protein can retard the degradation of hyaluronan in proteoglycan aggregates

OBJECTIVE

Loss of articular cartilage and intervertebral disc function in arthritis or disc degeneration is associated with degradation of the proteoglycan (PG) aggregates by either proteolysis of aggrecan or hyaluronan (HA) degradation. The aim of this work was to determine whether degradation of HA in PG aggregate degradation is influenced by link protein (LP) stabilization of the PG aggregates.

METHODS

Aggrecan and LP were prepared from fetal bovine epiphyseal cartilage, and PG aggregates were formed in the presence or absence of LP. The PG aggregates were exposed to hyaluronidase or free radicals to promote HA degradation. Degradation of HA, aggrecan and LP were assessed by gel filtration chromatography and polyacrylamide gel electrophoresis.

RESULTS

High concentrations of hyaluronidase cleaved both PG aggregates between each aggrecan molecule, whereas low concentrations gave much less cleavage of the LP-stabilized aggregate. High free radical concentrations gave extensive cleavage of all components of both PG aggregates, whereas low concentrations are more selective for HA damage and to a much lesser extent in the LP-stabilized aggregates. Thus the presence of LP caused a diminution in the capacity of both catabolic agents to degrade HA as long as levels of the degradative agents were not excessive.

CONCLUSION

In addition to stabilizing the PG aggregates towards dissociation, LP may also help protect the PG aggregates from degradation under conditions where tissue catabolism is promoted.

Versican G3 domain enhances cellular adhesion and proliferation of bovine intervertebral disc cells cultured in vitro

The functional role of versican in influencing intervertebral disc cell adhesion and proliferation was analyzed in bovine intervertebral disc. We have previously demonstrated the C-terminal globular G3 (or selectin-like) domain of versican to influence mesenchymal chondrogenesis and fibroblast proliferation in vitro. For this study, a versican G3 expression construct was generated to examine the role of the G3 domain of versican. Nucleus pulposus and annulus fibrosus cells were isolated from adult bovine caudal discs using sequential enzymatic digestion and versican expression characterized by RT-PCR. In cell proliferation assays, we observed that there was greater cellular proliferation in the presence of versican G3 for both disc cell types. The higher proliferation rate of annulus fibrosus cells when compared to nucleus pulposus cells seeded in monolayer supports heterogeneity of intervertebral disc cell populations. The presence of versican G3 construct enhanced the adhesion of isolated nucleus pulposus and annulus fibrosus cells approximately 4 to 6 fold, respectively. Cellular adhesion was greater in the presence of versican G3 in a dose dependent manner. G3 product was purified using affinity columns, and the purified G3 also enhanced cell adhesion.

A synthetic peptide of link protein stimulates the biosynthesis of collagens II, IX and proteoglycan by cells of the intervertebral disc

To date, there have been no reports on the effect on disc cells of the intervertebral disc (IVD) of the amino terminal peptide of link protein (DHLSDNYTLDHDRAIH) (link N) which is generated by the cleavage of human link protein by stromelysins 1 and 2, gelatinase A and B, and collagenase between His(16) and Ile(17). However, link N has been shown to act as a growth factor and stimulate synthesis of proteoglycans and collagen by chondrocytes of human articular cartilage. There are also no studies on the effect of link N on type IX collagen in any tissue. In the studies reported here, a serum-free pellet culture system has been used to examine whether link N can play a role in maintaining the integrity of disc matrix, specifically at the level of matrix assembly by cells of the IVD. Using this culture system, we determined the capacity of link N to stimulate accumulation of these matrix proteins in the annulus fibrosus (AF) and nucleus pulposus (NP). Gross inspection of separate AF and NP pellet cultures in the absence of link N revealed a progressive increase in size and a transition from "spherical" to "polygonal" pellets after centrifugation. Addition of 10 ng/ml link N resulted in increased pellet sizes for both AF and NP pellet cultures. Link N increased proteoglycan, type II and type IX collagen contents with an increase in DNA content over time. This study demonstrates that link N can act directly on disc cells to stimulate matrix production, which involves increased accumulation of proteoglycan, and types II and IX collagens. This study also identifies the value of pellet cultures for studies of the IVD cells in a serum-free chemically defined medium, in which pellets can continue growing in size in response to growth factors with minimal cell loss. Link N may have value in stimulating the growth and regeneration of the damaged IVD.

Age-related changes in the composition, the molecular stoichiometry and the stability of proteoglycan aggregates extracted from human articular cartilage

The heterogeneity of the components of proteoglycan aggregates, their stoichiometry within the aggregate and the aggregates' stability was investigated in normal human articular cartilage specimens (age-range newborn to 63 years). Proteoglycans were extracted from tissue by sequentially extracting them with PBS alone, PBS containing oligosaccharides of hyaluronan, and PBS containing solutions of increasing guanidinium chloride concentration (1 M, 2 M, 3 M and 4 M). A high proportion of each of the components of the proteoglycan aggregate, i.e. uronic acid, sulphated glycosaminoglycan, hyaluronan binding domain of aggrecan (G1-domain), link protein (LP) and hyaluronan, was extracted from immature cartilage by PBS alone and PBS containing oligosaccharides of hyaluronan. This was in marked contrast to adult cartilage, which required high concentrations of guanidinium chloride for the efficient extraction of these components. The molar ratios of total G1-domain:LP and the G1-domain associated with aggrecan:LP also differed markedly between immature and mature cartilage and between each of the sequential extracts. The concentration of LP was less than that of the G1-domain in all extracts of cartilage from individuals over 13 years, but this was particularly noticeable in the 1 M guanidinium chloride extracts, and it was surmised that a deficiency in LP produces unstable aggregates in situ. The fragmentation of LP, which is known to occur with advancing age, did not influence the extractability of LP, and fragments were present in each of the sequential extracts. Therefore the generally accepted model of proteoglycan aggregation presented in the literature, which is mostly derived from analysis of immature animal cartilage, cannot be used to describe the structure and organization of aggregates in adult human articular cartilage, where a heterogeneous population of complexes exist that have varying degrees of stability.

Subchondral bone of the human knee joint in aging and osteoarthritis

OBJECTIVE

Although most research investigating the pathogenesis of osteoarthritis (OA) has focused on cartilage, it has been suggested that the subchondral bone (SCB) plays an important role in the development of OA. The relationships between aging, severity of OA change and the SCB thickness and density in the human knee joint specimens from a wide range of ages were examined.

METHODS

One hundred forty knee joints from 72 individuals (25 females, 45 males and 2 unknowns; average age 54.8 years, range 17 to 91 years) were obtained. The surface of the articular cartilage of both the femur and tibia was evaluated for gross morphological changes with a 4-point grading scale. The lateral and medial femoral condyles were cut along a sagittal plane and the tibia along a coronal plane to make bone and cartilage strip specimens. The strips were X-rayed onto mammography film and then scanned into a computer for assessment of SCB thickness and density using image analysis software.

RESULTS

Medial tibial SCB thickness was significantly lower among the elderly (age>69 years) than among the young (age<40) or the middle-aged (40 to 69) (P< 0.001 via ANOVA). Lateral tibial SCB thickness also showed the same trend of decreasing thickness with increasing age, but differences between age groups were not statistically significant. Tibial SCB thicknesses were significantly lower in arthritic grades compared to normal grades (P=0.008 in lateral and 0.017 in medial via ANOVA); in contrast, no significant differences between normal and arthritic were found in femoral SCB thicknesses. The arthritic group tended to have lower SCB densities than the normal group, but this was statistically significant in only the lateral femoral condyle.

CONCLUSIONS

The results obtained in the present study are not consistent with generally accepted notions of the relationship between subchondral bone thickness or density and OA. Subchondral bone changes are not etiologic for OA but, more likely, are secondary to loss of articular cartilage which precedes the appearance of subchondral sclerosis.

Hyaluronate degradation as an alternative mechanism for proteoglycan release from cartilage during interleukin-1beta-stimulated catabolism

Data presented previously suggest that release of components of the cartilage matrix, in response to catabolic agents, cannot be accounted for by proteolytic mechanisms alone. In the present study, the release of glycosaminoglycan-containing components from bovine nasal cartilage cultured in the presence of interleukin-1beta, and from bovine nasal, fetal bovine epiphyseal and adult human articular cartilage cultured in the presence of retinoic acid, was accompanied by the loss of link protein and hyaluronate into the culture medium. Chromatographic analysis of the released hyaluronate showed it to be markedly reduced in size relative to that extracted from the corresponding tissue. It is proposed that, under stimulation by catabolic agents, two independent, but concurrent, mechanisms act to promote the release of aggrecan from the cartilage matrix. First, proteolytic cleavage of the aggrecan core protein results in the production of glycosaminoglycan-containing fragments that are free to diffuse from the tissue. Secondly, cleavage of hyaluronate renders portions of the proteoglycan aggregate small enough so that complexes of aggrecan (or fragments containing its G1 domain) and link protein are released from the tissue. It is likely that both mechanisms contribute to cartilage metabolism in normal physiology and pathology.

The characterization of versican and its message in human articular cartilage and intervertebral disc

Splicing variation of the versican message and size heterogeneity of the versican core protein were analyzed in human articular cartilage and intervertebral disc. Splicing variation of the message was studied by PCR analysis to detect the presence or absence of exons 7 and 8, which encode large chondroitin sulfate attachment regions. At all ages in normal cartilage from the third trimester fetus to the mature adult, the presence of the versican isoform possessing exon 8 but not exon 7 (V1) could be readily detected. The message isoforms possessing neither exon 7 nor 8 (V3) or both exons 7 and 8 (V0) were only detectable in the fetus, and the isoform possessing only exon 7 (V2) was never detected. In osteoarthritic cartilage and in adult intervertebral disc the versican message pattern was the same as that observed in the normal adult with only the isoform possessing exon 8 being detected. Core protein heterogeneity was studied by immunoblotting following enzymic removal of the glycosaminoglycan chains from the proteoglycan, using an antibody recognizing the globular G1 region of versican. All articular cartilage extracts from the fetus to the mature adult contained multiple core protein sizes of greater than 200 kDa. The adult cartilage extracts tended to have an increased proportion of the smaller sized core proteins and osteoarthritic cartilage possessed similar core protein sizes to the normal adult. In contrast, intervertebral disc at all post-natal ages showed a greater range of size heterogeneity with a prominent component of about 50 kDa. The abundance of this component increased if the samples were treated with keratanase prior to analysis, suggesting that the G1 region of versican in disc can be substituted with keratan sulfate. The increased presence of versican in the disc relative to articular cartilage may suggest a more pronounced functional role for this proteoglycan, particularly in the nucleus pulposus.

Age-associated changes in cartilage matrix: implications for tissue repair

The structure of the extracellular matrix of articular cartilage varies considerably with age. These changes are attributable to variations in molecular abundance and structure, and they can affect all the matrix components, but none more so than the proteoglycans. Some of these changes are attributable to variations in synthesis whereas others are attributable to variations in degradation, some of the changes occur during juvenile development whereas others occur throughout life, and some of the changes are beneficial to cartilage function whereas others are detrimental. These variations result in a cartilage that not only changes in its phenotype with age, but also in one whose functional properties are changing continuously throughout life. In a similar manner, the cartilage formed during repair also may show considerable variation in structure and function, depending on whether tissue is being replaced or regenerated and whether mature or immature cells are being used. Because all cartilage is not ceated equal, different repair techniques may not be equally efficacious.

A viscoelastic analysis of the tensile weakening of deep femoral head articular cartilage

Articular cartilage from below the surface of the femoral head of the hip joint shows a profound age-dependent weakening in its tensile mechanical properties. This ageing is also associated with a reduced viscoelastic response in the older tissue. A constitutive model of the viscoelastic behaviour of deep articular cartilage (as discussed by Egan in 1988) is used to generate a graphical pattern which represents the mechanical behaviour. This constitutive approach suggests that the tensile weakening of the older cartilage is due to an age-related reduction in the recruitment of load-carrying structures as the tissue is deformed. The viscoelastic constitutive model also predicts a reduction in the tensile strength of deep articular cartilage with rate of deformation. This prediction is supported by experimental fracture stress data. A weakening of the tensile integrity of the microstructure of articular cartilage could make the tissue less able to sustain normal compressive physiological loading without damage and thus make the tissue more susceptible to osteoarthritic degeneration. The constitutive approach indicates that the weakening of the older tissue may be related to changes within the microstructure which determine how applied mechanical energy is stored and dissipated.

Link peptide cartilage growth factor is degraded by membrane proteinases

The peptide DHLSDNYTLDHDRAIH (Link N), cleaved from the N-terminus of the link protein component of cartilage proteoglycan aggregates by the action of stromelysin, can act as a growth factor and stimulate synthesis of proteoglycans and collagen in articular cartilage [McKenna, Liu, Sansom and Dean (1998) Arthritis Rheum. 41, 157-161]. The mechanism by which this biologically active peptide is degraded and inactivated was investigated using U937 monocytes as a model cell. Time-course experiments showed that two major proteases, an initial serine proteinase followed by a metalloproteinase, acted in sequence. Analysis of the resulting fragments showed that the serine endopeptidase cleavage was at the Leu(3)-Ser(4) bond to produce the peptide SDNYTLDHDRAIH. The terminal serine could then be removed from the resulting peptide by an aminopeptidase. A second metallopeptidase liberated the peptides SDNYTL or DNYTL from DHDRAIH by cleavage at the Leu(9)-Asp(10) bond. The DNYTL peptide intermediate was degraded too rapidly to allow sequencing and sequential aminopeptidase cleavages removed further amino acids from the N-terminus of the remaining DHDRAIH peptide. The identical patterns of breakdown that occurred when either whole cells or purified plasma membranes were used indicated that proteolysis and inactivation of Link N was carried out entirely by membrane-associated enzymes.

An N-terminal peptide from link protein can stimulate biosynthesis of collagen by human articular cartilage

Previous studies have shown that a peptide identical in sequence to the N-terminal of link protein can function as a growth factor and up-regulate proteoglycan synthesis by human articular cartilage in explant culture (L. A. McKenna et al., Arthritis Rheum. 41, 157-162, 1998). The present study has extended these investigations to determine the effects of this peptide on the synthesis of collagen, another essential component of normal cartilage matrix. Explants from normal adult knee cartilage were maintained for periods of up to 8 days in medium with or without serum. Peptides were added during each day of culture. Synthesis of collagen was determined by the incorporation of [3H]proline into hydroxyproline and proteoglycans by incorporation of [35S]sulfate. The type of newly synthesized collagen was measured by SDS-polyacrylamide gel electrophoresis, fluorography, and immunoblotting. The link protein peptide stimulated synthesis of type II collagen in cartilage from a number of different subjects. Maximum up-regulation of synthesis was attained at a concentration of 100 ng/ml, similar to that observed previously for up-regulation of proteoglycan. Synthesis was up-regulated in both the presence and the absence of serum, although the overall rate of synthesis was greater when serum was added. The findings that this link peptide growth factor stimulated synthesis of proteins, including collagen, in a manner analogous to that shown previously for proteoglycans support the hypothesis that this peptide may have an important role in the feedback control of cartilage matrix synthesis.

The macromolecular characteristics of cartilage proteoglycans do not change when synthesis is up-regulated by link protein peptide

Previous studies have shown that a synthetic, unglycosylated analogue of the N-terminal peptide from link protein can function as a growth factor and up-regulate proteoglycan biosynthesis in explant cultures of normal human articular cartilage from a wide age range of subjects (McKenna et al., Arthritis Rheum. 41 (1998) 157-162). The present work further shows that link peptide increased proteoglycan synthesis by cartilage cultured in both the presence and absence of serum, suggesting that the mechanism of up-regulation may be different from that of insulin-like growth factors. The proteoglycans synthesised during stimulation with link peptide were of normal hydrodynamic size and the ratio of core protein to glycosaminoglycan side chains and the proportions of the large proteoglycan aggrecan to the small proteoglycans, decorin and biglycan, remained constant. Aggrecan molecules were equally capable of forming aggregates as those from control tissues and the relative proportions of decorin and biglycan were unchanged showing that both were co-ordinately up-regulated. These results confirmed that this novel peptide is a potent stimulator of proteoglycan synthesis by articular cartilage and showed that the newly synthesised proteoglycans were of normal composition.

Metabolic processing of newly synthesized link protein in bovine articular cartilage explant cultures

In explant cultures of articular cartilage from cattle of different ages radiolabeled leucine was shown to be incorporated into link proteins 1, 2 and 3. The newly synthesized link proteins were incorporated into and lost from the cartilage extracellular matrix with time. The levels of radiolabeled link proteins 1 and 2 remaining in the matrix declined over the culture period, but there was an initial increase in the amount of radiolabeled link protein 3, before its level declined. The turnover time of the radiolabeled link proteins 1 and 2 were similar, indicating that neither link protein was preferentially processed to generate link protein 3, nor lost from the extracellular matrix. The majority of the radiolabeled link protein lost from the cartilage matrix could not be recovered from the culture medium, suggesting that turnover of the radiolabeled aggrecan complexes involves the newly synthesized link protein being internalized by the chondrocytes. Inclusion of cytotoxic proteinase inhibitors to the culture medium resulted in a marked decrease in the rate of loss of link protein from the cartilage, suggesting that the catabolism of link protein is cell-mediated and dependent on metabolically active cells.

Age-related changes in the synthesis of link protein and aggrecan in human articular cartilage: implications for aggregate stability

The rates of incorporation of radiolabelled leucine into aggrecan and link protein have been measured in human articular cartilage of different ages. Aggrecan and link protein were purified in the A1 fraction of CsCl gradients as a result of their ability to form high-buoyant-density proteoglycan aggregates with hyaluronic acid. Separation of the aggrecan from the link protein was achieved by Mono Q anion-exchange chromatography. The rates of synthesis of both aggrecan and link protein decreased with age. The age-related decrease in synthesis of aggrecan was paralleled by a decrease in the rate of sulphate incorporation into glycosaminoglycan chains. The synthesis of link protein decreased with age to a greater extent than that of aggrecan such that the ratio of the rates of link protein to aggrecan synthesis decreased from 1 in immature cartilage to 0.2 in mature cartilage. The age-related decrease in link protein synthesis is controlled at least in part by transcriptional or post-transcriptional mechanisms, as shown by the accompanying age-related decrease in link-protein mRNA. The absence of any age-related decrease in aggrecan mRNA suggests that the decrease in synthesis of aggrecan core protein is controlled by a translational mechanism. Measurement of the total tissue content of aggrecan and link protein by radioimmunoassay revealed an age-related increase in the accumulation of these matrix proteins, even though their de novo synthesis was decreasing. This illustrates the importance that the regulation of extracellular post-translational modification also has in controlling the overall turnover of the cartilage matrix.

Ageing of equine articular cartilage: structure and composition of aggrecan and decorin

In order to identify the pathological processes involved in the destruction of articular cartilage in arthritic diseases, it is first necessary to characterise the normal homeostasis of cartilage in a healthy joint. In particular, normal age-related changes in the biochemistry of cartilage complicate any comparisons that are made between diseased and healthy tissue. There are, however, no reports in the literature detailing the influence of ageing on the biochemistry of proteoglycans in equine articular cartilage. This study addresses the absence of such information by investigating the structure of aggrecan and decorin extracted from a wide age-range of full thickness equine tissue. The total glycosaminoglycan content of articular cartilage from the metacarpophalangeal joint remained relatively constant throughout life. In contrast, specific components such as hyaluronan increased in concentration with advancing age as did the content of a structural epitope present on keratan sulphate chains. There were also significant age-related changes in the sulphation pattern of chondroitin sulphate chains. The structure of the large aggregating proteoglycan (aggrecan) became more heterogeneous in size with increasing age and each of the subspecies of aggrecan identified in the extracts was shown to carry a hyaluronan binding region (G1) domain. All subspecies of aggrecan also expressed specific epitopes to keratan sulphate, chondroitin-4-sulphate and chondroitin-6-sulphate glycosaminoglycan chains. The structure of the small nonaggregating proteoglycan decorin and the aggrecan stabilising molecule link protein were demonstrated to be similar in size and charge to that reported for other species.

Quantification of aggrecan and link-protein mRNA in human articular cartilage of different ages by competitive reverse transcriptase-PCR

A competitive reverse transcriptase-PCR (RT-PCR) assay has been developed for the quantification of particular mRNA species in human articular cartilage. Competitor RNA species were synthesized that differed from the amplified target sequence only by the central insertion of an EcoRI restriction site. By using known amounts of synthetic target and competitor RNA, it was shown that competitor RNA molecules designed in this way are reverse-transcribed and amplified with equal efficiency to the target of interest. Furthermore quantification could be performed during the plateau phase of the PCR, which was necessary when using ethidium bromide fluorescence as a detection system. The inhibition of aggrecan and link-protein mRNA expression by interleukin 1 or tumour necrosis factor in monolayers of human articular chondrocytes quantified by this competitive RT-PCR method compared favourably with Northern hybridization studies. The main advantage of this technique is that it can be used to quantify levels of mRNA with RNA extracted directly from 100 mg wet weight of human articular cartilage. Age-related changes in aggrecan and link-protein mRNA were therefore quantified in human articular cartilage directly after dissection from the joint. The concentration of link-protein mRNA was higher in immature cartilage than in mature cartilage when expressed relative to the amount of glyceraldehyde-3-phosphate dehydrogenase mRNA, but no age-related changes were observed in aggrecan mRNA expression. The ratio of aggrecan to link-protein mRNA was higher in mature cartilage than in immature tissue. These age-related differences in the molecular stoichiometry of aggrecan and link-protein mRNA might have implications with respect to the regulation of the formation and the stability of the proteoglycan aggregates in cartilage.

TSG-6 expression in human articular chondrocytes. Possible implications in joint inflammation and cartilage degradation

OBJECTIVE

The hyaluronan-binding protein TSG-6 (tumor necrosis factor-stimulated gene 6) forms a stable complex with the serine protease inhibitor, inter-alpha-inhibitor, potentiates the inhibition of plasmin activity, and has antiinflammatory effects in vivo. This study examines the expression of TSG-6 in human articular chondrocytes and cartilage.

METHODS

Human articular chondrocytes and cartilage explants were stimulated with cytokines, growth factors, and other agents. TSG-6 expression was analyzed by imaging-assisted Northern and Western blotting.

RESULT

TSG-6 messenger RNA (mRNA) expression was upregulated by cytokines and growth factors, predominantly interleukin-1 beta (IL-1 beta), tumor necrosis factor alpha (TNF alpha), platelet-derived growth factor AA (PDGF-AA), and transforming growth factor beta 1 (TGF beta 1). TSG-6 mRNA induction by TGF beta 1 was delayed as compared with IL-1beta. Treatment of the cells with the glucocorticoid dexamethasone neither induced TSG-6 mRNA nor did it affect IL-1 beta-induced transcript levels. TSG-6 mRNA induction may involve several signal transduction pathways. The strong transcriptional stimulation by phorbol myristate acetate suggests protein kinase C (PKC)-mediated signaling. In contrast, PKA- and Ca- dependent signals are only marginally involved as messengers leading to increased TSG-6 levels after IL-1beta and TNF alpha treatment. In chondrocyte and cartilage organ cultures, both free TSG-6 (35 kd) and the complex with inter-alpha-inhibitor (120 kd) were present and upregulated by IL-1 beta, TNF alpha, or TGF beta 1.

CONCLUSION

Chondrocytes are a source of TSG-6 which may play a role in cartilage remodeling and joint inflammation.

Human link protein gene: structure and transcription pattern in chondrocytes

We have examined the genomic organization and the transcription unit for the human link protein gene from genomic clones and RNA prepared from human cartilage over a wide age range. Five exons cover the gene which is greater than 60 kbp. Primer extension and S1 nuclease protection analysis revealed transcription initiation to be 315 bases upstream from the translation initiation codon in RNA derived from cartilage samples ranging from fetal to 53 years of age. The first exon size therefore is 289 bp and examination of the 5' flanking sequence indicated a lack of a TATA box in close proximity to the transcription start, although a TATAA-like motif (TCTAA) was present at -75 bp. Such a sequence at a similar distance can serve as a promoter in the chicken link protein gene. The large first exon of 289 bp is similar to that of the chicken but contrasts with that described previously for human (96 bp) and rat (62 bp). We also analysed human link protein mRNA by PCR for the presence of an alternatively spliced exon that is present in rat mRNA in low abundance, but could not detect such transcripts. Equine and porcine mRNA contained this spliced form but the results suggested that this was expressed as a rare transcript.

Effect of link protein and free hyaluronic acid binding region on spacing of proteoglycans in aggregates

Aging of articular cartilage results in accumulation of aggrecan fragments of various sizes that retain their ability to aggregate even though they may have relatively few glycosaminoglycan chains. Residual binding of partially degraded aggrecan may prevent binding of newly synthesized aggrecan subunits that have greater numbers of glycosaminoglycan chains. This study was undertaken to determine the effects of various relative molar ratios of intact aggrecan, link proteins, and hyaluronic acid binding region fragments on the structure of reconstituted aggregates. High molar ratios of link proteins relative to aggrecan decreased the spacing between adjacent aggrecan subunits; low molar ratios of hyaluronic acid binding region relative to aggrecan (4:1 or less) had no significant effect on spacing, and high molar ratios resulted in an increase in the spacing and a decrease in the percentage of aggrecan subunits found in aggregates. These data suggest that the density of aggrecan subunits on the aggregate is determined primarily by steric hindrance of the glycosaminoglycan chains of the aggrecan subunits and that, to a limited extent, partial degradation of aggrecan in an aggregate allows attachment of more aggrecan subunits.

Cartilage proteoglycans: structure and potential functions

Hyaline cartilage contains five well-characterized proteoglycans in its extracellular matrix, and it is likely that others exist. The largest in size and most abundant by weight is aggrecan, a proteoglycan that possesses over 100 chondroitin sulfate and keratan sulfate chains. Aggrecan is also characterized by its ability to interact with hyaluronic acid to form large proteoglycan aggregates. Both the high anionic charge on the individual aggrecan molecules endowed by the sulfated glycosaminoglycan chains and the localization within the matrix endowed by aggregate formation are essential for aggrecan function. The molecule provides cartilage with its osmotic properties, which give articular cartilage its ability to resist compressive loads. The other proteoglycans are characterized by their ability to interact with collagen. They are much smaller than aggrecan in size but may be present in similar molar amounts. Decorin, biglycan, and fibromodulin are closely related in protein structure but differ in glycosaminoglycan composition and function. Decorin and biglycan possess one and two dermatan sulfate chains, respectively, whereas fibromodulin bears several keratan sulfate chains. Decorin and fibromodulin both interact with the type II collagen fibrils in the matrix and may play a role in fibrillogenesis and interfibril interactions. Biglycan is preferentially localized in the pericellular matrix, where it may interact with type VI collagen. Finally, type IX collagen can also be considered as a proteoglycan, as its alpha 2(IX) chain may bear a glycosaminoglycan chain. It may serve as a bridge between the collagen fibrils or with the interspersed aggrecan network.

Age-related changes in cartilage proteoglycans: quantitative electron microscopic studies

Biochemical and biophysical studies have shown that the composition and sedimentation velocity of cartilage proteoglycans change with age, but these investigations cannot demonstrate the alterations in molecular structure responsible for these changes. Development of quantitative electron microscopic methods has made it possible to define the age-related structural changes in aggregating proteoglycans and to correlate the alterations in their structure with changes in tissue composition and morphology. Electron microscopic measurement of human and animal hyaline cartilage proteoglycans has shown that with increasing age the length of the chondroitin sulfate-rich region of aggregating proteoglycan monomers (aggrecan molecules) decreases, the variability in aggrecan length increases, the density of aggrecan keratan sulfate chains increases, the number of monomers per aggregate decreases, and the proportion of monomers that aggregate declines. Proteoglycans from the nucleus pulposus of the intervertebral disc show similar but more dramatic age-related alterations. At birth, nucleus pulposus aggrecan molecules are smaller and more variable in length than those found in articular cartilage. Within the first year of human life, the populations of aggregates and large aggrecan molecules analogous to those found in articular cartilage decline until few if any of these molecules remain in the central disc tissues of skeletally mature individuals. The mechanisms of the age-related changes in cartilage proteoglycans have not been fully explained, but measurement of proteoglycans synthesized by chondrocytes of different ages suggests that alterations in synthesis produce at least some of the age-related changes in aggrecan molecules. Degradation of aggrecan chondroitin sulfate-rich regions in the matrix probably also contributes to the structural changes seen by electron microscopy. Age-related changes in proteoglycan aggregation may be due to alterations in link protein function or inhibition of aggregation of newly synthesized aggrecan molecules by accumulation of degraded aggrecan molecules.

The expression of functional link protein in a baculovirus system: analysis of mutants lacking the A, B and B' domains

Functional recombinant human link protein has been produced using a baculovirus expression system. In addition to the intact link protein, three mutant forms have also been expressed. Each mutant bears a deletion equivalent to the protein encoded by one exon in the gene. These deletions represent the A domain, which is thought to be responsible for interaction with aggrecan, and the B or B' domains, which are associated with the interaction with hyaluronate. Such deletions split codons spanning exon boundaries, but maintain the reading frame of the protein and result in the correct amino acid being present at the splice junction. All the recombinant proteins appear as two components upon SDS/PAGe, though the abundance of the two forms does vary between preparations, as a result of variable substitution by N-linked oligosaccharides. The recombinant intact link protein was able to interact with both hyaluronate and aggrecan, showing that the baculovirus system is able to produce functional molecules. All of the recombinant mutant link proteins were also able to interact with hyaluronate, indicating that both the B and B' domains can function independently. The recombinant mutant link proteins were also able to interact with aggrecan, with the exception of the mutant lacking the A domain, confirming that this ability resides entirely within this domain.

Non-collagenous protein screening in the human chondrodysplasias: link proteins, cartilage oligomeric matrix protein (COMP), and fibromodulin

A gel-electrophoretic screening for link proteins, cartilage oligomeric matrix protein (COMP), and fibromodulin abnormalities was performed in fetuses, newborn infants, and children with various types of chondrodysplasia. Microdissected freeze-dried sections of upper tibial growth cartilage were extracted with 4M guanidinium chloride in the presence of proteolysis inhibitors. After dialysis against 8M urea, the extracts were submitted to stepwise ion-exchange chromatography to separate the large proteoglycans (aggrecans) from the other components. The latter were analyzed by gel electrophoresis, electrotransferred onto nitrocellulose membranes, and reacted with specific antibodies. Control samples from individuals with apparently normal growth were analyzed in the same runs. Two link protein bands with abnormal electrophoretic migration were found in a sporadic case of spondylometaphyseal dysplasia, Kozlowski type. Three link protein bands with the same migration as in the control samples were found in thanatophoric dysplasia, homozygous achondroplasia, achondrogenesis type II, hypochondrogenesis, Goldblatt syndrome, Desbuquois dysplasia, pseudoachondroplasia, and diastrophic dysplasia. In several pathologic cases with normal electrophoretic pattern of the link proteins, small link protein fragments appeared after reduction. The gel electrophoretic pattern of COMP was studied in thanatophoric dysplasia, diastrophic dysplasia, homozygous achondroplasia, fibrochondrogenesis, hypochondrogenesis, Goldblatt syndrome, and Kniest dysplasia. In all these cases the pattern was the same as in the control samples. The main band of fibromodulin had a normal migration rate in fibrochondrogenesis, Desbuquois dysplasia, Kniest dysplasia, and pseudoachondroplasia. It was delayed in diastrophic dysplasia.

Non-proteoglycan forms of biglycan increase with age in human articular cartilage

Polyclonal anti-peptide antibodies were raised to the C-terminal regions of human biglycan and decorin. These antibodies were used in immunoblotting to study structural variations with age in the proteoglycan core proteins present in extracts of human articular cartilage and intervertebral disc. Three forms of the biglycan core protein were identified. The largest form was detected only after chondroitinase treatment and represents the proteoglycan form of the molecule from which the glycosaminoglycan chains have been removed. However, chondroitinase treatment did not alter the electrophoretic mobility of the two smaller proteins, which appear to represent non-proteoglycan forms of the molecule, resulting either from a failure to substitute the intact proteoglycan core protein with glycosaminoglycan chains during its synthesis or from proteolytic processing of the intact proteoglycan causing removal of the N-terminal region bearing the glycosaminoglycan chains. The non-proteoglycan forms constituted a minor proportion of biglycan in the newborn, but were the major components in the adult. A similar trend was seen in both articular cartilage and intervertebral disc. In comparison, decorin appears to exist predominantly as a proteoglycan at all ages, with two core protein sizes being present after chondroitinase treatment. Non-proteoglycan forms were detected in the adult, but they were always a minor constituent.

Matrix metalloproteinases cleave at two distinct sites on human cartilage link protein

The actions of human recombinant stromelysins-1 and -2, collagenase, gelatinases A and B and matrilysin on neonatal human proteoglycan aggregates were examined. With the exception of gelatinase B, aggrecan was degraded extensively by most metalloproteinases studied, whereas link protein showed only limited proteolysis. Sequencing studies of modified link protein components revealed that stromelysins-1 and -2, gelatinases A and B and collagenase cleaved specifically between His16 and Ile17, and matrilysin, stromelysin-2 and gelatinase A cleaved between Leu25 and Leu26. Cleavage at the former bond generated a link protein component with the same N-terminus as that isolated from newborn human cartilage. Based on previously determined in situ cleavage sites it is evident that matrix metalloproteinases are not solely responsible for the accumulation of link protein degradation products in adult human cartilage, indicating that additional proteolytic agents are involved in the normal catabolism of human cartilage matrix.

Proteolytic degradation in human articular cartilage: its relationship to stromelysin

Proteolytic degradation of cartilage can in principle occur via the action of proteinases or free radicals, though current evidence favours the enzymic pathway. Link protein can be used as an in situ monitor to identify endogenous proteolytic agents, and its pattern of modification suggests that stromelysin may play a major role in the young, but that additional agents are active in the adult. In cartilage remaining on late-stage osteoarthritic joints, there is less evidence for the action of stromelysin than in normal cartilage, and the tissue appears to be undergoing repair rather than degeneration.

Studies of the articular cartilage proteoglycan aggrecan in health and osteoarthritis. Evidence for molecular heterogeneity and extensive molecular changes in disease

Changes in the structure of the proteoglycan aggrecan (PG) of articular cartilage were determined immunochemically by RIA and gel chromatography and related to cartilage degeneration documented histologically by the Mankin grading system. Monoclonal antibodies to glycosaminoglycan epitopes were used. In all cartilages, three chondroitin sulfate (CS)-rich populations of large size were observed in addition to a smaller keratan sulfate (KS)-rich population. In grades 7-13 OA cartilages (phase II), molecules were significantly larger than the equivalent molecules of grades 2-6 (phase I). CS chain lengths remained unchanged. In most OA cartilages, a CS epitope 846 was elevated in content, this being most marked in phase II (mean: fivefold). Loss of uronic acid, KS, and hyaluronic acid were only pronounced in phase II OA because of variations in normal contents. Aggregation of PG was unchanged (50-60%) or reduced in OA cartilages, but molecules bearing epitope 846 exhibited almost complete aggregation in normal cartilages. This study provides evidence for the capacity of OA cartilage to synthesize new aggrecan molecules to replace those damaged and lost by disease-related changes. It also defines two phases of PG change in OA: an early predominantly degenerate phase I followed by a net reparative phase II accompanied by net loss of these molecules.

Link protein shows species variation in its susceptibility to proteolysis

Human cartilage link protein exists as three native components, while equine, bovine, and porcine cartilage link protein exist as two and Swarm rat chondrosarcoma link protein exists as only one component. These nonhuman link protein components represent intact protein structures, and there is little evidence for proteolytically modified forms in nonhuman tissues. In human cartilage, the proteolytic production of modified link proteins increases with age, whereas high amounts of such products were not seen in the nonhuman tissues. However, the small amounts of link protein fragments that were observed in the nonhuman cartilages were of a similar size to their human counterparts. On digestion of human proteoglycan aggregate with stromelysin, rapid modification of the link protein components occurred, whereas the aggregates from nonhuman cartilages showed incomplete cleavage of their link protein components. The relative resistance of nonhuman link protein to stromelysin may in part be due to a unique amino acid substitution present near the enzymic cleave site.

Metalloproteinase digestion of cartilage proteoglycan. Pattern of cleavage by stromelysin and susceptibility to collagenase

The action of purified rabbit bone stromelysin was investigated on proteoglycan aggregates from pig laryngeal cartilage. The enzyme caused a rapid fall in viscosity of proteoglycan aggregate solution (6 mg/ml), and the products of a partial digest (60% loss of relative viscosity) and a complete digest (95% loss of relative viscosity) were characterized. Analysis by gel chromatography on Sepharose 2B under associative conditions showed that 95% of the glycosaminoglycans in the complete digest were in small-sized fragments, whereas most of the hyaluronan-binding G1 domain and link protein remained intact and bound to hyaluronan. In contrast, there was extensive digestion of the G2 domain which resulted in 76% loss in its detection by immunoassay. Analysis of the partial digest also showed considerable loss (40%) of detection of the G2 domain, but the glycosaminoglycan-rich fragments were much larger than in the complete digest. There was also much less cleavage to create small fragments containing the G1 domain. This was evident on SDS/PAGE analysis where a 58 kDa G1 domain fragment was abundant in the complete digest, but was only present in small amounts in the partial digest. There was also only very limited conversion of link protein from a 44 kDa form to a 40 kDa form. The digestion of proteoglycan aggregate (6 mg/ml) by stromelysin was unaffected by the addition of a high concentration of extra chondroitin sulphate chains (14 mg/ml), and the digestion of proteoglycan monomer showed that the G1 domain was resistant to stromelysin digestion even when not bound to hyaluronan and link protein. The results show that stromelysin degrades the proteoglycan protein core with major cleavages close to, but not within, the G1 domain, and extensive cleavage in other regions. Experiments with purified collagenase, a metalloproteinase structurally related to stromelysin, showed that it too cleaved proteoglycan at several sites within the glycosaminoglycan-rich region of the core protein. Metalloproteinase attack on proteoglycan thus not only occurs with stromelysin but also with collagenase.

Link protein as a monitor in situ of endogenous proteolysis in adult human articular cartilage

The link protein components of proteoglycan aggregates in adult human articular cartilage show heterogeneity due to proteolysis. Cleavages near the N-terminus of the intact link proteins, before residues 17, 19 and 24, generate three proteins of slightly diminished size (LP3). Cleavages within the N-terminal disulphide-bonded loop, before residues 66 and 73 of the intact link proteins, generate proteins that yield smaller degradation products upon reduction (LP fragments). In vitro, modified link protein components of a similar size to LP3 can be generated by a variety of proteinases, but of the physiologically relevant enzymes only stromelysin, cathepsin B and cathepsin G have the ability to yield modified link proteins with N-termini identical with those observed in situ. None of the proteolytic agents tested was able to produce LP fragments with N-termini identical with those observed in situ, and the majority of proteinases were not able to cleave within the disulphide-bonded loops. Cathepsin L and hydroxyl radicals can cleave within the N-terminal disulphide-bonded loop, and have the potential of initially opening the loop to allow further proteolytic processing by other agents to generate the native cleavage sites.

Identification of human intervertebral disc stromelysin and its involvement in matrix degradation

Human intervertebral disc when maintained in organ culture released a latent casein-degrading metalloproteinase into the medium in a manner analogous to cultures of human cartilage. This enzyme was demonstrated to be immunologically identical to prostromelysin. It was also found that the amount of procollagenase secreted by both cartilage and disc cells was considerably less than that of prostromelysin. Tissue extraction confirmed that the low level of procollagenase observed was not due to retention of the enzyme within the tissue. Human intervertebral disc link proteins were found to possess the same N-termini as those of their counterparts in human articular cartilage, where it appears that stromelysin is responsible for generating molecular heterogeneity. These results suggest that intervertebral disc cells are capable of secreting prostromelysin, which can become activated within the extracellular matrix and hence contribute to the age-related and degenerative changes in the disc.

A matrix protein of Mr 55,000 that accumulates in human articular cartilage with age

Adult human articular cartilage contains a protein of Mr 55,000 which is deficient in newborn cartilage. In the adult the molecule represents one of the most abundant non-collagenous, non-proteoglycan molecules in 4 M guanidinium chloride extracts of the tissue. The molecular size of the protein on SDS-PAGE remains constant under reducing and non-reducing conditions, suggesting that it does not exist as a disulphide-bonded multimer, nor do intramolecular disulphide bonds greatly influence its conformation. The protein has the ability to interact with some immunoglobulin preparations making its detection possible by Western blotting with some non-specific antisera. Labeling with [3H]leucine in organ culture indicates that protein of this size is being made by the chondrocytes. However, during purification the newly synthesized molecules do not behave as the resident protein on ion-exchange chromatography, suggesting that the protein may accumulate with age rather than being a major synthetic product of the adult chondrocytes. Amino terminal protein sequence analysis indicates that the N-terminus of the protein is blocked. Sequences derived from peptides generated with cyanogen bromide do not show homology with previously characterized proteins. Molecules of a similar size and composition have been described in bovine cartilage.

Complete amino acid sequence of human cartilage link protein (CRTL1) deduced from cDNA clones and chromosomal assignment of the gene

Little is known about the primary amino acid structure of human cartilage link protein (CRTL1). We screened a human genomic library with a cDNA encoding the 3' untranslated region and the adjoining B1 domain of chicken link protein. One clone was isolated and characterized. A 3.5-kb EcoRI-KpnI fragment from this genomic clone that contains the human B1 exon was used to map the gene to chromosome 5q13----q14.1. The same fragment was used to screen a cDNA library prepared from mRNA of Caco-2, a human colon tumor cell line. Two overlapping clones were isolated and shown to encode all of CRTL1. The deduced amino acid sequence is 354 residues long. The amino acid sequence shows a striking degree of identity to the porcine (96%), rat (96%), and chicken (85%) link protein sequences. Furthermore, there is greater than 86% homology between the 3' untranslated region of the genes encoding human and porcine link proteins. These results indicate that there has been strong evolutionary pressure against changes in the coding and 3' untranslated regions of the gene encoding cartilage link protein.

Cartilage proteoglycan aggregate is degraded more extensively by cathepsin L than by cathepsin B

The degradative actions of cathepsins L and B on human articular-cartilage proteoglycan aggregates were examined. Cathepsin L was found to be much more extensive than cathepsin B in degrading proteoglycan aggregates. It released products with size similar to that of single chondroitin sulphate chains, and a series of degraded link-protein fragments in the digestion mixtures. These proteolytically modified link-protein components (Mr 25,000 and 33,000) have similar Mr values to those of fragments observed in adult human cartilage. In contrast, cathepsin B exhibited a much more limited degradation on both proteoglycan subunits and link-protein components. Both cathepsins L and B generate multiple but distinct cleavage sites on human link proteins, and the hydrolysed bonds have been identified in the region between residues 18 and 29. Protein sequencing analysis of these modified link-protein components also provided evidence for the location of a second N-linked glycosylation site at residue 41 in human link proteins, in addition to that previously described at residue 6 on a proportion of the link proteins. Furthermore, it allows us to report the sequence of human link protein up to residue 65.

The effect of maturation and aging on the structure and content of link proteins in rabbit articular cartilage

We have examined extracts of articular cartilage from rabbits aged 3-100 weeks for evidence of age-related changes in the structure and content of link protein (LP) in this tissue, with the following findings: (a) Two major molecular weight forms of LP were seen on SDS-PAGE (41 and 48 kDa) and the proportion of these changed markedly with age. The 48 kDa species was predominant in young animals (representing about 78% of the total LP at 5 weeks) whereas the 41 kDa species increased in amount with age (representing 35% of the total LP at 100 weeks). A minor form of about 43 kDa, representing less than 20% of the total, was present only during the growth phase. A small amount of fragmented link protein (less than 5% of the total) of about 25-30 kDa was present in samples from mature and aged rabbits only. (b) The quantitation of LP in guanidinium: HCl extracts of cartilage, by radioimmunoassay with monoclonal antibody 8-A-4, was markedly influenced by the conditions of preparation and pretreatment of samples. Assays of dialyzed guanidine extracts following treatment at 80 degrees C for 15 min in 0.025% (w/v) SDS indicated that immature and mature cartilage contains about 50 and 180 micrograms of LP/g of tissue, respectively. On the other hand, assays following treatment at 100 degrees C for 20 min in 0.1% (w/v) SDS suggested that rabbit cartilage contains about 300 micrograms of LP/g of tissue at all ages; finally, assay of CsCl purified proteoglycan samples under these conditions indicated a content of about 500 micrograms of LP/g at all ages. (c) Calculations based on the analysis of proteoglycan preparations for aggregating monomer and link protein suggest that a LP:aggregating monomer molar ratio of about 0.9 is maintained in the articular cartilage throughout maturation and aging in the rabbit.

Degradation of proteoglycan aggregate by a cartilage metalloproteinase. Evidence for the involvement of stromelysin in the generation of link protein heterogeneity in situ

Cartilage proteoglycan aggregates were subjected to degradation by a metalloproteinase, capable of degrading proteoglycan, released from cartilage in culture. This proteinase was demonstrated to be immunologically identical with fibroblast stromelysin. An early release of hyaluronic acid-binding region and large glycosaminoglycan-attachment regions was observed. With increasing time the glycosaminoglycan-attachment regions were digested into smaller fragments and the hyaluronic acid-binding regions accumulated. The degradation of link proteins also occurred concomitantly with these events. Link proteins were converted into a component of similar size to that of the smallest native link protein component. N-Terminal sequence analysis of the three human link protein components indicated that they are all derived from the same protein core, which is closely homologous to that of the rat chondrosarcoma link protein. The two larger link proteins (Mr 48,000 and 44,000) contain the same N-terminal sequence, but they differ by the apparent presence of an N-linked oligosaccharide at residue 6 of the largest link protein component. The smallest link protein (Mr 41,000), however, has an N-terminal sequence equivalent to that commencing at residue 17 in the larger link proteins. It was found that the cartilage metalloproteinase cleaves link proteins in human neonatal cartilage proteoglycan aggregates at the His-16-Ile-17 bond, the same position at which the smallest link protein component appears to be derived naturally from the two larger link protein components. These results suggest that stromelysin secreted by chondrocytes can account for the increased accumulation of hyaluronic acid-binding regions and much of the degradation of link protein observed during aging within human articular cartilage.

The absence of type II collagen and changes in proteoglycan structure of hyaline cartilage in a case of Langer-Saldino achondrogenesis

Structural analysis of hyaline cartilage extracellular matrix components from the ribs and knee joint of a stillborn female with type II achondrogenesis was carried out. The absence of type II collagen, a decrease in the amount of proteoglycans (PG), and structural changes in PG, namely, increased electrophoretic mobility of PG, lower relative content of chondroitin 4-sulfate (Ch4-S), lower molecular weight and decreased total chondroitin sulfate (ChS) sulfation, were detected. Increased amounts of type I and type III collagens, atypical for hyaline cartilage, were revealed. Among the link proteins (LPs), a large protein with a mol. wt. of 48 kDa was predominant. Molecular and cellular mechanisms of the pathogenesis of achondrogenesis ("chondrogenesis imperfecta") are discussed. The data obtained suggest that the primary defect in type II achondrogenesis involves ChS or type II collagen synthesis.

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.

Effect of age on the abundance and fragmentation of link protein of the human intervertebral disc

The link proteins of the human intervertebral disc were studied in tissue extracts by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS/PAGE), followed by immunoblotting, using a specific monoclonal antibody. Three link proteins were detected, corresponding in electrophoretic mobility to those present in articular cartilage. As with articular cartilage, the largest link protein predominates in the young, whereas in the adult the smallest link protein is equally abundant and internal fragmentation of the link proteins occurs. Only in the newborn is the quantity of extractable link protein comparable to that from articular cartilage. In the adult, the disc contains much less link protein than is present in autologous articular cartilage. Neither the amount nor heterogeneity of the link protein differs among different levels within the lumbar spine, although the proportions of the three proteins can differ between the anulus fibrosus and nucleus pulposus. The anulus always contained more extractable link protein relative to tissue wet weight than the nucleus, and the nuclear link protein, at least in adolescents, contained a greater proportion of the smallest link protein. Such changes in the quantity and structure of the disc link proteins may affect the properties of the proteoglycan aggregates and, thus, could influence disc function.

Proteoglycans from bovine fetal epiphyseal cartilage. Sedimentation velocity and light scattering studies of the effect of link protein on proteoglycan aggregate size and stability

Proteoglycan monomer and link proteins were isolated from bovine fetal epiphyseal cartilage and characterized. The physical characteristics of proteoglycan monomer were: s0(20) = 21.3 S, D0t,z = 4.25 x 10(-8)cm2/sec, Mw = 3 x 10(6) and Rg,z = 980A. Link protein preparations contained link proteins 1 and 2, but little or none of the fragment, link protein 3. Link protein-stabilized and link protein-free proteoglycan aggregates were reassembled from proteoglycan monomer, link protein and hyaluronate. The effect of epiphyseal cartilage link protein on aggregate size and stability was examined in sedimentation velocity studies. Compared with link protein from mature bovine nasal and articular cartilages, which contain appreciable amounts of link protein 3, epiphyseal cartilage link protein dramatically stabilized aggregates at pH 5. In the presence of link protein, 92% of the proteoglycan monomers were bound as aggregate at pH 7, and 81% were bound at pH 5. In the absence of link protein, 51% of monomers were bound at pH 7, and only 32% were bound at pH 5. The progressive dissociation of link protein-free aggregates as a function of decreasing pH, and of increasing temperature, was also examined in dynamic light scattering studies. The results of the light scattering studies were in perfect accord with the results of the sedimentation velocity studies. However, compared with the sedimentation velocity studies, the dynamic light scattering studies provided a more detailed and informative description of the dissociation of the link-free aggregate as a function of pH, as a function of temperature, and of the capacity of link protein to stabilize aggregate against dissociation at decreased pH or elevated temperature.

Characterization of link protein(s) from human intervertebral-disc tissues

Proteoglycan aggregates (A1) were prepared from the anulus fibrosus, nucleus pulposus and cartilage-endplate tissues of postnatal (0-6-month-old)-and young-adult (20-30-year-old)-human intervertebral discs. The A1 fractions from young-adult disc contained a greater proportion of non-aggregating proteoglycans than did postnatal tissues. After dissociative CsCl-density-gradient fractionation of the A1, more than 90% of the uronic acid was found in the postnatal A1D1, whereas only 60-80% of the hexuronate was present in the A1D1 isolated from young-adult disc tissues. These results indicated that more lower-buoyant-density proteoglycans occur in the young-adult disc. Link-protein-rich fractions (A1D3) were subjected to SDS/polyacrylamide-gel electrophoresis and immunolocation analyses using monoclonal antibodies specific for epitopes on link protein or proteoglycan. Under non-reducing conditions, the major link protein present in postnatal disc tissues was link protein 1. By contrast, all three link proteins (1, 2 and 3) were detected in young-adult tissues, with the smaller link protein 3 predominating. Analyses of the A1D3 fractions under reducing conditions also indicated the presence of link-protein-degradation peptides (Mr approx. 26,000) from young-adult disc tissues, but not from postnatal tissues. Sequential Sepharose CL-6B and Sephacryl S-300 chromatography in 4 M-guanidinium chloride was employed to separate the link proteins of the A1D3 fraction from protein-rich proteoglycan. Immunolocation analyses indicated that postnatal samples contained no detectable contaminating proteoglycan fragments. However, young-adult link-protein preparations could not be separated from hyaluronic acid-binding region and other proteoglycan fragments by means of these chromatographic procedures. The studies indicate that, compared with hyaline articular cartilage, degraded link protein and proteoglycan accumulate at an early age in young-adult disc tissues. These partially degraded proteoglycan aggregate components may significantly alter the biomechanical properties of disc tissues.