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

The complex landscape of microRNAs in articular cartilage: biology, pathology, and therapeutic targets

The disabling degenerative disease osteoarthritis (OA) is prevalent among the global population. Articular cartilage degeneration is a central feature of OA; therefore, a better understanding of the mechanisms that maintain cartilage homeostasis is vital for developing effective therapeutic interventions. MicroRNAs (miRs) modulate cell signaling pathways and various processes in articular cartilage via posttranscriptional repression of target genes. As dysregulated miRs frequently alter the homeostasis of articular cartilage, modulating select miRs presents a potential therapeutic opportunity for OA. Here, we review key miRs that have been shown to modulate cartilage-protective or -destructive mechanisms and signaling pathways. Additionally, we use an integrative computational biology approach to provide insight into predicted miR gene targets that may contribute to OA pathogenesis, and highlight the complexity of miR signaling in OA by generating both unique and overlapping gene targets of miRs that mediate protective or destructive effects. Early OA detection would enable effective prevention; thus, miRs are being explored as diagnostic biomarkers. We discuss these ongoing efforts and the applicability of miR mimics and antisense inhibitors as potential OA therapeutics.

Link Protein N-Terminal Peptide as a Potential Stimulating Factor for Stem Cell-Based Cartilage Regeneration

Background

Link protein N-terminal peptide (LPP) in extracellular matrix (ECM) of cartilage could induce synthesis of proteoglycans and collagen type II in cartilaginous cells. Cartilage stem/progenitor cells (CSPCs), the endogenous stem cells in cartilage, are important in cartilage degeneration and regeneration. We hypothesized that LPP could be a stimulator for stem cell-based cartilage regeneration by affecting biological behaviors of CSPC.

Methods

CSPCs were isolated from rat knee cartilage. We evaluated the promoting effect of LPP on proliferation, migration, and chondrogenic differentiation of CSPCs. The chondrogenic differentiation-related genes and proteins were quantitated. Three-dimensional culture of CSPC was conducted in the presence of TGF-β3 or LPP, and the harvested pellets were analyzed to assess the function of LPP on cartilage regeneration.

Results

LPP stimulated the proliferation of CSPC and accelerated the site-directional migration. Higher expression of SOX9, collagen II, and aggrecan were demonstrated in CSPCs treated with LPP. The pellets treated with LPP showed more distinct characteristics of chondroid differentiation than those with TGF-β3.

Conclusion

LPP showed application prospect in cartilage regeneration medicine by stimulating proliferation, migration, and chondrogenic differentiation of cartilage stem/progenitor cells.

Aggrecan heterogeneity in articular cartilage from patients with osteoarthritis

Background

Aggrecan degradation is the hallmark of cartilage degeneration in osteoarthritis (OA), though it is unclear whether a common proteolytic process occurs in all individuals.

Methods

Aggrecan degradation in articular cartilage from the knees of 33 individuals with OA, who were undergoing joint replacement surgery, was studied by immunoblotting of tissue extracts.

Results

Matrix metalloproteinases (MMPs) and aggrecanases are the major proteases involved in aggrecan degradation within the cartilage, though the proportion of aggrecan cleavage attributable to MMPs or aggrecanases was variable between individuals. However, aggrecanases were more associated with the increase in aggrecan loss associated with OA than MMPs. While the extent of aggrecan cleavage was highly variable between individuals, it was greatest in areas of cartilage adjacent to sites of cartilage erosion compared to sites more remote within the same joint. Analysis of link protein shows that in some individuals additional proteolytic mechanisms must also be involved to some extent.

Conclusions

The present studies indicate that there is no one protease, or a fixed combination of proteases, responsible for cartilage degradation in OA. Thus, rather than targeting the individual proteases for OA therapy, directing research to techniques that control global protease generation may be more productive.

The role of aggrecan in normal and osteoarthritic cartilage

Aggrecan is a large proteoglycan bearing numerous chondroitin sulfate and keratan sulfate chains that endow articular cartilage with its ability to withstand compressive loads. It is present in the extracellular matrix in the form of proteoglycan aggregates, in which many aggrecan molecules interact with hyaluronan and a link protein stabilizes each interaction. Aggrecan structure is not constant throughout life, but changes due to both synthetic and degradative events. Changes due to synthesis alter the structure of the chondroitin sulfate and keratan sulfate chains, whereas those due to degradation cause cleavage of all components of the aggregate. These latter changes can be viewed as being detrimental to cartilage function and are enhanced in osteoarthritic cartilage, resulting in aggrecan depletion and predisposing to cartilage erosion. Matrix metalloproteinases and aggrecanases play a major role in aggrecan degradation and their production is upregulated by mediators associated with joint inflammation and overloading. The presence of increased levels of aggrecan fragments in synovial fluid has been used as a marker of ongoing cartilage destruction in osteoarthritis. During the early stages of osteoarthritis it may be possible to retard the destructive process by enhancing the production of aggrecan and inhibiting its degradation. Aggrecan production also plays a central role in cartilage repair techniques involving stem cell or chondrocyte implantation into lesions. Thus aggrecan participates in both the demise and survival of articular cartilage.

Physiological and behavioral evidence for focal nociception induced by epidural glutamate infusion in rats

STUDY DESIGN

Blinded animal study.

OBJECTIVES

To determine if an increased concentration of epidural glutamate can cause a focal nociceptive response in the lower extremities that is consistent with sciatica.

SUMMARY OF BACKGROUND DATA

It is believed that the origin of sciatic pain is related to more than physical pressure on the nerve roots. Recently, it was determined that disc material may be a significant source of free glutamate, resulting from the enzymatic degradation of matrix aggrecan proteins. We believe that this free glutamate acts as a neurotransmitter at glutamate receptors on the dorsal root ganglion (DRG) cell bodies, thereby initiating a nociceptive response.

METHODS

Rats were subject to a 72-hour epidural glutamate infusion via a mini osmotic pump. Von Frey behavioral testing was performed 24 hours before, and 24 and 72 hours after the onset of the infusion. DRG and dorsal horn tissues were analyzed for changes in receptor expression, which have been previously shown to correlate with a nociceptive state.

RESULTS

Von Frey behavioral tests showed focal hyperalgesia that was maximal at the 0.02 mmol/L glutamate concentration. Significant changes in DRG glutamate receptor expression were seen for alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid, kainite, and N-methyl-D aspartate receptors. Analysis of dorsal horn glutamate receptors also showed patterns in alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid and kainate receptor expression that were consistent with a nociceptive state.

CONCLUSIONS

Epidural glutamate elicits a focal nociceptive response. Free glutamate that has been liberated from the disc material may be an important factor in the development of sciatic pain.

Cleavage site specificity of cathepsin K toward cartilage proteoglycans and protease complex formation

Cathepsin K is a potent extracellular matrix-degrading protease that requires interactions with soluble glycosaminolycans for its collagenolytic activity in bone and cartilage. The major sources of glycosaminoglycans in cartilage are aggrecan aggregates. Therefore, we investigated whether cathepsin K activity is capable to hydrolyze aggrecan into fragments allowing the formation of glycosaminoglycan-cathepsin K complexes and determined the cleavage site specificity of cathepsin K toward the cartilage-resident link protein and aggrecan. The cleavage site specificity was compared with those of cathepsins S and L. All three cathepsins released glycosaminoglycans from native bovine cartilage at lysosomal pH and to a lesser degree at neutral extracellular pH. Cathepsin-predigested aggrecan complexes and cartilage provided suitable glycosaminoglycan fragments that allowed the formation of collagenolytically active cathepsin K complexes. A detailed analysis of the degradation of aggrecan aggregates revealed two cathepsin K cleavage sites in the link protein and several sites in aggrecan, including one site within the interglobular domain E1. In summary, these results demonstrate that cathepsin K is capable to degrade aggrecan complexes at specific cleavage sites and that cathepsin K activity alone is sufficient to self-provide the glycosaminoglycan fragments required for the formation of its collagenolytically active complex.

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.

Matrix metalloproteinases are involved in C-terminal and interglobular domain processing of cartilage aggrecan in late stage cartilage degradation

Monoclonal antibody (MAb) technology was used to examine aggrecan metabolites and the role of aggrecanases and matrix metalloproteinases (MMPs) in proteolysis of the interglobular domain (IGD) and C-terminus of aggrecan. An in vitro model of progressive cartilage degradation characterized by early proteoglycan loss and late stage collagen catabolism was evaluated in conjunction with a broad-spectrum inhibitor of MMPs. We have for the first time demonstrated that IGD cleavage by MMPs occurs during this late stage cartilage degeneration, both as a primary event in association with glycosaminoglycan (GAG) release from the tissue and secondarily in trimming of aggrecanase-generated G1 metabolites. Additionally, we have shown that MMPs were responsible for C-terminal catabolism of aggrecan and generation of chondroitin sulfate (CS) deficient aggrecan monomers and that this aggrecan truncation occurred prior to detectable IGD cleavage by MMPs. The onset of this later stage MMP activity was also evident by the generation of MMP-specific link protein catabolites in this model culture system. Recombinant MMP-1, -3 and -13 were all capable of C-terminally truncating aggrecan with at least two cleavage sites N-terminal to the CS attachment domains of aggrecan. Through analysis of aggrecan metabolites in pathological synovial fluids from human, canine and equine sources, we have demonstrated the presence of aggrecan catabolites that appear to have resulted from similar C-terminal processing of aggrecan as that induced in our in vitro culture systems. Finally, by developing a new MAb recognizing a linear epitope in the IGD of aggrecan, we have identified two novel aggrecan metabolites generated by an as yet unidentified proteolytic event. Collectively, these results suggest that C-terminal processing of aggrecan by MMPs may contribute to the depletion of cartilage GAG that leads to loss of tissue function in aging and disease. Furthermore, analysis of aggrecan metabolites resulting from both C-terminal and IGD cleavage by MMPs may prove useful in monitoring different stages in the progression of cartilage degeneration.

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.

Localization of cathepsins D, K, and L in degenerated human intervertebral discs

STUDY DESIGN

Localization of cathepsins D, K, and L in degenerated intervertebral discs was examined by immunohistochemistry.

OBJECTIVES

To determine the involvement of cathepsins in the pathomechanism of intervertebral disc degeneration by monitoring the immunolocalization of cathepsins in degenerated intervertebral disc tissue.

SUMMARY OF BACKGROUND DATA

Cathepsins D, K, and L are enzymes that contribute to the matrix destruction seen in the articular cartilage affected by osteoarthritis and rheumatoid arthritis. However, little is known about the contribution of these cathepsins to intervertebral disc degeneration.

METHODS

Paraffin-embedded sections of degenerated intervertebral disc tissue collected at the time of surgery (13 discs from 12 patients) were immunohistochemically stained with antibodies for cathepsins D, K, and L. For further characterization of the stained cells, immunohistochemical detection of CD68 and TRAP staining were performed.

RESULTS

Hematoxylin and eosin staining revealed obvious signs of degeneration in all sections. Cathepsins D and L were immunolocalized in disc fibrochondrocytes at various sites exhibiting degeneration. Cathepsins K were found in tartrate-resistant acid phosphatase-positive multinucleated cells, in particular near the cleft within the cartilaginous endplate. However, few cells were positive for these cathepsins in anulus fibrosus that maintained the lamellar structure of collagen fibers.

CONCLUSIONS

Marked expression of cathepsins D and L was observed at the site of degeneration. Cathepsins D and K localized in tartrate-resistant acid phosphatase-positive multinucleated cells existed at the cleft between the cartilaginous endplate and vertebral body. The site-specific localization of these cathepsins suggests the association of these proteinases with endplate separation and disorganization of the anulus fibrosus in degenerative spinal disorders.

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.

Articular cartilage and changes in arthritis: noncollagenous proteins and proteoglycans in the extracellular matrix of cartilage

Cartilage contains numerous noncollagenous proteins in its extracellular matrix, including proteoglycans. At least 40 such molecules have been identified, differing greatly in structure, distribution, and function. Some are present in only selected cartilages or cartilage zones, some vary in their presence with a person's development and age, and others are more universal in their expression. Some may not even be made by the chondrocytes, but may arise by absorption from the synovial fluid. In many cases, the molecules' function is unclear, but the importance of others is illustrated by their involvement in genetic disorders. This review provides a selective survey of these molecules and discusses their structure, function, and involvement in inherited and arthritic disorders.

Glycation induced crosslinking of link proteins, in vivo and in vitro

OBJECTIVE

Reducing sugars have the ability to crosslink proteins through creation of advanced glycosylated end products (AGE). In this study, we determined the ability of AGE to induce crosslinking of link proteins and aggrecan proteoglycans.

METHODS

Aggrecan proteoglycans and link proteins were purified from adult human articular cartilage and from young bovine nasal cartilage for in vivo and in vitro studies, respectively. In vitro studies concerned incubation of aggrecan aggregates or link proteins with ribose under physiological conditions. After 30 days, aggregates were centrifuged dissociatively to obtain aggrecan monomers and link proteins. Aggrecan monomers were analyzed by immunoblot assay. Incubated link proteins were analyzed by SDS-PAGE and Sephacryl-200 column chromatography.

RESULTS

After extensive purification, adult human cartilage aggrecan continued to show the presence of link protein antigens by immunoblot analysis. Immunoblot analysis of purified aggrecan derived from ribose-treated aggregates also showed the presence of link protein antigens. Ribose treatment of link protein lead to polymerization that was confirmed by Sephacryl-200.

CONCLUSIONS

These studies suggest that human link proteins tend to become crosslinked to aggrecan in adult cartilage. A likely cause of the crosslinking is formation of AGE due to reducing sugars.

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.

Herniated lumbar disc material as a source of free glutamate available to affect pain signals through the dorsal root ganglion

STUDY DESIGN

Combined prospective human cohort and prospective controlled animal model.

OBJECTIVES

To determine whether free glutamate is available in herniated disc material in concentrations sufficient to diffuse to glutamate receptors and affect the activity of neurons in the dorsal root ganglion that may transmit pain information.

SUMMARY OF BACKGROUND DATA

The severity of lumbar radicular pain cannot be fully explained by physical pressure on nerve roots or ganglions. In experimental models, inflammatory processes are relatively modest under conditions of disc herniation. The hypothesis for the current study was that the proteoglycan link and core proteins, which contain high fractions of acidic amino acids, may be a source of glutamate when enzymatically degraded in an environment without glutamate reuptake systems. Glutamate would be free to diffuse to the dorsal root ganglion to affect glutamate receptors.

METHODS

Disc material was harvested during surgery from herniated and nonherniated portions in patients undergoing elective lumbar disc surgery and subjected to immunohistochemistry and high-performance liquid chromatography for assessment of the presence of extracellular disc matrix glutamate. Miniosmotic pumps with differing concentrations of radiolabeled glutamate based on human data were implanted in the rat epidural space for 72 hours and dorsal root ganglion (DRG) in the region were harvested.

RESULTS

Densitometry of disc matrix demonstrated immunohistochemical evidence for significant extracellular glutamate (P < 0.002). High performance liquid chromatography showed significant concentrations of glutamate in disc material and significantly more in herniated than in nonherniated disc material (P < 0.05). Significant radiolabeling of the dorsal root ganglion after epidural glutamate infusion was found at concentrations two orders of magnitude below measured disc glutamate levels. Autoradiography demonstrated radiolabeling of adjacent DRG.

CONCLUSIONS

Glutamate originating from degenerated disc proteoglycan may diffuse to the dorsal root ganglion and effect glutamate receptors. Consideration may be given to treating disc radiculopathy with epidural glutamate receptor antagonists.

Structural analysis of cartilage proteoglycans and glycoproteins using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Cartilage extracellular matrix molecules synthesized and maintained by chondrocytes form a strong, elastic tissue functioning to cushion and protect the subchondral bone. Osteoarthritis is characterized by degradation of cartilage extracellular matrix molecules resulting in fibrillation, irreversible erosion, and eventual failure of the tissue. With recent interest in the degradation of cartilage extracellular matrix molecules, a need for more detailed structural information exists. Posttranslational modifications are believed to play a role in determining the susceptibility of these molecules to proteolytic degradation during the development of osteoarthritis. The purpose of this paper is to show how the application of matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry to extracellular matrix protein and proteoglycan structure will help elucidate problems in extracellular matrix biochemistry. Methodological issues relating to the high molecular weight, polydispersity, and high degree of posttranslational modification of these molecules are discussed. MALDI-TOF mass spectrometry provides an improved level of detail for extracellular matrix protein and proteoglycan structure and is useful in addressing issues surrounding the causes of degradation during osteoarthritis.

The use of cleavage site specific antibodies to delineate protein processing and breakdown pathways

The hydrolysis of peptide bonds is an integral part of most physiological and pathological processes, yet knowledge is often lacking as to which peptide bonds are cleaved, in which protein substrates, in which order, and by which proteolytic enzymes. An increase in our understanding of these processes will enhance understanding of the pathogenesis underlying many diseases and might aid in the recognition of new targets for therapeutic intervention. This article reviews the development, design, and use of antibodies for the detection of specific peptide bond cleavage events, and describes how the application of such antibodies can increase our understanding of the roles played by proteolytic enzymes in physiology and pathology.

Cathepsin B: an alternative protease for the generation of an aggrecan 'metalloproteinase' cleavage neoepitope

Previously, only matrix metalloproteinases were believed capable of cleaving the cartilage proteoglycan, aggrecan, between Asn341 and Phe342, to yield a small G1 fragment terminating in the residues VDIPEN. We show that the combined endo- and exopeptidase activities of the cysteine protease, cathepsin B, also generate this epitope, suggesting that it should no longer be considered as an exclusive marker of metalloproteinase activity.

Induction of arthritis in BALB/c mice by cartilage link protein: involvement of distinct regions recognized by T and B lymphocytes

Both type II collagen and the proteoglycan aggrecan are capable of inducing an erosive inflammatory polyarthritis in mice. In this study we provide the first demonstration that link protein (LP), purified from bovine cartilage, can produce a persistent, erosive, inflammatory polyarthritis when injected repeatedly intraperitoneally into BALB/c mice. We discovered a single T-cell epitope, located within residues 266 to 290 of bovine LP (NDGAQIAKVGQIFAAWKLLGYDRCD), which is recognized by bovine LP-specific T lymphocytes. We also identified three immunogenic regions in bovine LP that contain epitopes recognized by antibodies in hyperimmunized sera. One of these B-cell regions is found in the most species-variable domain of LP (residues 1 to 36), whereas the other epitopes are located in the most conserved regions (residues 186 to 230 and 286 to 310). The latter two regions contain an AGWLSDGSVQYP motif shared by the G1 globulin domain of aggrecan core protein, versican, neurocan, glial hyaluronan-binding protein, and the hyaluronan receptor CD44. Our data reveal that the induction of arthritis is associated with antibody reactivities to B-cell epitopes located at residues 1 to 19. Together, these observations show that another cartilage protein, LP, like type II collagen and the proteoglycan aggrecan, is capable of inducing an erosive inflammatory arthritis in mice and that the immunity to LP involves recognition of both T- and B-cell epitopes. This immunity may be of importance in the pathogenesis of inflammatory joint diseases, such as juvenile rheumatoid arthritis, in which cellular immunity to LP has been demonstrated.

Identification of a protein in squid cranial cartilage with link protein properties

Squid cranial cartilage extracts were found to contain a protein with a molecular mass of 35 kDa immunoreacting with an antiserum against sheep link protein. Because hyaluronan is not detected in this tissue and the structure of proteoglycans is different to that of aggrecan or versican, this observation was studied further. The 35 kDa protein was purified from cartilage extracts and immunolocalised in Western blots by both the polyclonal antibody and the mAb 8A4. It was found that it was able to bind to hyaluronan and to aggrecan. Direct and competitive microplate binding experiments showed that the squid protein binds to G1 domain of aggrecan, similarly to cartilage link protein and, therefore, it could be a link-like protein molecule of squid cranial cartilage. The 35 kDa protein was also able to bind to squid proteoglycan and this suggested that it might participate in squid cartilage proteoglycan aggregate formation.

A matrix metalloproteinase proenzyme activator produced by articular cartilage

Matrix metalloproteinases (MMPs) are involved in connective tissue turnover under physiological and pathological conditions. MMP activity is regulated by the requirement for zymogen activation. This report describes a proMMP-3 activator produced by articular cartilage. The activator initiates a step-wise processing of proMMP-3 to generate an array of active species. Sequencing of activation intermediates demonstrated cleavage on the NH2-terminal side of certain basic residues in the MMP-3 propeptide. Metal ion chelators inhibited activator-dependent proteolysis, and activity was restored by low levels of ZnCl2. These catalytic properties suggest similarity to members of the insulinase superfamily of metalloendopeptidases with in vitro specificity for single arginine or paired basic processing sites in a variety of prohormones. Dibasic sites also exist in the propeptides of several MMPs including proMMP-3. This is the first report that cartilage produces a potent MMP proenzyme activator, opening the possibility of a novel intrinsic activation pathway for catabolic processes in this avascular tissue.

Regulation of cartilage oligomeric matrix protein synthesis in human synovial cells and articular chondrocytes

OBJECTIVE

Cartilage oligomeric matrix protein (COMP) is a component of the extracellular matrix of articular cartilage. Its increased presence in synovial fluid and serum has been associated with accelerated joint damage in patients with rheumatoid arthritis (RA) and osteoarthritis. To fully understand the reasons for fluctuations of COMP levels, we studied the biosynthesis of this molecule in cells derived from joint tissues.

METHODS

Synovial cells were derived from synovial tissues of patients with RA, and human articular chondrocytes were prepared from normal articular cartilage. Analysis by Northern blotting was used to evaluate steady-state levels of COMP messenger RNA (mRNA), while secretion of the protein into culture media was analyzed by Western blotting. Expression of COMP in synovial tissues was studied by reverse transcriptase-polymerase chain reaction analysis and by in situ hybridization.

RESULTS

COMP was synthesized and secreted by synovial cells as well as by articular chondrocytes in culture. The basal rate of synthesis was very low; however, COMP biosynthesis in both cell populations was induced very strongly by transforming growth factor beta1 (TGFbeta1). Interleukin-1beta counteracted COMP induction by TGF-beta1. COMP was not detected in culture media of skin or fetal lung fibroblasts, either in the absence or the presence of TGFbeta1. COMP mRNA was also present in fresh synovial tissue specimens obtained from patients with RA.

CONCLUSION

COMP is synthesized and secreted not only by articular chondrocytes, but also by synovial fibroblasts. The demonstration of COMP expression in surgical specimens of synovial tissues suggests that the inflamed synovium may provide an additional source for the elevated levels of COMP observed in arthritis. Thus, increased COMP levels in body fluids may be indicative of active synovitis as well as of accelerated joint erosion.

An N-terminal peptide from link protein stimulates proteoglycan biosynthesis in human articular cartilage in vitro

OBJECTIVE

To determine the effects of a synthetic N-terminal peptide from link protein on the synthesis of proteoglycans by human articular cartilage.

METHODS

Explants from adult knee cartilage were maintained for 4 days in serum-free Dulbecco's modified Eagle's medium. Peptides were added for the final 2 days of culture. Synthesis of proteoglycans and proteins was measured by the incorporation of 35S-sulfate and 3H-serine. The sizes, sulfation patterns, and serine: sulfate ratios of newly synthesized glycosaminoglycans were measured by gel chromatography, high performance liquid chromatography, and ion-exchange chromatography.

RESULTS

The N-terminal peptide stimulated proteoglycan synthesis in cartilage from a wide age range of patients of both sexes. The newly synthesized glycosaminoglycans were identical in size and composition to those of control tissues, and their serine:sulfate ratios remained unchanged.

CONCLUSION

This N-terminal peptide, which can be liberated from proteoglycan aggregates by proteolysis, potently stimulated the synthesis of proteoglycans with normal glycosaminoglycan chains. The results suggest that the N-terminal peptide may have a regulatory role in maintaining the integrity of human cartilage matrix.

Ultrastructure of adult human articular cartilage matrix after cryotechnical processing

The ultrastructure of adult human articular cartilage matrix is reexamined in tissue processed according to recently improved cryotechniques [Studer et al. (1995) J. Microsc., 179:321-332]. In truely vitrified tissue, a network of fine cross-banded filaments (10-15 nm in diameter) with a periodicity characteristic of collagen fibrils is seen throughout the extracellular substance, even within the pericellular compartment, which has hitherto been deemed free of such components. Proteoglycans fill the interstices between these entities as a homogeneously distributed granular mass; they do not manifest a morphologically identifiable reticular structure. Longitudinally sectioned collagen fibrils exhibit variations in thickness and kinking; they tend to align with their periodic banding in register and are frequently seen to split or fuse along their longitudinal course. The tendency of fibrils to form bundles is greater in deeper zones than in more superficial ones. A duality in the orientation of fibrils and fibril bundles is observed within the interterritorial matrix compartment: superimposed upon the well-characterized arcade-like structure formed by one subpopulation is another, more randomly arranged one. The classical concepts of matrix organization thus need to be modified and refined to encompass these findings.

Suppression of cartilage matrix gene expression in upper zone chondrocytes of osteoarthritic cartilage

OBJECTIVE

To evaluate the anabolic activity of osteoarthritic chondrocytes in situ by investigating the messenger RNA (mRNA) expression of 3 major cartilage components, type II collagen, aggrecan, and link protein:

METHODS

In situ hybridization experiments and histochemical analysis for proteoglycan content were performed on parallel sections of normal and osteoarthritic (OA) cartilage specimens.

RESULTS

Most chondrocytes in the deeper.zones of OA cartilage showed an increase in mRNA expression, in particular, of type II collagen and to a lesser extent, aggrecan, compared with normal specimens. However, chondrocytes of the upper zone were largely negative for aggrecan or type II collagen mRNA. The expression of link protein mRNA was low in normal and OA specimens.

CONCLUSION

These observations suggest that suppression of the anabolic activity of chondrocytes in the upper zones contributes to the metabolic imbalance observed in OA cartilage. Stimulation of matrix anabolism in superficial chondrocytes might be a suitable target for therapeutic intervention.

Temporal pattern of cysteine endopeptidase (cathepsin B) expression in cartilage and synovium from rabbit knees with experimental osteoarthritis: gene expression in chondrocytes in response to interleukin-1 and matrix depletion

OBJECTIVE

To determine the temporal pattern of expression of cathepsin-B in chondrocytes and synovium in experimental osteoarthritis, and to determine possible mechanisms for upregulation and secretion of cathepsin-B from chondrocytes.

METHODS

Experimental osteoarthritis was induced with partial medial meniscectomy (PM); sham operated (SH) and normal (N) rabbits were used as controls. Cathepsin-B mRNA expression was assessed with northern blotting with a 32P labelled cDNA probe. Cathepsin-B was measured in conditioned media or cell extracts using a fluorogenic substrate Z-Arg-Arg-AMC. Chondrocyte monolayers were used to determine cathepsin-B expression in response to interleukin-1 beta (IL-1 beta). Cartilage explants were used to test the effect of matrix depletion on cathepsin-B release.

RESULTS

Chondrocytes obtained from experimental osteoarthritis knees did not show cathepsin-B mRNA upregulation. However, isolated chondrocytes secreted cathepsin-B into the culture medium. Enzyme release was significantly higher at 8 weeks relative to controls, but not at 12 weeks or 4 weeks. Enzyme released from synovium was significantly higher in PM group compared with SH group at 4 and 8 weeks. IL-1 beta was ineffective in upregulating steady state cathepsin-B mRNA in chondrocytes; however, it upregulated the intracellular enzyme, and this was blocked with cycloheximide. Enzymatic depletion of cartilage matrix after exposure of explants to IL-1 resulted in release of significantly higher amounts of cathepsin-B into the medium by matrix depleted chondrocytes compared with intact explants.

CONCLUSIONS

In experimental osteoarthritis, cathepsin-B is upregulated in synovial tissue during the early degenerative phase. Progression of experimental osteoarthritis is accompanied by upregulation of cathepsin-B in cartilage. Cartilage and synovial cathepsin-B levels decline as experimental osteoarthritis advances to more degenerative states. IL-1 upregulates intracellular cathepsin-B by increasing cathepsin-B protein synthesis; it is not an effective stimulus for enzyme secretion. Depletion of cartilage matrix during progression of experimental osteoarthritis may contribute to secretion of cathepsin-B and perpetuation of cartilage destruction.

Proteoglycan-degrading activity of human stromelysin-1 and leukocyte elastase in rabbit joints. Quantitation of proteoglycan and a stromelysin-induced HABR fragment of aggrecan in synovial fluid and cartilage

The objective of this study was to compare the specificity and potency of recombinant human SLN-1 (rhSLN) and human leukocyte elastase (HLE) as proteoglycan (PG)-degrading enzymes after intraarticular injection into rabbits. Another objective was to evaluate the elicitation of a rhSLN-induced hyaluronan-binding region (HABR) fragment from rabbit aggrecan in joints using a polyclonal antiserum (anti-FVDIPEN) against the synthetic peptide, Phe-Val-Asp-Ile-Pro-Glu-Asn (FVDIPEN). The intraarticular injection of either activated rhSLN or HLE resulted in enzyme-specific quantitative release of PG fragments into synovial fluid. Based on the criteria used herein, HLE appears to be a more potent PG-degrading enzyme than SLN. Intraarticular injection of rhSLN also resulted in time- and dose-dependent release of a new HABR fragment of aggrecan (HABR-FMDIPEN) into both articular cartilage and synovial fluid. HABR-FVDIPEN is likely to be a good marker of matrix metalloproteinase (MMP)-induced degradation of aggrecan.

Comparative analysis of cathepsin L, cathepsin D, and collagenase messenger RNA expression in synovial tissues of patients with rheumatoid arthritis and osteoarthritis, by in situ hybridization

OBJECTIVE

To compare the expression of cathepsin L, cathepsin D, and collagenase messenger RNA (mRNA) in synovial specimens from patients with rheumatoid arthritis (RA) and osteoarthritis (OA).

METHODS

The expression of cathepsins L and D as well as collagenase mRNA in synovial tissues from 8 patients with RA, 6 patients with OA, and 2 patients with noninflamed joints was evaluated using in situ hybridization with digoxigenin-labeled RNA probes.

RESULTS

Both RA and OA synovial tissue expressed cathepsins L and D as well as collagenase mRNA. The expression of the cathepsins was markedly higher in interstitial regions and, to some extent, in perivascular infiltrates of RA synovial tissue compared with OA specimens.

CONCLUSION

Cathepsins L and D mRNA are expressed differently in RA and OA synovial tissues, supporting the concept that these enzymes may contribute to the influx of mononuclear cells into RA synovium. Moreover, the data reveal that the expression of collagenase and cathepsins in RA and OA synovial lining is otherwise largely similar, and suggest that the adhesion of synovial cells to cartilage mediates the invasive destructive process in RA.

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.

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.

Cysteine proteinase inhibitors decrease articular cartilage and bone destruction in chronic inflammatory arthritis

OBJECTIVE

To determine the effects of peptidyl fluoromethyl ketones on the in vitro activity of purified cathepsins B and L, on tissue cysteine proteinase activity, and on cartilage and bone destruction in experimental arthritis.

METHODS

The effects of the fluoroketones on cathepsins B and L in vitro and the effects of oral administration of fluoroketones on ex vivo cysteine proteinase activity in tissue homogenates were determined by measuring the inhibition of fluorogenic substrate cleavage. To determine the effects on arthritis, animals were injected with adjuvant or type II collagen, treated orally with the fluoroketones, and the severity of arthritis was assessed by clinical, histologic, and radiologic methods.

RESULTS

All of the fluoroketones tested were potent inhibitors of purified cathepsins B and L activity. Oral administration of the fluoroketones reduced tissue cysteine proteinase activity by up to 77%. In addition, fluoroketone treatment significantly reduced the severity of clinical joint disease and decreased the destruction of articular cartilage and bone. Quantitative analysis of radiographic images indicated that treatment significantly reduced soft tissue changes, periosteal proliferation, and bone erosion, but only partially reduced juxtaarticular osteoporosis.

CONCLUSION

These studies suggest that cysteine proteinase inhibitors may limit tissue destruction in diseases such as rheumatoid arthritis.

The link proteins

Aggregates of chondroitin-keratan sulfate proteoglycan (aggrecan) and hyaluronic acid (hyaluronan) are the major space-filling components of cartilage. A glycoprotein, link protein (LP; 40-48 kDa) stabilizes the aggregate by binding to both hyaluronic acid and aggrecan. In the absence of LP, aggregates are smaller (as estimated by rotary shadowing of electron micrographs) and less stable (they dissociate at pH 5) than they are in the presence of LP. The proteoglycan aggregate, including LP, is dissociated in the presence of chaotropes such as 4 M guanidine hydrochloride. On removal of the chaotrope, the complex will reassociate. This forms the basis of the isolation of LP from cartilage and has been described in detail elsewhere. Tryptic digestion of the proteoglycan aggregates results in a high molecular weight product that consists of hyaluronic acid to which is bound LP and the N-terminal globular domain of aggrecan (hyaluronic acid binding region; HABR) in a 1:1 stoichiometry. The amino acid sequences of LP and HABR are surprisingly similar. The amino acid sequence can be divided into three domains; an N-terminal domain that falls into the immunoglobulin super-family and two C-terminal domains that are similar to each other. The DNA structure echoes this similarity, in that the major domains are reflected in three separate exons in both LP and HABR. The two C-terminal domains are largely responsible for the association with HA and are related to two recently described hyaluronate-binding proteins, CD44 and TSG-6. A variety of approaches, including analysis of the forms of LP in vivo, rotary shadowing and analysis of the sequence in the immunoglobulin-like domain, have shed considerable light on the structure-function relationships of LP. This review describes the structure and function of LP in detail, focusing on what can be inferred from the similarity of LP, HABR and related molecules such as immunoglobulins and lymphocyte HA-receptors.

Activation of cartilage stromelysin-1 at acid pH and its relation to enzyme pH optimum and osteoarthritis

Stromelysin-1 was purified from human articular cartilage and compared to synovial fibroblast enzyme and to recombinant enzyme. If the latent enzyme was incubated at pH 5.5 with substrates such as aggrecan, it spontaneously became active. Incubation of latent zymogen alone at pH 5.5 gave increasing activation over a period of at least 5 hours. However, this activation process was not accompanied by any shift in molecular weight even after continued incubation for 18 hours. Maximum activity observed was 45-60% of that seen with APMA activation. Stromelysin-1 has a pH optimum of 5.5-6.5 on various macromolecular and peptide substrates. Interaction with TIMP is reduced at pH 5.5 relative to that at 7.5. The hypothesis is presented that osteoarthritis may be initiated by acid activation of stromelysin-1.

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.

The link proteins

Aggregates of chondroitin-keratan sulfate proteoglycan (aggrecan) and hyaluronic acid (hyaluronan) are the major space-filling components of cartilage. A glycoprotein, link protein (LP; 40-48 kDa) stabilizes the aggregate by binding to both hyaluronic acid and aggrecan. In the absence of LP, aggregates are smaller (as estimated by rotary shadowing of electron micrographs) and less stable (they dissociate at pH 5) than they are in the presence of LP. The proteoglycan aggregate, including LP, is dissociated in the presence of chaotropes such as 4 M guanidine hydrochloride. On removal of the chaotrope, the complex will reassociate. This forms the basis of the isolation of LP from cartilage and has been described in detail elsewhere. Tryptic digestion of the proteoglycan aggregates results in a high molecular weight product that consists of hyaluronic acid to which is bound LP and the N-terminal globular domain of aggrecan (hyaluronic acid binding region; HABR) in a 1:1 stoichiometry. The amino acid sequences of LP and HABR are surprisingly similar. The amino acid sequence can be divided into three domains; an N-terminal domain that falls into the immunoglobulin super-family and two C-terminal domains that are similar to each other. The DNA structure echoes this similarity, in that the major domains are reflected in three separate exons in both LP and HABR. The two C-terminal domains are largely responsible for the association with HA and are related to two recently described hyaluronate-binding proteins, CD44 and TSG-6. A variety of approaches, including analysis of the forms of LP found in vivo, rotary shadowing and analysis of the sequence in the immunoglobulin-like domain, have shed considerable light on the structure-function relationships of LP. This review describes the structure and function of LP in detail, focusing on what can be inferred from the similarity of LP, HABR and related molecules such as immunoglobulins and lymphocyte HA-receptors.

An N-terminal peptide from link protein is rapidly degraded by chondrocytes, monocytes and B cells

A peptide cleaved from the link-protein component of human and pig proteoglycan aggregates by trypsin and stromelysin was taken up and degraded further by human monocytes, B cells, chondrocytes and by mouse peritoneal macrophages. Monocytes were able to process the peptide twice as rapidly as peritoneal macrophages and some 16 times more rapidly than articular chondrocytes. The B cell line Priess, which unlike the monocytes and macrophages could not take up or degrade whole proteoglycan aggregates, was able to degrade the peptide at a rapid rate. Synthetic, unglycosylated peptides consisting of the first 16 and 13 N-terminal amino acids of human link protein, corresponding to its stromelysin-cleavage and trypsin-cleavage products, were also taken up and degraded in a similar manner to the natural products and, in addition, were able to block uptake of the 125I-labelled natural peptides. The isoelectric points of the re-secreted breakdown fragment from both the synthetic and natural peptides were identical and each peptide was processed by the cells to produce a single radiolabelled fragment. Each of these fragments was eluted with the same retention time during HPLC, indicating that the natural peptides were derived from the N-terminal region of the link. Since a proportion of the link protein extracted from human and pig cartilage has already undergone proteolysis to remove peptides from its N-terminal region, these peptides may be produced in articular cartilage during the normal process of turnover and ageing. Although a physiological function for this protein has not been established, it may have a homeostatic role in the regulation of proteoglycan synthesis.

Metalloproteinases, tissue inhibitor, and proteoglycan fragments in knee synovial fluid in human osteoarthritis

OBJECTIVE

To determine the concentrations of human stromelysin-1, collagenase, tissue inhibitor of metalloproteinases (TIMP), and proteoglycan fragments in knee synovial fluid in patients with injury to the meniscus or anterior cruciate ligament, posttraumatic osteoarthritis, primary osteoarthritis, or pyrophosphate arthritis.

METHODS

Synovial fluid samples were collected from patients with knee disease diagnosed arthroscopically and radiologically. Concentrations of stromelysin-1, collagenase, and TIMP-1 were determined by sandwich immunoassay, using monoclonal and polyclonal antibodies. Fragments of cartilage proteoglycan containing the chondroitin sulfate-binding region were determined by immunoassay with a polyclonal antibody.

RESULTS

Average concentrations of metalloproteinases, TIMP, and proteoglycan fragments in joint fluid were significantly elevated in patients from all disease groups as compared with volunteers with healthy knees (reference group). Stromelysin concentrations in disease groups averaged 15-45 times that of the reference group. The molar ratios between stromelysin and collagenase varied between 10 and 150. The molar ratio between total stromelysin and free TIMP was 0.5 in the reference group and between 1.6 and 5.3 in the disease groups.

CONCLUSION

Stromelysin concentration in joint fluid is a parameter that distinguishes diseased joints from healthy joints, with a sensitivity of 84% and a specificity of 90%. The high concentrations of metalloproteinase relative to TIMP in joint fluid from patients with the conditions studied may be associated with cartilage matrix degradation in these arthritides.

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.

Comparison of the proteoglycanolytic activities of human leukocyte elastase and human cathepsin G in vitro and in vivo

In this study, we evaluated the in vitro and in vivo potency of human leukocyte elastase (HLE) and human cathepsin G (HCG) as proteoglycanases. In vitro evaluation was done using bovine nasal septum aggrecan and aggrecan/hyaluronan aggregate as substrates. Enzyme activity was assessed by the ability of the proteinases to abrogate the ability of aggrecan to aggregate with hyaluronan. In vivo activity of the proteinases was tested by injecting purified HLE and HCG intra-articularly into rabbit stifle joints and quantifying the levels of proteoglycan released into synovial fluids. On a molar basis, HCG was at least tenfold more potent than HLE as a proteoglycanase in vitro. Moreover, HCG was twofold more potent as a proteoglycanase in vivo. In contrast, HLE hydrolyzed elastin approximately 22-fold faster than HCG, but was only slightly more rapid than HCG when [3H]-transferrin was used as substrate. These data indicate that HCG is more potent than HLE as a proteoglycanase both in vitro and in vivo. Thus, HCG could be more important in the pathogenesis of rheumatoid arthritis than previously suspected.

Osteoarthritis in the human knee: a dynamic process of cartilage matrix degradation, synthesis and reorganization

The matrix of articular cartilage undergoes degenerative changes in osteoarthritis which involve a number of matrix molecules. The structural and mechanical integrity is organized around the composite collagen II, IX, XI fibrillar organization. The small proteoglycan decorin that binds to these fibrils may influence their structure and mechanical properties. Aggrecan interacts indirectly via hyaluronic acid and possibly directly through unknown mechanisms. When collagen is cleaved at the articular surface in early osteoarthritis, decorin and aggrecan are lost. Increases in decorin and aggrecan content occur deeper in the cartilage. This is accompanied by evidence for increased formation of collagen fibrils and increased degradation and synthesis of aggrecan and type II collagen. The net contents of these proteoglycan per tissue do not, however, change until advanced degeneration occurs. These degradative processes are likely catalyzed by metalloproteinases and cysteine proteases. Cartilage exhibits significant capacity for remodelling which may be enhanced by therapeutic management of this process.

High pressure gel-permeation assay for the proteolysis of human aggrecan by human stromelysin-1: kinetic constants for aggrecan hydrolysis

The adaptation of an analytical procedure for aggrecan based upon gel-permeation chromatography to an FPLC-based protocol has significantly sped up the analysis. The faster assay has permitted determination of the kinetic constants for digestion of human aggrecan by human stromelysin-1. Monomeric aggrecan appeared to be hydrolyzed by stromelysin-1 to multiple forms with lower molecular weight. The disappearance of high-molecular-weight aggrecan was first-order, showing Km much larger than 2 microM and kc/Km = 4000 M-1 s-1 at pH 7.5. The disappearance of high-molecular-weight aggrecan upon hydrolysis by stromelysin-1 at pH 5.5 was also first-order, with kc/Km = 10,700 M-1 s-1. The disappearance of high-molecular-weight aggrecan at pH 7.5 was first-order for digestion by human leukocyte elastase with kc/Km = 230,000 M-1 s-1, by human cathepsin G with kc/Km = 4200 M-1 S-1, and by human plasma plasmin with kc/Km = 2800 M-1 s-1, all with Km much larger than 2 microM.

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.