The heterogeneity of link proteins isolated from human articular cartilage proteoglycan aggregates

Comparison of sialylated and fucosylated N-glycans attached to Asn 6 and Asn 41 with different roles in hyaluronan and proteoglycan link protein 1 (HAPLN1)

Hyaluronan and proteoglycan link protein 1 (HAPLN1) is an extracellular matrix protein stabilizing interactions between hyaluronan and proteoglycan. Although HAPLN1 is being investigated for various biological roles, its N-glycosylation is poorly understood. In this study, the structure of N-glycopeptides of trypsin-treated recombinant human HAPLN1 (rhHAPLN1) expressed from CHO cells were identified by nano-liquid chromatography-tandem mass spectrometry. A total of 66 N-glycopeptides were obtained, including 16 and 12 N-glycans at sites Asn 6 (located in the N-terminal region) and Asn 41 (located in the Ig-like domain, which interacts with proteoglycan), respectively. The quantities (%) of each N-glycan relative to the totals (100 %) at each site were calculated. Tri- and tetra-sialylation (to resist proteolysis and extend half-life) were more abundant at Asn 6, and di- (core- and terminal-) fucosylation (to increase binding affinity and stability) and sialyl-Lewis X/a epitope (a major ligand for E-selectin) were more abundant at Asn 41. These results indicate that N-glycans attached to Asn 6 (protecting HAPLN1) and Asn 41 (supporting molecular interactions) play different roles in HAPLN1. This is the first study of site-specific N-glycosylation in rhHAPLN1, which will be useful for understanding its molecular interactions in the extracellular matrix.

A Novel Role of Hyaluronic Acid and Proteoglycan Link Protein 1 (HAPLN1) in Delaying Vascular Endothelial Cell Senescence

Cardiovascular diseases (CVDs) are the most common cardiovascular system disorders. Cellular senescence is a key mechanism associated with dysfunction of aged vascular endothelium. Hyaluronic acid and proteoglycan link protein 1 (HAPLN1) has been known to non-covalently link hyaluronic acid (HA) and proteoglycans (PGs), and forms and stabilizes HAPLN1-containing aggregates as a major component of extracellular matrix. Our previous study showed that serum levels of HAPLN1 decrease with aging. Here, we found that the HAPLN1 gene expression was reduced in senescent human umbilical vein endothelial cells (HUVECs). Moreover, a recombinant human HAPLN1 (rhHAPLN1) decreased the activity of senescence-associated β-gal and inhibited the production of senescence-associated secretory phenotypes, including IL-1β, CCL2, and IL-6. rhHAPLN1 also down-regulated IL-17A levels, which is known to play a key role in vascular endothelial senescence. In addition, rhHAPLN1 protected senescent HUVECs from oxidative stress by reducing cellular reactive oxygen species levels, thus promoting the function and survival of HUVECs and leading to cellular proliferation, migration, and angiogenesis. We also found that rhHAPLN1 not only increases the sirtuin 1 (SIRT1) levels, but also reduces the cellular senescence markers levels, such as p53, p21, and p16. Taken together, our data indicate that rhHAPLN1 delays or inhibits the endothelial senescence induced by various aging factors, such as replicative, IL-17A, and oxidative stress-induced senescence, thus suggesting that rhHAPLN1 may be a promising therapeutic for CVD and atherosclerosis.

Recombinant Human HAPLN1 Mitigates Pulmonary Emphysema by Increasing TGF-β Receptor I and Sirtuins Levels in Human Alveolar Epithelial Cells

Chronic obstructive pulmonary disease (COPD) will be the third leading cause of death worldwide by 2030. One of its components, emphysema, has been defined as a lung disease that irreversibly damages the lungs' alveoli. Treatment is currently unavailable for emphysema symptoms and complete cure of the disease. Hyaluronan (HA) and proteoglycan link protein 1 (HAPLN1), an HA-binding protein linking HA in the extracellular matrix to stabilize the proteoglycan structure, forms a bulky hydrogel-like aggregate. Studies on the biological role of the full-length HAPLN1, a simple structure-stabilizing protein, are limited. Here, we demonstrated for the first time that treating human alveolar epithelial type 2 cells with recombinant human HAPLN1 (rhHAPLN1) increased TGF-β receptor 1 (TGF-β RI) protein levels, but not TGF-β RII, in a CD44-dependent manner with concurrent enhancement of the phosphorylated Smad3 (p-Smad3), but not p-Smad2, upon TGF-β1 stimulation. Furthermore, rhHAPLN1 significantly increased sirtuins levels (i.e., SIRT1/2/6) without TGF-β1 and inhibited acetylated p300 levels that were increased by TGF-β1. rhHAPLN1 is crucial in regulating cellular senescence, including p53, p21, and p16, and inflammation markers such as p-NF-κB and Nrf2. Both senile emphysema mouse model induced via intraperitoneal rhHAPLN1 injections and porcine pancreatic elastase (PPE)-induced COPD mouse model generated via rhHAPLN1-containing aerosols inhalations showed a significantly potent efficacy in reducing alveolar spaces enlargement. Preclinical trials are underway to investigate the effects of inhaled rhHAPLN1-containing aerosols on several COPD animal models.

Persistent decrease in multiple components of the perineuronal net following status epilepticus

In the rodent model of temporal lobe epilepsy, there is extensive synaptic reorganization within the hippocampus following a single prolonged seizure event, after which animals eventually develop epilepsy. The perineuronal net (PN), a component of the neural extracellular matrix (ECM), primarily surrounds inhibitory interneurons and, under normal conditions, restricts synaptic reorganization. The objective of the current study was to explore the effects of status epilepticus (SE) on PNs in the adult hippocampus. The aggrecan component of the PN was studied, acutely (48 h post-SE), sub-acutely (1 week post-SE) and during the chronic period (2 months post-SE). Aggrecan expressing PNs decreased by 1 week, likely contributing to a permissive environment for neuronal reorganization, and remained attenuated at 2 months. The SE-exposed hippocampus showed many PNs with poor structural integrity, a condition rarely seen in controls. Additionally, the decrease in the aggrecan component of the PN was preceded by a decrease in hyaluronan and proteoglycan link protein 1 (HAPLN1) and hyaluronan synthase 3 (HAS3), which are components of the PN known to stabilize the connection between aggrecan and hyaluronan, a major constituent of the ECM. These results were replicated in vitro with the addition of excess KCl to hippocampal cultures. Enhanced neuronal activity caused a decrease in aggrecan, HAPLN1 and HAS3 around hippocampal cells in vivo and in vitro, leaving inhibitory interneurons susceptible to increased synaptic reorganization. These studies are the foundation for future experiments to explore how loss of the PN following SE contributes to the development of epilepsy.

Mechanism of proteoglycan aggregate degradation in cartilage stimulated with oncostatin M

OBJECTIVE

To investigate the potential synergistic and differential effects of cytokine combinations on proteoglycan aggregate catabolism in cartilage.

METHODS

Bovine articular cartilage explants were maintained in organ culture and subjected to stimulation with cytokine combinations including interleukin-1alpha (IL-1alpha), IL-1beta, IL-6, IL-17, tumor necrosis factor-alpha (TNFalpha) and oncostatin M (OSM). Aggrecan, link protein and hyaluronan (HA) release and degradation were analyzed, and the effect of the hyaluronidase inhibitor apigenin was investigated.

RESULTS

For all cytokine mixtures studied cleavage of aggrecan only by aggrecanase action was apparent. However, OSM acting synergistically with IL-1 or TNFalpha produced a rapid release of all proteoglycan aggregate components due to both aggrecan and HA degradation. This was abolished by the hyaluronidase inhibitor, apigenin. In addition, in the presence of OSM a low molecular weight aggrecan G3 product was observed, suggesting altered aggrecanase cleavage activity is induced by this cytokine.

CONCLUSIONS

Under cytokine stimulation, aggrecan release from cartilage may take place via proteolysis of the aggrecan core protein or via depolymerization of HA, with the latter mechanism being induced by OSM. OSM is associated with joint inflammation and its participation may account for the more rapid loss of aggrecan from articular cartilage in the inflammatory arthritides, compared to osteoarthritis.

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-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.

Cartilage and bone biological markers in the synovial fluid of osteoarthritic patients after hyaluronan injections in the knee

OBJECTIVE

To evaluate synovial fluid levels of cartilage and bone biological markers after repetitive intra-articular injections of sodium hyaluronate (HA) in knee osteoarthritis (OA) patients.

METHODS

Twenty patients with knee OA were evaluated before and after 5 weekly injections of HA. To study cartilage and bone biological markers, synovial fluid and urine samples were collected simultaneously with the first (FI=week 0) and before the last injection (LI=week 4) of HA. Not commercially available markers (cartilage oligomeric matrix protein (COMP), proteoglycan monomers and cyanogen bromide peptide 11 of the type II collagen chains (alpha (II) CA11B)) were determined by an indirect inhibition ELISA developed and standardized in our laboratory.

RESULTS

We found a significant reduction in levels of proteoglycan monomers (30+/-16 vs. 22+/-10 microg/ml, p<0.05), an increase in COMP concentration (2.9+/-0.9 vs. 3.6+/-0.9 microg/ml, p<0.05) and osteocalcin (BGP) levels (8.7+/-8 vs. 11.9+/-9 ng/ml, p<0.05). No significant changes were observed in the levels of alpha (II)CB11B), metalloproteinase-1 (MMP-1) or pyridinium cross-link/creatinine (Pyr/Cr).

CONCLUSIONS

HA could elicit an indirect response on the cartilage and bone metabolism due to the increased overuse of the joint caused by the analgesic effect of HA. However, a direct HA action on the metabolism of chondrocytes must not be ruled out.

Identification of link protein during follicle development and cumulus cell cultures in rats

Cumulus oocyte complex (COC) expansion is induced through hyaluronic acid production and accumulation of proteins of the inter-alpha-trypsin inhibitor family in the gonadotropin-stimulated cumulus cells. Link protein, a glycoprotein found in cartilage, interacts specifically with hyaluronic acid and stabilizes the binding of proteoglycan monomers to hyaluronic acid to form aggregates. The aim of this study was to investigate the expression of immunoreactive link protein during follicle development in rats and in cumulus cells in culture by immunohistochemistry and Western blot as well as by specific enzyme-linked immunosorbent assay. Immunohistochemical analysis revealed that the extracellular matrix of cumulus cells that were morphologically at a stage of COC expansion were markedly stained for link protein, whereas granulosa cells from immature follicles were not stained. Cumulus cells deposited link protein into the extracellular matrix in an in vitro culture system. The staining intensity was negated by the treatment with hyaluronidase, suggesting that the link protein is bound to hyaluronic acid. We have identified a 42-kDa immunoreactive link protein in rat ovary during the preovulatory period and in COC extracts. Addition of FSH to the medium of cumulus cells in culture supplemented with 10% FBS and oocyte-conditioned medium resulted in an increased rate of link protein synthesis. This work suggests that the cumulus cells synthesize the link protein that may stabilize the binding of inter-alpha-trypsin inhibitor or dermatan sulfate proteoglycan to hyaluronic acid to make up hyaluronic acid-rich matrix aggregate.

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.

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.

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.

Biotinylated hyaluronan: a versatile and highly sensitive probe capable of detecting nanogram levels of hyaluronan binding proteins (hyaladherins) on electroblots by a novel affinity detection procedure

Hyaluronan influences cellular proliferation and migration in developing, regenerating and remodelling tissues and in tissues undergoing malignant tumour-cell invasion. The widespread occurrence of hyaluronan-binding proteins indicates that the recognition of hyaluronan is important to tissue organisation and the control of cellular behaviour. A number of extracellular matrix and cellular proteins, which have been termed the hyaladherins, have specific affinities for hyaluronan. These include cartilage link-protein, hyaluronectin, neurocan, versican and aggrecan, which all bind to HA within the extracellular matrix. Cellular receptors for hyaluronan such as CD44 and RHAMM (receptor for hyaluronate-mediated motility) have also been identified. In the present study biotinylated hyaluronan (bHA) was prepared by reacting adipic dihydrazide with a 170 kDa hyaluronan sample using the bifunctional reagent 1-ethyl-3-[3-(dimethylamino) propyl] carbodiimide. The resultant free amine moeity of the hydrazido-hyaluronan was then reacted with biotin succinimidyl ester (sulfo-NHS-biotin) to prepare the bHA. After 4-20% gradient sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and electroblotting to nitrocellulose membranes, bHA and avidin alkaline phosphatase conjugate could be used in conjunction with nitroblue tetrazolium/5-bromo-4-chloro-3-indolyl phosphate substrates to specifically visualise with high sensitivity (> or = 2 ng), bovine nasal cartilage link-protein, aggrecan hyaluronan binding region, and human fibroblast hyaluronan receptors such as CD-44. Conventional Western blotting using specific monoclonal antibodies to these proteins was also used to confirm the identities of these proteins.

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.

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.

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.

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.

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.

Isolation and characterization of two glycoproteins from hyaline cartilage

Two acidic glycoproteins of molecular mass 92 kDa and 56 kDa were purified from 4 M guanidine hydrochloride extracts of chick sternal cartilage, by density gradient centrifugation, ion-exchange chromatography, gel chromatography and SDS/PAGE. The glycoproteins differed in their amino acid and carbohydrate compositions. They were identified by the immunoblotting technique in extracts of chick articular cartilage from various sites and in extracts of cartilage from other species. The proteins are synthesized by the chondrocytes and show a partial cross-reactivity between their antisera.

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.

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.

Transforming growth factor-beta is a potent inhibitor of IL-1 induced protease activity and cartilage proteoglycan degradation

Treatment of chondrocytes in culture with interleukin-1 results in the production of neutral proteases that cause the degradation of the large aggregating proteoglycan. TGF-beta is a pleiotropic growth factor that has been shown to induce differentiation of cartilage and, in some cases, was able to inhibit the IL-1-dependent processes. In this report, we examined whether TGF-beta could block the IL-1 induced catabolic effects on chondrocytes. After treatment with IL-1 beta (30 ng/ml), rabbit articular chondrocytes produced approximately 2 units of neutral protease activity. Under identical conditions, TGF-beta 1 alone did not induce any protease activity. However, a combination of IL-1 and TGF-beta resulted in a dramatic reduction in the level of protease activity. The inhibitory effect of TGF-beta was also observed at the level of proteoglycan incorporation into the extracellular matrix. The IL-1 treated chondrocytes failed to incorporate proteoglycans into their extracellular matrix. However, addition of TGF-beta in the presence of IL-1 resulted in partial reversal towards a normal extracellular matrix. These studies indicate that TGF-beta can block and at least partially inhibit the catabolic effects of IL-1 on chondrocytes.

Humoral immunity to link protein in patients with inflammatory joint disease, osteoarthritis, and in non-arthritic controls

Cartilage link protein of high purity was prepared and used in an enzyme linked immunosorbent assay (ELISA). Antibodies to link protein were sought in the sera of 98 patients with rheumatic disorders; 38 with rheumatoid arthritis (RA), 29 with osteoarthritis (OA), 13 with psoriatic arthritis (PA), nine with ankylosing spondylitis (AS), nine with systemic lupus erythematosus (SLE), and in 83 healthy controls. Antibodies were detected in all groups with the following prevalences: 21/83 normals, 9/38 RA, 7/29 OA, 7/13 PA, 3/9 AS, and 4/9 SLE. No statistically significant differences existed between the groups with regard to either prevalence or mean titre of anti-link antibodies. Serum antibodies to proteoglycan link protein appear to be no more common in patients with rheumatic disorders than in healthy controls.

The immunologic detection and characterization of cartilage proteoglycan degradation products in synovial fluids of patients with arthritis

Antibodies were used in radioimmunoassays with gel chromatography to detect the hyaluronic acid-binding region, core protein, and keratan sulfate of human cartilage proteoglycan in the synovial fluids of patients with rheumatoid arthritis, juvenile rheumatoid arthritis, and osteoarthritis. All fluids contained proteoglycan that was mainly included on Sepharose CL-4B; this result indicates cleavage of proteoglycan (which is normally excluded). The hyaluronic acid-binding region was the smallest and most commonly detected fragment. It was relatively free of keratan sulfate and core protein, and it could sometimes bind to hyaluronic acid. Other larger fragments containing core protein and/or keratan sulfate were detected in every fluid.

Light and electron microscopical localization of concanavalin A lectin binding sites in rat epiphyseal chondrocytes

Concanavalin A lectin binding sites have been detected within the cytoplasm of epiphyseal chondrocytes. Correlative light and electron microscopic results were obtained, indicating the presence of alpha-D-mannose and/or alpha-D-glucose residues detected by the lectin in the rough endoplasmic reticulum region. Quantitation of the electron microscopic cytochemical reaction also showed that the specific labelling was almost exclusively localized in the lumen of endoplasmic reticulum cisternae. No significant staining was found in other membrane compartments or extracellular matrix. This labelling pattern could be considered as the cytochemical evidence of N-glycosylation processes occurring during the biosynthesis of cartilage extracellular matrix components by chondrocytes.

The phosphorylation of human link proteins

Three link proteins of 48,44 and 40 kDa were purified from human articular cartilage and identified with monoclonal anti-link protein antibody 8-A-4. Two sets of lower molecular weight proteins of 30-31 kDa and 24-26 kDa also contained link protein epitopes recognized by the monoclonal antibody and were most likely degradative products of the intact link proteins. The link proteins of 48 and 40 kDa were identified as phosphoproteins while the 44 kDa link protein did not contain 32P. The phosphorylated 48 and 40 kDa link proteins contained approximately 2 moles PO4/mole link protein.

Resolution of cartilage proteoglycan and its proteolytic degradation products by high-performance liquid chromatography using a gel filtration system

Cartilage proteoglycan subunits are resolved from their various-size proteolytic degradation products by a gel filtration high-performance liquid chromatography system using a Bio-Gel TSK-60 column in tandem with a Bio-Gel TSK-50 column. Molecules ranging in size from the intact proteoglycan to single chondroitin sulfate chains are eluted in the included volume. Each analysis takes less than 30 min to complete, and with purified samples as little as 20 micrograms of proteoglycan is required. The method can be applied to the measurement of proteoglycan in mixtures, such as tissue culture media, by monitoring effluent fractions using the dimethylmethylene blue dye-binding assay.

Immunofluorescence studies on cartilage matrix synthesis. The synthesis of link protein, chondroitin sulfate proteoglycan monomer and type II collagen

A comparison of the synthesis and deposition of fibrous type II collagen and the constituents of chondroitin sulfate proteoglycan (CSPG) aggregates, CSPG monomer and link protein, was made for chicken sternal chondrocytes in culture, using simultaneous double immunofluorescence and lectin localization. Chondrocytes deposited only CSPG constituents--and not type II collagen--into the extracellular matrix (ECM). Intracellular precursors of CSPG monomer were localized primarily in perinuclear regions, but were observed in other cytoplasmic vesicles as well. Link protein antibodies stained the same intracellular structures, but stained the perinuclear cytoplasm less intensely. In contrast, type II procollagen was distributed in vesicles throughout the cytoplasm and was clearly absent from the distinctive, CSPG precursor-containing vesicles. Fluorescence-labelled lectins were used to further identify intracellular membrane compartments. Wheat germ agglutinin (WGA) and Ricinus lectins (which recognize carbohydrates added in the Golgi) stained the perinuclear cytoplasm, while concanavalin A (conA) (which recognizes mannose-rich oligosaccharides added co-translationally) stained vesicles throughout the rest of the cytoplasm and not the perinuclear cytoplasm. The distinctive CSPG-containing vesicles were not stained with WGA or Ricinus agglutinins. Data presented elsewhere demonstrate that the vesicles do not react with monoclonal antibodies which recognize chondroitin sulfate (CS) or keratan sulfate (KS) determinants. Thus, we conclude that the vesicles accumulate CSPG precursors which have not been modified by Golgi-mediated processes. The data indicate that matrix molecules may be segregated selectively prior to transit through the Golgi complex. The co-distribution of link protein and CSPG monomer precursors in vesicles prior to further, Golgi-mediated modification may reflect an as yet undetermined function of these vesicles in the processing or assembly of CSPG.

Isolation and characterization of a link protein from bovine aorta proteoglycan aggregate

Proteoglycan aggregates were isolated from bovine aorta by extraction with 0.5 M guanidine hydrochloride in the presence of proteinase inhibitors and purified by isopycnic CsCl centrifugation. The bottom two-fifths (A1) of the gradient contained 30% of proteoglycans in the aggregated form. The aggregate had 14.8% protein and 20.4% hexuronic acid with hyaluronic acid, dermatan sulfate and chondroitin sulfates in a proportion of 18:18:69. A link protein-containing fraction was isolated from the bottom two-fifths by dissociative CsCl isopycnic centrifugation. The link protein that floated to the top one-fifth of the gradient was purified by chromatography on Sephadex G-200 in the presence of 4 M guanidine hydrochloride. It moved as a single band in SDS-polyacrylamide gel electrophoresis with a molecular weight of 49 000. The amino acid composition of link protein resembled that of link protein from cartilage, but was strikingly different from that of the protein core of the proteoglycan monomer. The neutral sugar content of link protein was 3.5% of dry weight. Galactose, mannose and fucose constituted 21, 62 and 16%, respectively of the total neutral sugars. In aggregation studies the link protein was found to interact with both proteoglycan monomer and hyaluronic acid. Oligosaccharides derived from hyaluronic acid decreased the viscosity of link protein-free aggregates of proteoglycan and hyaluronic acid but not of link-stabilized aggregates, demonstrating that the link protein increases the stability of proteoglycan aggregates.

Identification of fibronectin in preparations of osteoarthritic human cartilage

Several high molecular weight proteins were observed in dissociative extracts of osteoarthritic, but not of normal, human cartilage. By gel electrophoresis, by DEAE-cellulose and gelatin-agarose chromatography, and immunologically, they were found to be identical to fibronectin. Incorporation of tritiated proline into these proteins indicated that this material was not a synovial fluid contaminant. Interactions with the proteoglycans suggested that, in articular cartilage, the role of fibronectin may be more closely associated with proteoglycans than with collagen. The appearance of fibronectin in the diseased cartilage suggests that this may be a feature of the chondrocyte's repair response to the loss of extracellular matrix.

Biochemical and morphologic studies of cartilage from the adult human sacroiliac joint

The cartilage of the adult sacroiliac joint contains type II collagen and a high concentration of glycosaminoglycan. Furthermore, large aggregating proteoglycans and link proteins were extracted from the sacral cartilage. In these respects the sacroiliac cartilage is similar to that in the peripheral joints. However, while the organization of the collagen in the sacral cartilage is typical of an articular cartilage, the organization of the ilial cartilage is very different, and throughout its depth it possesses narrow collagen fibrils arranged parallel to the articular surface.