Complete coding sequence and deduced primary structure of the human cartilage large aggregating proteoglycan, aggrecan. Human-specific repeats, and additional alternatively spliced forms

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.

Development and retranslational validation of an in vitro model to characterize acute infections in large human joints

Bacterial infections can destroy cartilage integrity, resulting in osteoarthritis. Goal was to develop an in vitro model with in vivo validation of acute joint inflammation. Inflammation in cocultivated human synovial fibroblasts (SFB), chondrocytes (CHDR), and mononuclear cells (MNC) was successively relieved for 10 days. Articular effusions from patients with (n = 7) and without (n = 5) postoperative joint infection in healthy patients (ASA 1-2) were used as model validation. Inflammation in vitro resulted in an enormous increase in IL-1 and a successive reduction in SFB numbers. CHDR however, maintained metabolic activity and proteoglycan synthesis. While concentrations of bFGF in vivo and in vitro rose consistently, the mRNA increase was only moderate. Concurring with our in vivo data, cartilage-specific IGF-1 steadily increased, while IGF-1 mRNA in the CHDR and SFB did not correlate with protein levels. Similarly, aggrecan (ACAN) protein concentrations increased in vivo and failed to correlate in vitro with gene expression in either the CHDR or the SFB, indicating extracellular matrix breakdown. Anabolic cartilage-specific BMP-7 with highly significant intra-articular levels was significantly elevated in vitro on day 10 following maximum inflammation. Our in vitro model enables us to validate early inflammation of in vivo cell- and cytokine-specific regulatory patterns. This trial is registered with MISSinG, DRKS 00003536.

An ARGS-aggrecan assay for analysis in blood and synovial fluid

OBJECTIVE

To validate a modified ligand-binding assay for the detection of aggrecanase generated aggrecan fragments with the ARGS neoepitope in synovial fluid (SF) and blood, and to verify the identity of aggrecan fragments found in blood.

DESIGN

An enzyme-linked immunosorbent assay (ELISA) on the Meso Scale Discovery (MSD) platform for detection of ARGS-aggrecan was validated, using a standard made from recombinant human aggrecan. Matched samples of SF, serum, plasma, and urine were obtained from 36 subjects at different time points after knee injury, and analysed for ARGS-aggrecan content. Aggrecan was purified from serum and plasma pools and analysed by Western blot.

RESULTS

The limits of quantification for the ARGS-aggrecan assay was between 0.2 and 0.025 pmol ARGS/ml, and the sensitivity of the assay was improved two-fold compared to when using a standard purified from human donors. The ARGS concentrations were highest in SF (mean, range; 3.02, 0.36-30.22 pmol/ml), 20 times lower in the blood samples (0.14, 0.055-0.28 pmol/ml serum and 0.13, 0.053-0.28 pmol/ml plasma), and 80 times lower in urine (0.036, below detection - 0.087 pmol/ml). Serum-ARGS and plasma-ARGS concentrations were similar, and correlated (r(S) = 0.773, P < 0.001). SF concentration correlated with serum concentrations (r(S) = 0.420, P = 0.011). In blood, we identified 129-138 kDa aggrecan fragments containing the ARGS neoepitope.

CONCLUSIONS

This novel ARGS-aggrecan assay is highly sensitive and suited for analysis of SF and blood samples. Both SF and blood contains ARGS-aggrecan, and ARGS concentrations in SF and serum are correlated.

Epiphycan from salmon nasal cartilage is a novel type of large leucine-rich proteoglycan

Chum salmon (Oncorhynchus keta) nasal cartilage was examined by next-generation DNA sequencing and mass spectrometric analyses, and 14 types of proteoglycans including epiphycan (EPY) were found. A cDNA encoding EPY was cloned and sequenced. The cDNA encoded 589 amino acids comprised a glycosaminoglycan (GAG) domain containing 55 potential GAG-modified sites (Ser-Gly and/or Gly-Ser), a cysteine cluster and 6 leucine-rich repeats. EPY was purified from salmon nasal cartilage and the structure of the GAG was characterized. As a result of unsaturated disaccharide analysis, GAG was found to be composed of chondroitin 6-sulfate (58.0%), chondroitin 4-sulfate (26.5%) and non-sulfated chondroitin (15.3%). The average molecular weight of GAG was estimated to be 3.0 × 10(4). Ser-100 and Ser-103 were identified as serine residues substituted by GAG chains by chemical modification and mass spectrometric analysis. More than 50 serine residues were assumed to be substituted by GAG chains. EPY is heavily substituted by chondroitin sulfate, giving an overall molecular weight of just under 2 × 10(6). EPY from salmon nasal cartilage is a novel type of large leucine-rich proteoglycan.

Chondrogenesis of periodontal ligament stem cells by transforming growth factor-β3 and bone morphogenetic protein-6 in a normal healthy impacted third molar

The periodontal ligament-derived mesenchymal stem cell is regarded as a source of adult stem cells due to its multipotency. However, the proof of chondrogenic potential of the cells is scarce. Therefore, we investigated the chondrogenic differentiation capacity of periodontal ligament derived mesenchymal stem cells induced by transforming growth factor (TGF)-β3 and bone morphogenetic protein (BMP)-6. After isolation of periodontal ligament stem cells (PDLSCs) from human periodontal ligament, the cells were cultured in Dulbecco's modified Eagle's medium (DMEM) with 20% fetal bovine serum (FBS). A mechanical force initiated chondrogenic differentiation of the cells. For chondrogenic differentiation, 10 µg·L⁻¹ TGF-β3 or 100 µg∙L⁻¹ BMP-6 and the combination treating group for synergistic effect of the growth factors. We analyzed the PDLSCs by fluorescence-activated cell sorting and chondrogenesis were evaluated by glycosaminoglycans assay, histology, immunohistochemistry and genetic analysis. PDLSCs showed mesenchymal stem cell properties proved by FACS analysis. Glycosaminoglycans contents were increased 217% by TGF-β3 and 220% by BMP-6. The synergetic effect of TGF-β3 and BMP-6 were shown up to 281% compared to control. The combination treatment increased Sox9, aggrecan and collagen II expression compared with not only controls, but also TGF-β3 or BMP-6 single treatment dramatically. The histological analysis also indicated the chondrogenic differentiation of PDLSCs in our conditions. The results of the present study demonstrate the potential of the dental stem cell as a valuable cell source for chondrogenesis, which may be applicable for regeneration of cartilage and bone fracture in the field of cell therapy.

Link protein hyaluronan-binding motif abrogates CD44-hyaluronan-mediated leukemia-liver cell adhesion

The liver is a frequent site for the metastasis of cancer cells originating from other sites. Leukemic liver metastasis is associated with poor prognosis. The ligation of CD44 with hyaluronan (HA) has been shown to contribute to the drug resistance of leukemic cells. In this study, a link protein HA-binding motif was genetically fused with enhanced green fluorescence protein (EGFP) to generate an EGFP-L fusion protein. Furthermore, a coculture system was established to investigate the interaction of leukemic cells with liver cells. CD44-positive Kasumi-1, but not CD44-negative HL-60 cells, were observed to adhere to the liver cell line L02. This cell-cell adhesion was significantly blocked by HA, indicating that Kasumi-L02 cell adhesion was mediated by the CD44-HA interaction. Compared to EGFP, EGFP-L fusion protein bound to L02 and BEL7404 liver cells. EGFP-L partially abrogated the Kasumi-L02 adhesion, suggesting that the link protein-binding motif is able to inhibit CD44-HA-mediated leukemia-liver adhesion. These results may help provide insight into novel therapeutic methods for leukemic patients diagnosed with liver metastasis.

Proteoglycans in the central nervous system: role in development, neural repair, and Alzheimer's disease

Proteoglycans (PGs) are major components of the cell surface and extracellular matrix and play critical roles in development and maintenance of the central nervous system (CNS). PGs are a family of proteins, all of which contain a core protein to which glycosaminoglycan side chains are covalently attached. PGs possess diverse physiological roles, particularly in neural development, and are also implicated in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD). The main functions of PGs in the CNS are reviewed as are the roles of PGs in brain injury and in the development or treatment of AD.

Normal and shear interactions between hyaluronan-aggrecan complexes mimicking possible boundary lubricants in articular cartilage in synovial joints

Using a surface force balance, normal and shear interactions have been measured between two atomically smooth surfaces coated with hyaluronan (HA), and with HA/aggrecan (Agg) complexes stabilized by cartilage link protein (LP). Such HA/Agg/LP complexes are the most abundant mobile macromolecular species permeating articular cartilage in synovial joints and have been conjectured to be present as boundary lubricants at its surface. The aim of the present study is to gain insight into the extremely efficient lubrication when two cartilage surfaces slide past each other in healthy joints, and in particular to elucidate the possible role in this of the HA/Agg/LP complexes. Within the range of our parameters, our results reveal that the HA/Agg/LP macromolecular surface complexes are much better boundary lubricants than HA alone, likely because of the higher level of hydration, due to the higher charge density, of the HA/Agg/LP layers with respect to the HA alone. However, the friction coefficients (μ) associated with the mutual interactions and sliding of opposing HA/Agg/LP layers (μ ≈ 0.01 up to pressure P of ca. 12 atm, increasing sharply at higher P) suggest that such complexes by themselves cannot account for the remarkable boundary lubrication observed in mammalian joints (up to P > 50 atm).

The different roles of aggrecan interaction domains

The aggregating proteoglycans of the lectican family are important components of extracellular matrices. Aggrecan is the most well studied of these and is central to cartilage biomechanical properties and skeletal development. Key to its biological function is the fixed charge of the many glycosaminoglycan chains, that provide the basis for the viscoelastic properties necessary for load distribution over the articular surface. This review is focused on the globular domains of aggrecan and their role in anchoring the proteoglycans to other extracellular matrix components. The N-terminal G1 domain is vital in that it binds the proteoglycan to hyaluronan in ternary complex with link protein, retaining the proteoglycan in the tissue. The importance of the C-terminal G3 domain interactions has recently been emphasized by two different human hereditary disorders: autosomal recessive aggrecan-type spondyloepimetaphyseal dysplasia and autosomal dominant familial osteochondritis dissecans. In these two conditions, different missense mutations in the aggrecan C-type lectin repeat have been described. The resulting amino acid replacements affect the ligand interactions of the G3 domain, albeit with widely different phenotypic outcomes.

MMP proteolysis of the human extracellular matrix protein aggrecan is mainly a process of normal turnover

Although it has been shown that aggrecanases are involved in aggrecan degradation, the role of MMP (matrix metalloproteinase) aggrecanolysis is less well studied. To investigate MMP proteolysis of human aggrecan, in the present study we used neoepitope antibodies against MMP cleavage sites and Western blot analysis to identify MMP-generated fragments in normal and OA (osteoarthritis/osteoarthritic) cartilage, and in normal, knee injury and OA and SF (synovial fluid) samples. MMP-3 in vitro digestion showed that aggrecan contains six MMP cleavage sites, in the IGD (interglobular domain), the KS (keratan sulfate) region, the border between the KS region and CS (chondroitin sulfate) region 1, the CS1 region, and the border between the CS2 and the G3 domain, and kinetic studies showed a specific order of digestion where the cleavage between CS2 and the G3 domain was the most preferred. In vivo studies showed that OA cartilage contained (per dry weight) 3.4-fold more MMP-generated FFGV fragments compared with normal cartilage, and although aggrecanase-generated SF-ARGS concentrations were increased 14-fold in OA and knee-injured patients compared with levels in knee-healthy reference subjects, the SF-FFGV concentrations did not notably change. The results of the present study suggest that MMPs are mainly involved in normal aggrecan turnover and might have a less-active role in aggrecan degradation during knee injury and OA.

Analysis of aggrecan catabolism by immunoblotting and immunohistochemistry

Aggrecan is essential for the normal function of articular cartilage and intervertebral disc, where it provides the ability for the tissues to withstand compressive loading. This property depends on both the high charge density endowed by its numerous chondroitin sulfate and keratan sulfate chains and its ability to form large molecular aggregates via interaction with hyaluronan. Degradation of aggrecan via the action of proteases takes place throughout life and the degradation products accumulate in the tissue and impair its function. Such degradation is exacerbated in degenerative or inflammatory joint disorders. The use of antibodies recognizing the various regions of aggrecan and the neoepitopes generated upon proteolytic cleavage has shown that matrix metalloproteinases and aggrecanases, members of the ADAMTS family, are responsible for aggrecan degradation, both throughout life and in disease. By using immunoblotting techniques, it is possible to determine the extent of aggrecan degradation and to identify the degradation products that have accumulated in the tissue, and immunohistochemistry allows the location of the aggrecan degradation to be established.

Pyrazinamide Effects on Cartilage Type II Collagen Amino Acid Composition

Introduction. Current therapeutic regimens with first-line antitubercular agents are associated to a high rate of adverse effects which could cause pronounced changes in collagen's contents and structure. Investigation of these changes is very important for optimization of antitubercular therapy and minimization of treatment-caused harm. The aim of present paper was to investigate potential effect of pyrazinamide on male rats' cartilage type II collagen amino acid composition. Materials and Methods. Wistar albino male rats (160–200 g b.w.) were divided into three groups: I—received pyrazinamide per os at a dose of 1000 mg/kg b.w./day; II—at a dose of 2000 mg/kg b.w./day, in both groups it was given for 60 days; III—control. After 60 days of the experiment, rats of the experimental (groups I and II) and control groups were sacrificed and the amino acids contents of male rat cartilage type II collagens were determined using amino acid analyzer. Results and Discussion. The study of pyrazinamide effects (administered in different doses) on rat cartilage type II collagen amino acid contents demonstrated presence of dose-dependent pyrazinamide-mediated quantitative and qualitative changes in these rat extracellular matrix proteins in comparison with control.

Synthesis and characterization of an aggrecan mimic

Aggrecan (AGG) is a large, aggregating proteoglycan present throughout the body, but predominantly found in articular cartilage. The principle features of AGG, its hyaluronan (HA) binding domain and its abundance of covalently attached glycosaminoglycans (GAGs), make it an essential component of the functional ability of articular cartilage. Current tissue engineering constructs have attempted to stimulate AGG production, but have been unable to produce adequate amounts of mature AGG, and hence have suffered a mismatch in mechanical properties. To address these deficiencies, an AGG mimic was synthesized to match AGG functional properties and provide greater control within tissue engineering constructs. Chondroitin sulfate was functionalized with HA-specific binding peptides to replicate both the GAG presence and HA-binding ability of AGG, respectively. Upon characterization and testing, the mimic was able to effectively bind to HA, increase the compressive strength of cartilage extracellular matrix-based constructs, and protect the other extracellular matrix (ECM) components from degradation, replicating the important functions of AGG. In particular, the mimic produced a 78% increase in compressive strength of the ECM-based constructs, and was able to significantly reduce the degradation of both HA and collagen. The initial characterization of the newly synthesized AGG mimic demonstrates its potential in tissue engineering constructs, and provides an essential basis for more explorative studies of the AGG mimic's abilities as an AGG substitute and beyond.

Structure and biology of the intervertebral disk in health and disease

The intervertebral disks along the spine provide motion and protection against mechanical loading. The 3 structural components, nucleus pulposus, annulus fibrosus, and cartilage endplate, function as a synergistic unit, though each has its own role. The cells within each of these components have distinct origins in development and morphology, producing specific extracellular matrix proteins that are organized into unique architectures fit for intervertebral disk function. This article focuses on various aspects of intervertebral disk biology and disruptions that could lead to diseases such as intervertebral disk degeneration.

MMPs are less efficient than ADAMTS5 in cleaving aggrecan core protein

Aggrecan degradation in articular cartilage occurs predominantly through proteolysis and has been attributed to the action of members of the matrix metalloproteinase (MMP) and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) families. Both families of enzymes cleave aggrecan at specific sites within the aggrecan core protein. One cleavage site within the interglobular domain (IGD), between Glu(373-374)Ala and five additional sites in the chondroitin sulfate-2 (CS-2) region of aggrecan were characterized as "aggrecanase" (ADAMTS) cleavage sites, while cleavage between Ser(341-342)Phe within the IGD of bovine aggrecan is attributed to MMP action. The objective of this study was to assess the cleavage efficiency of MMPs relative to ADAMTS and their contribution to aggrecan proteolysis in vitro. The analysis of aggrecan IGD degradation in bovine articular cartilage explants treated with catabolic cytokines over a 19-day period showed that MMP-mediated degradation of aggrecan within the IGD can only be observed following day 12 of culture. This delay is associated with the lack of activation of proMMPs during the first 12 days of culture. Analysis of MMP1, 2, 3, 7, 8, 9, 12, 13 and ADAMTS5 efficiencies at cleaving within the aggrecan IGD and CS-2 region in vitro was carried out by the digestion of bovine aggrecan with the various enzymes and Western blot analysis using aggrecan anti-G1 and anti-G3 antibodies. Of these MMPs, MMP12 was the most efficient at cleaving within the aggrecan IGD. In addition to cleavage in the IGD, MMP, 3, 7, 8 and 12 were also able to degrade the aggrecan CS-2 region. MMP3 and MMP12 were able to degrade aggrecan at the very C-terminus of the CS-2 region, cleaving the Glu(2047-2048)Ala bond which was previously shown to be cleaved by ADAMTS5. However, in comparison to ADAMTS5, MMP3 was about 100 times and 10 times less efficient at cleaving within the aggrecan IGD and CS-2 regions, respectively. Collectively, our results showed that the delayed activation of proMMPs and the relatively low cleavage efficiency of MMPs can explain the minor contribution of these enzymes to aggrecan catabolism in vivo. This study also uncovered a potential role for MMPs in the C-terminal truncation of aggrecan.

The association of aggrecan gene polymorphism with the risk of intervertebral disc degeneration

BACKGROUND

Intervertebral disc degeneration is now considered to be genetically determined in large part, with environmental factors also playing an important role. The human is known to uniquely exhibit variable numbers of tandem repeat polymorphism within the aggrecan CS1 domain. To date, the analysis of aggrecan's variable numbers of tandem repeat polymorphism has given inconsistent results with respect to the correlation between the allele's size and intervertebral disc degeneration. We wanted to investigate the patterns of the variable numbers of tandem repeat polymorphism in the aggrecan CS1 domain of Koreans, and we analyzed the association between the polymorphism and intervertebral disc degeneration.

METHOD

A total of 66 males and 38 females participated in this study. Their ages ranged from 13 to 73 years. Genomic deoxyribonucleic acid was extracted from blood samples and PCR was carried out to detect the alleles of the aggrecan gene. The subjects were evaluated on MRI and they were classified by the number, severity, and morphology of disc degeneration.

FINDINGS

The genotyping identified 11 alleles ranging from 21 to 36 repeats. Alleles 13, 18, 19, and 20 were not found in this study. Of the 104 subjects, 29 (28%) were homozygotes and 75 (72%) were heterozygotes. Allele 27 (39%) was the most common form together with alleles 26 (26%) and 28 (14%). The allele 36 is the longest among the alleles ever discovered. For the case that the analysis was limited to subjects with the fourth decades or less, the 21 allele was significantly overrepresented among the persons with multilevel disc degeneration (p < 0.006).

CONCLUSIONS

Carrying a copy of the allele with 21 repeats might increase the risk of multiple disc degeneration in the subjects below the age of 40 years.

Absorption-based assays for the analysis of osteogenic and chondrogenic yield

The typical characteristics of cartilage and bone tissue are their unique extracellular matrices on which our body relies for structural support. In the respective tissue, the cells that create these matrices are the chondrocyte and the osteoblast. During in vitro differentiation from an embryonic or any other stem cell, specific cell types must be unequivocally identifiable to be able to draw the conclusion that a specific cell type has indeed been generated. Here, gene expression profiling can be helpful, but examining functional properties of cells is a lot more conclusive. As proteoglycans are found in and are part of the function of cartilage tissue, their detection and quantification becomes an important diagnostic tool in tissue engineering. Likewise, in bone regeneration therapy and in research, alkaline phosphatase is a known marker to detect the degree of development and function of differentiating osteoblasts. Calcification of the maturing osteoblast is the last stage in its development, and thus, the quantification of deposited calcium can aid in determining how many cells in a given culture have successfully matured into fully functioning osteoblasts. This chapter describes methods ideal for testing of proteoglycan content, alkaline phosphatase activity, and calcium deposit during in vitro chondro- and osteogenesis.

Identification of proteoglycan from salmon nasal cartilage

There has been no structural information about the core protein of salmon nasal cartilage proteoglycan although its physiological activities have been investigated. Internal amino acid sequencing using nano-LC/MS/MS revealed that the salmon proteoglycan was aggrecan. Primer walk sequencing based on the amino acid information determined that the salmon aggrecan cDNA is comprised of 4207bp nucleotides predicted to encode 1324 amino acids with a molecular mass of 143,276. It exhibited significant similarities to predicted pufferfish aggrecan, zebrafish similar to aggrecan, zebrafish aggrecan, bovine aggrecan and human aggrecan isoform 2 precursor; whose amino acid identities were 56%, 55%, 49%, 31% and 30%, respectively. Salmon cartilage aggrecan had globular domains G1, G2 and G3 as in mammalian aggrecans. Neither the putative keratan sulfate attachment domain enriched with serine, glutamic acid and proline, nor the putative chondroitin sulfate attachment domain with repeating amino acid sequence containing serine-glycine, found in mammalian aggrecans were observed in salmon, however, random serine-glycine (or glycine-serine) sequences predicted to the sugar chain attachment sites were observed. Based on cDNA analysis and amino acid analysis after β-elimination, the ratio of serine attached to sugar chains was calculated to be approximately 37.7% of total serine, that is, 46 of 123 serine residues.

Association between the expression of aggrecan and the distribution of aggrecan gene variable number of tandem repeats with symptomatic lumbar disc herniation in Chinese Han of Northern China

STUDY DESIGN

Case-control study.

OBJECTIVE

To examine the association between the expression of aggrecan and the aggrecan gene variable number of tandem repeats (VNTR) polymorphism with symptomatic lumbar disc herniation (LDH) in Chinese Han of Northern China.

SUMMARY OF BACKGROUND DATA

Aggrecan fragments have been found in human degenerated discs, and an association between the aggrecan VNTR polymorphism and intervertebral disc degeneration has been previously reported in middle-aged Finnish men. However, the relationship between the munity of symptomatic LDH with the expression of aggrecan and aggrecan gene VNTR has not been well studied.

METHODS

The disease group consisted of 70 patients already diagnosed with symptomatic LDH. The control group consisted of 14 patients restricted to spinal trauma and 113 healthy blood donors without symptoms of LDH who were not diagnosed with LDH. Disc tissue samples were obtained from surgical operations, and blood samples were donated from all participants. The aggrecan expression in isolated tissues was assessed by Western blot using specific antibodies. The aggrecan gene VNTR region was analyzed by polymerase chain reaction.

RESULTS

The aggrecan expression positive rate of control group was statistically and significantly higher (P < 0.001) than that of the disease group. Moreover, there was a statistically significant higher frequency of allele 25 or allele 21 in disease group compared with controls (P(A25) = 0.003416 and P(A21) = 0.000716). Compared with the participants with 2 alleles > 25 repeats, subjects with 1 or 2 alleles < or = 25 repeats statistically and significantly overrepresented the disease group without the expression of aggrecan (P < 0.001).

CONCLUSION

The findings suggest a relation between aggrecan and symptomatic LDH, where symptomatic LDH has a lower tendency of allele repeats. In addition, this study observed an association between the distribution of aggrecan gene VNTR polymorphism and the expression of aggrecan in symptomatic LDH.

Expression of collagen types I and II on articular cartilage in a rat knee contracture model

The purpose of our study was to clarify the expression patterns of collagen types I and II on articular cartilage after immobilization in a rat knee contracture model in 3 specific areas (noncontact area, transitional area, contact area). The unilateral knee joints of adult male rats were rigidly immobilized at 150 degrees of flexion using screws and a rigid plastic plate. Sham-operated animals had holes drilled in the femur and the tibia and screws inserted but were not plated. The expression patterns of collagen types I and II in each area were evaluated by in situ hybridization (ISH), immunohistochemistry (IHC), and quantitative real-time polymerase chain reaction (qPCR). The expression of collagen type II in the noncontact area was decreased by ISH but appeared unchanged when examined by IHC. In the transitional and contact areas, the expression of collagen type II was initially shown to have decreased and then increased at the hypertrophic chondrocytes by ISH but appeared decreased by IHC. Quantitative PCR revealed the decreased expression of type II collagen in the contact area. Immunostaining of collagen type I was increased at the noncontact area and transitional areas. Alterations of collagen types I and II expression may also affect the degeneration of articular cartilage after immobilization and the changes were different in the three areas.

Calcineurin plays an important role in the shell formation of pearl oyster (Pinctada fucata)

Calcineurin (CN) is a multifunctional protein involved in many important physiological processes in mammalians, but the function of CN in mollusks is still largely unknown. In the present study, through the shell regeneration system, the changes of enzymatic activity of CN were determined in the process of shell regeneration in pearl oyster Pinctada fucata. CN was activated immediately and continuously in the shell regeneration process. The speed of shell regeneration was measured and the ultrastructure of inner shell surface was observed by scanning electron microscopy after inhibiting CN by intramuscular injection of immunosuppresant cyclosporine A (CsA). The results showed that the speed of shell regeneration was delayed and the morphology of calcite and aragonite in the inner shell surface became abnormal when CN was inhibited by CsA. Meanwhile, RT-PCR analysis revealed that the expression of P. fucata BMP-2 in mantle tissue decreased with CsA injection. In vitro secretion level of proteoglycans (PGs) in primary cultures of mantle cells was also decreased when mantle cells were exposed to CsA. Taken together, our results, for the first time, show that CN is involved in the shell formation through regulating the expression of Pf-BMP-2 in mantle tissue, which controls the secretion of PGs/GAGs of the mantle epithelial cells.

Fell-Muir Lecture: Proteoglycans and more--from molecules to biology

In this article the organization and functional details of the extracellular matrix, with particular focus on cartilage, are described. All tissues contain a set of molecules that are arranged to contribute structural elements. Examples are fibril-forming collagens forming major fibrillar networks in most tissues. The assembly process is regulated by a number of proteins (thrombospondins, LRR-proteins, matrilins and other collagens) that can bind to the collagen molecule and in many cases remain bound to the formed fibre providing additional stability and enhancing networking to other structural networks. One such network is formed by collagen VI molecules assembled to beaded filaments in the matrix catalysed by interactions with small proteoglycans of the LRR-family, which remain bound to the filament providing for interactions via a linker of a matrilin to other matrix constituents like collagen fibres and the large proteoglycans, e.g. aggrecan in cartilage. Aggrecan is contributing an extreme anionic charge density to the extracellular matrix, which by osmotic effects leads to water retention and strive to swelling, resisted by the tensile properties of the collagen fibres. Aggrecan is bound via one end to hyaluronan, including such molecules retained at the cell surface, to form very large molecular entities that interact with other constituents of the matrix, e.g. fibulins that can form their own network. Other important interactions are those with cell surface receptors such as integrins, heparan sulphfate proteoglycans, hyaluronan receptors and others. Many of the molecules with an ability to interact with these receptors can also bind to molecules in the matrix and provide a bridge from the matrix to the cell and induce various responses. In pathology, there is an imbalance in matrix turnover with often excessive proteolytic breakdown. This results in the formation of protein fragments, where cleavage provides information on the active enzyme. Those fragments released can be specifically detected employing antibodies specific to the cleavage site and used to diagnose and monitor e.g. joint disease at early stages.

Analysing the role of endogenous matrix molecules in the development of osteoarthritis

Osteoarthritis (OA) is the most common form of arthritis worldwide. In this condition, damage to the extracellular matrix (ECM) of cartilage occurs, resulting in joint destruction. Factors mediating cartilage damage include mechanical injury, cytokine and superoxide release on a background of genetic susceptibility and obesity. Studies of arthritic cartilage show increased production of ECM molecules including type II collagen, cartilage oligomeric matrix protein, fibronectin (FN) and fibromodulin. Recent reports suggest that ECM proteins may become endogenous catabolic factors during joint damage. Activation of pro-inflammatory pathways by ECM proteins has led to their description as damage-associated molecular patterns (DAMPs). The ECM proteins involved include fibromodulin, which activates the complement pathway and may promote the persistence of joint inflammation. Fragmentation of type II collagen, FN and hyaluronan reveals cryptic epitopes that stimulate proteolytic enzymes including matrix metalloproteinases and aggrecanases (ADAMTSs - a disintegrin and metalloproteinase with thrombospondin type 1 motifs). Proteolytic fragments also stimulate the release of nitric oxide, chemokines and cytokines and activation of the MAP kinases. Reports are emerging that the receptors for the fragments described involve interaction with integrins and toll-like receptors. In this review the contribution of endogenous ECM molecules to joint destruction will be discussed. A deeper understanding of the pathways stimulated by endogenous ligands could offer potential avenues for novel therapies in the future.

Chapter 2. Evolution of vertebrate cartilage development

Major advances in the molecular genetics, paleobiology, and the evolutionary developmental biology of vertebrate skeletogenesis have improved our understanding of the early evolution and development of the vertebrate skeleton. These studies have involved genetic analysis of model organisms, human genetics, comparative developmental studies of basal vertebrates and nonvertebrate chordates, and both cladistic and histological analyses of fossil vertebrates. Integration of these studies has led to renaissance in the area of skeletal development and evolution. Among the major findings that have emerged is the discovery of an unexpectedly deep origin of the gene network that regulates chondrogenesis. In this chapter, we discuss recent progress in each these areas and identify a number of questions that need to be addressed in order to fill key gaps in our knowledge of early skeletal evolution.

Structural analysis of glycosaminoglycans and proteoglycans by means of Raman microspectrometry

Raman spectra have been determined for hyaluronan, chondroitin-4-sulfate, chondroitin-6-sulfate, aggrecan monomers and aggregates. The nature of the saccharides and the pattern of sulfation can be discerned. There were only small spectral changes with pH and ionic composition. Differences between hydroxyl vibrations, bulk water and solution conditions are shown. The spectrum of aggrecan is dominated by chondroitin sulfate contribution. The sulfation pattern and ratio of protein to glycosaminoglycan and the secondary structure of the core protein were determined.

Characterization of an ADAMTS-5-mediated cleavage site in aggrecan in OSM-stimulated bovine cartilage

OBJECTIVE

In a previous study, we identified a 50-kDa G3-containing aggrecan degradation product in bovine cartilage, released from the tissue after interleukin-1 (IL-1) stimulation in the presence of oncostatin M (OSM). Our objective was to purify, determine the N-terminal sequence of this fragment and verify whether this cleavage could be attributed to a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-4 and ADAMTS-5 action in vitro.

METHODS

Collected media from bovine cartilage explant cultures stimulated with IL-1+OSM were subjected to anion-exchange chromatography. The N-terminal sequence of the fragment of interest in the purified fractions was determined by automated Edman sequencing. Fetal bovine aggrecan was digested with full-length recombinant ADAMTS-4 and ADAMTS-5 and resulting degradation products were analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS/PAGE) and immunoblotting using an anti-G3 antiserum and an anti-neoepitope antibody that had been generated to the new N-terminus of the G3 fragment.

RESULTS

Characterization of the 50-kDa fragment showed that it possesses chondroitin sulfate (CS) and is the result of a cleavage within the C-terminal portion of the CS-2 domain, adjacent to the G3 region. Sequence analysis identified the cleavage region as TQRPAE(2047)-(2048)ARLEIE, suggesting an aggrecanase-derived product. Using an anti-neoepitope antibody specific for the additional cleavage site, it was shown that the product is generated in vitro upon digestion of aggrecan by ADAMTS-5 and, to a much lesser extent, by ADAMTS-4.

CONCLUSIONS

The abundance and rapid rate of release of this degradation product in organ cultures in the presence of OSM suggest that it could result from a unique aggrecan proteolysis mediated by aggrecanases.

Tropoelastin regulates chemokine expression in fibroblasts in Costello syndrome

Costello syndrome is a multiple congenital anomaly associated with growth and mental retardation, cardiac and skeletal anomalies, and a predisposition to develop neoplasia. Comprehensive expression analysis revealed remarkable up-regulation of several cytokines and chemokines including Gro family proteins, interleukin-1beta (IL-1beta), IL-8 and MCP-1 but down-regulation of extracellular matrix components including collagens and proteoglycans of skin fibroblasts derived from a Japanese Costello syndrome patient characterized by significantly reduced tropoelastin mRNA, impaired elastogenesis and enhanced cell proliferation. In contrast, decreases in these chemokines and IL-1beta expression were observed in Costello fibroblastic cell lines stably expressing the bovine tropoelastin (btEln) gene and in restored elastic fibers. These results strongly suggest that the human TE gene (ELN) transfer could be applicable for the gene therapy of a group of Costello syndrome patients with reduced ELN gene expression.

Aggrecan components differentially modulate nerve growth factor-responsive and neurotrophin-3-responsive dorsal root ganglion neurite growth

Aggrecan is one of the major chondroitin sulfate proteoglycans (CSPGs) expressed in the central nervous system. The signaling pathways activated downstream of cell interaction with aggrecan and with CSPGs in general and the importance of chondroitin sulfate-glycosaminoglycan side chains in their inhibition are unclear. Therefore, to analyze the effect of different components of aggrecan in inhibiting neurite growth, neurite outgrowth was quantified in an in vitro model in which chick dorsal root ganglion (DRG) explants were grown on substrates containing aggrecan bound to hyaluronan and link protein as a macromolecular aggregate, aggrecan monomers, hyaluronan, or ChABC-treated aggrecan. Aggrecan aggregate, aggrecan monomer, and hyaluronan inhibited neurite outgrowth from nerve growth factor (NGF)- and neurotrophin-3 (NT3)-responsive DRG neurons. Aggrecan inhibition was dependent on its chondroitin sulfate-glycosaminoglycans, as ChABC digestion alleviated neurite inhibition because of aggrecan. Growth cones displayed full or partial collapse on aggrecan aggregate, hyaluronan, and ChABC-treated aggrecan. Inhibition of Rho kinase (ROCK) with Y27632 increased neurite growth on some but not all of the aggrecan components tested. With NGF in the culture medium, Y27632 increased neurite outgrowth on aggrecan aggregate, monomers, and ChABC-treated aggrecan, but not on hyaluronan. The ROCK inhibitor also increased NT3-responsive outgrowth on aggrecan aggregate and hyaluronan, but not on ChABC-treated aggrecan. This study showed that the matrix proteoglycan aggrecan and its components have multiple effects on neurite outgrowth and that some of these effects involve the Rho/ROCK pathway.

Rab23 regulates differentiation of ATDC5 chondroprogenitor cells

Insulin treatment of mouse ATDC5 chondroprogenitors induces these cells to differentiate into mature chondrocytes. To identify novel factors that are involved in this process, we carried out mutagenesis of ATDC5 cells through retroviral insertion and isolated two mutant clones incapable of differentiation. Inverse PCR analysis of these clones revealed that the retroviral DNA was inserted into the promoter region of the Rab23 gene, resulting in increased Rab23 expression. To investigate whether an elevated level of Rab23 protein led to inhibition of chondrogenic differentiation, we characterized ATDC5 cells that either overexpress endogenous Rab23 or stably express ectopic Rab23. Our results revealed that up-regulation of Rab23 can indeed inhibit chondrogenic differentiation with a concomitant down-regulation of matrix genes such as type II collagen and aggrecan. In addition, stable small interfering RNA knockdown of Rab23 also resulted in inhibition of chondrogenic differentiation as well as down-regulation of Sox9, a master regulator of chondrogenesis. Interestingly, Sox9 expression has recently been linked to Gli1, and we found that Rab23 knockdown decreased Gli1 expression in chondrocytes. Because the phenotypes of Rab23 mutations in mice and humans include defects in cartilage and bone development, our study suggests that Rab23 is involved in the control of Sox9 expression via Gli1 protein.

Functional differences of the catalytic and non-catalytic domains in human ADAMTS-4 and ADAMTS-5 in aggrecanolytic activity

ADAMTS-4 (aggrecanase-1) and ADAMTS-5 (aggrecanase-2) are multidomain metalloproteinases belonging to the ADAMTS family. We have previously reported that human ADAMTS-5 has much higher aggrecanolytic activity than human ADAMTS-4. To investigate the different proteolytic activity of the two enzymes, we generated a series of chimeras by exchanging various non-catalytic domains of the two proteinases. We found that the catalytic domain of ADAMTS-5 has higher intrinsic catalytic ability than that of ADAMTS-4. The studies also demonstrated that the non-catalytic domains of ADAMTS-5 are more effective modifiers than those of ADAMTS-4, making both catalytic domains more active against aggrecan, an Escherichia coli-expressed interglobular domain of aggrecan and fibromodulin. Addition of the C-terminal thrombospondin type I motif of ADAMTS-5 to the C terminus of ADAMTS-4 increased the activity of ADAMTS-4 against aggrecan and fibromodulin severalfold. In contrast to previous reports (Kashiwagi, M., Enghild, J. J., Gendron, C., Hughes, C., Caterson, B., Itoh, Y., and Nagase, H. (2004) J. Biol. Chem. 279, 10109-10119 and Gao, G., Plaas, A., Thompson, V. P., Jin, S., Zuo, F., and Sandy, J. D. (2004) J. Biol. Chem. 279, 10042-10051), our detailed investigation of the role of the C-terminal spacer domain of ADAMTS-4 indicated that full-length ADAMTS-4 is approximately 20-times more active against aggrecan than its spacer domain deletion mutant, even at the Glu373-Ala374 site of the interglobular domain. This discrepancy is most likely due to selective inhibition of full-length ADAMTS-4 by heparin, particularly for cleavage at the Glu373-Ala374 bond. However, removal of the spacer domain from ADAMTS-4 greatly enhanced more general proteolytic activity against non-aggrecan substrates, e.g. E. coli-expressed interglobular domain, fibromodulin, and carboxymethylated transferrin.

Proteoglycan expression patterns in human hair follicle

BACKGROUND

Proteoglycans (PGs) are known to play key roles in many cellular signalling pathways involved in hair follicle biology. Although some PG core proteins have previously been described in adult human hair follicles, their glycosaminoglycan (GAG) moieties have been less studied.

OBJECTIVES

To add knowledge about PG core protein and GAG distributions in human anagen hair follicle and, for selected follicles, during catagen.

METHODS

We used immunohistochemistry and immunohistofluorescence to revisit the expression pattern of GAG chains and core proteins in human hair follicle. The studied epitopes included CD44v3, syndecan-1, perlecan, versican, aggrecan, biglycan, heparan sulphate (HS), chondroitin sulphate (CS), dermatan sulphate (DS) and keratan sulphate (KS).

RESULTS

The membrane PGs syndecan-1 and CD44v3 were respectively detected in the epithelial part of whole hair and in the outer root sheath basal layer. The dermal part of the hair follicle contained high amounts of extracellular PGs such as perlecan, versican, aggrecan, biglycan and their saccharidic moieties, namely HS, CS, DS and KS. We also observed a variable distribution of these components along the hair follicle. Especially, we noted a PG impoverishment at the very bottom of the anagen bulb. Moreover, while type D chondroitin expression remained unaffected, 4C3-CS and PG4-CS/DS epitopes respectively decreased in the dermal papilla and the connective tissue sheath, at the onset of catagen.

CONCLUSIONS

GAG and PG expression along the human anagen hair follicle was characterized by (i) discontinuities mainly affecting the basement membrane and (ii) disappearance of some epitopes at catagen onset. These results are discussed in term of functionalities in nutrient diffusion, cell proliferation and differentiation, and hair protection.

Molecular signatures of metaplastic carcinoma of the breast by large-scale transcriptional profiling: identification of genes potentially related to epithelial-mesenchymal transition

Metaplastic carcinoma of the breast (MCB) is a poorly understood subtype of breast cancer. It is generally characterized by the coexistence of ductal carcinomatous and transdifferentiated sarcomatous components, but the underlying molecular alterations, possibly related to epithelial-mesenchymal transition (EMT), remain elusive. We performed transcriptional profiling using half-a-genome oligonucleotide microarrays to elucidate genetic profiles of MCBs and their differences to those of ductal carcinoma of breasts (DCBs) using discarded specimens of four MCBs and 34 DCBs. Unsupervised clustering disclosed distinctive expression profiles between MCBs and DCBs. Supervised analysis identified gene signatures discriminating MCBs from DCBs and between MCB subclasses. Notably, many of the discriminator genes were associated with downregulation of epithelial phenotypes and with synthesis, remodeling and adhesion of extracellular matrix, with some of them have known or inferred roles related to EMT. Importantly, several of the discriminator genes were upregulated in a mutant Snail-transfected MCF7 cell known to exhibit features of EMT, thereby indicating a crucial role for EMT in the pathogenesis of MCBs. Finally, the identification of SPARC and vimentin as poor prognostic factors reinforced the role of EMT in cancer progression. These data advance our understanding of MCB and offer clues to the molecular alterations underlying EMT.

Overexpression of SR proteins and splice variants modulates chondrogenesis

Fibronectin alternative exon EIIIA is largely included in undifferentiated mesenchymal cells of the developing limb bud, whereas the exon is excluded in differentiated chondrocytes. Inclusion of exon EIIIA in chondrocytic cells is increased by overexpression of SRp40, and, to a lesser extent, SRp75, but not SRp55. RT-PCR analysis using real-time PCR revealed that the levels of the mRNAs for these three proteins did not vary significantly in chick chondrocytes versus mesenchymal cells of the developing limb bud. However, a variant spliced form of SRp40, termed, SRp40LF, is detected preferentially in chondrocytes and in chondrifying mesenchymal cells. Forced overexpression of SRp40 or SRp75, but not SRp55, enhanced chondrogenic differentiation of chick limb mesenchymal cells in a high-density micromass assay. Overexpression of SRp40LF, which produces a truncated form of SRp40, also was strongly pro-chondrogenic. In a HeLa cell-based assay, SRp40LF fails to substitute for SRp40 in mediating an increase in exon EIIIA inclusion, suggesting that the latter event is not essential for the pro-chondrogenic effect. These results demonstrate the ability of these highly conserved splicing factors to modulate chondrogenesis and are consistent with earlier results that implicated exon EIIIA-containing isoforms of fibronectin in formation of chondrogenic condensations.

Localization and thyroid hormone influenced expression of collagen II in ovarian tissue

Collagen type II (Col II), one of the main components of the hyaline cartilage, is a member of the fibril-forming collagen family. Due to its amino acid composition, the extent of lysine hydroxylation of Col II is much higher than that of other fibril forming collagens. Since lysyl hydroxylase isoforms are less synthesized in hypothyroid ovarian tissue, Col II level is expected to be reduced here and contribute to the degradation of ovarian ECM in this condition. As there was no previous report, we have demonstrated Col II expression in rat ovary. Col2A1 mRNA shares significant part of the total collagens in ovary as shown by the relative expression of the major collagen genes present in this tissue. It has also been shown that Col II is down regulated in hypothyroid ovarian tissue and its expression is increased upon stimulation by thyroid hormone (T(3)). To know whether less Col II in hypothyroid ovarian tissue is due to less synthesis of the protein or its increased rate of degradation is also involved in it, we demonstrated the status of Collagen - degrading Matrix Metalloproteinases in this condition and found up regulation of MMP-1, -8 and -13 in hypothyroid rat ovary. The present study shows the reduced Col II expression in hypothyroid rat ovary, with the concomitant increase in Col II degradation. This information will be useful for further studies on reproductive disorders.

Age-related accumulation of pentosidine in aggrecan and collagen from normal and degenerate human intervertebral discs

During aging and degeneration, many changes occur in the structure and composition of human cartilaginous tissues, which include the accumulation of the AGE (advanced glycation end-product), pentosidine, in long-lived proteins. In the present study, we investigated the accumulation of pentosidine in constituents of the human IVD (intervertebral disc), i.e. collagen, aggrecan-derived PG (proteoglycan) (A1) and its fractions (A1D1-A1D6) in health and pathology. We found that, after maturity, pentosidine accumulates with age. Over the age range studied, a linear 6-fold increase was observed in pentosidine accumulation for A1 and collagen with respective rates of 0.12 and 0.66 nmol x (g of protein)(-1) x year(-1). Using previously reported protein turnover rate constants (k(T)) obtained from measurements of the D-isomer of aspartic residue in collagen and aggrecan of human IVD, we could calculate the pentosidine formation rate constants (k(F)) for these constituents [Sivan, Tsitron, Wachtel, Roughley, Sakkee, van der Ham, DeGroot, Roberts and Maroudas (2006) J. Biol. Chem. 281, 13009-13014; Tsitron (2006) MSc Thesis, Technion-Israel Institute of Technology, Haifa, Israel]. In spite of the comparable formation rate constants obtained for A1D1 and collagen [1.81+/-0.25 compared with 3.71+/-0.26 micromol of pentosidine x (mol of lysine)(-1) x year(-1) respectively], the higher pentosidine accumulation in collagen is consistent with its slower turnover (0.005 year(-1) compared with 0.134 year(-1) for A1D1). Pentosidine accumulation increased with decreasing buoyant density and decreasing turnover of the proteins from the most glycosaminoglycan-rich PG components (A1D1) to the least (A1D6), with respective k(F) values of 1.81+/-0.25 and 3.18+/-0.37 micromol of pentosidine.(mol of lysine)(-1) x year(-1). We concluded that protein turnover is an important determinant of pentosidine accumulation in aggrecan and collagen of human IVD, as was found for articular cartilage. Correlation of pentosidine accumulation with protein half-life in both normal and degenerate discs further supports this finding.

Aggrecan core protein of a certain length is protective against hand osteoarthritis

OBJECTIVE

To study the contribution of aggrecan VNTR (variable number of tandem repeats) polymorphism to clinically differing manifestations of hand osteoarthritis (OA).

DESIGN

Five hundred thirty Finnish females representing two academically similar occupations with completely diverse exposure to hand load were included. Radiographs of hands were analysed, the OA findings were graded and the subjects were divided into categories. Aggrecan VNTR alleles were identified by Southern hybridization. Statistical analyses were used to compare joint involvement and pathological findings with the prevalences of the alleles and genotypes.

RESULTS

Subjects homozygous for the most common aggrecan VNTR allele, A27 with 27 repeats, had a significantly lower risk of hand OA, with OR 0.46 (95% CI 0.27-0.78) for OA of grade 2 or more. Our results suggest that carrying two copies of the alleles with less than 27 repeats could predispose a subject to a severe hand OA (OR 2.45, 95% CI 1.17-5.12) and carrying two copies of the alleles with more than 27 repeats also increases the risk of the disease (OR 1.73, 95% CI 1.03-2.89).

CONCLUSIONS

These findings indicate that allele A27 provides protection from hand OA and that alleles shorter or longer than this may predispose subjects to the disease. Furthermore, they suggest that a certain number of tandem repeats provide for optimal functioning of the aggrecan molecule and that the contribution of genetic factors to the development of hand OA may be even more important than that of environmental factors.

Chondrocyte secreted CRTAC1: a glycosylated extracellular matrix molecule of human articular cartilage

Cartilage acidic protein 1 (CRTAC1), a novel human marker which allowed discrimination of human chondrocytes from osteoblasts and mesenchymal stem cells in culture was so far studied only on the RNA-level. We here describe its genomic organisation and detect a new brain expressed (CRTAC1-B) isoform resulting from alternate last exon usage which is highly conserved in vertebrates. In humans, we identify an exon sharing process with the neighbouring tail-to-tail orientated gene leading to CRTAC1-A. This isoform is produced by cultured human chondrocytes, localized in the extracellular matrix of articular cartilage and its secretion can be stimulated by BMP4. Of five putative O-glycosylation motifs in the last exon of CRTAC1-A, the most C-terminal one is modified according to exposure of serial C-terminal deletion mutants to the O-glycosylation inhibitor Benzyl-alpha-GalNAc. Both isoforms contain four FG-GAP repeat domains and an RGD integrin binding motif, suggesting cell-cell or cell-matrix interaction potential. In summary, CRTAC1 acquired an alternate last exon from the tail-to-tail oriented neighbouring gene in humans resulting in the glycosylated isoform CRTAC1-A which represents a new extracellular matrix molecule of articular cartilage.

Proteomic analysis of cartilage- and bone-associated samples

The skeleton of the human body is built of cartilage and bone, which are tissues that contain extensive amounts of extracellular matrix (ECM). In bone, inorganic mineral hydroxyapatite forms 50-70% of the whole weight of the tissue. Although the organic matrix of bone consists of numerous proteins, 90% of it is composed of type I collagen. In cartilage, ECM forms a major fraction of the tissue, type II collagen and aggrecans being the most abundant macromolecules. It is obvious that the high content of ECM components causes analytical problems in the proteomic analysis of cartilage and bone, analogous to those in the analysis of low-abundance proteins present in serum. The massive contents of carbohydrates present in cartilage proteoglycans, and hydroxyapatite in bone, further complicate the situation. However, the development of proteomic tools makes them more and more tempting also for research of musculoskeletal tissues. Application of proteomic techniques to the research of chondrocytes, osteoblasts, osteocytes, and osteoclasts in cell cultures can immediately benefit from the present knowledge. Here we make an overview to previous proteomic research of cartilage- and bone-associated samples and evaluate the future prospects of applying proteomic techniques to investigate key events, such as cellular signal transduction, in cartilage- and bone-derived cells.

Estimation of the identity of proteolytic aggrecan fragments using PAGE migration and Western immunoblot

OBJECTIVE

To develop calculation models, using Western immunoblot, as a tool for the estimation of proteolytic human aggrecan fragment identity.

METHOD

Seven human aggrecan fragments (calibrators), purified by CsCl gradient centrifugation and identified by Western immunoblot of N- and C-terminals, were used to develop calculation models. The models were used for identification of unknown aggrecan fragments each having one of their N- or C-terminals identified.

RESULTS

The calibrator molecular weights (Mw) from sodium dodecyl sulfate (SDS)-gels (m), the Mw of amino acids (a) and the Mw of their carbohydrate substitution (g) were expressed as K = m/(a+g), or as K = 1.085m/(a+g) when compensation for the G1 domain was required. Using these models together with average K-values, 12 out of the 17 immuno-detected aggrecan fragments were calculated to a known protease cleavage site, while five were identified to domain levels.

CONCLUSIONS

With six neoepitope antibodies together with antibodies against the G1- and G3-domain it was possible to predict the identity of several proteolytic fragments from different regions within the aggrecan monomer.

Link protein can retard the degradation of hyaluronan in proteoglycan aggregates

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Autoantibodies against aggrecan in systemic rheumatic diseases

OBJECTIVE

This study was undertaken to investigate the presence of autoantibodies against the main cartilage proteoglycan, aggrecan, in systemic rheumatic disease sera, and to identify substructure(s) responsible for the autoimmune response.

METHODS

Sera were obtained from 86 patients with various systemic rheumatic diseases, 14 with osteoarthritis (OA), 18 with cancer and 40 healthy individuals. The presence of autoantibodies against aggrecan was examined by a solid phase assay and by Western blotting, using proteoglycan aggregates treated with proteolytic enzymes. The positive bands were subjected to nanohigh performance liquid chromatography (nanoHPLC)-MS, in order to identify the aggrecan substructures involved in the autoimmune response.

RESULTS

Autoantibodies against aggrecan were identified in all systemic rheumatic disease sera at a high titre, almost three times that observed in healthy controls. OA and cancer sera produced a reaction equal to that of the healthy. Western blotting analysis of aggrecan proteolytic fragments revealed the presence of a triple band, reacting with the patients' sera, of about 37 kDa, which also reacted with a polyclonal antibody against hyaluronan-binding region. NanoHPLC-MS analysis suggested that this band belonged to the G2 domain of aggrecan.

CONCLUSION

At least a part of the autoimmune reaction to aggrecan, displayed by the systemic disease sera, involves the G2 domain. The significant difference observed between these sera and those from other diseases, especially cancer, may suggest a possible discriminatory role of anti-aggrecan antibodies. This may help in the differential diagnosis in complicated clinical cases. However, for this to be confirmed, studies in larger cohorts of patients should be performed.

The structure and degradation of aggrecan in human intervertebral disc

The ability of the intervertebral disc to resist compression is dependent on its high proteoglycan concentration. The disc proteoglycans are classified as aggregating or non-aggregating depending on their ability to interact with hyaluronan. The majority of the aggregating proteoglycans are derived from aggrecan, though their glycosaminoglycan substitution pattern has not been determined. In contrast, the origin of the non-aggregating proteoglycans is unclear, though it has been postulated that they are derived from aggrecan by proteolysis. The present work demonstrates that keratan sulfate (KS) in the glycosaminoglycan-binding region of disc aggrecan is confined to the KS-rich domain of the core protein and is not present in association with chondroitin sulfate (CS) in the CS1 and CS2 domains. It also shows that the non-aggregating disc proteoglycans are derived from aggrecan, with the large molecules possessing both the KS-rich and CS1 domains and the smaller molecules being generated from either the KS-rich or CS2 domain. The origin and spectrum of disc proteoglycan heterogeneity is the same in both the annulus fibrosus and nucleus pulposus.