Structure and function of aggrecan

N-terminal domains of CCN family 2/connective tissue growth factor bind to aggrecan

CCN2/CTGF (CCN family 2/connective tissue growth factor) is a multi-cellular protein with a broad range of activities. It modulates many cellular functions, including proliferation, migration, adhesion and extracellular matrix production, and it is thus involved in many biological and pathological processes. In particular, CCN2/CTGF is essential for normal skeletal development. To identify CCN2/CTGF-interactive proteins capable of modulating its action in cartilage, we carried out a yeast two-hybrid screening using CCN2/CTGF peptide as a bait and a cDNA library from a chondrocytic cell line, HCS-2/8. In the present paper, we report the identification of aggrecan, which is a major proteoglycan of the extracellular matrix in cartilage, as a CCN2/CTGF-binding protein. Among the four domains of CCN2/CTGF, the IGFBP [IGF (insulin-like growth factor)-binding protein-like] and/or VWC (von Willebrand factor type C) domains had a direct interaction with aggrecan in a yeast two-hybrid assay. The results of a solid-phase-binding assay using aggrecan-coated plates also showed binding to recombinant CCN2/CTGF in a dose-dependent manner. rIGFBP (recombinant IGFBP) and rVWC (recombinant VWC) module peptides had stronger binding to aggrecan compared with rTSP1 (recombinant thrombospondin type 1 repeat) and rCT (recombinant C-terminal cystine knot) module peptides. SPR (surface plasmon resonance) analysis showed the direct interaction between the CCN2/CTGF and aggrecan, and ectopically overexpressed CCN2/CTGF and AgG3 (G3 domain of aggrecan) confirmed their binding In vivo. Indirect immunofluorescence analysis indicated that CCN2/CTGF was extracellularly co-localized with aggrecan on HCS-2/8 cells. The rIGFBP-rVWC peptide effectively enhanced the production and release of aggrecan compared with the rTSP-rCT peptide in chondrocytes. These results indicate that CCN2/CTGF binds to aggrecan through its N-terminal IGFBP and VWC modules, and this binding may be related to the CCN2/CTGF-enhanced production and secretion of aggrecan by chondrocytes.

[Preventive and therapeutic effects of Yiqi Huayu Recipe on degeneration of articular cartilage in rats with osteoarthritis]

OBJECTIVE

To study the efficacy and possible mechanism of Yiqi Huayu Recipe (YQHYR), a compound traditional Chinese herbal medicine, in preventing and treating degeneration of the articular cartilage in rats with osteoarthritis (OA).

METHODS

A total of 90 one-month-old SD rats were randomly divided into normal control group, untreated group and YQHYR group, with 30 rats in each group. The osteoarthritis was induced by shoulder disarticulation plus upright posture in rats. The rats in YQHYR group were treated with YQHYR at 5-, 7- and 9-month old (4, 6 and 8 months after the surgery) for one month. Ten rats in each subgroup were sacrificed at 6-, 8- and 10-month old (5, 7 and 9 months after the surgery) respectively and the knee joint samples were harvested for detection. Safranin-O/fast green staining was performed to examine the morphology of articular cartilage. The expressions of type II collagen (Col2A1), aggrecan-1 (Agc1), matrix metalloproteinase-13 (MMP-13) and tissue inhibitor of metalloproteinase-1 (TIMP-1) mRNAs were evaluated by real-time fluorescent quantitation polymerase chain reaction.

RESULTS

Histological analysis showed that the structure of articular cartilage was seriously destroyed in rats in the untreated group. On the contrary, the degenerative changes of the articular cartilage in the YQHYR group were dropped off. Real-time fluorescent quantitation polymerase chain reaction showed that expressions of Agc1, TIMP-1 and Col2A1 mRNAs were up-regulated in 6- and 10-month-old rats in the YQHYR group as compared with those in the untreated group (P<0.01, P<0.05), but there was no significant difference in expression of MMP-13 mRNA between the YQHYR group and the untreated group. The expression of Agc1 mRNA was up-regulated and the expression of MMP-13 mRNA was down-regulated in the 8-month-old rats in YQHYR group as compared with those in the untreated group (P<0.01).

CONCLUSION

YQHYR can promote the synthesizing of aggrecan and type II collagen in chondrocytes and delay articular cartilage degradation in OA rats.

Egr-1 inhibits the expression of extracellular matrix genes in chondrocytes by TNFalpha-induced MEK/ERK signalling

Introduction

TNFα is increased in the synovial fluid of patients with rheumatoid arthritis and osteoarthritis. TNFα activates mitogen-activated kinase kinase (MEK)/extracellular regulated kinase (ERK) in chondrocytes; however, the overall functional relevance of MEK/ERK to TNFα-regulated gene expression in chondrocytes is unknown.

Methods

Chondrocytes were treated with TNFα with or without the MEK1/2 inhibitor U0126 for 24 hours. Microarray analysis and real-time PCR analyses were used to identify genes regulated by TNFα in a MEK1/2-dependent fashion. Promoter/reporter, immunoblot, and electrophoretic mobility shift assays were used to identify transcription factors whose activity in response to TNFα was MEK1/2 dependent. Decoy oligodeoxynucleotides bearing consensus transcription factor binding sites were introduced into chondrocytes to determine the functionality of our results.

Results

Approximately 20% of the genes regulated by TNFα in chondrocytes were sensitive to U0126. Transcript regulation of the cartilage-selective matrix genes Col2a1, Agc1 and Hapln1, and of the matrix metalloproteinase genes Mmp-12 and Mmp-9, were U0126 sensitive – whereas regulation of the inflammatory gene macrophage Csf-1 was U0126 insensitive. TNFα-induced regulation of Sox9 and NFκB activity was also U0126 insensitive. Conversely, TNFα-increased early growth response 1 (Egr-1) DNA binding was U0126 sensitive. Transfection of chondrocytes with cognate Egr-1 oligodeoxynucleotides attenuated the ability of TNFα to suppress Col2a1, Agc1 or Hapln1 mRNA expression.

Conclusions

Our results suggest that MEK/ERK and Egr1 are required for TNFα-regulated catabolic and anabolic genes of the cartilage extracellular matrix, and hence may represent potential targets for drug intervention in osteoarthritis or rheumatoid arthritis.

Inhibition of glycosaminoglycan incorporation influences collagen network formation during cartilage matrix production

To understand cartilage degenerative diseases and improve repair procedures, we investigate the influence of glycosaminoglycans (GAGs) on cartilage matrix biochemistry and functionality. Bovine articular chondrocytes were cultured in alginate beads with(out) para-nitrophenyl-beta-d-xyloside (PNPX) to inhibit GAG incorporation into newly formed proteoglycans. As expected, GAG deposition in alginate beads decreased with increasing PNPX concentration. Next to GAGs, collagen deposition and cross-linking also decreased. In the presence of PNPX, GAGs and collagen were deposited further away from the chondrocyte than in the control and increased amounts were found in the culture medium. These changes resulted in decreased functional properties of the construct. We conclude that in our culture system, intact proteoglycans play a role in deposition of collagen and thus the formation of a functional matrix. The effect of less proteoglycans on the collagen network could explain why cartilage repair is ineffective in osteoarthritis and help us with development of new therapies.

Evaluation of serum chondroitin sulfate and hyaluronan: biomarkers for osteoarthritis in canine hip dysplasia

Hip dysplasia (HD) is one of the most important bone and joint diseases in dogs. Making the radiographic diagnosis is sometime possible when the disease has markedly progressed. Chondroitin sulfate (CS) and hyaluronan (HA) are the most important cartilage biomolecules that are elevated in the serum taken from dogs with osteoarthritis. The serum CS and HA can be detected by an ELISA technique, with using monoclonal antibodies against CS epitope 3B3 and WF6 and the HA chain as the primary antibodies. The aim of this study was to compare the levels of serum CS (both epitopes) and HA in non-HD and HD dogs. All 123 dogs were categorized into 2 groups. The non-HD group was composed of 98 healthy dogs, while the HD group was comprised of 25 HD dogs. Blood samples were collected for analyzing the serum CS and HA levels with using the ELISA technique. The results showed that the average serum level of the CS epitope WF6 in the HD group (2,594 ± 3,036.10 ng/ml) was significantly higher than that in the non-HD group (465 ± 208.97 ng/ml) (p < 0.01) while the epitope 3B3 in the HD group (105 ± 100.05 ng/ml) was significantly lower than that in the non-HD group (136 ± 142.03 ng/ml) (p < 0.05). The amount of serum HA in the HD group (134.74 ± 59.71 ng/ml) was lower than that in the non HD group (245.45 ± 97.84 ng/ml) (p < 0.05). The results indicate that the serum CS and HA levels might be used as biomarkers for osteoarthritis in HD dogs.

Application of biomarkers in the clinical development of new drugs for chondroprotection in destructive joint diseases: a review

Emerging evidence supports the concept that biochemical markers are clinically useful non-invasive diagnostic tools for the monitoring of changes in cartilage turnover in patients with destructive joint diseases such as osteoarthritis (OA) and rheumatoid arthritis (RA). Epidemiological studies demonstrated that measurements of different degradation products of proteins in the extracellular matrix of hyaline cartilage in urine or serum samples are (1) increased in OA or RA patients compared with healthy individuals, (2) correlate with disease activity, and (3) are predictive for the rate of changes in radiographic measures of cartilage loss. The present review provides an updated list of available biomarkers and summarize the research data arguing for their clinical utility. In addition, it addresses the question whether or not the monitoring of biomarkers during different treatment modalities could be a useful approach to characterize the chondro-protective effects of approved and candidate drugs. Finally, it briefly reviews the in vitro/ex vivo experimental settings - isolated chondrocyte cultures and articular cartilage explants - that can assist in the verification of novel markers, but also studies assessing direct effects of drug candidates on chondrocytes. Collectively, biomarkers may acquire a function as established efficacy parameters in the clinical development of novel chondro-protective agents.

Cartilage degradation is fully reversible in the presence of aggrecanase but not matrix metalloproteinase activity

Introduction

Physiological and pathophysiological cartilage turnover may coexist in articular cartilage. The distinct enzymatic processes leading to irreversible cartilage damage, compared with those needed for continuous self-repair and regeneration, remain to be identified. We investigated the capacity of repair of chondrocytes by analyzing their ability to initiate an anabolic response subsequent to three different levels of catabolic stimulation.

Methods

Cartilage degradation was induced by oncostatin M and tumour necrosis factor in articular cartilage explants for 7, 11, or 17 days. The catabolic period was followed by 2 weeks of anabolic stimulation (insulin growth factor-I). Cartilage formation was assessed by collagen type II formation (PIINP). Cartilage degradation was measured by matrix metalloproteinase (MMP) mediated type II collagen degradation (CTX-II), and MMP and aggrecanase mediated aggrecan degradation by detecting the ³⁴²FFGVG and ³⁷⁴ARGSV neoepitopes. Proteoglycan turnover, content, and localization were assessed by Alcian blue.

Results

Catabolic stimulation resulted in increased levels of cartilage degradation, with maximal levels of ³⁷⁴ARGSV (20-fold induction), CTX-II (150-fold induction), and ³⁴²FFGVG (30-fold induction) (P < 0.01). Highly distinct protease activities were found with aggrecanase-mediated aggrecan degradation at early stages, whereas MMP-mediated aggrecan and collagen degradation occurred during later stages. Anabolic treatment increased proteoglycan content at all time points (maximally, 250%; P < 0.001). By histology, we found a complete replenishment of glycosaminoglycan at early time points and pericellular localization at an intermediate time point. In contrast, only significantly increased collagen type II formation (200%; P < 0.01) was observed at early time points.

Conclusion

Cartilage degradation was completely reversible in the presence of high levels of aggrecanase-mediated aggrecan degradation. After induction of MMP-mediated aggrecan and collagen type II degradation, the chondrocytes had impaired repair capacity.

Patients with rheumatoid arthritis have an altered circulatory aggrecan profile

Background

Rheumatoid arthritis (RA) is a chronic auto-immune disease with extensive articular cartilage destruction. Aggrecan depletion, mediated by aggrecanases is one of the first signs of early cartilage erosion. We investigated, whether measurement of aggrecan and fragments thereof in serum, could be used as biomarkers for joint-disease in RA patients and furthermore characterized the fragments found in the circulation.

Methods

The study consisted of 38 patients, 12 males (62.2 ± 16.0 years) and 26 females (59.8 ± 20.7 years) diagnosed with RA: 41.5 ± 27.5 mm/h erythrocyte sedimentation rate (ESR), 38.4 ± 34.7 mg/ml C-reactive protein (CRP) and 4.8 ± 1.7 disease activity score (DAS) and 108 healthy age-matched controls. Aggrecan levels were measured using two immunoassays, i.e. the ³⁷⁴ARGSVI-G2 sandwich ELISA measuring aggrecanase-mediated aggrecan degradation and the G1/G2 sandwich assay, detecting aggrecan molecules containing G1 and/or G2 (total aggrecan) We further characterized serum samples by western blots, by using monoclonal antibodies F-78, binding to G1 and G2, or by BC-3, detecting the aggrecanase-generated N-terminal ³⁷⁴ARGSVI neo-epitope.

Results

Total aggrecan levels in RA patients were significantly decreased from 824.8 ± 31 ng/ml in healthy controls to 570.5 ± 30 ng/ml (31% decrease, P < 0.0001), as measured by the G1/G2 ELISA. Western blot analysis with F-78 showed one strong band at 10 kDa, and weaker bands at 25 and 45 kDa in both healthy controls and RA patients. In contrast, staining for aggrecanase-activity revealed only one strong band in RA patients of 45 kDa.

Conclusion

This is the first study, which characterizes different aggrecan fragments in human serum. The data strongly suggests that total aggrecan levels, i.e. aggrecan molecules containing G1 and/or G2 are lower in RA patients, and that RA patients have at least one specific subpopulation of aggrecan fragments, namely aggrecanse generated ³⁷⁴ARGSVI fragments. Further clinical studies are needed to investigate the potential of G1/G2 as a structure-related biochemical marker in destructive joint-diseases.

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.

Indirect role of Ca2+ in the assembly of extracellular matrix proteins

We have investigated through molecular dynamics the binding properties at the interface between the C-type lectin subdomain (CLD) of aggrecan and the fibronectin type III domains ((4-5)FnIII) of tenasin, in particular the mechanistically unknown, but essential role of Ca(2+) in the binding. The binding between the CLD and (4-5)FnIII is critical in cross-linking aggrecan, hyaluronan, and tenascin to form extended protein networks found in tissues such as cartilage. None of the structurally resolved Ca(2+) ions in the complex of the CLD and (4-5)FnIII is directly bridging the two proteins. However, one of the Ca(2+) ions (Ca2) is found to play the role of maintaining the structure of the L4 loop at the CLD binding surface, thus facilitating a high affinity binding between the CLD and (4-5)FnIII. Removal of this Ca(2+) ion causes a drastic structural change in the L4 loop, which presumably hinders the binding of the CLD to the (4-5)FnIII. The other bound Ca(2+) ions (Ca1 and Ca3) have no significant effect on the structure of the CLD binding surface, and thus are not expected to affect the binding. Our results might also suggest that the role of Ca2 in maintaining the structure of the L4 loop is most important during the binding process. Once the complex is formed, the dependence of the complex on the structuring role of Ca2 is reduced. In response to tensile force, the CLD and (4-5)FnIII separate by breaking first the electrostatic interactions at the interface, followed by the hydrophobic ones. The sequence of the unbinding events and the maximum force required to separate the two proteins are independent of the presence of the Ca(2+) ions, underlying the indirect role of Ca(2+) in the binding.

Monte Carlo simulations reveal the straightening of an end-grafted flexible chain with a rigid side chain

We have studied the conformational properties of a flexible end-grafted chain (length N) with a rigid side chain (length S) by means of Monte Carlo simulations. Depending on the lengths N and S and the branching site b, we observe a considerable straightening of the flexible backbone as quantified via the gyration tensor. For b = N, i.e., when attaching the side chain to the free end of the flexible backbone, the effect was strongest.

Induction of increased cAMP levels in articular chondrocytes blocks matrix metalloproteinase-mediated cartilage degradation, but not aggrecanase-mediated cartilage degradation

OBJECTIVE

Calcitonin has been suggested to have chondroprotective effects. One signaling pathway of calcitonin is via the second messenger cAMP. We undertook this study to investigate whether increased cAMP levels in chondrocytes would be chondroprotective.

METHODS

Cartilage degradation was induced in bovine articular cartilage explants by 10 ng/ml oncostatin M (OSM) and 20 ng/ml tumor necrosis factor (TNF). In these cultures, cAMP levels were augmented by treatment with either forskolin (4, 16, or 64 microM) or 3-isobutyl-1-methyl xanthine (IBMX; 4, 16, or 64 microM). Cartilage degradation was assessed by 1) quantification of C-terminal crosslinking telopeptide of type II collagen fragments (CTX-II), 2) matrix metalloproteinase (MMP)-mediated aggrecan degradation by (342)FFGV- G2 assay, 3) aggrecanase-mediated degradation by (374)ARGS-G2 assay, 4) release of sulfated glycosaminoglycans (sGAG) into culture medium, 5) immunohistochemistry with a monoclonal antibody recognizing the CTX-II epitope, and 6) toluidine blue staining of proteoglycans. MMP expression and activity were assessed by gelatin zymography.

RESULTS

OSM and TNF induced an 8,000% increase in CTX-II compared with control (P < 0.001). Both forskolin and IBMX dose-dependently inhibited release of CTX-II (P < 0.001). OSM and TNF induced a 6-fold increase in (342)FFGV-G2, which was abrogated by forskolin and IBMX (by >80%). OSM and TNF stimulated MMP expression as visualized by zymography, and MMP expression was dose-dependently inhibited by forskolin and IBMX. The highest concentration of IBMX lowered cytokine-induced release of sGAG by 72%.

CONCLUSION

Levels of cAMP in chondrocytes play a key role in controlling catabolic activity. Increased cAMP levels in chondrocytes inhibited MMP expression and activity and consequently strongly inhibited cartilage degradation. Specific cAMP modulators in chondrocytes may be potential treatments for cartilage degenerative diseases.

Monitoring cartilage turnover

In arthritic diseases, the stability of the extracellular matrix of articular cartilage is compromised by extensive proteolytic breakdown associated with alterations of synthesis of the proteins of the tissue leading to cartilage loss. This article reviews developments in assays of biochemical markers of cartilage matrix turnover and studies investigating their use. Because type II collagen and aggrecan are the most abundant proteins of the cartilage matrix, current biochemical markers are based mainly on immunologic reagents detecting their synthesis and degradation. Clinical studies indicate that some markers of type II collagen may be useful to predict disease progression in osteoarthritis and rheumatoid arthritis. Conversely, major achievements have been made in the development of immunoassays detecting the various fragments of aggrecan released by matrix metalloproteases or aggrecanases, but their use has been limited mostly to investigating cartilage turnover in ex vivo experiments. Because of the complexity of the mechanisms involved in arthritic joint damage, only a combination of different biochemical markers reflecting the various aspects of synthesis and degradation of matrix molecules will likely provide efficient cartilage turnover monitoring.

The differential influence of colocalized and segregated dual protein signals on neurite outgrowth on surfaces

We present an in vitro micropatterning approach in which the density and spatial presentation of two separate protein layers can be independently controlled to form cell stripe assays through (1) the simultaneous application of microcontact printing (microCP) and microfluidic network (microFN) patterning to generate alternating stripes of pure single protein layers or (2) through microCP onto a pre-adsorbed homogeneous protein layer to generate alternating single and dual protein stripes. This approach enabled the creation of choice boundaries in which protein-protein interactions were limited and the effects of spatially segregated or colocalized dual protein signals on model primary neuronal behavior could be readily interrogated and compared on both glass and tissue culture polystyrene substrates. Dorsal root ganglion (DRG) cell body attachment was dictated largely by non-specific cell adhesion interactions and interactions between the guidance molecules laminin and aggrecan were insufficient to explain aggrecan inhibition on neurite outgrowth. The presentation of a specific laminin epitope stabilized by interactions with aggrecan and destabilized by microCP was a strong predictor of neurite promoting activity. These observations provide evidence that aggrecan is intrinsically inhibitory and that laminin-aggrecan interactions do not diminish laminin growth promoting properties.

Osteogenic nodule formation from single embryonic stem cell-derived progenitors

The process of bone formation can be approximated in vitro in the form of a mineralized nodule. Osteoprogenitors and mesenchymal stem cells (MSCs), the immediate precursors of the osteoprogenitor, proliferate and differentiate into osteoblasts when placed into culture. These osteoblasts secrete and mineralize a matrix during a period of 3-4 weeks. The differentiation potential of embryonic stem (ES) cells suggests that ES cells should also have the ability to form osteogenic nodules in vitro. ES cells were allowed to form embryoid bodies (EBs) and were cultured in suspension for 2 days; EBs were disrupted and plated as single cells at concentrations as low as 25 cells/cm(2). We provide five lines of evidence for osteogenesis in these ES cell-derived cultures: (1) cell and colony morphology as revealed by phase-contrast microscopy, (2) mineralization of extracellular matrix as revealed by von Kossa staining, (3) quantitative real-time PCR (QRT-PCR) analysis of cDNA from entire plates and individual colonies revealing expression of genes characteristic of, and specific for, osteoblasts, (4) confocal microscopy of nodules from osteocalcin-green fluorescent protein (GFP) ES cell lines demonstrating the appropriate stage and position of osteoblasts expressing the reporter, and (5) immunostaining of nodules with a type I collagen antibody. Our method of initiating osteogenesis from ES cell-derived cultures is the only described method that allows for the observation and manipulation of the commitment stage of mesengenesis from single embryonic progenitors.

Glucosamine regulates differentiation of a chondrogenic cell line, ATDC5

Osteoarthritis (OA) is a slowly progressing chronic joint disease. Glucosamine (GlcN) is a saccharide that is widely used to relieve symptoms associated with OA. However, the mechanism of the effects of GlcN on articular cartilage remains unclear. We studied the effects of GlcN and its analogues, including chitin derivatives included in health supplements containing GlcN, on a chondrogenic cell line, ATDC5. We examined the effects of these saccharides on the proliferation and differentiation of ATDC5 cells. Glucosamine analogues, such as N-acetyl glucosamine and chitobiose, did not affect the proliferation or differentiation of ATDC5 cells. While GlcN did not affect the proliferation of ATDC5 cells, it inhibited their differentiation. Next, we examined whether GlcN affects mineralization and glycosaminoglycan (GAG) production by ATDC5 cells. Mineralization was markedly inhibited by addition of GlcN to the cell culture medium. Moreover, GlcN induced the formation of sulfated GAG in ATDC5. We also analyzed the mRNA levels in ATDC5 cells. GlcN reduced the mRNA levels of Smad2, Smad4 and MGP. GlcN might inhibit expression of MGP mRNA and induce the production of chondroitin sulfate in ATDC5 cells. The mechanism by which GlcN inhibits mineralization may be by regulating the expression of mRNA for the Smad2 and Smad4 chondrogenic master genes.

MMP and non-MMP-mediated release of aggrecan and its fragments from articular cartilage: a comparative study of three different aggrecan and glycosaminoglycan assays

OBJECTIVE

Aggrecan is the major proteoglycan in articular cartilage and is known to be degraded by various proteases, including matrix metalloproteinases (MMPs). The present study was undertaken to develop immunoassays detecting aggrecan and its fragments generated by MMP and non-MMP-mediated proteolysis.

METHODS

Two immunoassays were developed: (1) the G1/G2 sandwich assay employing a monoclonal antibody (F-78) both as a capturing and a detecting antibody, and (2) the 342-G2 sandwich assay substituting the capturing antibody in the G1/G2 test with a monoclonal antibody, AF-28 recognizing the 342FFGVG neo-epitope generated by MMP cleavage. These assays were compared to the commercially available glycosaminoglycan (GAG) assay.

RESULTS

In supernatants of Oncostatin M and Tumor Necrosis Factor alpha (OSM/TNFalpha) stimulated explants, high levels of G1/G2 fragments and GAGs were released in the initial phase (days 2-5), followed by low levels in the intermediate (days 9-12) and late phase (days 12-21). MMP-generated fragments were detected in the late phase only. In the presence of the general MMP inhibitor GM6001, 342-G2 was not detected, whereas the G1/G2 profile remained virtually unchanged. In patients with rheumatoid arthritis (RA), the release of G1/G2 molecules was decreased (27.3%), and that of the 342-G2 fragments increased compared to healthy controls (33.3%).

CONCLUSION

The stimulation of bovine articular cartilage explants with OSM/TNFalpha released aggrecan fragments both in an MMP and non-MMP-mediated route. These immunoassays carry a potential as diagnostic tools for the quantitative assessment of the cartilage turnover in RA patients in addition to their utility in ex vivo explant cultures.

Biochemistry of amino acid racemization and clinical application to musculoskeletal disease

During aging, proteins are subject to numerous forms of damage. Several types of non-enzymatic post-translational modifications have been described in aging proteins, including oxidation, nitration, glycation, and racemization. Racemization of amino acids is the spontaneous conversion of L-enantiomers to the D-form, which is dependent on temperature, pH, and time. Because of the time-dependent nature of racemization, it can be used to determine the relative age and turnover rates of long-lived proteins. There are many such long-lived proteins within the body; they are found in the brain, eye, and heart, but are particularly abundant in proteins found in musculoskeletal tissues such as bone and cartilage. During disease, musculoskeletal tissues have pathologically altered turnover rates. Because turnover rates can be estimated from levels of racemization, racemized musculoskeletal protein fragments may serve as useful biomarkers of disease. This review discusses the biochemistry of amino acid racemization in proteins and its clinical application to musculoskeletal disease.

Gene expression in human keloids is altered from dermal to chondrocytic and osteogenic lineage

Keloids are a dermal fibrotic disease whose etiology remains totally unknown and for which there is no successful treatment. Here, we employed cDNA microarray analysis to examine gene expression in keloid lesions and control skin. We found that 32 genes among the 9000 tested were strongly up-regulated in keloid lesions, of which 21 were confirmed by Northern blotting. These included at least seven chondrocyte/osteoblast marker genes, and RT-PCR analysis revealed that transcription factors specific for these genes, SOX9 and CBFA1, were induced. Immunostaining and in situ hybridization further supported that these markers are expressed in keloid lesions. Intriguingly, scleraxis, a transcription factor known as a marker of tendons and ligaments, was also induced in keloid fibroblasts. We propose that reprogramming of gene expression or disordered differentiation from a dermal pattern to that of a chondrocytic/osteogenic lineage, probably closer to that of tendon/ligament lineage, may be involved in the etiology of keloids.

Drug delivery strategies for cathepsin inhibitors in joint diseases

Cathepsins play important roles in the development of joint and bone diseases such as osteoporosis, rheumatoid arthritis (RA) and osteoarthritis (OA). Cathepsin inhibitors are presently in development and clinical testing for use as novel disease-modifying drugs for the improved treatment of osteoporosis. They may also be applicable for the treatment of joint diseases. However, some barriers still hamper their clinical applications in these indications. Based on pathophysiological features of RA and OA, the authors discuss six potential drug delivery strategies for the effective delivery of cathepsin inhibitors or other antiarthritic drugs to the arthritic joint tissue. Successful application of these strategies may significantly contribute to a more effective and safe treatment of RA and OA.

Relative contribution of matrix metalloprotease and cysteine protease activities to cytokine-stimulated articular cartilage degradation

OBJECTIVE

Both matrix metalloprotease (MMP) activity and cathepsin K (CK) activity have been implicated in cartilage turnover. We investigated the relative contribution of MMP activity and CK activity in cartilage degradation using ex vivo and in vivo models.

METHODS

Bovine articular cartilage explants were stimulated with oncostatin M (OSM) 10 ng/ml and tumor necrosis factor-alpha (TNF-alpha) 20 ng/ml in the presence or absence of the broad-spectrum MMP inhibitor GM6001 and the cysteine protease inhibitor, E64. Cartilage degradation was evaluated in the conditioned medium by glycosaminoglycans (GAG), hydroxyproline, and cross-linked C-telopeptide fragments of type II collagen (CTX-II), which were compared to immunohistochemical evaluations of proteoglycans and CTX-II. We assessed MMP expression by gelatine zymography and CK expression by immunohistochemistry. In vivo, CTX-II release was measured from CK-deficient mice.

RESULTS

OSM and TNF-alpha combined induced significant (P<0.01) increase in cartilage degradation products measured by hydroxyproline and CTX-II compared to vehicle control. The cytokines potently induced MMP expression, assessed by zymography, and CK expression investigated by immunohistochemistry. Inhibition of MMP activity completely abrogated hydroxyproline and CTX-II release (P<0.01) and GAG release (P<0.05). In contrast, E64 resulted in increased CTX-II release by 100% (P<0.05) and inhibited GAG release by 30%. Up-regulation of CTX-II fragments was confirmed in vivo in CK null mice.

CONCLUSION

Inhibition of MMP activity reduced both proteoglycan loss and type II collagen degradation. In contrast, inhibition of cysteine proteases resulted in an increase rather than a decrease in MMP derived fragments of collagen type II degradation, CTX-II, suggesting altered collagen metabolism.

Development and characterization of a highly specific and sensitive sandwich ELISA for detection of aggrecanase-generated aggrecan fragments

OBJECTIVE

To develop an enzyme linked immunosorbent assay (ELISA) to quantify the levels of specific aggrecan fragments generated by aggrecanase-mediated cleavage at the 373Glu-374 Ala bond within the aggrecan interglobular domain.

METHODS

The ELISA employs a commercially available monoclonal antibody to capture aggrecan fragments containing keratan sulfate (KS). Aggrecan fragments generated by cleavage at the Glu-Ala bond were then detected using a monoclonal neoepitope antibody (mAb OA-1) that specifically recognizes the N-terminal sequence 'ARGSVIL'.

RESULTS

The mAb OA-1 antibody was highly specific for the immunizing neoepitope peptide since neither peptides spanning the cleavage site nor mutated peptides were detected. Aggrecan fragments generated by ADAMTS-4 digested human aggrecan monomers and from IL-1-stimulated human cartilage explants were quantified by the ELISA, and we observed increased sensitivity of the ELISA compared to mAb OA-1 Western analysis. We also observed that the basal, as well as IL-1-stimulated production of ARGS aggrecan fragments from human articular cartilage explants was blocked by a selective aggrecanase inhibitor, consistent with generation of the ARGS neoepitope in human articular cartilage being mediated by aggrecanase. Using purified human aggrecan digested by ADAMTS-4 as standard to quantify ARGS aggrecan fragments in human synovial fluids, we determined that the calculated amount of ARGSVIL-aggrecan fragments by ELISA measurement is in agreement with the published levels of these fragments, supporting its potential utility as a biomarker assay for osteoarthritis.

CONCLUSION

We have developed an assay that detects and quantifies specific aggrecan fragments generated by aggrecanase-mediated cleavage. Because aggrecanase mediates degradation of human articular aggrecan in joint disease, the KS/mAb OA-1 ELISA may serve as a biomarker assay for evaluation of preclinical and clinical samples.

Serglycin is the major secreted proteoglycan in macrophages and has a role in the regulation of macrophage tumor necrosis factor-alpha secretion in response to lipopolysaccharide

It has recently been shown that serglycin is essential for maturation of mast cell secretory granules. However, serglycin is expressed also by other cell types, and in this study we addressed the role of serglycin in macrophages. Adherent cells were prepared from murine peritoneal cell populations and from spleens, and analyzed for proteoglycan synthesis by biosynthetic labeling with [35S]sulfate. Conditioned media from serglycin-/- peritoneal macrophages and adherent spleen cells displayed a 65-80% reduction of 35S-labeled proteoglycans, compared with corresponding material from serglycin+/+ cells, indicating that serglycin is the dominant secretory proteoglycan in macrophages of these origins. In contrast, the levels of intracellular proteoglycans were similar in serglycin+/+ and serglycin-/- cells, suggesting that serglycin is not stored intracellularly to a major extent in macrophages. This is in contrast to mast cells, in which serglycin is predominantly stored intracellularly. Transmission electron microscopy revealed that the absence of serglycin did not cause any major morphological effects on peritoneal macrophages, in contrast to dramatic defects in intracellular storage vesicles in peritoneal mast cells. Several secretory products were not found to be affected by the lack of serglycin. However, the secretion of tumor necrosis factor-alpha in response to lipopolysaccharide stimulation was markedly higher in serglycin-/- cultures than in those of serglycin+/+. The present report thus demonstrates that serglycin is the major proteoglycan secreted by peritoneal macrophages and suggests that the macrophage serglycin may have a role in regulating secretion of tumor necrosis factor-alpha.

Lack of association between the aggrecan gene and familial idiopathic scoliosis

STUDY DESIGN

A study was conducted to determine the potential association between a specific aggrecan gene polymorphism and familial idiopathic scoliosis (FIS).

OBJECTIVES

To determine the allelic distribution of the exon 12 polymorphism within a sample of families with FIS.

SUMMARY OF BACKGROUND DATA

FIS is a structural curvature of the spine where the underlying genetic etiology has not been established. The aggrecan locus has been linked to multiple skeletal disorders. A polymorphism, within the aggrecan gene, was previously reported to be associated with curve severity in individuals with scoliosis.

METHODS

Fifty-eight families with FIS were genotyped for the aggrecan exon 12 polymorphism using a polymerase chain reaction method. Model-independent sib-pair linkage analyses and tests of association were performed to analyze the genetic effects of the exon 12 polymorphism.

RESULTS

Linkage analyses of a genomic screen performed on a subgroup of 48 families with a most likely to be X-linked dominant mode of inheritance of FIS showed marginally significant results on chromosome 15q25-26 (P < 0.05). The overall distribution of the alleles was consistent with previously reported literature; no evidence of association and marginal significance of linkage was found between the polymorphism and FIS or the degree of lateral curvature.

CONCLUSIONS

Despite the negative association reported here, further investigation of the gene and its potential association to FIS is required.

Tumor cells enhance their own CD44 cleavage and motility by generating hyaluronan fragments

Hyaluronan (HA) is an extracellular matrix glycosaminoglycan that interacts with cell-surface receptors, including CD44. Although HA usually exists as a high molecular mass polymer, HA of a much lower molecular mass that shows a variety of biological activities can be detected under certain pathological conditions, particularly in tumors. We previously reported that low molecular weight HAs (LMW-HAs) of a certain size range induce the proteolytic cleavage of CD44 from the surface of tumor cells and promote tumor cell migration in a CD44-dependent manner. Here, we show that MIA PaCa-2, a human pancreatic carcinoma cell line, secreted hyaluronidases abundantly and generated readily detectable levels of LMW-HAs ranging from approximately 10- to 40-mers. This occurred in the absence of any exogenous stimulation. The tumor-derived HA oligosaccharides were able to enhance CD44 cleavage and tumor cell motility. Inhibition of the CD44-HA interaction resulted in the complete abrogation of these cellular events. These results are consistent with the concept that tumor cells generate HA oligosaccha-rides that bind to tumor cell CD44 through the expression of their own constitutive hyaluronidases. This enhances their own CD44 cleavage and cell motility, which would subsequently promote tumor progression. Such an autocrine/paracrine-like process may represent a novel activation mechanism that would facilitate and promote the malignant potential of tumor cells.

The interaction of versican with its binding partners

Versican belongs to the family of the large aggregating chondroitin sulfate proteoglycans located primarily within the extracellular matrix (ECM). Versican, like other members of its family, has unique N- and C-terminal globular regions, each with multiple motifs. A large glycosaminoglycan-binding region lies between them. This review will begin by outlining these structures, in the context of ECM proteoglycans. The diverse binding partners afforded to versican by virtue of its modular design will then be examined. These include ECM components, such as hyaluronan, type I collagen, tenascin-R, fibulin-1, and -2, fibrillin-1, fibronectin, P- and L-selectins, and chemokines. Versican also binds to the cell surface proteins CD44, integrin beta 1, epidermal growth factor receptor, and P-selectin glycoprotein ligand-1. These multiple interactors play important roles in cell behaviour, and the roles of versican in modulating such processes are discussed.

Aggrecanase-1 (ADAMTS-4) interacts with alpha1-antitrypsin

In degradative articular diseases such as rheumatoid arthritis and osteoarthritis, loss of the extracellular matrix occurs, resulting in the destruction of joint cartilage. Proteolysis of aggrecan is one of the early events that leads to breakdown of the extracellular matrix. Aggrecanase-1 (ADAMTS--4) is considered to play a pivotal role in the abrasion of cartilage aggrecan in rheumatoid arthritis and osteoarthritis. To identify an endogenous inhibitor of aggrecanase-1, we performed a yeast two-hybrid screen using the catalytic domain of human aggrecanase-1 as a bait and transformed an EGY 48 yeast strain carrying the bait plasmid with a human liver cDNA library plasmid. This screen identified alpha1-antitrypsin, a member of the family of plasma serine proteinase inhibitors, as a prey. Recombinant aggrecanase-1 and alpha1-antitrypsin were expressed in mammalian cells and used in co-immunoprecipitation experiments, which showed that full-length aggrecanase-1 and alpha1-antitrypsin are also associated in vivo. However, aggrecanase-1 did not interfere with the inhibitory activity of alpha1-antitrypsin against elastase, and alpha1-antitrypsin had no effect on the proteolytic activity of aggrecanase-1. Taken together, these data suggest that aggrecanase-1 and alpha1-antitrypsin bind in vivo, although the physiological significance of the interaction between aggrecanase-1 and alpha1-antitrypsin remains unclear.

Focal cerebral ischemia induces changes in both MMP-13 and aggrecan around individual neurons

INTRODUCTION

To test the hypothesis that matrix metalloprotease-13 (MMP-13) and aggrecan may play roles in post-ischemic neuronal pathophysiology, we examined the impact of middle cerebral artery occlusion/reperfusion (MCAO/R) on the abundance of these proteins in different regions of the infarct by immunohistochemistry (IHC) and Western blotting (WB).

METHODS

The effect of MCAO/R on the abundance of MMP-13 and aggrecan was examined in 23 Wistar rats using antibodies against MMP-13 and aggrecan. BrdU was administered the last 2 days of the experiment. The cellular source of the respective antigens was examined with fluorescent double labeling using the neuronal marker NeuN. Sections were also stained for BrdU. The ischemic zone was defined by MRI on T2-weighted images and also on the tissue sections with the help of H and E counterstain. WB was performed for MMP-13.

RESULTS

MMP-13 protein is highly induced in ischemic brain and is associated with neurons, whereas aggrecan is associated with the perineuronal matrix in non-ischemic brain. After 3 days of cerebral ischemia, the number of MMP-13 positive neurons in the periphery of the ischemic lesion increased compared to the respective area in the non-ischemic brain with a peak on day 7. A stronger staining for aggrecan was observed around MMP-13 positive neurons compared with other neurons. The majority of the MMP-13 positive neurons in normal non-ischemic brain were also NeuN positive. BrdU was incorporated into MMP-13 positive neurons in the periphery of the infarct. WB confirmed this results by detecting MMP-13 bands in ischemic brains and activated MMP-13 up to 14 days after ischemia.

CONCLUSIONS

There is a close spatial association of MMP-13 and aggrecan around individual neurons. Both MMP-13 and aggrecan appear to be involved in perineuronal matrix remodeling suggesting a role in neuronal reorganization after cerebral ischemia.

Evaluation of hyaluronidase activity in equine and bovine sera and equine synovial fluid samples by use of enzyme zymography

OBJECTIVE

To investigate the activities of hyaluronidases in equine sera and synovial fluid samples and sera from fetal and adult bovids and evaluate the extent to which the degradation of hyaluronan is influenced by chondrocytes.

SAMPLE POPULATION

Commercial and noncommercial samples of equine (n = 6) and bovine (6) sera and 16 synovial fluid samples from horses.

PROCEDURE

Hyaluronidase activities in sera and synovial fluid samples were assessed via enzyme zymography (performed at pH 4, 5, 6, or 7). Chondrocytes were isolated from equine cartilage and cultured with or without hyaluronan (1 mg/mL); the degradation of hyaluronan was assessed via agarose gel electrophoresis.

RESULTS

[corrected] Hyaluronidase activity was detected in equine sera and synovial fluid samples at pH 4, but not at pH 7, and in bovine sera at both pH values. In all samples at pH 4, a major band of activity (molecular weight, approx 60 kd) and some additional higher molecular weight bands were detected; high- and low-molecular-weight activities were detected in bovine sera at pH 7 Hyaluronan in tissue culture medium with or without fetal calf serum was degraded in the presence, but not the absence, of equine chondrocytes.

CONCLUSIONS AND CLINICAL RELEVANCE

Hyaluronidase activity was detected in equine sera and synovial fluid at pH 4 and in bovine sera at pH 4 and 7. Primary chondrocytes in monolayer culture can degrade exogenous hyaluronan. Modulating native hyaluronidase activity may offer a new approach to improve the quantity and quality of hyaluronan in articular joints.

Lack of antigen-specific tissue remodeling in mice deficient in the macrophage galactose-type calcium-type lectin 1/CD301a

Macrophage galactose-type C-type lectins (MGLs), which were recently named CD301, have 2 homologues in mice: MGL1 and MGL2. MGLs are expressed on macrophages and immature dendritic cells. The persistent presence of granulation tissue induced by a protein antigen was observed in wild-type mice but not in mice lacking an endogenous, macrophage-specific, galactose-type calcium-type lectin 1 (MGL1) in an air pouch model. The anti-MGL1 antibody suppressed the granulation tissue formation in wild-type mice. A large number of cells, present only in the pouch of MGL1-deficient mice, were not myeloid or lymphoid lineage cells and the number significantly declined after administration of interleukin 1 alpha (IL-1alpha) into the pouch of MGL1-deficient mice. Furthermore, granulation tissue was restored by this treatment and the cells obtained from the pouch of MGL1-deficient mice were incorporated into the granulation tissue when injected with IL-1alpha. Taken together, MGL1 expressed on a specific subpopulation of macrophages that secrete IL-1alpha was proposed to regulate specific cellular interactions crucial to granulation tissue formation.

The concentration, gene expression, and spatial distribution of aggrecan in canine articular cartilage, meniscus, and anterior and posterior cruciate ligaments: a new molecular distinction between hyaline cartilage and fibrocartilage in the knee joint

The concentration, spatial distribution, and gene expression of aggrecan in meniscus, articular cartilage, and the anterior and posterior cruciate ligaments (ACL and PCL) was determined in the knee joints of five mature dogs. An anti-serum against peptide sequences specific to the G1 domain of aggrecan was employed in competitive-inhibition ELISA of guanidine HCl extracts and immunofluorescence microscopy. Gene expression was determined by Taqman real-time PCR. The concentration of aggrecan in articular cartilage (240.1 +/- 32 nMol/g dry weight) was higher than that in meniscus (medial meniscus: 33.4 +/- 4.3 nMol/g) and ligaments (ACL: 6.8 +/- 0.9 nMol/g). Aggrecan was more concentrated in the inner than the outer zone of the meniscus. Aggrecan in meniscus showed an organized, spatial network, in contrast to its diffuse distribution in articular cartilage. Thus, differences in the concentration, gene expression, and spatial distribution of aggrecan constitute another molecular distinction between hyaline cartilage and fibrocartilage of the knee.

Porcine dentin sialoprotein is a proteoglycan with glycosaminoglycan chains containing chondroitin 6-sulfate

Dentin sialoprotein (DSP) is a glycoprotein that is critical for proper tooth dentin formation, but little is known about the nature of its carbohydrate attachments and other post-translational modifications. We have isolated DSP from pig dentin and demonstrate that it is a proteoglycan. Polyclonal antibodies were raised in chicken against recombinant pig DSP, and used to identify native DSP in fractions of tooth dentin proteins extracted from developing pig molars. Amino acid analyses and characterization of lysylendopeptidase cleavage products confirmed that the purified protein was DSP, and that Arg391 is at the DSP C terminus. On SDS-PAGE and on urea gels, DSP appeared as a smear extending from 280 to 100 kDa, but in the presence of beta-mercaptoethanol the top of the DSP smear disappeared. The high molecular weight material was likely comprised of covalent DSP dimers connected by a disulfide bridge at Cys205. Oligosaccharides were released from DSP following N- and O-linked glycosidase digestions, but these digestions had little effect on the apparent molecular weight of DSP on SDS-PAGE, when compared with the significant reduction following chondroitinase ABC digestion. Glycosaminoglycanases with assorted glycosaminoglycan (GAG) cleavage specificities coupled with Western analyses of the cleaved GAG "stubs" demonstrated that the DSP GAG attachments contain chondroitin 6-sulfate, but not keratan sulfate, heparan sulfate, chondroitin, or chondroitin 4-sulfate. DSP binds biotin-labeled hyaluronic acid, and such binding is inhibited by the addition of unlabeled hyaluronic acid. We conclude that DSP is a proteoglycan and that GAG attachments are the predominant structural feature of porcine DSP.

PG-M/versican binds to P-selectin glycoprotein ligand-1 and mediates leukocyte aggregation

P-selectin glycoprotein ligand-1 (PSGL-1), a glycoprotein expressed on the cell surface of leukocytes, binds to selectins and mediates leukocyte rolling on the vascular endothelium. Here we report that PSGL-1 binds to the C-terminal (G3 domain) of the extracellular proteoglycan PG-M/versican. Cells transfected with PSGL-1 or a shorter form containing the binding site, or cells expressing endogenous PSGL-1 aggregate in the presence of versican or G3 product. The aggregation appears to be induced by G3 multimers that bind to PSGL-1 and form a network. Endogenous versican and/or G3-containing fragments also bind to PSGL-1 in human plasma. Removal of the endogenous G3-containing fragments reduces the effect of plasma on leukocyte aggregation. Finally, the roles of G3-containing fragments in leukocyte aggregation were confirmed in a mouse model. Taken together, our results strongly support a physiologically relevant role for PSGL-1/versican binding and may have implications in the immunoresponse.

Partitioning of 5alpha-dihydrotestosterone and 5alpha-androstane-3alpha, 17beta-diol activated pathways for stimulating human prostate cancer LNCaP cell proliferation

The growth and development of the prostate gland are regulated by androgens. Despite our understanding of molecular actions of 5alpha-dihydrotestosterone (5alpha-DHT) in the prostate through the trans-activation of the androgen receptor (AR), comprehensive analysis of androgen responsive genes (ARGs) has just been started. Moreover, expression changes induced by the androgen effects of 5alpha-androstane-3alpha,17beta-diol (3alpha-diol), a metabolite of 5alpha-DHT through the action of 3alpha-hydroxysteroid dehydrogenases (3alpha-HSDs), remain undefined. We demonstrated that both 5alpha-DHT and 3alpha-diol stimulated similar levels of androgen sensitive human prostate cancer LNCaP cell proliferation. However, consistent with the fact that 3alpha-diol has low affinity toward the AR, 3alpha-diol did not elicit the same levels of AR trans-activation activity as that of 5alpha-DHT. Since LNCaP cells respond to androgen stimulation by transcriptionally activating the AR downstream genes, gene expression patterns between 0 and 48 h following 3alpha-diol and 5alpha-DHT stimulation were analyzed using cDNA-based membrane arrays to define the temporal regulation of ARGs. Array analysis identified 217 and 219 androgen-modulated genes in at least one time point following 3alpha-diol and 5alpha-DHTstimulation, respectively, including key regulators of cell proliferation. Only a subset of these genes (143) was regulated by both androgens. These data suggest that 3alpha-diol exerts androgenic effects independent of the action of 5alpha-DHT in steroid target tissues. Accordingly, 3alpha-diol might activate cell proliferation cascades independent of AR pathway in the prostate.

Cartilage link protein interacts with neurocan, which shows hyaluronan binding characteristics different from CD44 and TSG-6

The interaction of neurocan with hyaluronan was qualitatively characterized with alkaline phosphatase fusion proteins secreted by mammalian cells. The wild type neurocan hyaluronan binding domain fused to alkaline phosphatase bound to immobilized hyaluronan under physiological as well as moderately hypertonic conditions, whereas its ability to bind to immobilized chondroitin sulfate dropped rapidly with increasing salt concentration. Strong hyaluronan binding ability was still evident when in both link modules within the hyaluronan binding domain a basic amino acid was mutated, which is well conserved among link modules of hyaluronan binding proteins. A strong enhancement of the binding of neurocan to immobilized hyaluronan was observed after preincubation of the immobilized hyaluronan with cartilage link protein. Moreover, this preincubation mediated also the binding of a fusion protein representing only the immunoglobulin module of neurocan linked to alkaline phosphatase, which showed no binding to immobilized hyaluronan alone. The interaction of the neurocan immunoglobulin module with link protein could also be shown by overlay blot analysis. These observations suggest that the hyaluronan binding characteristics of paired link modules are different from those of single link modules, and that the reported temporal co-expression of cartilage link protein and of neurocan in developing brain implicates the possibility of a cooperative function of these molecules.

From Cell-ECM interactions to tissue engineering

The extracellular matrix (ECM) consists of a complex mixture of structural and functional macromolecules and serves an important role in tissue and organ morphogenesis and in the maintenance of cell and tissue structure and function. The great diversity observed in the morphology and composition of the ECM contributes enormously to the properties and function of each organ and tissue. The ECM is also important during growth, development, and wound repair: its own dynamic composition acts as a reservoir for soluble signaling molecules and mediates signals from other sources to migrating, proliferating, and differentiating cells. Approaches to tissue engineering center on the need to provide signals to cell populations to promote cell proliferation and differentiation. These "external signals" are generated from growth factors, cell-ECM, and cell-cell interactions, as well as from physical-chemical and mechanical stimuli. This review considers recent advances in knowledge about cell-ECM interactions. A description of the main ECM molecules and cellular receptors with particular care to integrins and their role in stimulation of specific types of signal transduction pathways is also explained. The general principles of biomaterial design for tissue engineering are considered, with same examples.

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

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

Articular cartilage biology

Articular cartilage is a complex tissue maintained by chondrocytes, which undergo metabolic changes as a result of aging, disease, and injury. These changes may hinder tissue maintenance and repair, resulting in accelerated loss of articular surface and leading to end-stage arthritis. Researchers are investigating both normal and pathologic cellular and molecular processes as well as the development of chondroprotective agents to improve the metabolic function of articular cartilage. Current research is helping to clarify the mechanisms by which a variety of agents, such as glucosamine, chondroitin sulfate, hyaluronic acid, green tea, glucocorticoids, and nonsteroidal anti-inflammatory drugs, can modify the symptoms and course of osteoarthritis. Also under investigation are methods of stimulating repair or replacing damaged cartilage, such as matrix metalloproteinase inhibitors, gene therapy, growth factors, cytokine inhibitors, and artificial cartilage substitutes. Tissue engineering, the combining of artificial matrices with cells and growth factors or genes, offers great potential for improving patient care.

Organization and adhesive properties of the hyaluronan pericellular coat of chondrocytes and epithelial cells

Hyaluronan is a megadalton glycosaminoglycan composed of repeating units of D-N-acetylglucosamine-beta-D-Glucuronic acid. It is known to form a highly hydrated pericellular coat around chondrocytes, fibrosarcoma, and smooth muscle cells. Using environmental scanning electron microscopy we detected fully hydrated hyaluronan pericellular coats around rat chondrocytes (RCJ-P) and epithelial cells (A6). Hyaluronan mediates early adhesion of both chondrocytes and A6 cells to glass surfaces. We show that chondrocytes in suspension establish early "soft contacts" with the substrate through a thick, hyaluronidase-sensitive coat (4.4 +/- 0.7 microm). Freshly-attached cells drift under shear stress, leaving hyaluronan "footprints" on the surface. This suggests that chondrocytes are surrounded by a multilayer of entangled hyaluronan molecules. In contrast, A6 cells have a 2.2 +/- 0.4- microm-thick hyaluronidase-sensitive coat, do not drift under shear stress, and remain firmly anchored to the surface. We consider the possibility that in A6 cells single hyaluronan molecules, spanning the whole thickness of the pericellular coat, mediate these tight contacts.

G3 domains of aggrecan and PG-M/versican form intermolecular disulfide bonds that stabilize cell-matrix interaction

The extracellular matrix plays a critical role in maintaining tissue integrity. Among the matrix molecules, the large aggregating chondroitin sulfate proteoglycans are the major structural molecules and are the primary contributors to the stability for some tissues such as cartilage. The notable exceptions are nanomelic cartilage and arthritic cartilage: the former contains a point mutation leading to a stop codon before translating to the C-terminal G3 domain; the latter contains a large proportion of aggrecan from which the G3 domain has been cleaved. These phenomena suggest that the G3 domain may be important in cartilage stability. Here, we demonstrated for the first time that the G3 domains of aggrecan and another proteoglycan, PG-M/versican, formed intermolecular disulfide bonds, and all subdomains were involved. Further studies indicated that each of the 10 cysteine residues of the aggrecan G3 domain could potentially form intermolecular disulfide bonds in vitro. The disulfide bonds were disrupted in the presence of reducing reagent beta-mercaptoethanol and dithiothreitol. As a result, normal chondrocyte-matrix interaction was disrupted, and the structure of the extracellular matrix was altered. Furthermore, disruption of disulfide bonds also reduced the role of PG-M/versican G3 domain in mediating cell adhesion. Our study provides strong evidence of the importance of proteoglycan interactions through intermolecular disulfide bonds in cartilage firmness and cell-matrix stability.

Identification of the motifs and amino acids in aggrecan G1 and G2 domains involved in product secretion

Members of the large aggregating chondroitin sulfate proteoglycans are characterized by an N-terminal fragment known as G1 domain, which is composed of an immunoglobulin (IgG)-like motif and two tandem repeats (TR). Previous studies have indicated that the expressed product of aggrecan G1 domain was not secreted. Here we demonstrated that the inability of G1 secretion was associated with the tandem repeats but not the IgG-like motif, and specifically with TR1 of aggrecan. We also demonstrated that the G2 domain, a domain unique to aggrecan, had a similar effect on product secretion. The sequence of TR1 of G1 is highly conserved across species, which suggested similar functions played by these motifs. In a yeast two-hybrid assay, TR1 interacted with the calcium homeostasis endoplasmic reticulum protein. Deletion/mutation experiments indicated that the N-terminal fragment of TR1, in particular, the amino acids H(2)R(4) of this motif were key to its effect on product secretion. However, the N-terminal 55 amino acids were required to exert this function. Taken together, our study suggests a possible molecular mechanism for the function of the tandem repeats in product processing.

beta 1-Integrin-mediated glioma cell adhesion and free radical-induced apoptosis are regulated by binding to a C-terminal domain of PG-M/versican

Integrins are cell-surface glycoproteins that mediate cell activities, including tissue morphogenesis, development, immune response, and cancer, through interaction with extracellular proteins. Here we report a novel means by which integrin signaling and functions are regulated. In pull-down assays and immunoprecipitation, beta(1)-integrin bound to the C-terminal domain of PG-M/versican, an extracellular chondroitin sulfate proteoglycan. This was confirmed by cell-surface binding assays. Binding was calcium- and manganese-dependent. Upon native gel electrophoresis, beta(1)-integrin comigrated with the C-terminal domain of PG-M/versican. The interaction of beta(1)-integrin with the C-terminal domain of PG-M/versican activated focal adhesion kinase, enhanced integrin expression, and promoted cell adhesion. As a result, cells expressing the C-terminal domain of PG-M/versican were resistant to free radical-induced apoptosis. As the PG-M/versican peptide used in this study does not contain the RGD consensus-binding motif for integrins, the mechanism of the observed binding represents an entirely new function.