Proteoglycans synthesized by an osteoblast-like cell line (UMR 106-01)

Development of new methods for determining the heparanase enzymatic activity

INTRODUCTION

Heparanase is a mammalian endo-β-glucuronidase. Notwithstanding its importance in various pathological and non-pathological events few straightforward methods for heparanase enzymatic activity has been stated. The aim of this study was to develop two heparanase activity assays to cover a whole range of applications. First, a fast and easy method based on commercial homogenous substrate, fondaparinux, was described. The other method is a quantitative assay based on biotinylated heparan sulfate that uses an easier technique to immobilize the substrate in a 96-well plate.

METHODS

1): The heparanase recombinant enzyme and fondaparinux were incubated overnight. After incubation, a fluorescent redox marker, resazurin, was added. The reduction of resazurin depends on the amount of glucuronic acid released by heparanase digestion. Fluorescence measurements were done using excitation and emission wavelengths of 560 nm and 590 nm, respectively.

METHODS

2): The 96-well plate was incubated with protamine sulfate. Subsequently, biotinylated heparan sulfate was immobilized. The enzymatic assay was performed using chimeric recombinant heparanase at different concentrations. In sequence, the immobilized biotinylated heparan sulfate that was not digested by recombinant heparanase was bound to streptavidin conjugated with europium. Fluorescence was measured using a time-resolved fluorometer.

CONCLUSION

Both methods have high sensitivity and can be used to detect heparanase activity. Fondaparinux assay is a quick and easy method for screening of heparanase inhibitors using recombinant enzyme or bacterial crude extract. Biotinylated heparan sulfate assay can be used for quantitative analysis in biological samples and protamine sulfate showed been capable to immobilized heparan sulfate.

Hyaluronan, a crucial regulator of inflammation

Hyaluronan (HA), a major component of the extracellular matrix (ECM), plays a key role in regulating inflammation. Inflammation is associated with accumulation and turnover of HA polymers by multiple cell types. Increasingly through the years, HA has become recognized as an active participant in inflammatory, angiogenic, fibrotic, and cancer promoting processes. HA and its binding proteins regulate the expression of inflammatory genes, the recruitment of inflammatory cells, the release of inflammatory cytokines, and can attenuate the course of inflammation, providing protection against tissue damage. A growing body of evidence suggests the cell responses are HA molecular weight dependent. HA fragments generated by multiple mechanisms throughout the course of inflammatory pathologies, elicit cellular responses distinct from intact HA. This review focuses on the role of HA in the promotion and resolution of inflammation.

Expression, localisation and synthesis of versican by the enamel organ of developing mouse molar tooth germ: an in vivo and in vitro study

OBJECTIVE

Versican is a large, aggregating chondroitin sulphate proteoglycan. In dental tissue, versican expression occurs primarily in mesenchymal tissue but rarely in epithelial tissue. We investigated the expression, localisation and synthesis of versican in the enamel organ of the developing tooth germ.

DESIGN

To elucidate versican localisation in vivo, in situ hybridisation and immunohistochemistry were conducted in foetal ICR mice at E11.5-E18.5. Epithelium and mesenchyme from the lower first molars at E16.0 were enzymatically separated and versican mRNA expression was investigated by semi-quantitative RT-PCR. Organ culture of the separated samples combined with metabolic labelling with [(35)S], followed by gel filtration, was performed to analyse secreted proteoglycans.

RESULTS

Versican mRNA was first expressed in the thickened dental epithelium at E12.0 and continued to be expressed in the enamel organ until the bell stage. Versican immunostaining was detected in the stellate reticulum areas from the bud stage to the apposition stage. The enamel organ at E16.0 expressed versican mRNA at a level comparable to that in dental mesenchyme. Furthermore, when compared to dental mesenchyme, about 1/2-3/4 of the [(35)S]-labelled versican-like large proteoglycan was synthesised and released into tissue explants by the enamel organ.

CONCLUSIONS

The dental epithelium of developing tooth germ is able to synthesise significant amounts of versican.

Characterization of heparanase from a rat parathyroid cell line

Cell surface heparan sulfate proteoglycans undergo unique intracellular degradation pathways after they are endocytosed from the cell surface. Heparanase, an endo-beta-glucuronidase capable of cleaving heparan sulfate, has been demonstrated to contribute to the physiological degradation of heparan sulfate proteoglycans and therefore regulation of their biological functions. A rat parathyroid cell line was found to produce heparanase with an optimal activity at neutral and slightly acidic conditions suggesting that the enzyme participates in heparan sulfate proteoglycan metabolism in extralysosomal compartments. To elucidate the detailed properties of the purified enzyme, the substrate specificity against naturally occurring heparan sulfates and chemically modified heparins was studied. Cleavage sites of rat heparanase were present in heparan sulfate chains obtained from a variety of animal organs, but their occurrence was infrequent (average, 1-2 sites per chain) requiring recognition of both undersulfated and sulfated regions of heparan sulfate. On the other hand intact and chemically modified heparins were not cleaved by heparanase. The carbohydrate structure of the newly generated reducing end region of heparan sulfate cleaved by the enzyme was determined, and it represented relatively undersulfated structures. O-Sulfation of heparan sulfate chains also played important roles in substrate recognition, implying that rat parathyroid heparanase acts near the boundary of highly sulfated and undersulfated domains of heparan sulfate proteoglycans. Further elucidation of the roles of heparanase in normal physiological processes would provide an important tool for analyzing the regulation of heparan sulfate-dependent cell functions.

Biosynthesis of versican by rat dental pulp cells in culture

The biosynthesis of proteoglycans by these cultured pulp cells was investigated by metabolic labelling, using [(35)S]sulphate, [(3)H]glucosamine and [(3)H]leucine as precursors. Versican-like large proteoglycan, decorin- and biglycan-like small proteoglycans and a small amount of sulphated protein were released into the culture medium. Heparan sulphate species were also identified in cell-layer extracts. Versican-like proteoglycan had an average molecular mass of approximately 800kDa. The molecular mass of chondroihnase ABC-digested core protein exhibited heterogeneity, ranging from 250 to 400kDa, and the glycosaminoglycan chains had an average molecular mass of approximately 42kDa. These results indicate the presence of 10-13 glycosaminoglycan chains per core protein, consistent with the characteristics of versican. This glycosaminoglycan chain contained approximately 63% 4-sulphated disaccharides.

Expression of heparanase in oral cancer cell lines and oral cancer tissues

In the process of metastasis, cancer cells secrete several enzymes which degrade extracellular matrices (ECMs) and basement membranes (BMs) of blood vessels. One of them, heparanase, has been reported to be an important enzyme when metastatic cancer cells invade blood vessels. The enzyme cleaves heparan sulfate (HS), a main component of ECM and BM. In the present study, HS-degrading ability of several human oral cancer cell lines (HSC2, HSC3, HSC4, Ca9-22, NA, ACC3 and Ab-J) and tissues derived from human oral squamous cell carcinomas (both metastatic and non-metastatic) were investigated by measuring heparanase activities and levels of heparanase mRNA by a quantitative reverse transcriptase-polymerase chain reaction. The catalytic activities and the mRNA levels of heparanase showed a good agreement. Clinical demonstration of cancer metastasis generally correlated with high levels of heparanase activity and its mRNA. The results suggest that heparanase activity and its mRNA level are good diagnostic parameters for evaluating the metastatic properties of human oral cancer cells.

Growth hormone-regulated intracellular signaling in UMR 106 osteosarcoma cells

Growth hormone (GH) and insulin-like growth factor 1 (IGF-1) are important growth factors for postnatal longitudinal bone growth. Although many effects of GH on bone growth are mediated by IGF-1, GH can directly influence bone cells. Limited knowledge exists regarding specific intracellular signaling pathways and genes activated by GH in bone cells. GH is known to activate several intracellular signaling pathways, among them the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway. GH mainly activates JAK2 and both isoforms of STAT5, A and B. STAT5 gene deletion experiments have shown the importance of these transcription factors for growth. To understand the molecular mechanism(s) behind this, different experimental models are needed. The UMR 106 cell line is a rat clonal osteosarcoma cell line with osteoblast-like phenotypic properties, one is the endogenous expression of GH receptor (GHR). The present study focused on whether these cells express a functional GH-responsive JAK2/STAT5 pathway. Analysis of cell extracts by immunoprecipitation and Western blot showed that physiological concentrations of GH activated JAK2. Western blot analysis of nuclear extracts from GH-stimulated UMR 106 cells showed that physiological concentrations of GH induced nuclear translocation of both STAT5 isoforms, but with STAT5A being predominant. Both isoforms displayed similar nuclear turnover after GH stimulation of cells. Gel electrophoretic mobility shift assay (GEMSA) of nuclear extract revealed that both STAT5A and STAT5B obtained DNA-binding capacity after GH stimulation. Thus, we have shown, for the first time, the expression and GH-induced activation of JAK2 and STAT5A/B in UMR 106 osteoblast-like cells. This study also shows that this cell line is a suitable experimental model to study unique GH effects in osteoblasts mediated by STAT5.

Significant role of adhesion properties of primary osteoblast-like cells in early adhesion events for chondroitin sulfate and dermatan sulfate surface molecules

The purpose of this study was to characterize the role of cell surface adhesive macromolecules through enzyme modulation and metabolic recovery prior to and during a kinetic cell adhesion assay. Primary rat calvarial osteoblast-like cells were derived from Sprague-Dawley calvarial plates. Cell adhesion kinetics was evaluated with the definition of first-order adhesion kinetics. Osteoblasts were incubated in an adhesion buffer for 1 h prior to a cell attachment assay using various enzymes to remove cell surface glycosaminoglycans (GAGs). A subtractive adhesion analysis was performed by plating cells at 5 x 10(4)/well for variable periods through 2 h. The medium was collected, the well surface washed and pooled, and the number of cells enumerated with a Coulter Counter. Cell adhesion demonstrated first-order logarithmic adhesion kinetics in the first 60 min. Scatchard analysis demonstrated a linear relationship. Preexposure of cells to various enzyme combinations demonstrated that 50% of the equilibrium adhesion was dependent on chondroitin sulfate or dermatan sulfate surface macromolecules. These results were confirmed with pretreatment with a metabolic inhibitor of GAG synthesis (beta-D-xyloside). These results suggest an important role for cell associated chondroitin sulfate and dermatan sulfate in cell adhesion in addition to Arg-Gly-Asp or integrin mediated adhesion events.

Differential regulation of syndecan expression by osteosarcoma cell lines in response to cytokines but not osteotropic hormones

Bone cells are regulated by interactions with both growth factors and components of the extracellular matrix (ECM). Syndecans are cell-surface heparan sulfate proteoglycans known to play a role in cell adhesion and migration, and binding of growth factors. This study was performed to investigate the expression of syndecans by osteoblasts. Reverse transcription-linked polymerase chain reaction (RT-PCR) and Northern analysis detected syndecan transcripts in the human osteosarcoma cell lines MG-63, TE-85, SaOS-2, and U2OS; human osteoblast-like cells; rat calvarial osteoblasts; and in human bone. Western blot analysis of proteoglycans from MG-63 and TE-85 cells detected multiple heparan sulfate proteoglycan core proteins consistent with syndecan expression. Regulation of syndecan-1, -2, and -4 expression was investigated in TE-85, MG-63, and SaOS-2 cells, in response to interleukin (IL)-1beta, and IL-6, parathyroid hormone [PTH(1-34)], and 1,25(OH)2-vitamin D3. Northern analysis demonstrated that in the osteosarcoma cell lines there was no regulation of syndecan transcript levels in response to PTH(1-34) or 1,25(OH)2-vitamin D3 for 24 or 48 h. In contrast, when MG-63 and SaOS-2 cells were incubated with IL-1beta (0.01-10 ng/mL) and IL-6 (0.1-50 ng/mL) there was a dose-dependent decrease in mRNA levels for syndecan-1 and -2 at 24 and 48 h, but in response to IL-1beta upregulation in the levels of syndecan-4 transcripts. In addition, Northern analysis was performed on RNA isolated from neonatal rat calvarial osteoblasts cultured under conditions that promote osteogenesis for 0, 5, 13, 21, and 35 days. Syndecan-1 expression was observed to decrease during the culture period, syndecan-2 transcript levels increased, and there appeared to be no overall change in syndecan-4 levels. Controlled expression of syndecans by cells of the osteoblast lineage may be important in the regulation of osteoblastic proliferation and differentiation.

The soft tissue cover of the mandibular condyle. Differentiation in histological forms and age-related changes of aggrecan- and versican-like proteoglycans

Age-related changes of the composition of the extracellular matrix of the soft tissue cover of the mandibular condyle (STC), especially of the large proteoglycans, have been investigated. Proteoglycans were extracted from the STC of neonatal, juvenile and adult domestic pigs, fractionated by density gradient centrifugation and analyzed by electrophoresis/Western blotting. Experiments revealed firstly that a large CS/KS proteoglycan (aggrecan) is an essential constituent of the STC at all ages. This proteoglycan is required for nutrition of avascular tissues, and age-related changes in its average size and substitution with KS (keratan sulfate) may be a response to altered functional loading and tissue architecture of the STC. Secondly it was shown that a large CS/DS (chondroitin sulfate/dermatan sulfate) proteoglycan characterized by a doublet of core proteins at 200 and 250 kDa, thereby resembling perlecan, is present in the tissue of adults, but not of neonates and juveniles. Thirdly a large CS/DS proteoglycan characterized by core proteins at 350, 450 and 550 kDa, thereby resembling versican, was present in juveniles. It was detectable only weakly in neonates and not in adults. Results of core protein analysis were confirmed by results of agarose gel electrophoresis/Western blotting of the undigested proteoglycans isolated directly from the tissue extracts. Versican is believed to destabilize cell-matrix interactions required for cell proliferation and differentiation. In this context, presence of versican-like proteoglycans in the STC of growing individuals and its disappearance in adults appears to be related to the growth potential of the mandibular condyle.

Proteoglycans at the bone-implant interface

The widespread success of clinical implantology stems from bone's ability to form rigid, load-bearing connections to titanium and certain bioactive coatings. Adhesive biomolecules in the extracellular matrix are presumably responsible for much of the strength and stability of these junctures. Histochemical and spectroscopic analyses of retrievals have been supplemented by studies of osteoblastic cells cultured on implant materials and of the adsorption of biomolecules to titanium powder. These data have often been interpreted to suggest that proteoglycans permeate a thin, collagen-free zone at the most intimate contact points with implant surfaces. This conclusion has important implications for the development of surface modifications to enhance osseointegration. The evidence for proteoglycans at the interface, however, is somewhat less than compelling due to the lack of specificity of certain histochemical techniques and to possible sectioning artifacts. With this caveat in mind, we have devised a working model to explain certain observations of implant interfaces in light of the known physical and biological properties of bone proteoglycans. This model proposes that titanium surfaces accelerate osseointegration by causing the rapid degradation of a hyaluronan meshwork formed as part of the wound-healing response. It further suggests that the adhesive strength of the thin, collagen-free zone is provided by a bilayer of decorin proteoglycans held in tight association by their overlapping glycosaminoglycan chains.

Neurotrophin stimulation of human melanoma cell invasion: selected enhancement of heparanase activity and heparanase degradation of specific heparan sulfate subpopulations

Heparanase is an endo-beta-D-glucuronidase whose enzymatic targets are the glycosaminoglycan chains of heparan sulfate proteoglycans (50). Elevated levels of heparanase are associated with the metastatic potential of melanoma cells, and treatment of murine and human melanoma cells with the prototypic neurotrophin nerve growth factor (NGF) increases the production of heparanase by melanoma cells. We previously reported that physiological concentrations of NGF increased invasion of early passage human brain-metastatic 70W melanoma cells but not melanoma cells metastatic to other sites or nonmetastatic melanoma cells as measured in Matrigel invasion assays. Here we found that treatment of 70W melanoma cells with neurotrophin-3 (NT-3) increased Matrigel invasion, whereas treatment with neurotrophins other than NGF or NT-3 did not influence invasion. Mutants of NGF that do not bind to the neurotrophin receptor p75NTR or other nonneuronal growth factors were not able to enhance the invasion of 70W melanoma cells. When 70W cells were exposed to antisense oligonucleotides directed against p75NTR mRNA, there was a reduction in NGF and NT-3 binding, and the neurotrophins failed to enhance Matrigel invasion. To study the properties of heparanase in neurotrophin-regulated malignant melanoma invasive processes, we developed a sensitive heparanase assay consisting of purified [35S]HS subpopulations separated by agarose gel electrophoresis. Incubation of 70W cells with NGF or NT-3 but not brain-derived neurotrophic factor, neurotrophin-4/5 or mutant NGF resulted in increased release of heparanase activity that was capable of degrading a subpopulation of heparan sulfate molecules.

Cloning of mouse diastrophic dysplasia sulfate transporter gene induced during osteoblast differentiation by bone morphogenetic protein-2

Although intensive studies have been directed at understanding osteoblastic differentiation, the molecular mechanisms are still unclear. In this study, we describe a cDNA that encodes a sulfate transporter that was cloned as a gene induced in osteoblast precursor cells in association with osteoblastic differentiation. Based on the fact that bone morphogenetic protein-2 (BMP-2) induces osteoblastic phenotypes in immature mouse fibroblastic C3H10T1/2 cells, we performed a subtraction hybridization between BMP-2-treated and untreated cells, and have isolated one clone (designated as st-ob for sulfate transporter in osteoblast) induced by BMP-2 that is constantly expressed in osteoblastic cells. The deduced amino acid sequence and proposed structure of st-ob are mostly identical to those of the human diastrophic dysplasia sulfate transporter gene product (DTDST). St-ob mRNA was abundantly expressed in the thymus, testis, calvaria and osteoblastic MC3T3-E1 cells, whereas its expression was faint in C3H10T1/2 cells. Expression of st-ob in C3H10T1/2 cells was increased by transforming growth factor-beta1 (TGF-beta1), retinoic acid and dexamethasone as well as BMP-2. Furthermore, BMP-2 increased sulfate incorporation in C3H10T1/2 cells about twice as high as the baseline level. Osteoblasts actively take up sulfate to synthesize proteoglycans, which are one of the major components of the extracellular matrix of bone and cartilage. The present study demonstrates that st-ob induced during osteoblastic differentiation is an important phenotype of osteoblasts for characterizing their function.

Cloning and characterization of the 5'-flanking region of the mouse diastrophic dysplasia sulfate transporter gene

Dyastrophic dysplasia sulfate transporter (DTDST) plays an important role in proteoglycan synthesis in the extracellular matrix of bone and cartilage. Recently, we found that the mouse DTDST gene was induced in pluripotent C3H10T1/2 cells during differentiation by bone morphogenetic protein-2 (BMP-2). To clarify the transcriptional regulation of the DTDST gene, we have cloned the 5'-flanking region of the mouse DTDST gene by the PCR based gene walking method. Sequence analysis revealed the presence of the TATA box followed by GC rich sequences containing two Sp-1 binding sites and a CBFA1 binding site. Transient transfection assays demonstrated that the basal transcriptional activity in osteoblastic MC3T3-E1 cells was mainly present between -309 and -275 bp upstream of the transcription start site (Segment -309/-275) which contained the consensus sequence for the xenobiotic-responsible element (XRE). Nuclear proteins from MC3T3-E1 cells and C3H10T1/2 cells could bind to this short segment in vitro. BMP-2 increased the promoter activity as well as the nuclear protein binding to the sequence in C3H10T1/2 cells. The present data suggest that the DTDST gene expression in osteoblasts and differentiating precursor cells to osteoblast/chondrocyte lineage would be mainly regulated by undetermined XRE binding transcription factors.

Heparanase and a synthetic peptide of heparan sulfate-interacting protein recognize common sites on cell surface and extracellular matrix heparan sulfate

Heparanase is an endo-beta-D-glucuronidase that degrades the glycosaminoglycan chains of heparan sulfate (HS) proteoglycans at specific sites. Elevated levels of heparanase are associated with the metastatic potential of melanoma and other types of tumor cells. We previously reported heparanase degradation of cell surface HS subpopulations of the human adenocarcinoma cell line RL95. In the present study, heparanase activity was examined on RL95 cell surface HS subpopulations in the presence of a synthetic peptide (CRPKAKAKAKAKDQTK) of heparin/heparan sulfate-interacting protein (HIP; Liu, S., Smith, S. E., Julian, J., Rohde, L. H., Karin, N. J., and Carson, D. D. (1996) J. Biol. Chem. 271, 11817-11823). Heparanase digestion generated HS fragments from cell surface- or extracellular matrix-derived HS of approximately 25 and 9 kDa, respectively. In contrast, HS of various size classes isolated from proteoglycans secreted or released by RL95 and endothelial cells in culture were not susceptible to heparanase digestion. Incubation of heparanase-containing melanoma cellular extracts or partially purified heparanase preparations with cell surface- or ECM-derived HS and HIP peptide, but not a scrambled sequence of this peptide or other HS-binding proteins present in ECM, completely inhibited heparanase action. Conversely, predigestion of cell surface HS with either heparanase-containing cellular extracts or with secreted or partially purified heparanase destroyed binding to HIP peptide. Preincubation of HS with HIP peptide prevented subsequent heparanase digestion. Collectively, these data demonstrate that HIP peptide and heparanase recognize specific, common motifs within HS chains at cell surfaces and in ECM and may mutually modulate HS-dependent activities.

Rapidly forming apatitic mineral in an osteoblastic cell line (UMR 106-01 BSP)

This study evaluated a rapid biomineralization phenomenon exhibited by an osteoblastic cell line, UMR 106-01 BSP, when treated with either organic phosphates [beta-glycerophosphate (beta-GP), Ser-P, or Thr-P], inorganic phosphate (P(i)), or calcium. In a dose-dependent manner, these agents (2-10 mM) stimulated confluent cultures to deposit mineral in the cell layer (ED50 of approximately 4.6 mM for beta-GP (30 +/- 2 nmol Ca2+/microgram DNA) and approximately 3.8 mM (29 +/- 2 nmol Ca2+/microgram DNA) for P(i)) with a plateau in mineral formation by 20 h (ET50 approximately 12-15 h). beta-GP or P(i) treatment yielded mineral crystals having an x-ray diffraction pattern similar to normal human bone. Alizarin red-S histology demonstrated calcium mineral deposition in the extracellular matrix and what appeared to be intracellular paranuclear staining. Electron microscopy revealed small, needle-like crystals associated with fibrillar, extracellular matrix deposits and intracellular spherical structures. Mineral formation was inhibited by levamisole (ED50 approximately 250 microM), pyrophosphate (ED50 approximately 1-10 microM), actinomycin C1 (500 ng/ml), cycloheximide (50 micrograms/ml), or brefeldin A (1 microgram/ml). These results indicate that UMR 106-01 BSP cells form a bio-apatitic mineralized matrix upon addition of supplemental phosphate. This process involves alkaline phosphatase activity, ongoing RNA and protein synthesis, as well as Golgi-mediated processing and secretion.

Attachment and proliferation of osteoblasts and fibroblasts on biomaterials for orthopaedic use

Using a variety of cell types, cell attachment and growth was studied on prospective (polyethersulphone (PES) and polyetheretherketone) and currently used (titanium 318 alloy, cobalt chrome molybdenum alloy and ultra-high molecular weight polyethylene (UHMWPE)) orthopaedic biomaterials. Proliferation of fibroblasts and osteoblasts was measured using incorporation of tritiated thymidine into total DNA. Attachment of cells was assessed by indirect immunofluorescent labelling of vinculin, a component of the cell's focal adhesion plaque. The degree of cell attachment was quantified on the materials by determining the mean number of adhesion plaques and using an image analysis system to determine the mean total area of plaques per cell. Fibroblasts and osteoblasts responded differently to the materials tested. When grown on PES surfaces, rat tail fibroblasts synthesized significantly greater amounts of DNA than cells on all other surfaces, whilst fibroblasts on UHMWPE synthesized significantly less DNA than cells on all other materials. Interestingly, there was no significant difference between the amounts of DNA synthesized by osteoblasts grown on the various materials. Determination of the number of vinculin adhesion plaques per cell and the mean total area of the plaques per cell showed that the attachment of fibroblasts to UHMWPE was significantly reduced compared with other materials. In contrast there was no significant difference in the adhesion of osteoblasts to different materials. Scanning electron microscope (SEM) observations of cells on the materials correlated with the morphometric data. Cells with the greatest number and area of adhesion plaques were well spread and flattened whilst those with the least number of adhesion plaques were more rounded and less spread.

Isolation and purification of proteoglycans

Purification of a protein typically involves development of a quantitative assay to track protein integrity (e.g. enzyme activity) during subsequent isolation steps. The generalized procedure involves choosing the source of the protein, defining extraction conditions, developing bulk purification methods followed by refined, more selective methods. The purification of proteoglycans is often complicated by a) limited source quantities, b) necessity of chaotrophic solvents for efficient extraction, c) their large molecular size and d) lack of defined functions to enable purity (i.e. activity, conformation) to be assessed. Because the usual goal of proteoglycan purification is physical characterization (intact molecular weight, core protein and glycosaminoglycan class and size), the problems of a suitable assay and/or native conformation are avoided. The 'assay' for tracking proteoglycan isolation typically utilizes uronic acid content or radiolabel incorporation as a marker. Once extracted from their cellular/extracellular environment, proteoglycans can be isolated by density gradient centrifugation and/or column chromatography techniques. Recent advances in the composition of chromatographic supports have enabled the application of ion-exchange, gel permeation, hydrophobic interaction and affinity chromatography resins using efficient high-pressure liquid chromatography to proteoglycan purification.

Function of proteoglycans in the extracellular matrix

Proteoglycans are glycosylated proteins which have covalently attached highly anionic glycosaminoglycans. Many forms of proteoglycans are present in virtually all extracellular matrices of connective tissues. The major biological function of proteoglycans derives from the physicochemical characteristics of the glycosaminoglycan component of the molecule, which provides hydration and swelling pressure to the tissue enabling it to withstand compressional forces. This function is best illustrated by the most abundant proteoglycan in cartilage tissues, aggrecan. During the past decade, diverse species of proteoglycans have been identified in many connective tissues, on cell surfaces and in intracellular compartments. These proteoglycans have distinct biological functions apart from their hydrodynamic functions, and their involvement in many aspects of cell and tissue activities has been demonstrated. For example, decorin, which is widely distributed in many connective tissues, may have functions in regulating collagen fibril formation and in modifying the activity of transforming growth factor-beta; perlecan, the major heparan sulfate proteoglycan in the glomerular basement membrane, may play an important role as the major anionic site responsible for the charge selectivity in glomerular filtration. Specific interactions between proteoglycans (through both their glycosaminoglycan and core protein components) and macromolecules in the extracellular matrix are the key factors in the functions of proteoglycans. Exciting biological functions of proteoglycans are now gradually emerging.

Isolation and purification of proteoglycans

Purification of a protein typically involves development of a quantitative assay to track protein integrity (e.g. enzyme activity) during subsequent isolation steps. The generalized procedure involves choosing the source of the protein, defining extraction conditions, developing bulk purification methods followed by refined, more selective methods. The purification of proteoglycans is often complicated by a) limited source quantities, b) necessity of chaotropic solvents for efficient extraction, c) their large molecular size and d) lack of defined functions to enable purity (i.e. activity, conformation) to be assessed. Because the usual goal of proteoglycan purification is physical characterization (intact molecular weight, core protein and glycosaminoglycan class and size), the problems of a suitable assay and/or native conformation are avoided. The 'assay' for tracking proteoglycan isolation typically utilizes uronic acid content or radiolabel incorporation as a marker. Once extracted from their cellular/extracellular environment, proteoglycans can be isolated by density gradient centrifugation and/or column chromatography techniques. Recent advances in the composition of chromatographic supports have enabled the application of ion-exchange, gel permeation, hydrophobic interaction and affinity chromatography resins using efficient high-pressure liquid chromatography to proteoglycan purification.

Versican gene expression in human articular cartilage and comparison of mRNA splicing variation with aggrecan

The chondrocytes in human articular cartilage from subjects of all ages express mRNAs for both of the aggregating proteoglycans aggrecan and versican, although the level of expression of versican mRNA is much lower than that of aggrecan mRNA. Aggrecan shows alternative splicing of the epidermal growth factor (EGF)-like domain within its C-terminal globular region, but there is no evidence for a major difference in situ in the relative expression of this domain with age. At all ages studied from birth to the mature adult, a greater proportion of transcripts lacked the EGF domain. The relative proportions of the two transcripts did not change upon culture and passage of isolated chondrocytes. In contrast, the neighbouring complement regulatory protein (CRP)-like domain was predominantly expressed irrespective of age, but cell culture did result in variation of the splicing of this domain. Versican possesses two EGF-like domains and one CRP-like domain, but at all ages the three domains were predominantly present in all transcripts. This situation persisted upon culture and passage of the chondrocytes. Thus, unlike aggrecan, the versican expressed by human articular cartilage does not appear to undergo alternative splicing of its C-terminal globular region, either in cartilage in situ or in chondrocytes in culture.

Plasma-membrane-intercalated heparan sulphate proteoglycans in an osteogenic cell line (UMR 106-01 BSP)

The heparan sulphate (HS) proteoglycans associated with the cell layer of a rat osteosarcoma cell line [UMR 106-01 (BSP)] were compared with similar cell-associated proteoglycans from other cells, and their interaction with the plasma membrane was studied. HS proteoglycans were metabolically labelled by incubation of cell cultures with [3H]glucosamine or [3H]leucine and [35S]sulphate. HS proteoglycan core protein preparation generated by heparitinase digestion of the major species from UMR 106-01 (BSP) cells co-migrated on PAGE with identical preparations from ovarian granulosa cells and parathyroid cells (at approximately 70 kDa). The hydrophobic nature of the major HS proteoglycans from these diverse cell lines, based on elution position from octyl-Sepharose, were also comparable. Linkages of the HS proteoglycan to the cell membrane were investigated by labelling plasma-membrane preparations with a lipid soluble photoactivatable reagent, 3-(trifluoromethyl)-3- (m-[125I]iodophenyl)diazirine (TID), which selectively labels plasma-membrane-spanning peptide domains. Purified HS proteoglycan from UMR 106-01 (BSP) cells was shown to be accessible to the [125I]TID, and the core protein portion of the molecule was labelled, confirming its close association with the plasma membrane. Approx. 36% of 35S-labelled HS proteoglycans were released from the cell surface by phospholipase C (Bacillus thuringiensis), which specifically cleaves phosphatidylinositol-linked proteins. In the presence of insulin, the metabolism of the phospholipase C-sensitive population was unaltered; however, release of the phospholipase C-insensitive population into the medium was increased. These data indicate that a subpopulation of HS proteoglycans are covalently bound to the plasma membrane by a glycosylphosphatidylinositol structure, with the remainder representing those species directly inserted into the plasma membrane via a hydrophobic peptide domain. These observations are similar to those reported for ovarian granulosa cells [Yanagishita & McQuillan (1989) J. Biol. Chem. 264 17551-17558], and thus may represent a general phenomenon for many cell types.