Proteoglycan biosynthesis in chondrocytes: protein A-gold localization of proteoglycan protein core and chondroitin sulfate within Golgi subcompartments

Tumor Abnormality-Oriented Nanomedicine Design

Anticancer nanomedicines have been proven effective in mitigating the side effects of chemotherapeutic drugs. However, challenges remain in augmenting their therapeutic efficacy. Nanomedicines responsive to the pathological abnormalities in the tumor microenvironment (TME) are expected to overcome the biological limitations of conventional nanomedicines, enhance the therapeutic efficacies, and further reduce the side effects. This Review aims to quantitate the various pathological abnormalities in the TME, which may serve as unique endogenous stimuli for the design of stimuli-responsive nanomedicines, and to provide a broad and objective perspective on the current understanding of stimuli-responsive nanomedicines for cancer treatment. We dissect the typical transport process and barriers of cancer drug delivery, highlight the key design principles of stimuli-responsive nanomedicines designed to tackle the series of barriers in the typical drug delivery process, and discuss the "all-into-one" and "one-for-all" strategies for integrating the needed properties for nanomedicines. Ultimately, we provide insight into the challenges and future perspectives toward the clinical translation of stimuli-responsive nanomedicines.

What Are the Potential Roles of Nuclear Perlecan and Other Heparan Sulphate Proteoglycans in the Normal and Malignant Phenotype

The recent discovery of nuclear and perinuclear perlecan in annulus fibrosus and nucleus pulposus cells and its known matrix stabilizing properties in tissues introduces the possibility that perlecan may also have intracellular stabilizing or regulatory roles through interactions with nuclear envelope or cytoskeletal proteins or roles in nucleosomal-chromatin organization that may regulate transcriptional factors and modulate gene expression. The nucleus is a mechano-sensor organelle, and sophisticated dynamic mechanoresponsive cytoskeletal and nuclear envelope components support and protect the nucleus, allowing it to perceive and respond to mechano-stimulation. This review speculates on the potential roles of perlecan in the nucleus based on what is already known about nuclear heparan sulphate proteoglycans. Perlecan is frequently found in the nuclei of tumour cells; however, its specific role in these diseased tissues is largely unknown. The aim of this review is to highlight probable roles for this intriguing interactive regulatory proteoglycan in the nucleus of normal and malignant cell types.

Purification and Partial Characterization of Agglutinin Lectin from Heamolymph of German Cockroach, Blattella germanica

Background:

Lectin molecules have crucial biological role in insects’ immune system. The aim of present study was to find the agglutinin activities in haemolymph of German cockroach, Belatella germanica with appropriate screening and purification.

Methods:

The heamolymph of cockroach was collected and agglutinin test performed against different animal and human red blood cells (RBC). Then sugar inhibition assay was carried out to find carbohydrate specific binding lectin. The proteins of haemolymph was purified using ion-exchange chromatography (HPLC) and each fraction was tested for agglutinin activity. Finally the molecular weight of the agglutinin protein was determined using SDS-page.

Results:

The most agglutinin activity of haemolymph was found against RBC of mouse at titer 1/128ml/L dilution and sugar inhibition assay showed that fucos, N-acetyglucoseamine and galactose reduced titer of agglutinin to ½ml/L. Only one fraction of heamolymph at rotation time of 36 minute showed agglutinin activity. The molecular weight of this lectin was measured as 120Kds.

Conclusion:

The range of agglutinin activities against different RBC indicates that the isolated lectin is not specific for a particular carbohydrate. In addition, the isolated lectin at low concentration present in heamolymph should be an innate lactin not secreted, because we found it without any trigger immunity of the insect.

Emerging tools to study proteoglycan function during skeletal development

In the past 20years, appreciation for the varied roles of proteoglycans (PGs), which are specific types of sugar-coated proteins, has increased dramatically. PGs in the extracellular matrix were long known to impart structural functions to many tissues, especially articular cartilage, which cushions bones and allows mobility at skeletal joints. Indeed, osteoarthritis is a debilitating disease associated with loss of PGs in articular cartilage. Today, however, PGs have a demonstrated role in cell biological processes, such as growth factor signalling, prompting new perspectives on the etiology of PG-associated diseases. Here, we review diseases associated with defects in PG synthesis and sulfation, also highlighting current understanding of the underlying genetics, biochemistry, and cell biology. Since most research has analyzed a class of PGs called heparan sulfate PGs, more attention is paid here to studies of chondroitin sulfate PGs (CSPGs), which are abundant in cartilage. Interestingly, CSPG synthesis is tightly linked to the cell biological processes of secretion and lysosomal degradation, suggesting that these systems may be linked genetically. Animal models of loss of CSPG function have revealed CSPGs to impact skeletal development. Specifically, our work from a mutagenesis screen in zebrafish led to the hypothesis that cartilage PGs normally delay the timing of endochondral ossification. Finally, we outline emerging approaches in zebrafish that may revolutionize the study of cartilage PG function, including transgenic methods and novel imaging techniques. Our recent work with X-ray fluorescent imaging, for example, enables direct correlation of PG function with PG-dependent biological processes.

Toxic effects of a modified montmorillonite clay on the human intestinal cell line Caco-2

The incorporation of the natural mineral clay montmorillonite into polymeric systems enhances their barrier properties as well as their thermal and mechanical resistance, making them suitable for a wide range of industrial applications, e.g., in the food industry. Considering humans could easily be exposed to these clays due to migration into food, toxicological and health effects of clay exposure should be studied. In the present work, the cytotoxic effects induced by two different clays (the unmodified clay Cloisite(®) Na(+) , and the organically modified Cloisite(®) 30B) on Caco-2 cells were studied after 24 and 48 h of exposure. The basal cytotoxicity endpoints assessed were total protein content, neutral red uptake and a tetrazolium salt reduction. Our results showed that only Cloisite(®) 30B induced toxic effects. Therefore, the effects of subcytotoxic concentrations of this clay on the generation of intracellular reactive oxygen species, glutathione content and DNA damage (comet assay) were investigated. Results indicate that oxidative stress may be implicated in the toxicity induced by Closite(®) 30B, in regards of the increases in intracellular reactive oxygen species production and glutathione content at the highest concentration assayed, while no damage was observed in DNA. The most remarkable morphological alterations observed were dilated cisternae edge in the Golgi apparatus and nucleolar segregation, suggesting impairment in the secretory functions, which could be related to inhibition in the synthesis of proteins.

HIF1α is a central regulator of collagen hydroxylation and secretion under hypoxia during bone development

Collagen production is fundamental for the ontogeny and the phylogeny of all multicellular organisms. It depends on hydroxylation of proline residues, a reaction that uses molecular oxygen as a substrate. This dependency is expected to limit collagen production to oxygenated cells. However, during embryogenesis, cells in different tissues that develop under low oxygen levels must produce this essential protein. In this study, using the growth plate of developing bones as a model system, we identify the transcription factor hypoxia-inducible factor 1 α (HIF1α) as a central component in a mechanism that underlies collagen hydroxylation and secretion by hypoxic cells. We show that Hif1a loss of function in growth plate chondrocytes arrests the secretion of extracellular matrix proteins, including collagen type II. Reduced collagen hydroxylation and endoplasmic reticulum stress induction in Hif1a-depleted cells suggests that HIF1α regulates collagen secretion by mediating its hydroxylation and consequently its folding. We demonstrate in vivo the ability of Hif1α to drive the transcription of collagen prolyl 4-hydroxylase, which catalyzes collagen hydroxylation. We also show that, concurrently, HIF1α maintains cellular levels of oxygen, most likely by controlling the expression of pyruvate dehydrogenase kinase 1, an inhibitor of the tricarboxylic acid cycle. Through this two-armed mechanism, HIF1α acts as a central regulator of collagen production that allows chondrocytes to maintain their function as professional secretory cells in the hypoxic growth plate. As hypoxic conditions occur also during pathological conditions such as cancer, our findings may promote the understanding not only of embryogenesis, but also of pathological processes.

SOX9 transduction increases chondroitin sulfate synthesis in cultured human articular chondrocytes without altering glycosyltransferase and sulfotransferase transcription

The transcription factor SOX9 (Sry-type high-mobility-group box 9) is expressed in all chondrocytes and is essential for the expression of aggrecan, which during biosynthesis is substituted with more than 10 times its weight of CS (chondroitin sulfate) and is secreted by chondrocytes to form the characteristic GAG (glycosaminoglycan)-rich ECM (extracellular matrix) of cartilage. SOX9 expression rapidly falls during monolayer culture of isolated chondrocytes and this turns off aggrecan and associated CS synthesis. We therefore investigated whether SOX9 transduction of cultured human articular chondrocytes had any effect on the gene expression of the glycosyltransferases and sulfotransferases necessary for GAG biosynthesis. Retroviral SOX9 transduction of passaged chondrocytes increased the endogenous rate of GAG synthesis and the total capacity for GAG synthesis assessed in monolayer culture with β-xyloside. Both the endogenous rate and the total capacity of GAG biosynthesis were increased further in chondrogenic cell aggregate cultures. The GAG synthesized was predominantly CS and the hydrodynamic size of the newly synthesized chains was unchanged by SOX9 transduction. Aggrecan gene expression was increased in the SOX9-transduced chondrocytes and increased further in chondrogenic culture, but no comparable effects were found in SOX9 transduced dermal fibroblasts. However, the expression of CS glycosyltransferase and sulfotransferase genes in chondrocytes was unaffected by SOX9 transduction. Therefore SOX9 transduction in chondrocytes increased their CS synthetic capacity, but this was not accompanied by changes in the transcription of the CS biosynthetic enzymes and must occur by indirect regulation of enzyme activity through control of enzyme protein translation or enzyme organization.

Biosynthesis and processing of proteodermatan sulphate

The biosynthesis and processing of the small iduronic acid-rich proteodermatan sulphate (PDS) was studied in cultured human skin fibroblasts and arterial smooth muscle cells (SMC) with the aid of core-directed antibodies and various inhibitors of protein synthesis, intracellular transport, and glycosylation. Components of the linkage region became attached to the core protein most likely in a pre-Golgi compartment. Phosphorylation of PDS precursors also occurred in the endoplasmic reticulum with a minor contribution by the Golgi complex. Serine residues and the linkage region were identified as phosphorylated species in secreted PDS. Blockade of transport by monensin did not affect 6-sulphation but affected uronic acid epimerization and 4-sulphation. On relief from the monensin block, additional sulphation along the glycosaminoglycan chain was possible, whereas chain elongation was as in the continuous presence of the drug. Asparagine-bound oligosaccharides or glycosaminoglycan chains were not required for secretion of PDS or core protein. PDS from fibroblast and SMC secretions differed markedly in the composition of the glycosaminoglycan chains. No significant difference, however, was found on isoelectric focusing of core protein and after limited proteolysis of chondroitin ABC lyase-treated core protein. Tryptic and chymotryptic peptide maps of iodinated core proteins were similar.

The folded protein modules of the C-terminal G3 domain of aggrecan can each facilitate the translocation and secretion of the extended chondroitin sulfate attachment sequence

Aggrecan is a multidomain proteoglycan containing both extended and folded protein modules. The C-terminal G3 domain contains a lectin-like, complement regulatory protein-like, and two alternatively spliced epidermal growth factor-like modules. It has been proposed that the lectin module alone has a necessary role in the intracellular translocation and secretion of proteins expressed containing G3. Constructs containing human aggrecan G3 together with 1155 bases of the adjacent chondroitin sulfate attachment region (CS-2) were prepared with different combinations and deletions of the protein modules and transfected into mammalian cells of monkey or hamster origin. The results showed that the products containing only the unfolded protein sequences (CS-2 with or without the C-terminal tail sequence) were translated and accumulated intracellularly but were not secreted. In contrast the constructs containing any of the folded protein modules and the extended CS-2 region were translated and secreted from the cells. The results show that the lectin module was not unique in facilitating the intracellular translocation and secretion of the G3 domain. The conservation of G3-like domains within the aggrecan family of proteoglycans may therefore result from their participation in other extracellular functions.

In vitro synthesis of sulfated glycosaminoglycans coupled to inter-compartmental Golgi transport

We have used isolated rat liver Golgi membranes to reconstitute the synthesis of sulfated glycosaminoglycans (GAGs) onto the membrane-permeable, external acceptor xyloside. Biosynthetic labeling of GAGs with [35S]sulfate in vitro is shown to have an absolute requirement for ATP and cytosolic proteins and is inhibited by dismantling the Golgi apparatus with okadaic acid or under mitotic conditions suggesting that inter-compartmental transport between Golgi cisternae is a prerequisite for the successful completion of the initiation, polymerization, and sulfation of GAGs. Accordingly, we show that in vitro synthesis of 35S-GAGs utilizes the same machinery employed in Golgi transport events in terms of vesicle budding (ADP-ribosylation factor and coatomer), docking (Rabs), targeting (SNAREs), and fusion (N-ethylmaleimide-sensitive factor). This provides compelling evidence that GAGs synthesis is linked to Golgi membrane traffic and suggests that it can be used as a complementation-independent method to study membrane transport in Golgi preparations from any source. We have applied this system to show that intra-Golgi traffic requires the function of the Golgi target-SNARE, syntaxin 5.

Post-translational processing of the inter-alpha-trypsin inhibitor in the human hepatoma HepG2 cell line

In human hepatoma HepG2 cells, the serum inter-alpha-trypsin inhibitor (ITI)-like protein is synthesized from two protein precursors, the heavy chain (H) H2 and the light chain (L). Both of them carry sulphate groups involved in the chondroitin sulphate glycosaminoglycan (GAG) linkage, as demonstrated by [35S]sulphate labelling, chondroitinase digestion and inhibition with beta-D-xyloside, an artificial GAG acceptor. While inhibition of N-glycosylation prevented neither the maturation nor the secretion of the ITI-related entities, brefeldin A induced the accumulation of H and L precursors in the cells, therefore blocking subsequent association and maturation of the precursors before their secretion. The enzyme system involved in the ester linkage between H and L chains is localized in the trans-Golgi network since no ITI-like protein could be obtained in the presence of monensin; instead free heavy-chain protein forms and bikunin were secreted in culture supernatants. The ITI-like protein synthesized by HepG2 cells is therefore composed of two heavy chains HC2 linked to two bikunin chains by chondroitin sulphate bridges, although the GAG linkage between HC2 chains is presumably different. Further, a different maturation route leading to restricted heavy-chain forms, Hm and Hd, could be shown.

The teratogenic effects of valproic acid in human chondrogenesis in vitro

The anticonvulsant drug valproic acid (VPA) is a known teratogen in humans. In general, anticonvulsants effect major systems in the embryo causing craniofacial, cardiovascular, neurological, urogenital, and major and minor skeletal defects. The limb defects associated with in utero VPA exposure include digital hypoplasia, ectrodactyly, radial ray aplasia, and proximal phocomelia. Human studies are limited to case reports and to retrospective and/or prospective studies. Although animal studies have demonstrated a teratogenic effect of VPA on skeletogenesis, these doses were well above the human therapeutic dose which makes extrapolation from these studies to humans difficult. The purpose of this research was to evaluate the potential deleterious effects of VPA on chondrogenesis, a process that occurs in human limb formation. To accomplish this goal, human chondrocytes were cultured in a three dimensional agarose gel and treated with VPA. The use of this model system was a novel approach to evaluate the teratogenic potential of VPA during chondrogenesis. The influence of VPA on human chondrocytes was monitored using histochemical, immunocytochemical, and morphological techniques. There was a decrease in mitotic activity and the extracellular matrix was modified. At human therapeutic doses, immunofluorescence revealed that type II collagen was reduced, while type I collagen increased. In addition, the alcian blue-staining matrices (i.e., sulfated proteoglycans) were reduced. Moreover, the Golgi apparatus had swelling in the trans-face cisternae suggesting that proteoglycan synthesis may be altered.

Differential rates of aggrecan synthesis and breakdown in different zones of the bovine growth plate

This study examines the basic metabolic events of aggrecan synthesis and breakdown in the growth plate at different depths and at different stages of development. Growth plate was harvested from the distal tibia of fetal and calf tissue and maintained as explants in serum-free-conditions. The tissue was sectioned into three equal depths (resting/proliferative zone, upper hypertrophic zone, and lower hypertrophic zone) and (a) cultured for three days with daily media change for studies of proteoglycan breakdown rates, or (b) incubated with [35S]-sulfate to determine relative rates of proteoglycan synthesis. Rates of both aggrecan synthesis and turnover were highest in the resting/proliferative zone compared to the upper or lower hypertrophic zones, and was greater in the calf compared to the fetal tissue. In situ hybridization studies showed that aggrecan gene expression in the cells of the resting/proliferative zone and the upper hypertrophic zones were similar, and was reduced in the deepest cells of the lower hypertrophic zone, adjacent to the zone of calcification. Proteoglycan structure was characterized by associative and dissociative Sepharose CL2B chromatography. These results showed that approximately 90% of the newly synthesized proteoglycan, and the total proteoglycan population, was able to aggregate and that the monomers were relatively large. The proteoglycan released into the media had a reduced ability to aggregate and the monomers were of a more variable size. These data support the hypothesis that the matrix proteoglycan content is controlled both by the rate of synthesis and breakdown, but in the lower regions the rate of synthesis may play a more dominant role. The higher metabolic activity of aggrecan in the calf than fetal growth plate may be a result of environmental stimuli (i.e., soluble mediators, loading) during different stages of development.

The quantitation of a native chondroitin sulfate epitope in synovial fluid lavages and articular cartilage from canine experimental osteoarthritis and disuse atrophy

OBJECTIVE

Previous studies have shown the presence of a native chondroitin sulfate epitope in articular cartilage proteoglycans from canine knee joints with experimental early osteoarthritis (OA), but not in normal cartilage. The objective of this study was to quantitate the native epitope recognized by monoclonal antibody 3-B-3 in synovial fluids and articular cartilage of diseased joints.

METHODS

An immunoassay with monoclonal antibody 3-B-3, which recognizes a native chondroitin-6-sulfate structure, was developed and used to analyze synovial fluid lavage material and extracts of articular cartilage from canine knee joints with early experimental OA or with mild disuse atrophy, and from control animals.

RESULTS

The concentration of epitope in the OA fluids was elevated 33-35-fold, and in the OA articular cartilage extracts it was elevated > 200-fold, compared with samples from the control group. No significant difference was detected in the levels of 3-B-3 epitope in the synovial fluid lavage material or cartilage extracts from the joints of the disuse group versus the control group.

CONCLUSION

The native 3-B-3 epitope in articular cartilage and synovial fluids may be a specific marker of ongoing anabolic events in early degenerative joint disease.

Increased release of matrix components from articular cartilage in experimental canine osteoarthritis

The release rates of specific components of the proteoglycan aggregates (G1 domain, the chondroitin sulfate and keratan sulfate containing portion of the protein core, and link protein) of the articular cartilage of mature beagles were studied at early stages of canine experimental osteoarthritis (OA), generated by transection of the anterior cruciate ligament. Analysis of cartilage explants and synovial fluids indicates that at early stages of experimental OA, there is increased release of the proteoglycan aggregates of the articular cartilage. This involves a release from the tissue of the components of the proteoglycan that are specifically involved with aggregation together with the glycosaminoglycans of the proteoglycan. These components were detected at elevated levels in the media of explants of cartilage from the operated joint, and in the synovial fluids of the operated joints.

Differential transport kinetics of chondroitin sulfate and dermatan sulfate proteoglycan by monkey aorta smooth muscle cells

Pulse-chase studies were performed to study the kinetics of chondroitin sulfate proteoglycan (CSPG) and dermatan sulfate proteoglycan (DSPG) transport in monkey aorta smooth muscle cells. During a short pulse (5 min) with [35S]Na2SO4 (500 microCi/ml), the cells synthesized 59% DSPG, 38% CSPG, and 3% heparan sulfate proteoglycan. Both DSPG and CSPG were transported out of the cell very rapidly after sulfate incorporation. At various chase times, proteoglycans (PGs) were isolated from four cellular compartments: (a) medium, (b) total cell lysate, (c) intracellular pool, and (d) extracellular pool. The PGs from the different pools were analyzed by Sepharose CL-2B column chromatography. The data of intracellular DSPG loss fitted a double exponential decay model: approximately 90% was secreted quickly with a t1/2 of 7 min, and the remaining 10% had a dramatically slower rate of secretion (t1/2 of 130 min). DSPG was rapidly secreted into the medium without prior accumulation in the extracellular matrix. In contrast, the loss of intracellular CSPG fitted a single exponential decay model with a t1/2 of 8 min; however, there was a significant accumulation of CSPG in the extracellular matrix compartment before release into the medium, resulting in a relatively slower secretion of CSPG into the medium (t1/2 of about 31 min). This delay in CSPG secretion into the medium is probably due to aggregation in the extracellular matrix, since addition of short hyaluronan oligomers (8-14 oligosaccharides) to the medium during the chase increased the rate of CSPG being secreted into the medium. We concluded that in aortic smooth muscle cell cultures, CSPG and DSPG are secreted via two distinct pathways through the cellular compartments.

Effects of cyclofenil diphenol, an agent which disrupts Golgi structure, on proteoglycan synthesis in chondrocytes

1. Cyclofenil diphenol (F6060), a weak non-steroidal oestrogen, was shown previously to inhibit [35S]proteoglycan synthesis [Mason, Lineham, Phillipson & Black (1984) Biochem. J. 223, 401-412] and to induce fragmentation of the Golgi apparatus into small vesicles [Lancaster, Fryer, Griffiths & Mason (1989) J. Cell Sci. 92, 271-280] in cultures of Swarm chondrosarcoma chondrocytes. Two structurally related compounds, F6204 and F6091, show a similar concentration-related effect, with complete inhibition of [35S]proteoglycan synthesis at 90 micrograms/ml. The apparent [3H]protein synthesis is only approx. 40% inhibited with [3H]lysine as precursor. Stilboestrol, clomiphene and tamoxiphen are also potent inhibitors of [35S]proteoglycan synthesis. 2. Syntheses of chondroitin 4-[35S]sulphate and chondroitin 6-[35S]sulphate, which are Golgi-mediated events, are inhibited 40-68% and 3-48% respectively by concentrations of cyclofenil between 50 and 70 micrograms/ml. [3H]Hyaluronan synthesis, which occurs by a different mechanism at the plasma membrane, is inhibited by 47-66%. These results suggest that cyclofenil may act via more than one inhibitory mechanism. Cyclofenil diphenol inhibits polymerization of chondroitin sulphate on to p-nitrophenyl beta-xyloside even when the chondrocytes are loaded with the initiator prior to treatment. 3. Cyclofenil diphenol interferes with the cellular uptake of amino acids via the system A carrier, as shown by inhibition of uptake of methylaminoisobutyric acid, a specific substrate for this system. The drug had no effect on the uptake of 2-deoxyglucose by the cells. 4. Cyclofenil diphenol (90 micrograms/ml) caused a decrease in the pool size of UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine and UDP-hexoses, but this was insufficient to account for the accompanying profound inhibition of [35S]proteoglycan synthesis. Entry of [3H]glucosamine into the cell and into the UDP-N-acetylhexosamine pool did not appear to be affected. 5. Cyclofenil diphenol inhibited the substitution of 3H-labelled proteoglycan core protein with chondroitin sulphate chains. Core protein was identified in treated cultures on the basis of immunoprecipitation with an antiserum against the hyaluronate-binding region and distinguished from precipitated proteoglycan on SDS/PAGE.

Cartilage and diarthrodial joints as paradigms for hierarchical materials and structures

The anatomic forms of diarthrodial joints are important structural features which provide and limit the motions required for the joint. Typically, the length scale of topographic variation of anatomic forms ranges from 0.5 to 15 cm. Articular cartilage is the thin layer of hydrated soft tissue (0.5-5.0 mm thick) covering the articulating bony ends in diarthrodial joints. This tissue has a set of unique mechanical and physicochemical properties which are responsible for its load-carrying capabilities and near-frictionless qualities. The mechanical properties of articular cartilage are determined at the tissue-scale level and these properties depend on the composition of the tissue, mainly collagen and proteoglycan, and their molecular and ultrastructural organization (ultra-scale: 10(-8)-10(-6) m). Because proteoglycans possess a high density of fixed negative charges, articular cartilage exhibits a significant Donnan osmotic pressure effect. This physicochemically derived osmotic pressure is an important component of the total swelling pressure; the other component of the total swelling pressure stems from the charge-to-charge repulsive force exerted by the closely spaced (1-1.5 nm) negative charge groups along the proteoglycan molecules. Thus these interactions take place at a nano-scale level: 10(-10)-10(-9) m. Finally, cartilage biochemistry and organization are maintained by the chondrocytes which exist at a micro-scale level (10(-7)-10(-6) m). Significant mechanoelectrochemical transduction occurs within the extracellular matrix at the micro-scale level which affects and modulates cellular anabolic and catabolic activities. At present, the exact details of these transduction mechanisms are unknown. In this review, we present a summary of the hierarchical features for articular cartilage and diarthrodial joints and tables of known material properties for cartilage. Also we summarize how the multi-scale interactions in articular cartilage provide for its unique material properties and tribological characteristics.

Uncoupling of chondroitin sulfate glycosaminoglycan synthesis by brefeldin A

Brefeldin A has dramatic, well-documented, effects on the structural and functional organization of the Golgi complex. We have examined the effects of brefeldin A (BFA) on the Golgi-localized synthesis and addition of chondroitin sulfate glycosaminoglycan carbohydrate side chains. BFA caused a dose-dependent inhibition of chondroitin sulfate glycosaminoglycan elongation and sulfation onto the core proteins of the melanoma-associated proteoglycan and the major histocompatibility complex class II-associated invariant chain. In the presence of BFA, the melanoma proteoglycan core protein was retained in the ER but still acquired complex, sialylated, N-linked oligosaccharides, as measured by digestion with endoglycosidase H and neuraminidase. The initiation of glycosaminoglycan synthesis was not affected by BFA, as shown by the incorporation of [6-3H]galactose into a protein-carbohydrate linkage region that was sensitive to beta-elimination. The ability of cells to use an exogenous acceptor, p-nitrophenyl-beta-D-xyloside, to elongate and sulfate core protein-free glycosaminoglycans, was completely inhibited by BFA. The effects of BFA were completely reversible in the absence of new protein synthesis. These experiments indicate that BFA effectively uncouples chondroitin sulfate glycosaminoglycan synthesis by segregating initiation reactions from elongation and sulfation events. Our findings support the proposal that glycosaminoglycan elongation and sulfation reactions are associated with the trans-Golgi network, a BFA-resistant, Golgi subcompartment.

Metabolic effects of forskolin in chick chondrocytes

The effects of forskolin on parameters of energy metabolism and proteoglycan synthesis have been investigated in chick embryo sternal chondrocyte cultures. After 8 h exposure to 100 microM forskolin, ATP levels and oxygen consumption were unaltered. Protein synthesis was unaffected up to 50 microM forskolin and protein degradation was unaffected by forskolin up to 100 microM. In contrast, incorporation of the proteoglycan precursors, 35SO4 and [3H]glucosamine, was more sensitive to forskolin. Inhibition was linear with dose between 10 and 100 microM, reaching 70% at 100 microM. Incorporation of 35SO4 into glycosaminoglycan chains initiated on an artificial beta-xyloside acceptor was inhibited in the same manner. cAMP accumulation was maximal at 10 microM forskolin, a concentration which did not alter proteoglycan synthesis. We conclude that a major, acute effect of forskolin in these short-term experiments is inhibition of proteoglycan synthesis in a cAMP-independent manner.

Precursors of chondroitin sulfate proteoglycan are segregated within a subcompartment of the chondrocyte endoplasmic reticulum

Immunocytochemical methods were used at the levels of light and electron microscopy to examine the intracellular compartments of chondrocytes involved in extracellular matrix biosynthesis. The results of our studies provide morphological evidence for the compartmentalization of secretory proteins in the ER. Precursors of the large chondroitin sulfate proteoglycan (CSPG), the major proteoglycan species produced by chondrocytes, were present in the Golgi complex. In addition, CSPG precursors were localized in specialized regions of the ER. Link protein, a separate gene product which functions to stabilize extracellular aggregates of CSPG monomers with hyaluronic acid, was segregated similarly. In contrast, type II procollagen, another major secretory molecule produced by chondrocytes, was found homogeneously distributed throughout the ER. The CSPG precursor-containing ER compartment exhibits a variable tubulo-vesicular morphology but is invariably recognized as an electronlucent, smooth membrane-bounded region continuous with typical ribosome-studded elements of the rough ER. The observation that this ER structure does not stain with antibodies against resident ER proteins also suggests that the compartment is a specialized region distinct from the main part of the ER. These results support recent studies that consider the ER as a compartmentalized organelle and are discussed in light of the possible implications for proteoglycan biosynthesis and processing.

Immuno-electron microscopy of chondrocyte-derived cells in the rheumatoid cartilage-pannus junction

Immuno-electron microscopy has been utilised to examine the cartilage-pannus junction in seven patients with rheumatoid arthritis. Using a monoclonal antibody, keratan sulphate was localised to cells with the ultra-structural appearance of fibroblasts within a transitional fibroblastic zone in the pannus in two cases. This confirms previous light microscopy evidence that this area (which is clearly separate from articular cartilage) contains cells which produce keratan sulphate, and strengthens the hypothesis that these cells are derived from cartilage rather than from the adjacent synovial membrane.

Tyrosine O-sulfate ester in proteoglycans

Tyrosine O-sulfate residues were detected in the protein core of sulfated proteoglycans. When cultured skin fibroblasts and arterial smooth muscle cells were incubated in the presence of [35S]sulfate, dermatan sulfate proteoglycan and chondroitin sulfate proteoglycan isolated from the culture medium contained tyrosine [35S]sulfate ester which accounted for 0.03%-0.82% of total 35S radioactivity incorporated into the sulfated proteoglycans. This corresponds to a tyrosine sulfation of every second (fibroblasts) and every 10th (smooth muscle cells) dermatan sulfate proteoglycan molecule. [3H]Tyrosine labeling of fibroblast dermatan sulfate proteoglycan gave a similar stoichiometry. However, the relative proportion of tyrosine [35S]sulfate in proteoglycans from arterial tissue was about 10 times higher than in that from cultured arterial cells. Pulse chase experiments with [35S]sulfate revealed that tyrosine sulfation is a late event in the biosynthesis of dermatan sulfate proteoglycan from fibroblasts and occurs immediately prior to secretion. Cultured skin fibroblasts from a patient with a progeroid variant (Kresse et al. 1987, Am. J. Hum. Gen. 41, 436-453) which exhibit a partial deficiency to synthesize dermatan sulfate proteoglycan were shown to form and to secrete a tyrosine-sulfated but glycosaminoglycan-free protein core, thus confirming a selective and independent [35S]sulfate labeling of the protein core.

Subcellular localization of the sulphation reaction of heparan sulphate synthesis and transport of the proteoglycan to the cell surface in rat liver

We report on the incorporation of radiolabelled sulphate into proteoglycan in the 'in situ'-perfused rat liver. After 5 min virtually all of the [35S]sulphate was incorporated into heparan sulphate; no partially sulphated precursors were detected. Pulse-chase experiments, followed by centrifugation in gradients of sucrose and metrizamide, showed that, at 5 min, the heparan sulphate was associated predominantly with the Golgi membranes. Over the next 20 min, intact proteoglycan appeared at the plasma membrane. At intermediate times the heparan sulphate was detected simultaneously in two distinct populations of membrane vesicles. Whether the heparan sulphate in these two populations has two different destinies (e.g. plasma membrane or secretion) is not yet clear. Subfractionation of the Golgi membranes showed that the N-sulphotransferase co-purified with the heparan [35S]sulphate and was separable from the galactosyltransferase of glycoprotein synthesis, confirming that the Golgi membrane system is functionally segregated. Subfractionation also permitted an almost 100-fold purification of the N-sulphotransferase over the homogenate: this will provide an excellent starting material for isolation and further characterization of the enzyme.

Proteoglycans of joint cartilage. Structure, function, turnover and role as markers of joint disease

Joint cartilage consists of cells embedded in a matrix of fibrous collagen within a concentrated water-proteoglycan gel. The integrity of this matrix is crucial for the biomechanical properties of the joint cartilage. The different components of the matrix are synthesized and degraded by the cartilage cells, a process regulated by the amount of mechanical stress applied to the chondrocytes as well as by peptide factors and hormones present in synovial fluid. The proteoglycans are large macromolecules consisting of a protein core to which are attached multiple chains of glycosaminoglycans and oligosaccharides. During normal and pathological turnover, degradation products are released to the synovial fluid and to the circulation. Newly developed assays allow the sensitive and specific detection of these fragments in joint fluid and serum. Results of experimental and clinical investigations suggest that these assays will be of value in efforts to diagnose, grade and predict the outcome of inflammatory and degenerative joint disease.

Ultrastructural demonstration of lectin binding sites in the Golgi apparatus of rat epiphyseal chondrocytes

Binding sites for wheat germ agglutinin (WGA), Dolichos biflorus agglutinin (DBA), Ricinus communis I agglutinin (RCA I) and Limax flavus agglutinin (LFA) have been ultrastructurally detected in rat epiphyseal chondrocytes by a post-embedding cytochemical technique using colloidal gold as marker. The four lectins labelled exclusively the Golgi apparatus of chondrocytes embedded in Lowicryl K4M resin by two different methods. WGA binding sites were localized in medial and trans cisternae as well as in immature secretory vesicles, whereas those for DBA were seen concentrated in cis and medial cisternae. Labelling with both RCA I and LFA lectins was distributed throughout all the cisternae of the Golgi stack, and the latter also in vesicles and tubules at the trans face. Neuraminidase pretreatment of the sections abolished LFA staining, decreased reaction with WGA and increased that with RCA I, while it did not affect DBA staining. After chondroitinase ABC treatment only the RCA I reaction was modified, revealing new binding sites in the trans Golgi face, secretory granules and extracellular matrix. These results indicate that the distribution of subcompartments in the Golgi apparatus of chondrocytes is different from that in cells secreting glycoproteins as major products.

Immunochemical studies on the synthesis and secretion of link protein and aggregating proteoglycan by chondrocytes

Chondrocytes from pig laryngeal cartilage were maintained in culture, and the biosynthesis and secretion of link protein and proteoglycan were studied using immunochemical, biochemical and immunolocalisation techniques. In the presence of monensin there was a dose-dependent inhibition of link protein secretion which was very similar to that of aggregating proteoglycan, and suggested that they followed the same intracellular pathway during biosynthesis. In the presence of cycloheximide there was a similar dose-dependent inhibition of the secretion of both link protein and proteoglycan. Kinetics of secretion following inhibition of synthesis by cycloheximide showed that both proteins had similar intracellular pool sizes. Analysis of protein core and glycosaminoglycan biosynthesis showed that the time for synthesis and glycosylation of proteoglycan was 22 minutes, and this was quickly followed (within 6 minutes) by secretion. Intracellular electron microscopic immunolocalisation using protein A-gold showed link protein to be present in the Golgi cisternae and vesicles, and double-labelling experiments showed link protein only to be detected in vesicles that also labelled for proteoglycan protein core. When chondrocytes were maintained in monolayer culture for 10 days the rate of biosynthesis and secretion of proteoglycan increased although that of link protein remained constant. The control of their biosynthesis was thus shown to be independent. Within 4 hours of secretion a high proportion of link protein was incorporated into proteoglycan aggregates.

The detection of substructures within proteoglycan molecules. Electron-microscopic immuno-localization with the use of Protein A-gold

Proteoglycan monomers from pig laryngeal cartilage were examined by electron microscopy with benzyldimethylalkylammonium chloride as the spreading agent. The proteoglycans appeared as extended molecules with a beaded structure, representing the chondroitin sulphate chains collapsed around the protein core. Often a fine filamentous tail was present at one end. Substructures within proteoglycan molecules were localized by incubation with specific antibodies followed by Protein A-gold (diameter 4 nm). After the use of an anti-(binding region) serum the Protein A-gold (typically one to three particles) bound at the extreme end of the filamentous region. A small proportion of the labelled molecules (10-15%) showed the presence of gold particles at both ends. A monoclonal antibody specific for a keratan sulphate epitope (MZ15) localized a keratan sulphate-rich region at one end of the proteoglycan, but gold particles were not observed along the extended part of the protein core. This distribution was not changed by prior chondroitin AC lyase digestion of the proteoglycan. Localization with a different monoclonal antibody to keratan sulphate (5-D-4) caused a change in the spreading behaviour of a proportion (approx. 20%) of the proteoglycan monomers that lost their beaded structure and appeared with the chondroitin sulphate chains projecting from the protein core. In these molecules the Protein A-gold localized antibody (5-D-4) along the length of the protein core whereas in those molecules with a beaded appearance it labelled only at one end. Labelling with either of the monoclonal antibodies was specific, as it was inhibited by exogenously added keratan sulphate. The differential localization achieved may reflect structural differences within the proteoglycan population involving keratan sulphate and the protein core to which it is attached. The results showed that by this technique substructures within proteoglycan molecules can be identified by Protein A-gold labelling after the use of specific monoclonal or polyclonal antibodies.

Physical properties of chondroitin sulphate/dermatan sulphate proteoglycans from bovine aorta

Bovine aortic chondroitin sulphate/dermatan sulphate proteoglycans (PG-25, PG-35 and PG-50) were differentially precipitated with ethanol and analysed by a variety of chemical and physical techniques. The glycosaminoglycan chains of PG-25 and PG-35 contained a mixture of glucuronic acid and iduronic acid, whereas the uronic acid component of PG-50 was primarily glucuronic acid. In addition, various amounts of oligosaccharides containing small amounts of mannose, a galactose/hexosamine ratio of 1:1 and an absence of uronic acid were covalently linked to the core protein of all proteoglycans. The weight-average Mr (Mw) values of the proteoglycans determined by light-scattering in 4 M-guanidinium chloride were 1.3 X 10(6) (PG-25), 0.30 X 10(6) (PG-35) and 0.88 X 10(6) (PG-50). The s0 values of the proteoglycans were distributed between 7 and 8 S, and the reduced viscosities, eta sp./c, of all proteoglycans were dependent on the shear rate and polymer concentration. Electron microscopy of spread molecules revealed that PG-25 contained small structural units that appeared to self-associate into large aggregates, whereas PG-35 and PG-50 appeared mainly as monomers consisting of a core with various numbers of side projections. Hyaluronic acid-proteoglycan complexes occurred only with a small proportion of the molecules present in PG-35, and their formation could be inhibited by oligosaccharides. These results suggest the presence in the aorta of subspecies of chondroitin sulphate and dermatan sulphate proteoglycans, which show large variations in their physicochemical and inter- and intra-molecular association properties.