The intracellular localisation of proteoglycans and their accumulation in chondrocytes treated with monensin

Metabolism of cartilage proteoglycans in health and disease

Cartilage proteoglycans are extracellular macromolecules with complex structure, composed of a core protein onto which a variable number of glycosaminoglycan chains are attached. Their biosynthesis at the glycosaminoglycan level involves a great number of sugar transferases well-orchestrated in Golgi apparatus. Similarly, their degradation, either extracellular or intracellular in lysosomes, involves a large number of hydrolases. A deficiency or malfunction of any of the enzymes participating in cartilage proteoglycan metabolism may lead to severe disease state. This review summarizes the findings regarding this topic.

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 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.

Morphological and electrophysiological changes induced by calcium ionophores (A23187 and X-537A) in spontaneously beating rabbit sino-atrial node cells

1. Effects of calcium ionophores (A23187 and X-537A) on the spontaneously beating sino-atrial (SA) node cells of rabbit heart were examined using electron microscopic and an electrophysiological techniques. 2. During exposure to A23187 or X-537A (2 x 10(-5) M), the cycle length was significantly prolonged by 11% (n = 12) or 118% (n = 11), respectively. But neither ionophore affected other action potential parameters. 3. X-537A (2 x 10(-5) M) induced irregular rhythm (dysrhythmia), probably due to cellular calcium overload. Similarly, ouabain (3 x 10(-7) M) also elicited dysrhythmia. In the presence of isoproterenol (ISP, 10(-7) M), X-537A potentiated dysrhythmia, and A23187 newly induced it. 4. In ultrastructural analyses, X-537A caused swelling of the cisternae of Golgi apparatus within 10 min, whereas A23187 and ouabain did not produce any changes even after 30 min-application. 5. Addition of high Ca2+ (10 mM) and/or ISP (10(-7) M) to X-537A produced a further dilation and vacuolization. In A23187 or ouabain, however, the addition of Ca2+ and ISP did not cause any changes, even during dysrhythmia. 6. These results indicate that X-537A elicited a more potent calcium overload than A23187, and that a discrepancy between ultrastructural damages and electrical changes exists.

Effect of monensin on the sulfation of heparan sulfate proteoglycan from endothelial cells

Monensin is a monovalent metal ionophore that affects the intracellular translocation of secretory proteins at the level of trans-Golgi cisternae. Exposure of endothelial cells to monensin results in the synthesis of heparan sulfate and chondroitin sulfate with a lower degree of sulfation. The inhibition is dose dependent and affects the ratio [35S]-sulfate/[3H]-hexosamine of heparan sulfate from both cells and medium, with no changes in their molecular weight. By the use of several degradative enzymes (heparitinases, glycuronidase, and sulfatases) the fine structure of the heparan sulfate synthesized by control and monensin-treated cells was investigated. The results have shown that among the six heparan sulfate disaccharides there is a specific decrease of the ones bearing a sulfate ester at the 6-position of the glucosamine moiety. All other biosynthetic steps were not affected by monensin. The results are indicative that monensin affects the hexosamine C-6 sulfation, and that this sterification is the last step of the heparan sulfate biosynthesis and should occur at the trans-Golgi compartment.

Immunoglobulin fold and tandem repeat structures in proteoglycan N-terminal domains and link protein

Detailed primary sequence and secondary structure analyses are reported for the hyaluronate binding region (G1 domain) and link protein of proteoglycan aggregates. These are based on six full or partial sequences from the chicken, pig, human, rat and bovine proteins. Determinations of a full pig and a partial human link protein sequence are reported in the Appendix. Five sequences at the N terminus in both proteins were compared with the structures of 11 variable immunoglobulin (Ig) fold domains for which crystal structures are available. Despite only modest sequence homology, a clear alignment could be proposed. Analysis of this shows that the equivalents of the first and second hypervariable segments are now significantly longer, and both proteins have N-terminal extensions that are up to 23 residues in length. Secondary structure predictions showed that these sequences could be identified with available crystal structures for the variable Ig fold. However the hydrophobic residues involved in interactions between the light and heavy chains in Igs are replaced by hydrophilic charged groups in both proteins. These results imply that both proteins are members of the Ig superfamily, but exhibit structural differences distinct from other members of this superfamily for which crystal structures are known. The proteoglycan tandem repeat (PTR) is a repeat of 99 residues that is found twice in the amino acid sequence of link protein and the proteoglycan G1 domain adjacent to the Ig fold, and also twice in the proteoglycan G2 domain. A total of 16 PTRs was available for analysis. Compositional analyses show that these are positively charged if these originate from link protein, and negatively charged if from the G1 or G2 domains. The 16 Robson secondary structure predictions for the PTRs were averaged to improve the statistics of the prediction, and checked by comparison with Chou-Fasman calculations. A strong alpha-helix prediction was found at residues 13 to 25, and several beta-strands were predicted. The overall content is 18% alpha-helix and 28% beta-sheet, with 44% of the remaining sequence being predicted as turns. These analyses show that both the proteoglycan G1 domain and link protein are constructed from two distinct globular components, which may provide the two functional roles of these proteins in proteoglycan aggregation.

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.

Localization of the incorporation of 3H-galactose and 3H-sialic acid into thyroglobulin in relation to the block of intracellular transport induced by monensin. Studies with isolated porcine thyroid follicles

The Na+/K+ ionophore monensin is known to arrest the intracellular transport of newly synthesized proteins in the Golgi complex. In the present investigation the effect of monensin on the secretion of 3H-galactose-labeled and 3H-sialic acid-labeled thyroglobulin was studied in open thyroid follicles isolated from porcine thyroid tissue. Follicles were incubated with 3H-galactose at 20 degrees C for 1 h; at this temperature the labeled thyroglobulin remains in the labeling compartment (Ring et al. 1987a). The follicles were then chased at 37 degrees C for 1 h in the absence or presence of 1 microM monensin. Without monensin substantial amounts of labeled thyroglobulin were secreted into the medium, whereas in the presence of the ionophore secretion was inhibited by 80%. Since we have previously shown (Ring et al. 1987b) that monensin does not inhibit secretion of thyroglobulin present on the distal side of the monensin block we conclude that galactose is incorporated into thyroglobulin on the proximal side of this block. Secretion was also measured in follicles continuously incubated with 3H-galactose for 1 h at 37 degrees C in the absence or presence of monensin. In these experiments secretion of labeled thyroglobulin was inhibited by about 85% in the presence of monensin. Identically designed experiments with 3H-N-acetylmannosamine, a precursor of sialic acid, gave similar results, i.e., almost complete inhibition of secretion of labeled thyroglobulin in the presence of monensin. The agreement between the results of the galactose and sialic acid experiments indicates that sialic acid, like galactose, is incorporated into thyroglobulin on the proximal side of the monensin block.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural localization of the major proteoglycan and type II procollagen in organelles and extracellular matrix of cultured chondroblasts

The mechanisms of synthesis and intracellular routing of the various cartilage matrix macromolecules are still unclear. We have studied this problem in cultured chondroblasts at the ultrastructural level using monospecific antibodies against the core protein of the keratan sulfate/chondroitin sulfate-rich cartilage proteoglycan (KS:CS-PG) or Type II procollagen, and cuprolinic blue, a cationic dye that binds to the glycosaminoglycan chains of proteoglycans. Intracellularly, the proteoglycan antibodies localized KS:CS-PG and its precursors primarily in the Golgi complex and secretory vesicles. In contrast, the bulk of Type II procollagen was found within the rough endoplasmic reticulum (ER). While devoid of collagen, the extracellular matrix was rich in KS:CS-PG molecules some of which studded the chondroblast plasmalemma. Cuprolinic blue staining indicated that the proteoglycans present in the Golgi complex fell into a predominant class of large proteoglycans, probably representing KS:CS-PG, and a minor class of smaller proteoglycans. Groups of these divergent proteoglycans often occupied distinct Golgi subcompartments; moreover, single large proteoglycans appeared to align along the luminal surface of Golgi cisternae and secretory vesicles. These results suggest that in cultured chondroblasts KS:CS-PG and Type II procollagen are differentially distributed both in organelles and in the extracellular matrix, and that different proteoglycan types may occupy distinct subcompartments in trans Golgi.

Binding of lectin-fluorescein conjugates to intracellular compartments of growth-plate chondrocytes in situ

In this study, lectin-binding techniques are applied to growth-plate cartilage to analyze the intracellular localization of lectin-binding glycoconjugates of chondrocytes in situ. The binding of ten fluorescein-conjugated lectins is analyzed on 1-micron-thick Epon-embedded, nondecalcified sections of growth plates from Yucatan swine. Comparisons are made to intracellular binding in chondrocytes of tracheal, articular, and auricular cartilage. Ear epithelium, tracheal epithelium, and kidney are used as positive control tissues for the specificity of lectin binding. Only the mannose-binding lectins had affinity for the RER and nuclear envelope. Eight lectins reacted within the Golgi complex with characteristic patterns which ranged from localized fine linear strands to extensive vesicular accumulations. When cartilage slabs were exposed before embedment to the ionophore monensin to disrupt intracellular transport through the Golgi, it was possible to define differential subcompartments of the Golgi complex, based upon sites of sugar addition. Also, it was possible to characterize the cytoplasmic deposits of reserve-zone chondrocytes which were positive with concanavalin A as glycogen, based upon their sensitivity to amylase. This method allows resolution at the light-microscopic level of lectin-binding glycoconjugates with localization to specific organelles. Patterns of intracellular binding were consistent with biochemical data relating to the subcellular localization of processing steps of complex carbohydrates prior to secretion.

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

The intracellular pathway of cartilage proteoglycan biosynthesis was investigated in isolated chondrocytes using a protein A-gold electron microscopy immunolocalization procedure. Proteoglycans contain a protein core to which chondroitin sulfate and keratan sulfate chains and oligosaccharides are added in posttranslational processing. Specific antibodies have been used in this study to determine separately the distribution of the protein core and chondroitin sulfate components. In normal chondrocytes, proteoglycan protein core was readily localized only in smooth-membraned vesicles which co-labeled with ricin, indicating them to be galactose-rich medial/trans-Golgi cisternae, whereas there was only a low level of labeling in the rough endoplasmic reticulum. Chondroitin sulfate was also localized in medial/trans-Golgi cisternae of control chondrocytes but was not detected in other cellular compartments. In cells treated with monensin (up to 1.0 microM), which strongly inhibits proteoglycan secretion (Burditt, L.J., A. Ratcliffe, P. R. Fryer, and T. Hardingham, 1985, Biochim. Biophys. Acta., 844:247-255), there was greatly increased intracellular localization of proteoglycan protein core in both ricin-positive vesicles, and in ricin-negative vesicles (derived from cis-Golgi stacks) and in the distended rough endoplasmic reticulum. Chondroitin sulfate also increased in abundance after monensin treatment, but continued to be localized only in ricin-positive vesicles. The results suggested that the synthesis of chondroitin sulfate on proteoglycan only occurs in medial/trans-Golgi cisternae as a late event in proteoglycan biosynthesis. This also suggests that glycosaminoglycan synthesis on proteoglycans takes place in a compartment in common with events in the biosynthesis of both O-linked and N-linked oligosaccharides on other secretory glycoproteins.

Influence of monensin on biosynthesis, processing and secretion of proteodermatan sulfate by skin fibroblasts

The influence of monensin on biosynthesis, processing and secretion of proteodermatan sulfate from human skin fibroblasts was studied with the aid of a specific immunological procedure. Double-labeling experiments with [3H]leucine and [35S]sulfate indicated that monensin caused a dose-dependent parallel decrease of sulfate incorporation into total and of secretion of 3H-labeled proteodermatan sulfate. Compared with the untreated control, a greater proportion of incorporated [35S]sulfate than of incorporated [3H]leucine became secreted. Other monensin effects were a moderate intracellular accumulation of glycosaminoglycan-free core protein, a reduced chain length and a greatly reduced epimerization of D-glucuronic to L-iduronic acid residues. In contrast to the formation of N-acetylgalactosamine 4-sulfate residues 6-sulfation was not affected. Conversion of high-mannose-type oligosaccharides to complex-type N-glycans which normally occurred concomitantly with glycosaminoglycan biosynthesis was inhibited. Withdrawal of monensin made possible an additional sulfation of intracellularly accumulated proteodermatan sulfate. The newly formed sulfate esters did not cluster at the non-reducing ends of the glycosaminoglycan chains. Cells preexposed to monensin and labeled with [3H]glucosamine either in the absence or continuous presence of the drug incorporated similar amounts of 3H radioactivity into proteodermatan sulfate. The results suggest that epimerization of D-glucuronic acid residues and 4-sulfation occur predominantly in the trans cisternae of the Golgi apparatus whereas chain polymerisation and 6-sulfation take place predominantly in the cis Golgi complex.