Intracellular features of type II procollagen and chondroitin sulfate proteoglycan synthesis in chondrocytes

Role of TSP-5/COMP in pseudoachondroplasia

Pseudoachondroplasia (PSACH) is a well-characterized dwarfing condition associated with disproportionate short stature, abnormal joints and osteoarthritis requiring joint replacement. PSACH is caused by mutations in cartilage oligomeric matrix protein (COMP). COMP, the fifth member of the thrombospondin (TSP) gene family, is a pentameric protein found primarily in the extracellular matrix of musculoskeletal tissues. Functional studies have shown that COMP binds types II and IX collagens but the role of COMP in the extracellular matrix remains to be defined. Mutations in COMP interfere with calcium-binding and protein conformation. PSACH growth plate and growth plate chondrocytes studies indicate that COMP mutations have a dominant negative effect with both COMP and type IX collagen being retained in large rER cisternae. This massive retention causes impaired chondrocyte function with little COMP secreted into the matrix and premature loss of chondrocytes. Deficiency of linear growth results from loss of chondrocytes from the growth plate. Secondarily, the matrix contains minimal COMP, which may be normal and/or mutant, and little type IX collagen. This deficiency results in abnormal joints that are easily eroded and cause painful osteoarthritis. Unlike other misfolded proteins that are targeted for degradation, much of the retained COMP escapes degradation, compromises cell function, and causes cell death. Gene therapy will need to target the reduction of COMP in order to restore normal chondrocyte function and longevity.

The differential distribution of acetylated and detyrosinated alpha-tubulin in the microtubular cytoskeleton and primary cilia of hyaline cartilage chondrocytes

The primary cilium is a ubiquitous cytoplasmic organelle of unknown function. Ultrastructural evidence of primary cilia in chondrocytes, and their colocalisation with the Golgi apparatus, has led to speculation that these structures are functionally linked. To investigate the relationship between these organelles, we examined the molecular anatomy of the microtubular cytoskeleton in the chondrocytes of chick embryo sterna. Thick cryosections were immunolabelled with antibodies directed against acetylated alpha-tubulin (C3B9), detyrosinated alpha-tubulin (ID5) and total alpha-tubulin (TAT), and imaged at high magnification using confocal laser scanning microscopy. Transmission electron microscopy confirmed the ultrastructure of the chondrocyte primary cilium and its structural relationship to the Golgi apparatus. Detyrosinated and acetylated alpha-tubulins were concentrated in the centrioles, centrosome and microtubule organising centre adjacent to the nucleus, with total alpha-tubulin distributed throughout the cytoplasm. ID5 stained the primary cilium at an incidence of 1 per cell, its colocalisation with C3B9 identifying the primary cilium as one of the most stable features of the microtubular cytoskeleton. Primary cilia varied from 1 to 4 microm in length, and 3 patterns of projection into the extracellular matrix were identified; (1) full extension and matrix contact, with minor undulations along the length; (2) partial extension and matrix contact, with a range of bending deflections; (3) cilium reclined against the cell surface with minimal matrix contact. Ultrastructural studies identified direct connections between extracellular collagen fibres and the proteins which decorate ciliary microtubules, suggesting a matrix-cilium-Golgi continuum in hyaline chondrocytes. These results strengthen the hypothesis that the primary cilium acts as a 'cellular cybernetic probe' capable of transducing environmental information from the extracellular matrix, communicating this information to the centrosome. and regulating the exocytosis of Golgi-derived secretory vesicles.

Glycoconjugate distribution in early human notochord and axial mesenchyme

Glycosylation patterns of cells and tissues give insights into spatially and temporally regulated developmental processes and can be detected histochemically using plant lectins with specific affinities for sugar moieties. The early development of the vertebral column in man is a process which has never been investigated by lectin histochemistry. Therefore, we studied binding of several lectins (AIA, Con A, GSA II, LFA, LTA, PNA, RCA I, SBA, SNA, WGA) in formaldehyde-fixed sections of the axial mesenchyme of 5 human embryos in Carnegie stages 12-15. During these developmental stages, an unsegmented mesenchyme covers the notochord. Staining patterns did not show striking temporal variations except for SBA which stained the cranial axial mesenchyme only in the early stage 12 embryo and for PNA, of which the staining intensity in the mesenchyme decreased with age. The notochord appeared as a highly glycosylated tissue. Carbohydrates detected may correspond to adhesion molecules or to secreted substances like proteoglycans or proteins which could play an inductive role, for example, for the neural tube. The axial perinotochordal unsegmented mesenchyme showed strong PNA binding. Therefore, its function as a PNA-positive "barrier" tissue is discussed. The endoderm of the primitive gut showed a lectin-binding pattern that was similar to that of the notochord, which may correlate with interactions between these tissues during earlier developmental stages.

A comparative study of lectin binding to cultured chick sternal chondrocytes and intact chick sternum

Cultured chondrocytes derived from the caudal and cephalic ends of embryonic chick sterna have been compared with each other and with whole sternum, by using a panel of 21 lectins to probe the distribution of oligosaccharides in glycoconjugates of cells and matrix at various times of culture or development. On culture in collagen gels, the cells changed their morphology with time, degrading glycan in the surrounding culture medium and depositing new matrix, the glycan content of which reflected the site of origin of the cells, indicating that the glycan phenotype of both cells and matrix ('glycotype') was predetermined and persistent. Sterna of embryonic chicks showed unexpected complexity in their distribution pattern of glycan, containing at least six distinct regions. Major regional temporal differences were evident among saccharides terminating in alpha-N-acetyl galactosamine and beta-galactose, while changes in glycans terminating in fucose, sialic acid and alpha-mannose were somewhat less marked. Subsets of complex N-glycans changed little.

The chondrodystrophy, nanomelia: biosynthesis and processing of the defective aggrecan precursor

The lethal chicken mutation nanomelia leads to severe skeletal defects because of a deficiency of aggrecan, which is the largest aggregating chondroitin sulphate proteoglycan of cartilage. In previous work, we have demonstrated that nanomelic chondrocytes produce a truncated aggrecan precursor that fails to be secreted, and is apparently arrested in the endoplasmic reticulum (ER). In this study, we investigated the biosynthesis and extent of processing of the abnormal aggrecan precursor. The truncated precursor was translated directly in cell-free reactions, indicating that it does not arise post-translationally. Further studies addressed the processing capabilities of the defective precursor. We found that the mutant precursor was modified by N-linked, mannose-rich oligosaccharides and by the addition of xylose, but was not further processed; this is consistent with the conclusion that it moves no further along the secretory pathway than the ER. Using brefeldin A we demonstrated that the defective precursor can function as a substrate for Golgi-mediated glycosaminoglycan chains, but does not do so in the nanomelic chondrocyte because it fails to be translocated to the appropriate membrane compartment. These studies illustrate how combined cell biological/biochemical and molecular investigations may contribute to our understanding of the biological consequences and molecular basis of genetic diseases, particularly those involving errors in large, highly modified molecules such as proteoglycans.

Nanomelic chondrocytes synthesize, but fail to translocate, a truncated aggrecan precursor

Cartilage extracellular matrix (ECM) is composed primarily of type II collagen and large, link stabilized aggregates of hyaluronic acid and chondroitin sulfate proteoglycan (aggrecan). Maturation and function of these complex macromolecules are dependent upon sequential processing events which occur during their movements through specific subcellular compartments in the constitutive secretory pathway. Failure to complete these events successfully results in assembly of a defective ECM and may produce skeletal abnormalities. Nanomelia is a lethal genetic mutation of chickens characterized by shortened and malformed limbs. Previous biochemical studies have shown that cultured nanomelic chondrocytes synthesize a truncated aggrecan core protein precursor that disappears with time; however, the protein does not appear to be processed by the Golgi or secreted. The present study investigates the intracellular trafficking of the defective aggrecan precursor using immunofluorescence, immunoelectron microscopy and several inhibitors. Results indicate that nanomelic chondrocytes assemble an ECM that contains type II collagen, but lacks aggrecan. Instead, aggrecan precursor was localized intracellularly, within small cytoplasmic structures corresponding to extensions of the endoplasmic reticulum (ER). At no time were precursor molecules observed in the Golgi. In contrast, normal and nanomelic chondrocytes exhibited no difference in the intracellular or extracellular distribution of type II procollagen. Therefore, retention of the aggrecan precursor appears to be selective. Incubation of chondrocytes at 15 degrees C resulted in the retention and accumulation of product in the ER. After a return to 37 degrees C, translocation of the product to the Golgi was observed for normal, but not for nanomelic, chondrocytes, although the precursors disappeared with time. Ammonium chloride, an inhibitor of lysosomal function, had no effect on protein loss, suggesting that the precursor was removed by a non-lysosomal mechanism, possibly by ER-associated degradation. Based on these studies, we suggest that nanomelic chondrocytes are a useful model for examining cellular trafficking and sorting events and the processes by which abnormal products are targeted for retention or degradation. Further investigations should provide insight into the mechanisms underlying chondrodystrophies and other related diseases.

Electron microscopic observations and electrophoresis of the glycosaminoglycans in the epiphyseal cartilage of the congenital osteochondrodysplasia rat (ocd/ocd)

The osteochondrodysplasia rat, inherited by a single autosomal recessive lethal gene ocd, shows a typical dwarfing syndrome with systemic subcutaneous edema. The skeletal system is most severely affected. The affected newborn also demonstrates abnormal kidney position and respiratory system anomalies and central nervous malfunction. Previous light microscopic observations show that the chondrocytes are expanded and destroyed, and the amounts of extracellular matrix (ECM) and glycosaminoglycans (GAGs) are decreased. The present studies describe ultrastructural appearances, and measurement and electrophoretic analysis of the major components of the cartilaginous GAGs. Decrease in amounts of ECM and swollen chondrocytes with the expanded organelles were reconfirmed in the ocd/ocd by electron microscopic observation. The large expanded vesicles contained unevenly distributed granular materials and large ruthenium red (RR) granules. The RR granules in the ECM were small and most parts of the collagen fibers did not associate with the granule in the ocd/ocd, while the RR granules attached to all the collagen fibers in the phenotypically normal (+/?). There were large collagen bundles in the region where the chondrocytes were committed to self-destruction. The biochemical analysis of the cartilage showed that noncollagenous proteins were increased and the GAGs were decreased in amount in the ocd/ocd, although the hydroxyproline content was comparable to that of the +/?. The hyaluronic acid was close to the limit of detection by electrophoresis of the cartilaginous GAGs in the ocd/ocd. These results suggest that the ocd gene affects GAG metabolism. The decrease in amounts of GAGs, especially hyaluronic acid, may be responsible for the anomalies of the cartilage in the ocd/ocd.

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.

Intrinsic and extrinsic controls of the hypertrophic program of chondrocytes in the avian columella

Immunohistochemical studies of the chick columella have shown that the extracellular matrix of this ossicular cartilage template is composed largely of type II collagen. As development proceeds, synthesis of type X collagen, a hypertrophic cartilage-specific molecule, is initiated by endochondral chondrocytes within the zone of cartilage cell hypertrophy. Subsequently, these cells and their surrounding extracellular matrix are removed, resulting in marrow cavity formation. We have examined which of these processes are programmed within the columella chondrocytes themselves, and which require involvement of exogenous factors. Prehypertrophic columella from 12-day chick embryos were grown either in organ culture on Nuclepore filters or as explants on the chorioallantoic membrane of host embryos. Chondrocytes from the same source were grown in monolayer cell cultures. In both organ culture and cell culture, chondrocytes developed to the stage at which some of them entered the hypertrophic program and initiated the production of type X collagen as determined by immunofluorescence histochemistry with a monoclonal antibody specific for that collagen type. The organ cultures, however, did not progress to the next stage, in which detectable removal of the type X collagen-containing matrix occurs. When identical columella were grown on the chorioallantoic membrane of host chicks, the type X collagen-containing matrix which formed was rapidly removed, resulting in the formation of a marrow cavity. Thus, progression of endochondral chondrocytes to the deposition of type X collagen-containing matrix seems to be programmed within the cells themselves. Subsequent removal of this matrix requires the involvement of exogenous factors.

Rabbit articular chondrocytes: an in vitro model for studying the effect of sodium aurothiopropanol sulfonate on proliferation kinetics, type II collagen phenotype and mitochondrial activity

Despite the benefits of chrysotherapy the responsible mechanism of action of gold compounds remains unclear. At a concentration of 5 x 10(-4) M, sodium aurothiopropanol sulfonate (SAS) modified the in vitro proliferation kinetics of articular chondrocytes by reducing growth, viability and plating efficiency. Flow cytometry analysis, using propidium iodide DNA staining, revealed slight but significant cell arrest in G2+M which, in fact, represents an increase in the proportion of binucleate cells. SAS did not induce any variations in chondrocyte phenotype stability as far as the biosynthesis of type II collagen was concerned, and no appreciable changes in overall mitochondrial activity reflected by rhodamine 123 incorporation.

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.

The structure and distribution of proteochondroitin sulphate during the formation of chick embryo feather germs

The dorsal skin of the chick embryo, in which feather germ forms, was found to synthesize two proteochondroitin sulphates, PCS-I and PCS-II and a proteoheparan sulphate, PHS. A monoclonal antibody (I3B9) was prepared against PCS-I, a higher molecular weight proteochondroitin sulphate. Distribution of PCS-I was immunohistochemically studied using I3B9. PCS-I was found in the epidermis, basement membrane and superficial dermis prior to formation of feather rudiments. As the feather rudiments formed, PCS-I was noted in a condensed area of dermal cells and in the basement membrane, while PCS-I decreased remarkably in the epidermal placode. The formation of feather buds resulted in a decrease in PCS-I in the region of dermal condensation and the basement membrane situated above this region. PCS-I was asymmetrically distributed in the feather filaments. The turnover of proteochondroitin sulphate was studied using autoradiography of [35S]sulphate. Proteochondroitin sulphate in the basement membrane and condensed dermis of the feather rudiments showed very slow turnover. On the other hand, the outgrowth of feather buds caused rapid turnover of proteochondroitin sulphate in the region of dermal condensation and basement membrane situated above this region. The mechanism for the uneven distribution of PCS-I during feather germ formation is discussed.

Biosynthetic precursors of cartilage chondroitin sulfate proteoglycan

Early steps in the biosynthesis of chondroitin sulfate proteoglycan (CSPG) and collagenous cartilage matrix molecules were examined by the comparison of products translated in mRNA-directed cell-free reactions and those synthesized by intact cartilage cells. RNA isolated from embryonic chicken sterna was used to direct cell-free translation reactions. Chicken sternal chondrocytes in culture were pulse-labeled with [35S]-methionine. The CSPG core protein was identified by immunoprecipitation. The Mr of the cartilage cell-synthetized core protein was determined to be 370K, approximately 10-15K greater than that of the comparable cell-free translation product. Experimental results strongly support the view that the observed difference in Mr reflects the cotranslational addition of mannose-rich, N-asparagine-linked oligosaccharides to the cell-synthesized core protein: 1) the cell-synthesized product was labeled with [3H]-mannose and precipitated by concanavalin A-sepharose beads; 2) the incorporated [3H]-mannose could be subsequently removed by digestion with endoglycosidase H (Endo H); 3) the Mr of the cell-synthesized core protein was reduced by Endo H digestion to that of the comparable cell-free translation product; 4) the core protein synthesized by tunicamycin-treated chondrocytes (inhibited in their ability to add N-asparagine-linked mannose-rich oligosaccharides to proteins) was comparable in electrophoretic mobility to that of the core protein cell-free translation product; and 5) the core protein translated in microsome-coupled cell-free reactions had an Mr 8-10K greater than that of the core protein translated in the absence of microsomes. For the purpose of examining biosynthetic intermediates, chondrocytes were labeled continuously or pulse-chase labeled for varying times. No biosynthetic CSPG intermediates migrating between the core protein and the CSPG monomer were detected. However, a band of 355Kdal appeared to share certain characteristics with the 307Kdal core protein (including its immunoprecipitability with CSPG antibodies), and a 340Kdal band was noted. Type II procollagen and other collagenase-sensitive products of 205Kdal and 110Kdal were observed among translation and chondrocyte-synthesized products. In chondrocytes, all three products exhibited labeling or chase time-dependent increases in Mr which were accelerated by ascorbate supplements and inhibited by the addition of alpha, alpha'-dipyridyl. These results suggest that the observed time-dependent increases in Mr are a consequence of collagen hydroxylation. The 110Kdal and 205Kdal collagenous proteins may be related to the minor collagens recently described in cartilage.

Light and electron microscopical localization of concanavalin A lectin binding sites in rat epiphyseal chondrocytes

Concanavalin A lectin binding sites have been detected within the cytoplasm of epiphyseal chondrocytes. Correlative light and electron microscopic results were obtained, indicating the presence of alpha-D-mannose and/or alpha-D-glucose residues detected by the lectin in the rough endoplasmic reticulum region. Quantitation of the electron microscopic cytochemical reaction also showed that the specific labelling was almost exclusively localized in the lumen of endoplasmic reticulum cisternae. No significant staining was found in other membrane compartments or extracellular matrix. This labelling pattern could be considered as the cytochemical evidence of N-glycosylation processes occurring during the biosynthesis of cartilage extracellular matrix components by chondrocytes.

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.

Immunofluorescence studies on cartilage matrix synthesis. The synthesis of link protein, chondroitin sulfate proteoglycan monomer and type II collagen

A comparison of the synthesis and deposition of fibrous type II collagen and the constituents of chondroitin sulfate proteoglycan (CSPG) aggregates, CSPG monomer and link protein, was made for chicken sternal chondrocytes in culture, using simultaneous double immunofluorescence and lectin localization. Chondrocytes deposited only CSPG constituents--and not type II collagen--into the extracellular matrix (ECM). Intracellular precursors of CSPG monomer were localized primarily in perinuclear regions, but were observed in other cytoplasmic vesicles as well. Link protein antibodies stained the same intracellular structures, but stained the perinuclear cytoplasm less intensely. In contrast, type II procollagen was distributed in vesicles throughout the cytoplasm and was clearly absent from the distinctive, CSPG precursor-containing vesicles. Fluorescence-labelled lectins were used to further identify intracellular membrane compartments. Wheat germ agglutinin (WGA) and Ricinus lectins (which recognize carbohydrates added in the Golgi) stained the perinuclear cytoplasm, while concanavalin A (conA) (which recognizes mannose-rich oligosaccharides added co-translationally) stained vesicles throughout the rest of the cytoplasm and not the perinuclear cytoplasm. The distinctive CSPG-containing vesicles were not stained with WGA or Ricinus agglutinins. Data presented elsewhere demonstrate that the vesicles do not react with monoclonal antibodies which recognize chondroitin sulfate (CS) or keratan sulfate (KS) determinants. Thus, we conclude that the vesicles accumulate CSPG precursors which have not been modified by Golgi-mediated processes. The data indicate that matrix molecules may be segregated selectively prior to transit through the Golgi complex. The co-distribution of link protein and CSPG monomer precursors in vesicles prior to further, Golgi-mediated modification may reflect an as yet undetermined function of these vesicles in the processing or assembly of CSPG.