Cloning and sequence analysis of a partial cDNA for chicken cartilage proteoglycan core protein

FGF signaling in mandibular skeletogenesis

OBJECTIVE

To examine the functions of FGF/FGFR signaling during mandibular skeletogenesis in ovo.

DESIGN

We examined the effects of inhibition of FGF signaling during mandibular skeletogenesis by overexpressing replication-competent RCAS virus encoding a truncated form of FGFR3 in the chicken mandibular process between stages 17 and 26.

RESULTS

Injection of RCAS-dnFGFR3 into the developing mandible resulted in abnormalities in a stage- and region-dependent manner. Injection at early stages of development resulted in the truncation of Meckel's cartilage, severely reduced outgrowth of the mandibular process and absence of five of the mandibular bones. Injection at later stages did not affect the outgrowth of the mandibular process and Meckel's cartilage but resulted in abnormalities in mandibular osteogenesis in a region-specific manner. The bones in the more caudal region were frequently truncated whereas bones in the more rostral regions such as dentary and splenial bones were frequently absent.

CONCLUSION

Together these experiments have revealed essential roles for FGF/FGFR signaling in the elongation of Meckel's cartilage, development of osteogenic condensations and appositional growth of mandibular bones.

The MEK-ERK Signaling Pathway Is a Negative Regulator of Cartilage-specific Gene Expression in Embryonic Limb Mesenchyme

The extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase pathway, also known as the MEK-ERK kinase cascade, has recently been implicated in the regulation of embryonic cartilage differentiation. However, its precise role in this complex process remains controversial. To more thoroughly examine the role of the MEK-ERK kinase cascade in chondrogenesis, we analyzed the effects of two structurally different pharmacological inhibitors of MEK, the upstream kinase activator of ERK, on chondrocyte differentiation in micromass cultures of embryonic chick limb mesenchyme cells. We found that the MEK inhibitors, U0126 and PD98059, promote increased accumulation of cartilage-characteristic mRNA transcripts for type II collagen, aggrecan, and the transcription factor, Sox9. PD98059 treatment stimulated increased deposition of sulfated glycosaminoglycan into both Alcian blue-stainable cartilage matrix and the surrounding culture medium, whereas U0126 elevated glycosaminoglycan secretion into the medium fraction alone. Both MEK inhibitors increased total type II collagen protein accumulation in micromass culture and elevated the activity of a transfected type II collagen enhancer-luciferase reporter gene. Thus, pharmacological MEK inhibition induced increased expression of multiple chondrocyte differentiation markers. Conversely, transfection of limb mesenchyme cells with a constitutively active MEK1 plasmid resulted in a prominent decrease in the activity of a co-transfected type II collagen enhancer-luciferase reporter gene. Collectively, these findings support the hypothesis that signaling through the MEK-ERK kinase cascade may function as an important inhibitory regulator of embryonic cartilage differentiation.

The folded modules of aggrecan G3 domain exert two separable functions in glycosaminoglycan modification and product secretion

Aggrecan is the major proteoglycan in the extracellular matrix of cartilage. A notable exception is nanomelic cartilage, which lacks aggrecan in its matrix. The example of nanomelia and other evidence leads us to believe that the G3 domain plays an important role in aggrecan processing, and it has indeed been confirmed that G3 allows glycosaminoglycan (GAG) chain attachment and product secretion. However, it is not clear how G3, which contains at least a carbohydrate recognition domain (CRD) and a complement binding protein (CBP) motif, plays these two functional roles. The present study was designed to dissect the mechanisms of this phenomenon and specially 1) to determine the effects of various cysteine residues in GAG modification and product secretion as well as 2) to investigate which of the two processing events is the critical step in the product processing. Our studies demonstrated that removal of the two amino-terminal cysteines in the CRD motif and the single cysteine in the amino terminus of CBP inhibited secretion of CRD and CBP. Use of the double mutant CRD construct also allowed us to observe a deviation from the usual strict coupling of GAG modification and product secretion steps. The presence of a small chondroitin sulfate fragment overcame the secretion-inhibitory effects once the small chondroitin sulfate fragment was modified by GAG.

Roles of insulin-like growth factor-I (IGF-I) and IGF-I binding protein-2 (IGFBP2) and -5 (IGFBP5) in developing chick limbs

Insulin-like growth factor-I (IGF-I) and the IGF-I binding proteins (IGFBPs) which modulate IGF-I action have been implicated in the development of the vertebrate limbs and skeleton. We have examined the distribution of IGF-I, IGFBP2 and IGFBP5 in developing chick limb buds and have investigated their functional roles and relationships during chick limb development. IGF-I and IGFBP2 are co-expressed throughout the lateral plate from which limbs form, although IGFBP2, unlike IGF-I, does not promote formation of rudimentary limb buds from non-limb-forming flank regions in vitro. During limb outgrowth, IGF-I is present in non-AER limb ectoderm, but little IGF-I is present in the AER itself, suggesting that restriction of endogenous IGF-I activity may be required for proper AER function. Consistent with this possibility, the ectoderm of mutant limbless and wingless wing buds, which fail to form an AER, continues to express IGF-I. We also found that the AER contains abundant IGFBP2 but that IGFBP2 is not present in limb subridge mesoderm. In contrast, IGFBP2 is present in the distal mesoderm of mutant limbless or wingless limb buds, which fail to grow out. This suggests that attenuation of IGFBP2 expression is controlled by the AER and that cessation of IGFBP2 expression may be necessary for the proliferation and suppression of differentiation of subridge mesoderm that is required for limb outgrowth to occur. Consistent with this possibility, we found that exogenous IGFBP2 inhibits the anti-differentiative activity of the AER in vitro. We also found that regions of cell death in the limb contain abundant IGF-I-immunoreactive cells, consistent with a role for IGF-I in apoptosis. During skeletogenesis, IGF-I and IGFBP2 are co-localized to the condensing central core of the limb, implicating these factors as potential regulators of the onset of chondrogenic differentiation. Intriguingly, we found that IGF-I and IGFBP2 have opposing effects on chondrogenesis, as IGF-I stimulates but IGFBP2 inhibits accumulation of cartilage matrix by micromass cultures in vitro. Long [R(3)] IGF-I, an analog of IGF-I that cannot bind IGFBPs, is more effective than IGF-I in stimulating matrix accumulation, consistent with a negative role for IGFBP2 in chondrogenesis. As the chondrocytes of the limb mature, IGF-I is present only in terminal hypertrophic chondrocytes, which undergo programmed cell death, while IGFBP2 becomes localized to prehypertrophic and hypertrophic chondrocytes, suggesting involvement in chondrocyte maturation. Consistent with this possibility, we found that exogenous IGFBP2 induces precocious expression of Indian hedgehog, a marker of prehypertrophy, in maturing chondrocytes in vitro. IGF-I and IGFBP2 are also present in the osteoblasts, clasts and nascent matrix of the long bones, consistent with roles in endochondral bone formation. Unlike in rodent limbs, IGFBP5 is not expressed by chick limb ectoderm or AER. IGFBP5 expression is highly localized to developing limb musculature and, later, to the developing skeletal elements where it is expressed by osteoblast precursers and osteoblasts. The results of this study suggest potential novel roles for IGF-I and IGFBP2 in several aspects of limb development including limb outgrowth and AER activity, programmed cell death, chondrogenesis and chondrocyte maturation.

Domain organization, genomic structure, evolution, and regulation of expression of the aggrecan gene family

Proteoglycans are complex macromolecules, consisting of a polypeptide backbone to which are covalently attached one or more glycosaminoglycan chains. Molecular cloning has allowed identification of the genes encoding the core proteins of various proteoglycans, leading to a better understanding of the diversity of proteoglycan structure and function, as well as to the evolution of a classification of proteoglycans on the basis of emerging gene families that encode the different core proteins. One such family includes several proteoglycans that have been grouped with aggrecan, the large aggregating chondroitin sulfate proteoglycan of cartilage, based on a high number of sequence similarities within the N- and C-terminal domains. Thus far these proteoglycans include versican, neurocan, and brevican. It is now apparent that these proteins, as a group, are truly a gene family with shared structural motifs on the protein and nucleotide (mRNA) levels, and with nearly identical genomic organizations. Clearly a common ancestral origin is indicated for the members of the aggrecan family of proteoglycans. However, differing patterns of amplification and divergence have also occurred within certain exons across species and family members, leading to the class-characteristic protein motifs in the central carbohydrate-rich region exclusively. Thus the overall domain organization strongly suggests that sequence conservation in the terminal globular domains underlies common functions, whereas differences in the central portions of the genes account for functional specialization among the members of this gene family.

Roles of aggrecan domains in biosynthesis, modification by glycosaminoglycans and product secretion

Aggrecan is a member of the chondroitin sulphate (CS) proteoglycan family, which also includes versican/PG-M, neurocan and brevican. Members of this family exhibit structural similarity: a G1 domain at the N-terminus and a G3 domain at the C-terminus, with a central sequence for modification by CS chains. A unique feature of aggrecan is the insertion of three additional domains, an inter-globular domain (IGD), a G2 domain and a keratan sulphate (KS) domain (sequence modified by KS chains), between the G1 domain and the CS domain (sequence modified by CS chains). The G1 and G3 domains have been implicated in product secretion, but G2, although structurally similar to the tandem repeats of G1, performs an unknown function. To define the functions of each aggrecan domain in product processing, we cloned and expressed these domains in various combinations in COS-7 cells. The results indicated that the G3 domain enhanced product secretion, alone or in combination with the KS or CS domain, and promoted glycosaminoglycan (GAG) chain attachment. Constructs containing the G1 domain were not secreted. Addition of a CS domain sequence to G1 reduced this inhibition, but GAG chain attachment was still decreased. The potential GAG chain attachment site in the IGD was occupied by GAGs, and IGD product was secreted efficiently. The KS domain was modified by GAG chains and secreted. Finally, the G2 domain was expressed but not secreted, and inhibited secretion of the IGD when expressed as an IGD-G2 combination.

Sox9 expression during chondrogenesis in micromass cultures of embryonic limb mesenchyme

Sox9 plays a crucial role in chondrogenesis. It encodes an HMG-domain transcription factor that activates an enhancer in the gene for type II collagen (Col2a1), a principal cartilage matrix protein. We have characterized the temporal pattern of Sox9 RNA expression in micromass culture, a widely used in vitro model for the analysis of embryonic cartilage differentiation. Cultures were prepared from distal subridge mesenchyme of the stage 24/25 chick embryo wing bud, which undergoes uniform chondrogenic differentiation in vitro. The early "prechondrogenic" phase of culture was characterized by the activation of Sox9 RNA expression, which preceded detectable upregulation of Col2a1 transcription. Sox9 RNA levels peaked between 20 and 65 h of culture, a phase of progressive Col2a1 transcript accumulation, then declined in the mature cartilage of 120-h cultures. Staurosporine treatment enhanced chondrogenesis in micromass culture by inducing a rapid quantitative increase in Sox9 transcript levels. However, PMA, a phorbol ester that inhibits Col2a1 expression and chondrocyte differentiation, had an unexpectedly modest effect on Sox9 RNA accumulation.

Chondrogenic potential of skeletal cell populations: selective growth of chondrocytes and their morphogenesis and development in vitro

Most vertebrate embryonic and post-embryonic skeletal tissue formation occurs through the endochondral process in which cartilage serves a transitory role as the anlage for the bone structure. The differentiation of chondrocytes during this process in vivo is characterized by progressive morphological changes associated with the hypertrophy of these cells and is defined by biochemical changes that result in the mineralization of the extracellular matrix. The mechanisms, which, like those in vivo, promote both chondrogenesis in presumptive skeletal cell populations and endochondral progression of chondrogenic cells, may be examined in vitro. The work presented here describes mechanisms by which cells within presumptive skeletal cell populations become restricted to a chondrogenic lineage as studied within cell populations derived from 12-day-old chicken embryo calvarial tissue. It is found that a major factor associated with selection of chondrogenic cells is the elimination of growth within serum-containing medium. Chondrogenesis within these cell populations appears to be the result of permissive conditions which select for chondrogenic proliferation over osteogenic cell proliferation. Data suggest that chondrocyte cultures produce autocrine factors that promote their own survival or proliferation. The conditions for promoting cell growth, hypertrophy, and extracellular matrix mineralization of embryonic chicken chondrocytes in vitro include ascorbic acid supplementation and the presence of an organic phosphate source. The differentiation of hypertrophic chondrocytes in vitro is associated with a 10-15-fold increase in alkaline phosphatase enzyme activity and deposition of mineral within the extracellular matrix. Temporal studies of the biochemical changes coincident with development of hypertrophy in vitro demonstrate that proteoglycan synthesis decreases 4-fold whereas type X collagen synthesis increases 10-fold within the same period. Ultrastructural examination reveals cellular and extracellular morphology similar to that of hypertrophic cells in vivo with chondrocytes embedded in a well formed extracellular matrix of randomly distributed collagen fibrils and proteoglycan. Mineral deposition is seen in the interterritorial regions of the matrix between the cells and is apatitic in nature. These characteristics of chondrogenic growth and development are very similar in vivo and in vitro and they suggest that studies of chondrogenesis in vitro may provide a valuable model for the process in vivo.

Divergent secretory behavior of the opposite ends of aggrecan

The proteoglycan, aggrecan has a globular domain, G1, at the N terminus and a different globular domain, G3, at the C terminus. Aggrecan produced by mutant nanomelic chickens is truncated due to a premature stop codon and consequently lacks G3 and a minor portion of its chondroitin sulfate domain (Li, H., Schwartz, N. B., and Vertel, B. M.(1993) J. Biol. Chem. 268, 23504-23511). The mutant protein is retained in the endoplasmic reticulum and fails to enter the Golgi stacks (Vertel, B. M., Walters, L. M., Grier, B., Maine, N. , and Goetinck, P. F.(1993) J. Cell Sci. 104, 939-948). The homozygous mutant is lethal because of failure of chondrogenesis and osteogenesis, while the heterozygous mutant is dwarfed. To further elucidate the pathogenetic mechanisms underlying nanomelia and to determine if G1 and G3 are themselves secreted, we expressed them in transfected host cells. Expression was performed in wild type Chinese hamster ovary (CHO) cells and in mutant CHO cells which are unable to link glycosaminoglycan (GAG) chains to core proteins. We compared: (a) secretion of expressed G1 and G3 constructs containing contiguous GAG chain consensus sites and (b) GAG chain modification of the secreted proteins. We find that: 1) G3 is 24-100 times more rapidly secreted than G1; 2) secreted G3 contains contiguous chondroitin sulfate GAG chains, while secreted G1 lacks contiguous GAG chains; 3) G3 secretion is not coupled to xylosylation of contiguous GAG chain consensus sites. These results imply that G1 and G3 intrinsically differ in passage through the cell secretory route.

Evidence of a defined spatial arrangement of hyaluronate in the central filament of cartilage proteoglycan aggregates

Aggregates of proteoglycans from the Swarm rat chondrosarcoma reassembled in vitro have been studied by rotary-shadowing electron microscopy, and shown to be similar to native structures that have never been dissociated [Mörgelin, Engel, Heinegård and Paulsson (1992) J. Biol. Chem. 267, 14275-14284]. A hyaluronate with defined chain length (HAshort) has now been prepared by autoclaving high-Mr hyaluronate and fractionation to a narrow size distribution by gel filtration. Proteoglycan monomers, core protein, hyaluronate-binding region and link protein were combined with HAshort. Free chains of HAshort and reconstituted complexes with proteoglycan, link protein and aggrecan fragments were examined by electron microscopy after rotary shadowing. Length measurements showed that the hyaluronate was condensed to about half of its original length on binding intact aggrecan monomers, any aggrecan fragment or link protein alone. This strongly implies that hyaluronate adopts a defined spatial arrangement within the central filament of the aggregate, probably different from its secondary structure in solution. No differences in length were observed between link-free and link-stabilized aggregates.

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.

Transforming growth factor-beta and bone morphogenetic protein-2 act by distinct mechanisms to promote chick limb cartilage differentiation in vitro

A number of studies suggest that several members of the transforming growth factor-beta (TGF-beta) family of peptide growth factors may be involved in the regulation of cartilage differentiation. It has been previously reported that TGF-beta 1 and TGF-beta 2 promote the chondrogenic differentiation of chick limb mesenchymal cells in high density micromass cultures (Kulyk et al. [1989a] Dev. Biol. 135:424-430). In this study we report that chick limb mesenchymal cells express mRNA for chicken TGF-beta 1, TGF-beta 2, and TGF-beta 3 during cartilage differentiation in vitro. In addition, the time course of their expression during cartilage differentiation is consistent with their playing a role in the initiation of this differentiation process. We also report that two members of the TGF-beta family, TGF-beta 3 and bone morphogenetic protein-2 (BMP-2), are capable of promoting the accumulation of cartilage extracellular matrix molecules by differentiating chick limb mesenchymal cells in micromass culture. Significant differences, however, were noted between the specific effects on matrix production elicited by these two growth factors which suggest that they may be acting by distinct mechanisms to regulate cartilage matrix production. TGF-beta appears to be most effective on cells which have not yet undergone cell condensation, a critical event in early cartilage differentiation, whereas BMP-2 is most effective after cells have condensed or differentiated. These observations suggest that TGF-beta 3 and BMP-2 may be acting in a sequential manner to regulate chick limb mesenchymal cells through the different stages of cartilage differentiation.

The cartilage proteoglycan aggregate: assembly through combined protein-carbohydrate and protein-protein interactions

In vitro reassembled aggregates of cartilage proteoglycan (aggrecan) were studied by glycerol spraying/rotary shadowing electron microscopy and compared to the corresponding native (i.e. never dissociated) structures. In both cases a tightly packed central filament structure was observed consisting of the hyaluronate binding region (HABR) of the proteoglycan, link protein (LP) and hyaluronate (HA). This differs from earlier results where a discontinuous central filament structure was seen after spreading proteoglycan aggregates at a water/air interphase. Binding of isolated HABR to HA is random but upon addition of link protein a clustering of the HA-binding proteins is observed, indicating a cooperativity. In a fully saturated aggregate the HA is covered by a continuous protein shell consisting of HABR and LP. When added in amounts below saturation HABR and LP bind to the HA in clusters which are interrupted by free strands of HA. The proteoglycan aggregate is thus an example for a structure where a polysaccharide forms a template for a supramolecular assembly largely stabilized by protein-protein interactions.

Molecular cloning and analysis of the protein modules of aggrecans

The large aggregating chondroitin sulfate proteoglycan of cartilage, aggrecan, has served as a prototype of proteoglycan structure. Molecular cloning has elucidated its primary structure and revealed both known and unknown domains. To date the complete structures of chicken, rat and human aggrecans have been deduced, while partial sequences have been reported for bovine aggrecan. A related proteoglycan, human versican, has also been cloned and sequenced. Both aggrecan and versican have two lectin domains, one at the amino-terminus which binds hyaluronic acid and one at the carboxyl-terminus whose physiological ligand is unknown. Both lectins have homologous counterparts in other types of proteins. Within the aggrecans the keratan sulfate domain may be variably present and also has a prominent repeat in some species. The chondroitin sulfate domain has three distinct regions which vary in their prominence in different species. The complex molecular structure of aggrecans is consistent with the concept of exon shuffling and aggrecans serve as suitable prototypes for comprehending the evolution of multi-domain proteins.

Alternate exon usage is a commonly used mechanism for increasing coding diversity within genes coding for extracellular matrix proteins

Extracellular matrix proteins are a diverse family of secreted proteins and glycoproteins that are responsible for a variety of critical functions in different tissues. A large number of multiexon genes encode these proteins of the extracellular matrix. Over the last few years, it has become evident that the processing of the pre-mRNA from several of these genes involves alternative splicing. This review summarizes the known examples of alternative splicing in genes coding for the extracellular matrix and attempts to relate the increase in coding diversity generated by alternate exon usage to the function(s) of individual extracellular matrix proteins.

Molecular cloning and analysis of the protein modules of aggrecans

The large aggregating chondroitin sulfate proteoglycan of cartilage, aggrecan, has served as a prototype of proteoglycan structure. Molecular cloning has elucidated its primary structure and revealed both known and unknown domains. To date the complete structures of chicken, rat and human aggrecans have been deduced, while partial sequences have been reported for bovine aggrecan. A related proteoglycan, human versican, has also been cloned and sequenced. Both aggrecan and versican have two lectin domains, one at the amino-terminus which binds hyaluronic acid and one at the carboxyl-terminus whose physiological ligand is unknown. Both lectins have homologous counterparts in other types of proteins. Within the aggrecans the keratan sulfate domain may be variably present and also has a prominent repeat in some species. The chondroitin sulfate domain has three distinct regions which vary in their prominence in different species. The complex molecular structure of aggrecans is consistent with the concept of exon shuffling and aggrecans serve as suitable prototypes for comprehending the evolution of multi-domain proteins.

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

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

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.

Molecular cloning of chicken aggrecan. Structural analyses

The large, aggregating chondroitin sulphate proteoglycan of cartilage, aggrecan, has served as a generic model of proteoglycan structure. Molecular cloning of aggrecans has further defined their amino acid sequences and domain structures. In this study, we have obtained the complete coding sequence of chicken sternal cartilage aggrecan by a combination of cDNA and genomic DNA sequencing. The composite sequence is 6117 bp in length, encoding 1951 amino acids. Comparison of chicken aggrecan protein primary structure with rat, human and bovine aggrecans has disclosed both similarities and differences. The domains which are most highly conserved at 70-80% identity are the N-terminal domains G1 and G2 and the C-terminal domain G3. The chondroitin sulphate domain of chicken aggrecan is smaller than that of rat and human aggrecans and has very distinctive repeat sequences. It has two separate sections, one comprising 12 consecutive Ser-Gly-Glu repeats of 20 amino acids each, adjacent to the other which has 23 discontinuous Ser-Gly-Glu repeats of 10 amino acids each; this latter region, N-terminal to the former one, appears to be unique to chicken aggrecan. The two regions contain a total of 94 potential chondroitin sulphate attachment sites. Genomic comparison shows that, although chicken exons 11-14 are identical in size to the rat and human exons, chicken exon 10 is the smallest of the three species. This is also reflected in the size of its chondroitin sulphate coding region and in the total number of Ser-Gly pairs. The putative keratan sulphate domain shows 31-45% identity with the other species and lacks the repetitive sequences seen in the others. In summary, while the linear arrangement of specific domains of chicken aggrecan is identical to that in the aggrecans of other species, and while there is considerable identity of three separate domains, chicken aggrecan demonstrates unique features, notably in its chondroitin sulphate domain and its keratan sulphate domain. Thus different variants of chondroitin sulphate and keratan sulphate domains may have evolved separately to fulfil specific biochemical and physiological functions.

The cartilage proteoglycan deficient mutation, nanomelia, contains a DNA polymorphism in the proteoglycan core protein gene that is genetically linked to the nanomelia phenotype

The avian mutation, nanomelia (nm), is an autosomal recessive embryonic lethal. Homozygous embryos show hypoplasia of the limbs and a parrot-like beak. Biochemical studies have associated this phenotype with the absence of the major cartilage specific proteoglycan core protein (Argraves et al., 1981). Stirpe et al. (1987) demonstrated a reduction in core protein transcripts in nanomelic embryos. Southern analyses did not detect a rearrangement of the core protein gene or a restriction fragment length polymorphism (RFLP) in the core protein gene linked to the nanomelia mutation. These data suggest that the genetic lesion associated with the nanomelia mutation is either a subtle alteration in the core protein gene affecting the biosynthesis of core protein transcript or a defect in a regulatory gene that produces a trans-acting factor requisite for the proper expression of the core protein gene. To distinguish between these two alternative molecular mechanisms for the nanomelia mutation, experiments were conducted to demonstrate genetic linkage or non-linkage of the core protein gene to the nanomelia mutation. Using denaturing gradient gel electrophoresis (DGGE) technology, a DNA polymorphism has been identified at the 3' end of the core protein gene. The polymorphism defines two alleles, one allele is associated with the normal core protein gene, while the other allele always segregates with the nanomelia mutation. These results suggest that the identified DNA polymorphism in the core protein gene is genetically linked to the inheritance of the nanomelic phenotype and the nanomelia mutation contains a lesion in the core protein gene.

Studies of mineralization in tissue culture: optimal conditions for cartilage calcification

The optimal conditions for obtaining a calcified cartilage matrix approximating that which exists in situ were established in a differentiating chick limb bud mesenchymal cell culture system. Using cells from stage 21-24 embryos in a micro-mass culture, at an optimal density of 0.5 million cells/20 microliters spot, the deposition of small crystals of hydroxyapatite on a collagenous matrix and matrix vesicles was detected by day 21 using X-ray diffraction, FT-IR microscopy, and electron microscopy. Optimal media, containing 1.1 mM Ca, 4 mM P, 25 micrograms/ml vitamin C, 0.3 mg/ml glutamine, no Hepes buffer, and 10% fetal bovine serum, produced matrix resembling the calcifying cartilage matrix of fetal chick long bones. Interestingly, higher concentrations of fetal bovine serum had an inhibitory effect on calcification. The cartilage phenotype was confirmed based on the cellular expression of cartilage collagen and proteoglycan mRNAs, the presence of type II and type X collagen, and cartilage type proteoglycan at the light microscopic level, and the presence of chondrocytes and matrix vesicles at the EM level. The system is proposed as a model for evaluating the events in cell mediated cartilage calcification.

Stage-related chondrogenic potential of avian mandibular ectomesenchymal cells

We have examined the in vitro stage-related chondrogenic potential of avian mandibular ectomesenchymal cells using micromass cultures. Our results indicate that mandibular ectomesenchymal cells as early as stage 16, soon after the formation of the mandibular arches and well before the initiation of in vivo chondrogenesis, have chondrogenic potential which is expressed in micromass culture. There is an increase in the total area of the cultures occupied by cartilage when cells from increasing stages of development are used. The nodular pattern of chondrogenesis in these cultures indicates that mandibular ectomesenchymal cells are a heterogenous population from the time of mandibular arch formation. In addition, we studied the temporal expression of the genes for extracellular matrix proteins during in vitro chondrogenesis and correlated the morphological changes with the pattern of gene expression. Low levels of type II collagen mRNA are present in the cultures prior to detection of any stainable cartilage matrix and increase 5 fold just before the onset of chondrogenesis in vitro. On the other hand mRNA for cartilage proteoglycan core protein was not detected until the second day of culture when stainable cartilage matrix was present and progressively increased thereafter. Messenger RNA for type I collagen was present at the time of initiation of cultures and continuously increased during the culture period. Our experiments also indicated that embryonic epithelia can inhibit the in vitro chondrogenesis of mandibular ectomesenchymal cells and that the inhibitory effect of embryonic epithelia is independent of its age and site of origin.

Temporal and spatial expression of genes for cartilage extracellular matrix proteins during avian mandibular arch development

We have examined the temporal expression of genes for extracellular matrix proteins (type I collagen, type II collagen, and the cartilage specific proteoglycan core protein) during the development of the avian mandibular arch. We detected low levels of type II collagen mRNA in the mandibular arch as early as stage 15. Type II collagen mRNA remained low but increased slightly as development progressed from stage 15 to stage 25. More dramatic increases occurred after stage 25 coincident with overt chondrogenesis. In contrast, mRNA for the core protein of cartilage specific proteoglycan was not detected prior to the onset of chondrogenesis, appeared at stage 25, and increased thereafter. Type I collagen mRNA was also present as early as stage 15 and dramatically increased after stage 28/29, coincident with initiation of osteogenesis. Using in situ hybridization, we found that type II collagen mRNA became detectable in the center of the mandible around stage 24/25 coincident with the initiation of chondrogenesis. At later stages (26-32) type II collagen mRNA was localized in the cartilaginous rudiment. The pattern of hybridization observed with the proteoglycan core protein probe at later stages of development was essentially identical to that observed with the type II collagen probe. In contrast, the probe for the alpha 1 (I) collagen mRNA was localized over the perichondrium, over differentiated bone, and in areas within the mandibular arch where bone formation had been initiated.

Type IX collagen gene expression during limb cartilage differentiation

Changes in the steady-state levels of mRNAs for the alpha 1(IX) and alpha 2(IX) polypeptide chains of cartilage-characteristic type IX collagen were examined during the course of chick limb chondrogenesis in vitro and in vivo. Cytoplasmic type IX collagen mRNAs begin to accumulate at the onset of overt chondrogenesis in high density micromass culture coincident with the crucial condensation phase of the process, in which prechondrogenic mesenchymal cells become closely juxtaposed prior to depositing a cartilage matrix. The initiation of type IX collagen mRNA accumulation at condensation coincides with the initiation of accumulation of cartilage proteoglycan core protein mRNA and with a striking increase in type II collagen mRNA accumulation. Following condensation in vitro, there is a concomitant progressive increase in cytoplasmic type IX collagen, core protein, and type II collagen mRNA levels which parallels the progressive accumulation of cartilage matrix. Type IX collagen mRNAs also begin to accumulate at the initiation of overt chondrogenesis in vivo in the chondrogenic central core of the developing limb bud. In contrast, little, or no type IX collagen mRNAs are detectable in the nonchondrogenic peripheral regions of the developing limb bud.

A tetranectin-related protein is produced and deposited in extracellular matrix by human embryonal fibroblasts

Tetranectin is a tetrameric human plasma protein that binds to plasminogen kringle 4. Its amino acid sequence is homologous with the C-terminal parts of asialoglycoprotein receptors and proteoglycan core proteins. In the present study, we have demonstrated that the human embryonal fibroblast cell line WI-38 produce a tetranectin-related molecule, which might, by several criteria, be similar to tetranectin from plasma. These criteria include immunoblotting analysis of conditioned cell medium revealing a protein band with Mr 17,000, indistinguishable from the Mr of plasma tetranectin. A preparation obtained by purification of conditioned medium by affinity chromatography on an anti-(plasma tetranectin) IgG column also contained the Mr 17,000 protein. This protein (partly purified from the conditioned medium) was shown by crossed immunoelectrophoresis to bind to heparin, CaCl2 and plasminogen kringle 4, as previously described for tetranectin in plasma. Importantly, this tetranectin-related protein is not only present in conditioned culture medium, but the Mr 17,000 protein reacting with anti-(plasma tetranectin) IgG was also present in the extracellular material, remaining after removal of WI-38 cells from the culture dishes, as demonstrated by immunoblotting analysis and immunocytochemical staining. We conclude that WI-38 cells produce a tetranectin-related protein and secrete it into the extracellular matrix.

Gene expression and extracellular matrix ultrastructure of a mineralizing chondrocyte cell culture system

Conditions were defined for promoting cell growth, hypertrophy, and extracellular matrix mineralization of a culture system derived from embryonic chick vertebral chondrocytes. Ascorbic acid supplementation by itself led to the hypertrophic phenotype as assessed by respective 10- and 15-fold increases in alkaline phosphatase enzyme activity and type X synthesis. Maximal extracellular matrix mineralization was obtained, however, when cultures were grown in a nutrient-enriched medium supplemented with both ascorbic acid and 20 mM beta-glycerophosphate. Temporal studies over a 3-wk period showed a 3-4-fold increase in DNA accompanied by a nearly constant DNA to protein ratio. In this period, total collagen increased from 3 to 20% of the cell layer protein; total calcium and phosphorus contents increased 15-20-fold. Proteoglycan synthesis was maximal until day 12 but thereafter showed a fourfold decrease. In contrast, total collagen synthesis showed a greater than 10-fold increase until day 18, a result suggesting that collagen synthesis was replacing proteoglycan synthesis during cellular hypertrophy. Separate analysis of individual collagen types demonstrated a low level of type I collagen synthesis throughout the 21-d time course. Collagen types II and X synthesis increased during the first 2 wk of culture; thereafter, collagen type II synthesis decreased while collagen type X synthesis continued to rise. Type IX synthesis remained at undetectable levels throughout the time course. The levels of collagen types I, II, IX, and X mRNA and the large proteoglycan core protein mRNA paralleled their levels of synthesis, data indicating pretranslational control of synthesis. Ultrastructural examination revealed cellular and extracellular morphology similar to that for a developing hypertrophic phenotype in vivo. Chondrocytes in lacunae were surrounded by a well-formed extracellular matrix of randomly distributed collagen type II fibrils (approximately 20-nm diam) and extensive proteoglycan. Numerous vesicular structures could be detected. Cultures mineralized reproducibly and crystals were located in extracellular matrices, principally associated with collagen fibrils. There was no clear evidence of mineral association with extracellular vesicles. The mineral was composed of calcium and phosphorus on electron probe microanalysis and was identified as a very poorly crystalline hydroxyapatite on electron diffraction. In summary, these data suggest that this culture system consists of chondrocytes which undergo differentiation in vitro as assessed by their elevated levels of alkaline phosphatase and type X collagen and their ultrastructural appearance.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of interleukin-1 and insulin like growth factor-1 on the release of proteoglycan components and hyaluronan from pig articular cartilage in explant culture

The turnover of proteoglycans was investigated in articular cartilage in explant culture by analysing the components released into the culture medium. The effect of IL-1 alpha on the release of fragments derived from different proteoglycan domains and hyaluronan (HA) was determined over 4 days in culture. The effect of IGF-1 (100 ng/ml) on matrix degradation of proteoglycan and its ability to inhibit the effects of IL-1 (10 ng/ml) was also assessed. The rate of release of G1 and G2 globular domains of proteoglycans into the culture medium was determined by radioimmunoassay. In unstimulated control cartilage there was a greater release of proteoglycan G2 domain than of G1 domain suggesting that cleavage occurred between them and that some G1 was preferentially retained bound in the matrix. Compared with control cartilage IL-1 stimulated the release of all proteoglycan components and hyaluronan. IL-1 had a greater effect on the release of G1 than on G2 domain, but also resulted in some net loss of these proteins (approximately 45% as detected in the immunoassays). In explants treated with both IL-1 and IGF-1 there was much less release of proteoglycan fragments and evidence for less extensive degradation. IGF-1 was particularly affective in preventing any increase in HA release and also preventing the apparent loss of G1 and G2 domains. It also partially inhibited the release of G1 and G2 domains and the sulphated glycosaminoglycan fragments. IGF-1 was therefore an effective antagonist of IL-1 action on cartilage. It is not known at what level it blocks the chondrocyte response to IL-1, but it clearly results in the suppression of matrix degradative activity.

Immunological detection of the EGF-like domain of the core proteins of large proteoglycans from human and baboon cartilage

Recent data from the literature have shown that cDNA clones for the carboxyterminal domain of the core protein of large proteoglycan monomers from human cartilage contain an EGF-like domain, which appears to undergo alternative splicing. In the present study we have found that articular proteoglycans from human and baboon separated on agarose flat-bed gels and blotted onto nitrocellulose react with a rabbit antiserum to mouse EGF. In addition both forms of the proteoglycans (band I and band II) seen on these gels are reactive. Reactivity is seen with proteoglycans extracted from human articular cartilage of various ages (fetal, newborn, young and aged) and with proteoglycans extracted from cartilage of thanatophoric dysplasia and homozygous achondroplasia. Reactivity is dependent on prior digestion of the nitrocellulose blot with Chase ABC, suggesting masking of epitope by chondroitin sulfate. Reactivity of the EGF antiserum with cartilage proteoglycan core protein was also demonstrated in an ELISA system with core protein as coating antigen. The reactivity appears to reside in a tryptic peptide generated from Chase/keratanase digested core protein. The immunoreactive species migrates as a 68 KDa species on gradient gels. Immunological detection and quantitative analysis of the EGF-like domain could be useful for analysis of various proteoglycan samples.

Proteoglycan core protein and type II collagen gene expressions are not correlated with cell shape changes during low density chondrocyte cultures

Chondrocytes isolated from chicken embryo sterna were cultivated in low density monolayer cultures to induce their dedifferentiation. At different stages of the long-term cultures, changes in expression of a cartilage-specific sulfated proteoglycan and cartilage-characteristic type II collagen have been examined and related to the shape change of cells using in situ hybridization and immunocytochemistry. At the beginning of the culture, all cells exhibit a round shape and express the cartilage phenotype. Then, during the course of the culture, chondrocytes flatten and become fibroblast-like, but this morphological modification does not start for all the cells at the same time. Interestingly, the loss of cartilage proteoglycan or type II collagen expression did not occur for all polygonal or fibroblast-like cells. Moreover, we observed a variability in the steady state levels of RNA or protein accumulation among chondrocytes exhibiting a similar shape, as judged by the intensity of hybridization signal or immunofluorescence over the cells. These observations support the hypothesis that the shape change does not have a causative role in the chondrocyte phenotype expression, but is rather a secondary effect of the dedifferentiation process. Furthermore, the disappearance of hybridizable core protein or type II collagen mRNA during the dedifferentiation process was coincident with the disappearance of the proteins for which they code as detected by immunohistochemical staining. This suggest that core protein and type II collagen gene expressions are controlled primarily at the transcriptional level in long-term chondrocyte cultures.

Characterization of the human gene that encodes the peptide core of secretory granule proteoglycans in promyelocytic leukemia HL-60 cells and analysis of the translated product

Based upon the deduced amino acid sequence of a cDNA (cDNA-H4) that had been proposed to encode the peptide core of an eosinophil and a HL-60 cell secretory granule proteoglycan, a 16-amino acid peptide was synthesized. This peptide was then used to elicit rabbit antibodies for study of the translation and post-translational modification of this gene product in hematopoietic cells. When HL-60 cells were radiolabeled for 2 min with [35S]methionine, a protein that migrated in a sodium dodecyl sulfate-polyacrylamide electrophoresis gel with a Mr of 20,000 was immunoprecipitated with the IgG fraction of the anti-peptide serum. Kinetic experiments revealed that within 10 min this radiolabeled precursor protein was converted in HL-60 cells into an Mr approximately 150,000 chondroitin sulfate proteoglycan intermediate. After a 20-min to 1-h chase, this [35S]methionine- or [35S]sulfate-labeled proteoglycan intermediate lost its antigenicity, presumably due to proteolysis of its N terminus. A human genomic library was probed under conditions of high stringency with cDNA-H4 to isolate genomic clones that contain the gene that encodes this proteoglycan peptide core. This gene spans approximately 15 kilobases and consists of three exons. The first exon encodes the 5'-untranslated region of the mRNA transcript, as well as the entire 27-amino acid signal peptide of the translated molecule. The second exon encodes a 49-amino acid region of the peptide core, predicted to be the N terminus of the molecule after its proteolytic processing in the endoplasmic reticulum. The third exon encodes the remainder of the molecule, including its glycosaminoglycan attachment, serine-glycine repeat region. As assessed by S1 nuclease mapping and primer extension analysis, the transcription-initiation site in HL-60 cells for this gene resides 53 base pairs upstream from the translation-initiation site.

In situ cross-linking of cartilage proteoglycans

Although the in vitro interactions between purified cartilage matrix components have been studied extensively, little is known about these interactions in situ. In this study, cartilage was treated with a cross-linking reagent with a span of 1.2 nm between its reactive terminal groups in order to preserve the native relationships between closely associated matrix components throughout extraction, purification, and preparation for electron microscopy. After in situ cross-linking, electron microscopy and gel chromatography revealed that about one-half of the guanidine hydrochloride extractable proteoglycans were polymeric, usually with two to five proteoglycan subunits in each polymer. Cross-linking consistently involved the thin segments of the proteoglycan subunits. Some of the proteoglycan polymers were capable of binding hyaluronic acid and were parts of aggregates under associative conditions. SDS-polyacrylamide gel electrophoresis revealed that link proteins were present within the polymers, and studies in which purified proteoglycans were cross-linked in vitro confirmed that the link proteins increased the proportion of polymeric proteoglycans. These findings suggest that individual proteoglycans within cartilage have intimate associations with other proteoglycans that are mediated by link proteins.

Interaction of tetranectin with sulphated polysaccharides and trypan blue

Tetranectin is a recently discovered plasminogen binding protein from human plasma, with a known primary structure. Its interactions with various sulphated polysaccharides and trypan blue has been investigated by crossed immunoelectrophoresis, immunosorbent assay (ELISA) and gel filtration experiments. Interaction of tetranectin with chondroitin sulphate A, B, and C, heparan sulphate and trypan blue could be demonstrated by crossed immunoelectrophoresis against monospecific rabbit anti-tetranectin. Interaction of tetranectin with fucoidan could be demonstrated by all three methods, and this interaction could be prevented by heparin and to a lesser degree by chondroitin sulphate A and C, but not by fucose. These findings, together with the recently published findings, that tetranectin is present in exocrine and endocrine glands as well as in secreting surface epithelia and mesenchyme, suggest that tetranectin, in conjunction with proteoglycans, may have a function in the packaging of granules or as a participant in exocytocic processes.

Abnormal ambient glucose levels inhibit proteoglycan core protein gene expression and reduce proteoglycan accumulation during chondrogenesis: possible mechanism for teratogenic effects of maternal diabetes

Using a tissue culture system based on a nearly pure population of avian precartilage mesenchymal cells, we have found that ambient glucose levels as little as 50% lower, or 100% higher, than normally present in embryonic sera are deleterious to cartilage development, as measured by the accumulation of highly sulfated proteoglycan and the corresponding cartilage-specific chondroitin sulfate core protein mRNA. Abnormal glucose concentrations in the ranges studied did not selectively influence cell replication, and the effects on chondrogenesis were not due to differences in overall protein synthesis or glucose utilization in the treatment groups. Core protein gene expression was more severely affected than accumulation of extracellular product, suggesting the existence of posttranscriptional compensatory mechanisms. The sensitivity to ambient glucose levels of both expression of the cartilage-specific chondroitin sulfate core protein gene and the accumulation of the corresponding extracellular matrix macromolecules during chondrogenesis suggest a molecular mechanism for the well-known adverse effect of maternal diabetes on embryonic skeletogenesis. The results further suggest that hypoglycemia resulting from stringent control of diabetes may also be deleterious to skeletal development.