A large chondroitin sulfate proteoglycan (PG-M) synthesized before chondrogenesis in the limb bud of chick embryo

Chondroitin sulfate proteoglycans of the endothelia of human umbilical vein and arteries and assessment for the adherence of Plasmodium falciparum-infected erythrocytes

Infection with Plasmodium falciparum during pregnancy leads to chondroitin 4-sulfate-mediated adhesion of the infected red blood cells (IRBCs) in the placenta, causing severe health complications to fetus and the mother. The IRBCs are also frequently found in low density in the umbilical cord of infected placentas. In this study, the CSPGs of umbilical vein and arteries were purified, characterized, and their localization and IRBC-binding abilities were studied. While a versican type CSPG was found both in the vein and arteries, a serglycin type CSPG was present exclusively in the vein. The CSPGs were present at significant level on the endothelial surface of the umbilical vein but not on that of arteries. Although the purified versican and serglycin type CSPGs could bind IRBCs, their binding abilities were significantly less compared to the low sulfated CSPGs of the placenta because of the predominance of 6-sulfated disaccharide moieties in the CS chains. Therefore, IRBCs were unable to bind efficiently onto the umbilical cord endothelial surface. Unexpectedly, however, the IRBCs adhered densely in the blood vessels of fetal villi in the placental tissue sections and sparingly in the blood spaces of the umbilical cord vein, presumably because the CSPG that can efficiently bind IRBCs is present at high levels in the fetal blood vessels and at very low levels in the umbilical cord blood vessels. Since the C4S-adherent IRBCs that enter the fetal blood vessels cannot adhere to the cord endothelial surface and parasites cannot efficiently grow due to fetal hemoglobin toxicity and protection by maternal antibodies, transplacental infection may be quickly cleared without clinical episodes.

Versican G3 domain enhances cellular adhesion and proliferation of bovine intervertebral disc cells cultured in vitro

The functional role of versican in influencing intervertebral disc cell adhesion and proliferation was analyzed in bovine intervertebral disc. We have previously demonstrated the C-terminal globular G3 (or selectin-like) domain of versican to influence mesenchymal chondrogenesis and fibroblast proliferation in vitro. For this study, a versican G3 expression construct was generated to examine the role of the G3 domain of versican. Nucleus pulposus and annulus fibrosus cells were isolated from adult bovine caudal discs using sequential enzymatic digestion and versican expression characterized by RT-PCR. In cell proliferation assays, we observed that there was greater cellular proliferation in the presence of versican G3 for both disc cell types. The higher proliferation rate of annulus fibrosus cells when compared to nucleus pulposus cells seeded in monolayer supports heterogeneity of intervertebral disc cell populations. The presence of versican G3 construct enhanced the adhesion of isolated nucleus pulposus and annulus fibrosus cells approximately 4 to 6 fold, respectively. Cellular adhesion was greater in the presence of versican G3 in a dose dependent manner. G3 product was purified using affinity columns, and the purified G3 also enhanced cell adhesion.

G3 domains of aggrecan and PG-M/versican form intermolecular disulfide bonds that stabilize cell-matrix interaction

The extracellular matrix plays a critical role in maintaining tissue integrity. Among the matrix molecules, the large aggregating chondroitin sulfate proteoglycans are the major structural molecules and are the primary contributors to the stability for some tissues such as cartilage. The notable exceptions are nanomelic cartilage and arthritic cartilage: the former contains a point mutation leading to a stop codon before translating to the C-terminal G3 domain; the latter contains a large proportion of aggrecan from which the G3 domain has been cleaved. These phenomena suggest that the G3 domain may be important in cartilage stability. Here, we demonstrated for the first time that the G3 domains of aggrecan and another proteoglycan, PG-M/versican, formed intermolecular disulfide bonds, and all subdomains were involved. Further studies indicated that each of the 10 cysteine residues of the aggrecan G3 domain could potentially form intermolecular disulfide bonds in vitro. The disulfide bonds were disrupted in the presence of reducing reagent beta-mercaptoethanol and dithiothreitol. As a result, normal chondrocyte-matrix interaction was disrupted, and the structure of the extracellular matrix was altered. Furthermore, disruption of disulfide bonds also reduced the role of PG-M/versican G3 domain in mediating cell adhesion. Our study provides strong evidence of the importance of proteoglycan interactions through intermolecular disulfide bonds in cartilage firmness and cell-matrix stability.

Phytoestrogen genistein acts as an estrogen agonist on human osteoblastic cells through estrogen receptors alpha and beta

Genistein, a soybean isoflavone, has estrogen-like activity in mammals, including the prevention of bone loss. However, whether its mechanism of action on bone turnover is distinct from that of estrogen or raloxifene is unknown. Although genistein has been reported to bind both estrogen receptor (ER) isoforms (alpha and beta), little is known concerning differential activation of gene expression via these ER isoforms. To examine this question, comparison of the responses of normal fetal osteoblast (hFOB) cells stably expressing either ERalpha (hFOB/ERalpha9) or ERbeta (hFOB/ERbeta6), to treatment with genistein, 17beta-estradiol (E(2)) or raloxifene were conducted. In hFOB/ERalpha9 cells, both genistein and E(2) increased the endogenous gene expression of the progesterone receptor (PR), the proteoglycan versican, and alkaline phosphatase (AP), but inhibited osteopontin (OP) gene expression and interleukin-6 (IL-6) protein levels. Raloxifene had no effect on these bone markers. Genistein, but not raloxifene, also mimicked E(2) action in the hFOB/ERbeta6 cells increasing PR gene expression and inhibiting IL-6 production. To determine whether the gene regulatory actions of genistein in human osteoblast cells occur at the level of transcription, its action on the transcriptional activity of a PR-A promoter-reporter construct was assessed. Both genistein and E(2) were found to stimulate the PR promoter in the hFOB cell line when transiently co-transfected with either ERalpha or ERbeta. Whereas hFOB cell proliferation was unaffected by E(2), raloxifene or genistein at low concentrations, higher concentrations of genistein, displayed significant inhibition. Together, these findings demonstrate that genistein behaves as a weak E(2) agonist in osteoblasts and can utilize both ERalpha and ERbeta.

Hyaluronan and CD44: strategic players for cell-matrix interactions during chondrogenesis and matrix assembly

Embryonic induction, soluble and insoluble factors, receptors, and signal transduction are orchestrated for the morphogenesis of the cartilage elements. The interaction of cells with the extracellular matrix (ECM) may lead to altered cellular response to morphogens based on the formation of new adhesive contacts, or the uncoupling of cell-matrix interactions. Hyaluronan's influence on cell behavior, and its intimate association with cells are accomplished by a wide variety of specific binding proteins for hyaluronan. The temporal expression of the hyaluronan receptor CD44 (which is expressed as several alternatively spliced variants) may be strategic to many of these cell-matrix interactions during chondrogenesis. CD44 expression is temporally coincident with the reduction of intercellular spaces at the regions of future cartilage deposition. The spatial organization of CD44 at the cell surface may function to establish or regulate the structure of the pericellular matrix dependent on a hyaluronan scaffold. As the ECM is modified during embryogenesis, the cellular response to inductive signals may be altered. An uncoupling of chondrocyte-hyaluronan interaction leads to chondrocytic chondrolysis. Thus, consideration of cell-matrix interactions during chondrogenesis, in the light of our current understanding of the temporal and spatial expression of signaling morphogens, should become a promising focus of future research endeavors.

Distribution of PG-M/versican variants in human tissues and de novo expression of isoform V3 upon endothelial cell activation, migration, and neoangiogenesis in vitro

We have carried out a comprehensive molecular mapping of PG-M/versican isoforms V0-V3 in adult human tissues and have specifically investigated how the expression of these isoforms is regulated in endothelial cells in vitro. A survey of 21 representative tissues highlighted a prevalence of V1 mRNA; demonstrated that the relative frequency of expression was V1 > V2 > V3 >or= V2; and showed that <15% of the tissues transcribed significant levels of all four isoforms. By employing novel and previously described anti-versican antibodies we verified a ubiquitous versican deposition in normal and tumor-associated vascular structures and disclosed differences in the glycanation profiles of versicans produced in different vascular beds. Resting endothelial cells isolated from different tissue sources transcribed several of the versican isoforms but consistently failed to translate these mRNAs into detectable proteoglycans. However, if stimulated with tumor necrosis factor-alpha or vascular endothelial growth factor, they altered their versican expression by de novo transcribing the V3 isoform and by exhibiting a moderate V1/V2 production. Induced versican synthesis and de novo V3 expression was also observed in endothelial cells elicited to migrate in a wound-healing model in vitro and in angiogenic endothelial cells forming tubule-like structures in Matrigel or fibrin clots. The results suggest that, independent of the degree of vascularization, human adult tissues show a limited expression of versican isoforms V0, V2, and V3 and that endothelial cells may contribute to the deposition of versican in vascular structures, but only following proper stimulation.

Annexin 6 is a putative cell surface receptor for chondroitin sulfate chains

Chondroitin sulfate proteoglycans, including PG-M/versican, inhibit cell-substratum adhesion. They achieve this through their chondroitin sulfate chains. In order to define the molecular mechanism for this inhibition, we investigated the influence of these chains on cell attachment to substratum,the first step in cell adhesion. Chondroitin sulfate chains did not prevent cell attachment. In fact, a variety of cells attached to chondroitin sulfate,implying the existence of putative receptors and/or binding proteins for this extracellular matrix glycosaminoglycan. Detergent-extracted human fibroblast membrane protein extracts were examined by affinity chromatography in the presence of Ca2+ on chondroitin sulfate immobilized on agarose CL-6B. A 68 kDa and a 35 kDa protein were isolated, sequenced and demonstrated to be annexin 6 and annexin 4, respectively. Next we used A431 cells devoid of annexin 6 expression to verify that annexin 6 is the receptor for this glycosaminoglycan. We confirmed that A431 cells were unable to attach to the chondroitin sulfate substratum and that the stable transfectants expressing annexin 6 conferred the ability to attach to chondroitin sulfate chains. Further, the presence of annexin 6 on the cell surface was confirmed by fluorescence-activated cell sorting analysis using the annexin 6 antibody;annexin 4 is not present on the cell surface. In summary, annexin 6 is a candidate receptor for chondroitin sulfate chains.

Molecular cloning and characterization of chick sialoprotein associated with cones and rods, a developmentally regulated glycoprotein of interphotoreceptor matrix

MY-174 is an IgM class monoclonal antibody originally established against chick PG-M/versican. The antibody specifically stains the photoreceptor layer, where we recently reported an absence of PG-M/versican. In this study, we re-characterized the antibody and identified the molecule that reacts to MY-174 at the photoreceptor layer. Immunohistochemistry localized the antigen to the matrix surrounding photoreceptors. A variety of glycosidase digestions showed that the antigen is the 150-kDa glycoprotein that has sialylated N- and O-linked glycoconjugates having a molecular mass of more than 30-kDa. The peptide sequences obtained from purified MY-174 antigen showed we had sequenced a full-length cDNA with an open reading frame of 2787 base pairs, encoding a polypeptide of 928 amino acids, with 56 and 54% identities to human and mouse sialoprotein associated with cones and rods (SPACRs), respectively, and with the structural features observed in SPACRs. The specific sialylated O-glycoconjugates here are involved in the epitope structure for MY-174. SPACR first appeared by embryonic days 15-16, and expression increased with developmental age, paralleling the adhesion between neural retina and retinal pigment epithelium. Thus, we concluded that the MY-174 antigen at the photoreceptor layer, a developmentally regulated glycoprotein, is identical to chick SPACR and may be involved in a novel system mediating adhesion between neural retina and retinal pigment epithelium.

Biosynthesis of versican by rat dental pulp cells in culture

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

beta 1-Integrin-mediated glioma cell adhesion and free radical-induced apoptosis are regulated by binding to a C-terminal domain of PG-M/versican

Integrins are cell-surface glycoproteins that mediate cell activities, including tissue morphogenesis, development, immune response, and cancer, through interaction with extracellular proteins. Here we report a novel means by which integrin signaling and functions are regulated. In pull-down assays and immunoprecipitation, beta(1)-integrin bound to the C-terminal domain of PG-M/versican, an extracellular chondroitin sulfate proteoglycan. This was confirmed by cell-surface binding assays. Binding was calcium- and manganese-dependent. Upon native gel electrophoresis, beta(1)-integrin comigrated with the C-terminal domain of PG-M/versican. The interaction of beta(1)-integrin with the C-terminal domain of PG-M/versican activated focal adhesion kinase, enhanced integrin expression, and promoted cell adhesion. As a result, cells expressing the C-terminal domain of PG-M/versican were resistant to free radical-induced apoptosis. As the PG-M/versican peptide used in this study does not contain the RGD consensus-binding motif for integrins, the mechanism of the observed binding represents an entirely new function.

Chondrodysplasias due to proteoglycan defects

The proteoglycans, especially the large chondroitin sulfate proteoglycan aggrecan, have long been viewed as important components of the extracellular matrix of cartilage. The drastic change in expression during differentiation from mesenchyme to cartilage, the loss of tissue integrity associated with proteoglycan degradation in several disease processes and, most important, the demonstration of abnormalities in proteoglycan production concomitant with the aberrant growth patterns exhibited by the brachymorphic mouse, the cartilage matrix deficient mouse, and the nanomelic chick provide the strongest evidence that the proteoglycan aggrecan is essential during differentiation and for maintenance of the skeletal elements. More recently, mutations associated with proteoglycans other than aggrecan, especially the heparan sulfate proteoglycans, glypican and perlecan, suggest an important role for these molecules in skeletal development as well. This review focuses on the molecular bases of the hereditary proteoglycan defects in animal models, as well as of some human chondrodysplasias, that collectively are providing a better understanding of the role of proteoglycans in the development and maintenance of the skeletal elements.

Versican interacts with fibrillin-1 and links extracellular microfibrils to other connective tissue networks

Fibrillin-containing microfibrils are polymeric structures that are difficult to extract from connective tissues. Proteolytic digestion of tissues has been utilized to release microfibrils for study. Few of the molecules that connect microfibrils to other elements in the matrix have been identified. In this study, electron microscopic immunolocalization of anti-versican antibodies in tissues and in extracted microfibrils demonstrated that the C-terminal region of versican is found associated with fibrillin microfibrils. Extraction of microfibrils followed by treatment of microfibrils under dissociating conditions suggested that the versican C terminus is covalently bound to microfibrils. Binding assays using recombinant fibrillin-1 polypeptides and recombinant lectican lectin domains indicated that the versican lectin domain binds to specific fibrillin-1 polypeptides. The versican lectin domain also bound to molecules comigrating with authentic fibrillin-1 monomers in an assay using cell culture medium. In assays using microfibrils, the versican lectin domain demonstrated preferential binding compared with other lecticans. Binding was calcium-dependent. The binding site for versican in microfibrils is most likely within a region of fibrillin-1 between calcium-binding epidermal growth factor-like domains 11 and 21. Human mutations in this region can result in severe forms of the Marfan syndrome ("neonatal" Marfan syndrome). The connection between versican and fibrillin microfibrils may be functionally significant, particularly in cardiovascular tissues.

Identification of the motif in versican G3 domain that plays a dominant-negative effect on astrocytoma cell proliferation through inhibiting versican secretion and binding

This study was designed to investigate the mechanisms by which mutant versican constructs play a dominant-negative effect on astrocytoma cell proliferation. Although a mini-versican or a versican G3 construct promoted growth of U87 astrocytoma cells, a mini-versican lacking epidermal growth factor (EGF) motifs (versicanDeltaEGF) and a G3 mutant (G3DeltaEGF) exerted a dominant-negative effect on cell proliferation. G3DeltaEGF-transfected cells formed smaller colonies, arrested cell cycle at G(1) phase, inhibited expression of cell cycle proteins cdk4 and cyclin D1, and contained multiple nucleoli. In cell surface binding assays, G3 products expressed in COS-7 cells and bacteria bound to U87 cell surface. G3DeltaEGF products exhibited decreased binding activity, but higher levels of G3DeltaEGF products were able to inhibit the binding of G3 to the cell surface. G3DeltaEGF expression inhibited secretion of endogenous versican in astrocytoma cells and also inhibited the secretion of mini-versican in COS-7 cells co-transfected with the mini-versican and G3DeltaEGF constructs. The effect seems to depend on the expression efficiency of G3DeltaEGF, and it occurred via the carbohydrate recognition domain.

Age differences in immunohistochemical localizations of large proteoglycan, PG-M/versican, and small proteoglycan, decorin, in the dermis of rats

Proteoglycans were localized immunohistochemically in the dermis of Donryu rats, using monoclonal antibodies raised against large proteoglycan (PG-M/versican) and small proteoglycan (decorin). The localizations of these proteoglycans in the dermis were compared between young rats (22-day old) and old ones (24 or 30 months of age), and distinct age differences were observed. In the young dermis, PG-M/versican was observed to be abundant in almost all fibroblastic cells (both cytoplasm and cell processes) whose cellularity was very rich compared with the dermis of old rats. Decorin was only faintly visible in the interstitial fibrous elements of young dermis. In the old dermis, however, decorin was distinctly detected on the fibrous elements, which were diffusely distributed as a fibrous network, and likewise PG-M/versican was visible in only a few fibrous elements which seemed to be the fine processes of fibroblastic cells. In the border layer between epidermis and dermis as well as the basal layer surrounding hair follicles, both large and small proteoglycans could be observed in old dermis. Since decorin, which was abundant in old dermis, has been found to have a growth inhibitory effect, it is conceivable that decorin may be one of the Cell Growth Inhibitory Factors in aging as proposed by Tauchi et al. [17, 18].

Versican modulates embryonic chondrocyte morphology via the epidermal growth factor-like motifs in G3

This investigation was designed to characterize the effect of the extracellular matrix molecule versican on chondrocyte morphology, using the well-studied chondrocyte cell culture system. When cultured chondrocytes reverted or "dedifferentiated" to a fibroblast-like morphology, we found that versican expression was significantly enhanced. Transfection of chondrocytes, isolated from embryonic chicken sterna, with a chicken miniversican construct accelerated the reversion process, while expression of an antisense construct inhibited it. A mutant miniversican lacking two epidermal growth factor-like motifs (versicanDeltaEGF) promoted differentiation, as shown by morphological changes and changes in the expression of other extracellular matrix molecules. A truncated versican mutant, the G3DeltaEGF, a G3 domain lacking its two epidermal growth factor-like motifs, also enhanced differentiation. This effect is related to G3DeltaEGF-induced change in cytoskeleton, since transfected cells exhibited misassembly of actin filaments. This article thus provides the first evidence that versican modulates chondrocyte morphology via changes in cytoskeletal structure, and may imply that extracellular matrix molecules play an important role in cell differentiation.

The proteoglycans aggrecan and Versican form networks with fibulin-2 through their lectin domain binding

Aggrecan, versican, neurocan, and brevican are important components of the extracellular matrix in various tissues. Their amino-terminal globular domains bind to hyaluronan, but the function of their carboxyl-terminal globular domains has long remained elusive. A picture is now emerging where the C-type lectin motif of this domain mediates binding to other extracellular matrix proteins. We here demonstrate that aggrecan, versican, and brevican lectin domains bind fibulin-2, whereas neurocan does not. As expected for a C-type lectin, the interactions are calcium-dependent, with K(D) values in the nanomolar range as measured by surface plasmon resonance. Solid phase competition assays with previously identified ligands demonstrated that fibulin-2 and tenascin-R bind the same site on the proteoglycan lectin domains. Fibulin-1 has affinity for the common site on versican but may bind to a different site on the aggrecan lectin domain. By using deletion mutants, the interaction sites for aggrecan and versican lectin domains were mapped to epidermal growth factor-like repeats in domain II of fibulin-2. Affinity chromatography and solid phase assays confirmed that also native full-length aggrecan and versican bind the lectin domain ligands. Electron microscopy confirmed the mapping and demonstrated that hyaluronan-aggrecan complexes can be cross-linked by the fibulins.

Characterization of proteoglycans of human placenta and identification of unique chondroitin sulfate proteoglycans of the intervillous spaces that mediate the adherence of Plasmodium falciparum-infected erythrocytes to the placenta

In pregnant women infected with Plasmodium falciparum, the infected red blood cells (IRBCs) selectively accumulate in the intervillous spaces of placenta, leading to poor fetal outcome and severe health complications in the mother. Although chondroitin 4-sulfate is known to mediate IRBC adherence to placenta, the natural receptor has not been identified. In the present study, the chondroitin sulfate proteoglycans (CSPGs) of human placenta were purified and structurally characterized, and adherence of IRBCs to these CSPGs investigated. The data indicate that the placenta contains three distinct types of CSPGs: significant quantities of uniquely low sulfated, extracellular CSPGs localized in the intervillous spaces, minor amounts of two cell-associated CSPGs, and major amounts of dermatan sulfate-like CSPGs of the fibrous tissue. Of the various CSPGs isolated from the placenta, the low sulfated CSPGs of the intervillous spaces most efficiently bind IRBCs. Based on IRBC adherence capacities and localization patterns of various CSPGs, we conclude that the CSPGs of the intervillous spaces are the receptors for placental IRBC adherence. The identification and characterization of these CSPGs provide a valuable tool for understanding the precise molecular interactions involved in placental IRBC adherence and for the development of therapeutic strategies for maternal malaria. In the accompanying paper (Alkhalil, A., Achur, R. N., Valiyaveettil, M., Ockenhouse, C. F., and Gowda, D. C. (2000) J. Biol. Chem. 275, 40357-40364), we report the structural requirements for the IRBC adherence.

Cell specific-chondroitin sulfate proteoglycan expression during CNS morphogenesis in the chick embryo

There is increasing evidence that proteoglycans, particularly chondroitin sulfate proteoglycans (CSPGs), are integral components in the assembly of the extracellular matrix during early stages of histogenesis. The differential expression of several CSPGs in the developing CNS has raised questions on their origin, phenotype (chemical and structural characteristics), regulation of expression and function. The S103L monoclonal antibody has been an invaluable specific reagent to identify and study a large and abundant CSPG in embryonic chick brain. In the present study we demonstrate that during embryogenesis of the chick CNS, the S103L CSPG (B-aggrecan) is synthesized by neurons of all major neuronal cell types but not by astrocytes, is developmentally regulated, and is associated predominantly with neuronal somata, suggesting that neuronal-specific regulatory mechanisms control the expression of the S103L CSPG in culture. Neurons also exhibit differential expression of glycosaminoglycan type (i.e., KS) and sulfation patterns on different CSPGs when compared to astrocytes, meningial cells or chondrocytes, implying the existence of additional, cell type-specific modes of regulation of the final CSPG phenotype (chemical and structural posttranslational characteristics). A specific temporal pattern of expression of the S103L-CSPG was observed which may contribute to conditions that induce or stabilize specific cell phenotypes during CNS development. In contrast, the other major CSPG in the CNS recognized by the HNK-1 antibody, is synthesized by all cell types of different cell lineages over the entire embryonic period, suggesting a more global cell maintenance function for this CSPG.

Tenascin-C induced stimulation of chondrogenesis is dependent on the presence of the C-terminal fibrinogen-like globular domain

The relationship between structure of tenascin-C (Tn-C), a multi-domain extracellular matrix protein, and its stimulation of chondrogenesis was examined using recombinant Tn-C isoforms (full length or with specific domains deleted) as substrata for undifferentiated chicken mesenchymal cells. Of the Tn-C variants tested, only Tn-C lacking the fibrinogen-like domain or Tn-C comprised solely of fibrinogen-like domains failed to stimulate chondrogenesis. The ability of variants to stimulate chondrogenesis was not dependent on their ability to support adhesion or stimulate proliferation. These results demonstrate that the fibrinogen-like domain of Tn-C is necessary but not sufficient for induction of chondrogenesis.

Tandem repeats are involved in G1 domain inhibition of versican expression and secretion and the G3 domain enhances glycosaminoglycan modification and product secretion via the complement-binding protein-like motif

The large aggregating chondroitin sulfate proteoglycans, including aggrecan, versican (PG-M), neurocan, and brevican, are characterized by N-terminal and C-terminal globular (or selectin-like) domains known as the G1 and G3 domains, respectively. For this study, we generated a series of expression constructs containing various combinations of chicken versican/PG-M domains and a leading peptide of link protein in order to examine the roles of the G1 and G3 domains in versican function. In transfection studies, we observed that the presence of the G1 domain was sufficient to inhibit product secretion, while the G3 domain enhanced this process. We also demonstrated that the G1 domain inhibited the attachment of glycosaminoglycan chains to the core proteins, while the G3 domain enhanced this process. Further studies revealed that inhibition of secretion by G1 was mediated by its two tandem repeats, while G3's promotion of glycosaminoglycan chain attachment was apparently dependent on G3's complement-binding protein (CBP)-like motif. The modulatory effects of these two molecular domains may contribute to versican's biological activities.

A heparin-binding growth factor, midkine, binds to a chondroitin sulfate proteoglycan, PG-M/versican

Midkine is a heparin-binding growth factor with survival-promoting and migration-enhancing activities. In order to understand the regulation of midkine signaling, we isolated midkine-binding proteoglycans from day 13 mouse embryos, when midkine is intensely expressed. Deglycosylation followed by SDS/PAGE revealed various protein bands; one of these was identified as PG-M/versican by in gel trypsin digestion and sequencing the resulting peptides. PG-M/versican isolated from day 13 mouse embryos bound midkine with a Kd of 1.0 nM. Pleiotrophin/heparin-binding growth-associated molecule, which has a structure related to midkine, was also bound similarly. Digestion with chondroitinase ABC, AC-I or B abolished the binding to midkine. Heparin as well as chondroitin sulfate D and E inhibited the binding. After chondroitinase ABC digestion, the midkine-binding PG-M/versican released 4-sulfated, 6-sulfated, 2, 6-disulfated and 4,6-disulfated unsaturated disaccharides. These results suggest that midkine binds to a polysulfated domain in the chondroitin sulfate chain with a region of dermatan sulfate structure. This proteoglycan may modulate the midkine activity, as binding to midkine can enhance midkine action by concentrating it to the cell periphery or inhibit the action by competing with the binding to a signaling receptor.

Avian neural crest cell migration is diversely regulated by the two major hyaluronan-binding proteoglycans PG-M/versican and aggrecan

It has been proposed that hyaluronan-binding proteoglycans play an important role as guiding cues during neural crest (NC) cell migration, but their precise function has not been elucidated. In this study, we examine the distribution, structure and putative role of the two major hyaluronan-binding proteoglycans, PG-M/versicans and aggrecan, during the course of avian NC development. PG-M/versicans V0 and V1 are shown to be the prevalent isoforms at initial and advanced phases of NC cell movement, whereas the V2 and V3 transcripts are first detected following gangliogenesis. During NC cell dispersion, mRNAs for PG-M/versicans V0/V1 are transcribed by tissues lining the NC migratory pathways, as well as by tissues delimiting nonpermissive areas. Immunohistochemistry confirm the deposition of the macromolecules in these regions and highlight regional differences in the density of these proteoglycans. PG-M/versicans assembled within the sclerotome rearrange from an initially uniform distribution to a preferentially caudal localization, both at the mRNA and protein level. This reorganization is a direct consequence of the metameric NC cell migration through the rostral portion of the somites. As suggested by previous in situ hybridizations, aggrecan shows a virtually opposite distribution to PG-M/versicans being confined to the perinotochordal ECM and extending dorsolaterally in a segmentally organized manner eventually to the entire spinal cord at axial levels interspacing the ganglia. PG-M/versicans purified from the NC migratory routes are highly polydispersed, have an apparent M(r) of 1,200-2,000 kDa, are primarily substituted with chondroitin-6-sulfates and, upon chondroitinase ABC digestion, are found to be composed of core proteins with apparent M(r)of 360–530, 000. TEM/rotary shadowing analysis of the isolated PG-M/versicans confirmed that they exhibit the characteristic bi-globular shape, have core proteins with sizes predicted for the V0/V1 isoforms and carry relatively few extended glycosaminoglycan chains. Orthotopical implantation of PG-M/versicans immobilized onto transplantable micromembranes tend to ‘attract’ moving cells toward them, whereas similar implantations of a notochordal type-aggrecan retain both single and cohorts of moving NC cells in close proximity of the implant and thereby perturb their spatiotemporal migratory pattern. NC cells fail to migrate through three-dimensional collagen type I-aggrecan substrata in vitro, but locomote in a haptotactic manner through collagen type I-PG-M/versican V0 substrata via engagement of HNK-1 antigen-bearing cell surface components. The present data suggest that PG-M/versicans and notochordal aggrecan exert divergent guiding functions during NC cell dispersion, which are mediated by both their core proteins and glycosaminoglycan side chains and may involve ‘haptotactic-like’ motility phenomena. Whereas aggrecan defines strictly impenetrable embryonic areas, PG-M/versicans are central components of the NC migratory pathways favoring the directed movement of the cells.

Fibulin-1 is a ligand for the C-type lectin domains of aggrecan and versican

The aggregating proteoglycans (aggrecan, versican, neurocan, and brevican) are important components of many extracellular matrices. Their N-terminal globular domain binds to hyaluronan, but the function of their C-terminal region containing a C-type lectin domain is less clear. We now report that a 90-kDa protein copurifies with recombinant lectin domains from aggrecan and versican, but not from the brain-specific neurocan and brevican. Amino acid sequencing of tryptic peptides from this protein identified it as fibulin-1. This extracellular matrix glycoprotein is strongly expressed in tissues where versican is expressed (blood vessels, skin, and developing heart), and also expressed in developing cartilage and bone. It is thus likely to interact with these proteoglycans in vivo. Surface plasmon resonance measurements confirmed that aggrecan and versican lectin domains bind fibulin-1, whereas brevican and neurocan do not. As expected for a C-type lectin, the interactions with fibulin-1 are Ca2+-dependent, with KD values in the low nanomolar range. Using various deletion mutants, the binding site for aggrecan and versican lectin domains was mapped to the epidermal growth factor-like repeats in domain II of fibulin-1. No difference in affinity was found for deglycosylated fibulin-1, indicating that the proteoglycan C-type lectin domains bind to the protein part of fibulin-1.

Selective activation of the versican promoter by epithelial- mesenchymal interactions during hair follicle development

Interaction between the epithelium and the mesenchyme is an essential feature of organogenesis, including hair follicle formation. The dermal papilla (DP), a dense aggregate of specialized dermis-derived stromal cells located at the bottom of the follicle, is a major component of hair that signals the follicular epithelial cells to prolong the hair growth process. However, little is known about DP-specific gene activation with regard to hair induction. In this study we demonstrate that a short fragment (839 bp) of the human versican (a core protein of one of the matrix chondroitin sulfate proteoglycans) promoter is sufficient to activate lacZ reporter gene expression in the DP of postnatal transgenic mice and also in the condensed mesenchyme (the origin of the DP) beneath the hair placode during hair follicle embryogenesis. Using the same versican promoter with green fluorescent protein (GFP), large numbers of fresh pelage DP cells were isolated from newborn transgenic skin by high-speed cell sorting. These GFP-positive DP cells showed abundant versican mRNA, confirming that the reporter molecules reflected endogenous versican gene expression. These sorted GFP-positive cells showed DP-like morphology in culture, but both GFP and versican expression was lost during primary culture. In vivo hair growth assays showed that GFP-positive cells could induce hair when grafted with epithelial cells, whereas GFP-negative cells grafted with epithelium or GFP-positive cells alone did not. These results suggest that versican may play an essential role both in mesenchymal condensation and in hair induction.

Dynamic expression of proteoglycans during chicken skeletal muscle development and maturation

Skeletal muscle development is a complex process in which cell migration and adhesion play important roles. Because these cellular activities involve cell surface and extracellular matrix molecules, proteoglycan analysis was performed for developing chick skeletal muscle. Proteoglycans are macromolecular conjugates of protein and carbohydrate found in the extracellular matrix and at the cell surface. In developing muscle, both in vivo and in vitro, there is a development-related progression from synthesis of primarily large proteoglycans at earlier stages to mainly small proteoglycans at later stages. This progression was demonstrated by radiolabeling developing muscle and extracting and characterizing the proteoglycans. The large proteoglycans synthesized earlier in myogenesis have been identified as the large chondroitin sulfate proteoglycan, versican. Among the small proteoglycans synthesized at later stages is the small dermatan sulfate proteoglycan, decorin. Immunolocalization of these proteoglycans shows that versican is initially present in pericellular locations around developing myotubes, whereas decorin is observed in the epimysium early in development, and then its distribution gradually spreads to also include the perimysium and endomysium. Studies of regenerating muscle show that there is a recapitulation of the embryonic pattern of proteoglycan synthesis, which, coupled with the results from embryonic muscle development, suggests a role for versican in some early aspect of myogenesis.

Transcriptional and posttranscriptional regulation of proteoglycan gene expression

Proteoglycans are among the most complex and sophisticated molecules of mammalian systems in terms of their protein and carbohydrate moieties. These macromolecules are in a continuous interplay with each other and the cell surface signal-transducing pathways, some of which are beginning to be elucidated. Because of their domain structure, catalytic potential, and diversity, these molecules appear to be designed for integrating numerous signaling events. For example, some proteoglycans interact with hyaluronan and lectins, thereby linking cell surfaces and distant matrix molecules. Some interact with collagen during the complex process of fibrillogenesis and regulate this biological process fundamental to animal life. Others interact with growth factors and serve as depot available during growth or tissue remodeling. In this review, we center on the most recent developments of proteoglycan biology, focusing primarily on genomic organization and transcriptional and posttranscriptional control. We discuss only those proteoglycans whose gene and promoter elements have been characterized and proved to be functional. When possible, we correlate the effects of growth factors and cytokines on proteoglycan gene expression with the topology of cis-acting elements in their genomic control regions. The analysis leads to a comprehensive critical appraisal of the principles that underlie the regulation of proteoglycan gene expression and to the delineation of common regulatory mechanisms.

Cell adhesion and proliferation mediated through the G1 domain of versican

We have demonstrated previously that versican stimulated cell proliferation through the G3 domain. In these experiments, we show that versican mini-gene-transfected cell lines exhibited decreased cell-substratum interaction and increased cell proliferation. Exogenous addition of growth medium containing the versican gene product produced the same results. Because the G1 domain of versican is structurally similar to the G1 domain of aggrecan and to link protein, both of which play role in cell adhesion, we hypothesized that versican's proliferative effects may be a consequence of its ability to reduce cell adhesion, and may be mediated through the G1 domain. To investigate this, we expressed a G1 construct in NIH3T3 cells and showed that it reduced cell adhesion and enhanced cell proliferation. We then demonstrated that deletion of the G1 domain from versican greatly, but not completely, reversed the effects of versican: G1-deletion mutants of versican show slightly reduced amounts of cell adhesion and slightly increased rates of proliferation. We concluded that versican can stimulate cell proliferation via two mechanisms: through two EGF-like motifs in the G3 domain which play a role in stimulating cell growth, and through the G1 domain, which destabilizes cell adhesion and facilitates cell growth. We purified the G1 product with an affinity column and demonstrated that it reduced cell adhesion and enhanced cell proliferation.

Identification of mouse CPX-1, a novel member of the metallocarboxypeptidase gene family with highest similarity to CPX-2

The recent finding that Cpe(fat)/Cpe(fat) mice, which lack carboxypeptidase E (CPE) activity because of a point mutation, are still capable of a reduced amount of neuroendocrine peptide processing suggested that additional carboxypeptidases (CPs) participate in this processing reaction. Searches for novel members of the CPE gene family led to the discovery of CPD, CPZ, AEBP1, and CPX-2. In the present report, we describe mouse CPX-1, another novel member of this gene family. Like AEBP1 and CPX-2, CPX-1 contains an N-terminal region of 160 amino acids with sequence similarity to the discoidin domain of a variety of proteins. The 410-residue CP-like domain of CPX-1 has 54% to 62% amino acid sequence identity with AEBP1 and CPX-2 and 33% to 49% amino acid identity with other members of the CPE subfamily. However, several active-site residues that are important for catalytic activity of other CPs are not conserved in CPX-1. Furthermore, CPX-1 expressed in either the baculovirus system or the mouse AtT-20 cell line does not cleave standard CP substrates. Northern blot analysis showed the highest levels of CPX-1 mRNA in testis and spleen and lower levels in salivary gland, brain, heart, lung, and kidney. In situ hybridization of CPX-1 mRNA in embryonic and fetal mouse tissue showed expression throughout the head and thorax, with abundance in primordial cartilage and skeletal structures. In the head, high levels of CPX-1 mRNA were associated with the nasal mesenchyme, primordial cartilage structures in the ear, and the meninges. In the thorax, CPX-1 mRNA was expressed in multiple developing skeletal structures, including chondrocytes and perichondrial cells of the rib, vertebral, and long-bone primordia. Taken together, these findings suggest that it is unlikely that CPX-1 functions in the processing of neuroendocrine peptides. Instead, CPX-1 may have a role in development, possibly mediating cell interactions via its discoidin domain.

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

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

The G3 domain of versican inhibits mesenchymal chondrogenesis via the epidermal growth factor-like motifs

Versican is a highly expressed proteoglycan in zones of developing tissues. To investigate whether versican plays a role in cell differentiation, we studied its role in mesenchymal condensation and chondrogenesis. Here we report that a mini-versican gene product inhibits mesenchymal chondrogenesis but not condensation. The mini-versican-treated mesenchymal cultures form fewer, smaller cartilaginous nodules and produced lower levels of link protein and type II collagen. The versican G3 domain alone, but not G1, was sufficient to inhibit mesenchymal chondrogenesis. Deletion of two epidermal growth factor (EGF)-like motifs in the G3 domain abolished the effect of versican. The G3 domain of aggrecan, which does not contain an EGF-like motif, did not inhibit mesenchymal chondrogenesis. We also generated a chimera construct containing the two EGF-like motifs of versican and the G3 domain of aggrecan, and we observed that this chimera construct inhibited chondrogenesis to a lesser extent than did the full-length versican G3 construct. Direct transfection of mesenchymal cells with different constructs produced similar results. Furthermore, treatment with versican antisense oligonucleotides and transfection with a versican antisense construct promoted chondrogenesis. Taken together, our results strongly suggest that versican inhibits mesenchymal chondrogenesis via its EGF-like motifs.

The Cspg2 gene, disrupted in the hdf mutant, is required for right cardiac chamber and endocardial cushion formation

The heart defect (hdf) mouse is a recessive lethal that arose from a transgene insertional mutation on chromosome 13. Embryos homozygous for the transgene die in utero by embryonic day 10.5 postcoitus and exhibit specific defects along the anterior-posterior cardiac axis. The future right ventricle and conus/truncus of the single heart tube fail to form and the endocardial cushions in the atrioventricular and conus/truncus regions are absent. Because the hdf mouse mutation provided the opportunity to identify a gene required for endocardial cushion formation and for specification or maintenance of the anterior most segments of the heart, we initiated studies to further characterize the phenotype, clone the insertion site, and identify the gene disrupted. Chromosome mapping studies first identified the gene, Cspg2 (versican), as a candidate hdf gene. In addition, an antibody recognizing a glycosaminoglycan epitope on versican was found to be positive by immunohistochemistry in the extracellular matrix of normal wild-type embryonic hearts, but absent in homozygous hearts. Expression analysis of the Cspg2 gene showed that the 6/8, 6/9, and 7/9 Cspg2 exon boundaries were present in mRNA of normal wild-type embryonic hearts but absent in the homozygous mutant embryos. DNA sequence flanking the transgene was used to isolate from a normal mouse library overlapping genomic DNA segments that span the transgene insertion site. The contiguous genomic DNA segment was found to contain exon 7 of the Cspg2 in a position 3' to the transgene insertion site. These four separate lines of evidence support the hypothesis that Cspg2 is the gene disrupted by the transgene insertion in the hdf mouse line. The findings of this study and our previous studies of the hdf insertional mutant mouse have shown that normal expression of the Cspg2 gene is required for the successful development of the endocardial cushion swellings and the embryonic heart segments that give rise to the right ventricle and conus/truncus in the outlet of the looped heart.

The G3 domain of versican enhances cell proliferation via epidermial growth factor-like motifs

Versican is a member of the large aggregating chondroitin sulfate proteoglycan family. We have expressed in NIH3T3 fibroblasts a recombinant versican mini-gene comprising the G1 and G3 domains and 15% of the CS domain. We observed that expression of the mini-versican gene stimulated cell proliferation as determined by cell counting and cell cycle analysis. Addition of exogenous mini-versican protein to cultured cells produced the same result. The effects of the mini-versican were greatly reduced when the G3 domain was deleted. Expression of the G3 domain alone promotes cell proliferation, and addition of purified G3 gene products to NIH3T3 fibroblasts and cultured chicken fibroblasts enhances cell growth. Further, deletion of the epidermal growth factor (EGF)-like motifs in the versican G3 domain reduced the effects of the mini-versican on cell proliferation. In the presence of the purified mini-versican protein, antisense oligonucleotides to the EGF receptor inhibited proliferation of NIH3T3 fibroblasts, compared with control sense oligonucleotides. Taken together, these results imply that versican enhances cell proliferation, and this effect is mediated, at least in part, by the action of versican EGF-like motifs on endogenous EGF receptor.

Dermatan sulfate proteoglycans from the mineralized matrix of the avian eggshell

The eggshell of the chicken is a useful model to study matrix components which affect biomineralization. As an extension of our previous immunohistochemical work which suggested the presence of dermatan sulfate proteoglycans in the mineralized region of the eggshell, a study was undertaken to characterize these molecules biochemically. After demineralization with HCl and extraction with 4 M guanidinium chloride containing protease inhibitors, the extract was partitioned by anion exchange chromatography. Step elution with 0.25 M and 1.0 M sodium chloride resulted in the generation of two fractions, both of which contain chondroitinase-sensitive proteoglycans with molecular weights estimated at 200,000 by gel electrophoresis. The proteoglycans in each fraction have core proteins with molecular weights of approximately 120,000 and glycosaminoglycans with average molecular weights of 22,000. Based on differential sensitivity to chondroitinase ABC and AC II, these glycosaminoglycans contain a small proportion of dermatan sulfate. The disaccharide compositions of these glycosaminoglycans differ for the proteoglycans eluted with 0.25 M and 1.0 M sodium chloride. Those eluted with lower sodium chloride are enriched in unsulfated chondroitin and have much more 4-sulfated than 6-sulfated disaccharides; those eluted with 1.0 M sodium chloride contain primarily 4-sulfated disaccharides, a small amount of 6-sulfated disaccharides, and less unsulfated disaccharides than the proteoglycans eluted with 0.25 M sodium chloride. The large difference in the proportions of unsulfated chondroitin may be the reason for the elution at different sodium chloride concentrations. Both of the anion exchange column fractions contain other proteins in addition to the proteoglycans. These proteins are not separated from the proteoglycans by a second anion exchange column or by molecular sieve chromatography under dissociative conditions. Of particular interest is the observation that the eggshell proteoglycans and their core proteins are recognized by a monoclonal antibody which recognizes an epitope on the core protein of avian versican. This suggests that, in spite of the large differences in the sizes of the core proteins of versican and the eggshell proteoglycans, these core proteins share some homology. Because anionic molecules are thought to be important regulators of biomineralization, and because preparations like those analyzed in this study have been shown to influence in vitro calcium carbonate crystallization, the eggshell proteoglycans may play a role in eggshell mineralization.

Isolation and characterization of chondroitin sulfate proteoglycans from embryonic quail that influence neural crest cell behavior

The movement of neural crest cells is controlled in part by extracellular matrix. Aggrecan, the chondroitin sulfate proteoglycan from adult cartilage, curtails the ability of neural crest cells to adhere, spread, and move across otherwise favorable matrix substrates in vitro. Our aim was to isolate, characterize, and compare the structure and effect on neural crest cells of aggrecan and proteoglycans purified from the tissues through which neural crest cells migrate. We metabolically radiolabeled proteoglycans in E2.5 quail embryos and isolated and characterized proteoglycans from E3.3 quail trunk and limb bud. The major labeled proteoglycan was highly negatively charged, similar in hydrodynamic size to chick limb bud versican/PG-M, smaller than adult cartilage aggrecan but larger than reported for embryonic sternal cartilage aggrecan. The molecular weight of the iodinated core protein was about 400 kDa, which is more than reported for aggrecan but less than that of chick versican/PG-M. The proteoglycan bore chondroitin sulfate glycosaminoglycan chains of 45 kDa, which is larger than those of aggrecan. It lacked dermatan sulfate, heparan sulfate, or keratan sulfate chains. It bound to collagen type I, like aggrecan, but not to fibronectin (unlike versican/PG-M), collagen type IV, or laminin-1 in solid-phase assays and it bound to hyaluronate in gel-shift assays. When added at concentrations between 10 and 30 microg/ml to substrates of fibronectin, trunk proteoglycan inhibited neural crest cell spreading and migration. Attenuation of cell spreading was shown to be the most sensitive and titratable measure of the effect on neural crest cells. This effect was sensitive to digestion with chondroitinase ABC. Similar cell behavior was also produced by aggrecan and the small dermatan sulfate proteoglycan decorin; however, 30-fold more aggrecan was required to produce an effect of similar magnitude. When added in solution to neural crest cells which were already spread and migrating on fibronectin, the embryonic proteoglycan rapidly and reversibly caused complete rounding of the cells, being at least 30-fold more potent than aggrecan in this activity.

Production of a monoclonal antibody by in vitro immunization that recognizes a native chondroitin sulfate epitope in the embryonic chick limb and heart

We report the production of a monoclonal antibody (d1C4) by in vitro immunization that has immunoreactivity with a native chondroitin sulfate epitope in embryonic chick limb and heart. Murine lymphocytes were stimulated by direct exposure to unfixed, unsolubilized precartilage mesenchymal aggregates in high-density micromass culture derived from Stage 22-23 chick limb buds. Specificity of d1C4 reactivity was demonstrated by sensitivity of immunohistochemical staining to pretreatment with chondroitinase ABC or AC, preferential immunoreactivity with chondroitin-6-sulfate glycosaminoglycan (CS-C GAG) in ELISA, and competition of immunohistochemical staining with CS-C GAG. Immunohistochemical analysis of the expression of the d1C4 epitope revealed a striking localization of immunoreactivity in the extracellular matrix (ECM) of precartilage aggregates of chick limb mesenchyme in high-density micromass culture by 16 hr and the prechondrogenic limb core at Stage 23 in vivo. Immunoreactivity in both cultured limb mesenchyme and the embryonic limb continued through differentiation of prechondrogenic condensations into cartilage tissue. In the developing chick heart, d1C4 staining was found throughout the ECM of atrioventricular cushion tissue by Stage 25, but was localized to mesenchyme adjacent to the myocardium in the outflow tract cushions. There was an abrupt demarcation between d1C4-reactive intracardiac mesenchyme and unreactive extracardiac mesenchyme of the dorsal mesocardium in the Stage 22 embryo. This study demonstrates the efficacy of in vitro immunization of lymphocytes for the production of MAbs to native ECM constituents, such as CS-GAGs. Immunohistochemical data utilizing d1C4 suggest that CS-GAGs bearing this epitope may be important in early morphogenetic events leading to cartilage differentiation in the limb and valvuloseptal morphogenesis in the heart.

Reduced epidermal expression of a PG-M/versican-like proteoglycan in embryos of the white mutant axolotl

Axolotl embryos have previously been used to study neural crest cell migration. In embryos of the normal wild type, neural crest cells migrate subepidermally to form pigment cells. In the trunk of the white mutant embryo, these cells are unable to migrate, possibly due to an inherited delay in the maturation of the local extracellular matrix. The present investigation reveals a reduced incorporation of [35S]sulfate into PG-M/versican-like proteoglycans synthesized in epidermal explants from the dorsal trunk of white mutant embryos during stages pertinent to migration. This is the major form of proteoglycans in the subepidermal matrix, where they are assembled in large disulfide-stabilized supramolecular complexes. The reduction in [35S]sulfate incorporation is not due to qualitative differences between wild-type and white mutant proteoglycans but is paralleled by a reduced expression of mRNA for the core protein of the PG-M/versican-like proteoglycan. We conclude that a reduced amount of these proteoglycans is produced by the white mutant embryo during the period critical for migration.

Alternative splicing of the unique "PLUS" domain of chicken PG-M/versican is developmentally regulated

We investigated the occurrence of alternatively spliced forms (V0, V1, V2, and V3) of PG-M/versican, a large chondroitin sulfate proteoglycan in developing chicken retinas, using the reverse transcription-polymerase chain reaction. We characterized the PLUS domain, which is apparently unique to the chicken molecule and is regulated by alternative splicing. PG-M in chicken retinas consisted of four forms with (V0, V1, V2, and V3) and two forms without (V1 and V3) the PLUS domain (PG-M+ and PG-M-, respectively). The four forms of PG-M+ were found in all samples examined, but the occurrence of the two PG-M- forms was regulated developmentally. Genomic analysis has revealed that the PLUS and CS-alpha domains are encoded by a single exon, and this exon has an internal alternative 5'-splice donor site, allowing alternative spliced forms that do not include the 3'-end of the exon. Sequences corresponding to the chicken PLUS domain (plus) were not found in mouse and human and may have disappeared during evolution. Sequence similarity suggests that the PLUS domain corresponds to the keratan sulfate attachment domain of aggrecan and that it has a distinct function in the chicken eye.

In vitro formation of mineralized cartilagenous tissue by articular chondrocytes

Study of the deep articular cartilage and adjacent calcified cartilage has been limited by the lack of an in vitro culture system which mimics this region of the cartilage. In this paper we describe a method to generate mineralized cartilagenous tissue in culture using chondrocytes obtained from the deep zone of bovine articular cartilage. The cells were plated on Millipore CMR filters. The chondrocytes in culture accumulated extracellular matrix and formed cartilagenous tissue which calcified when beta-glycerophosphate was added to the culture medium. The cartilagenous tissue generated in vitro contains both type II and type X collagens, large sulfated proteoglycans, and alkaline phosphatase activity. Ultrastructurally, matrix vesicles were seen in the extracellular matrix. Selected area electron diffraction confirmed that the calcification was composed of hydroxyapatite crystals. The chondrocytes, as characterized thus far, appear to maintain their phenotype under these culture conditions which suggests that these cultures could be used as a model to examine the metabolism of cells from the deep zone of cartilage and mineralization of cartilagenous tissue in culture.

Specific properties of the extracellular chondroitin sulphate proteoglycans in the mandibular condylar growth centre in pigs

The developing condylar cartilage of the temporomandibular joint responds to changes in load by adaptive growth. Because local regulatory events taking place during growth processes are not well understood, investigation of extracellular matrix composition could provide new information about which matrix molecules are involved in the regulation of growth processes in this avascular tissue. The large chondroitin sulphate-rich proteoglycans in the mandibular condyle were compared to the proteoglycans in the weight-bearing femoral condyle of juvenile domestic pigs with respect to their buoyant density, chemical composition and immunological identity after isolation by dissociative extraction and CsCl density-gradient centrifugation. The distribution of these proteoglycans was studied in cryosections of mandibular condyle by immunohistochemistry using polyclonal antibodies produced against pig large proteoglycans. In the mandibular condyle, predominantly in the articular zone, the relative amount of proteoglycans with a low glycosaminoglycan content was greater than in femoral cartilage. The large proteoglycan immunologically related to aggrecan gave a protein core of 450 kDa after enzymatic deglycosylation and clearly possessed less keratan sulphate than in femoral aggrecan. Furthermore, the mandibular tissue contained another large proteoglycan with a protein core of 550 kDa after enzymatic deglycosylation, which was immunologically related to the fibroblast-like versican. Immunohistochemistry showed aggrecan increasing in amount inferiorly. In contrast, "versican' was exclusively found in the fibrous and differentiation layers. Aggrecan is mainly responsible for shock absorption and versican and its homologues may be involved in the control of cell proliferation and differentiation. Thus the matrix components of the mandibular condyle seem to be adapted to its special functional needs including parallel articulation and growth.

Inhibition of in vitro limb cartilage differentiation by syndecan-3 antibodies

The transmembrane heparan sulfate proteoglycan syndecan-3 is transiently expressed in high amounts during the cellular condensation process that characterizes the onset of limb cartilage differentiation. During condensation, limb mesenchymal cells become closely juxtaposed and undergo cell-cell and cell-matrix interactions that are necessary to trigger cartilage differentiation and cartilage-specific gene expression. To test directly the possible involvement of syndecan-3 in regulating the onset of limb chondrogenesis, we examined the effect of polyclonal antibodies against a syndecan-3 fusion protein on the chondrogenic differentiation of chick limb mesenchymal cells in micromass culture. Syndecan-3 antiserum elicits a dose-dependent inhibition of the accumulation of Alcian blue-stainable cartilage matrix by high density limb mesenchymal cell micromass cultures (2 x 10(5) cells/10 microliters) and a corresponding reduction in steady-state levels of mRNAs for cartilage-characteristic type II collagen and the core protein of the cartilage proteoglycan aggrecan. In preimmune serum-treated control cultures proliferating cells are limited to the periphery of areas of cartilage matrix deposition, whereas large numbers of proliferating cells are uniformly distributed throughout the undifferentiated cultures supplemented with syndecan-3 antiserum. Limb mesenchymal cells cultured at lower densities (1 x 10(5) cells/10 microliters) in the presence of preimmune serum form extensive precartilage condensations characterized by the close juxtaposition of rounded cells by day 2 of culture. In contrast, in the presence of syndecan-3 antiserum, the cells fail to aggregate but rather remain flattened and spatially separated from one another, suggeting that syndecan-3 antibodies impair the formation of precartilage condensations. These results indicate that syndecan-3 plays an important role in regulating the onset of limb chondrogenesis, perhaps by mediating the cell-cell and cell-matrix interactions required for condensation and subsequent cartilage differentiation.

Tenascin-C is associated with early stages of chondrogenesis by chick mandibular ectomesenchymal cells in vivo and in vitro

Tenascin-C is an extracellular matrix protein thought to be involved in skeletogenesis. We have examined the distribution of tenascin-C in the developing chick mandibular arch between stages 18-36, and during in vitro chondrogenesis of mandibular ectomesenchymal cells in micromass cultures using a probe and antibody that correspond to the portion of the tenascin-C transcript conserved in all of the three known chick splice variants. In situ hybridization and immunohistochemical analyses demonstrate that tenascin-C is predominantly expressed in the condensing mesenchyme of developing cartilage, and in the perichondrium of differentiated cartilage. Tenascin-C expression, although detected in differentiating chondroblasts, was not detected in differentiated cartilage. Tenascin-C was also expressed in the developing membranous bones. In addition, the expression of tenascin-C transcripts during in vitro chondrogenesis of mandibular ectomesenchymal cells in micromass cultures was compared to the patterns of expression of aggrecan core protein and alpha 1(I) collagen transcripts. Our in situ hybridization analyses of micromass cultures demonstrate the expression of tenascin-C and aggrecan core protein mRNAs by pre-chondrogenic aggregates in the 1-day cultures and by chondroblasts in differentiating cartilage nodules in 2-day cultures. In 4- and 9-day cultures, the pattern of expression of tenascin-C mRNA was different from the patterns of expression of aggrecan core protein mRNA, and appeared to be more closely related to the expression of alpha 1(I) collagen mRNA. Aggrecan core protein mRNA was expressed by chondrocytes in cartilage nodules in 4- and 9-day cultures. On the other hand, tenascin-C and alpha 1(I) collagen mRNAs, in addition to being expressed in the loose connective tissues in the inter-nodular spaces, were predominantly expressed by the elongated, flattened, and fibroblast-like cells around the cartilage nodules. These results indicate that during the in vitro chondrogenesis of mandibular ectomesenchymal cells, expression of tenascin-C mRNA identifies chondrocytes in their early stages of differentiation. The patterns of expression of tenascin-C mRNA in 4- and 9-day cultures further suggest that tenascin-C is expressed in the perichondrium-like structures that form around the cartilage nodules in micromass cultures. Therefore, our in vitro studies, in agreement with our in vivo studies, suggest an association of tenascin-C with the initial or early stages of chondrogenesis in the chicken mandibular arch.

Versican is selectively expressed in embryonic tissues that act as barriers to neural crest cell migration and axon outgrowth

Chondroitin sulfate proteoglycans have been implicated in the regulation of cell migration and pattern formation in the developing peripheral nervous system. To identify whether the large aggregating proteoglycan versican might be mediating these processes, we prepared monospecific antibodies against a recombinant core protein fragment of chick versican. The purified antibodies recognize the predominant versican splice-variants V0 and V1. Using these antibodies, we revealed a close correlation between the spacio-temporal expression of versican and the formation of molecular boundaries flanking or transiently blocking the migration pathways of neural crest cells or motor and sensory axons. Versican is present in the caudal sclerotome, the early dorsolateral tissue underneath the ectoderm, the pelvic girdle precursor and to a certain extent in the perinotochordal mesenchyme. Versican is completely absent from tissues invaded by neural crest cells and extending axons. Upon completion of neural crest cell migration and axon outgrowth, versican expression is shifted to pre-chondrogenic areas. Since versican inhibits cellular interactions with fibronectin, laminin and collagen I in vitro, the selective expression of versican within barrier tissues may be linked to a functional role of versican in the guidance of migratory neural crest cells and outgrowing axons.

The gene structure and organization of mouse PG-M, a large chondroitin sulfate proteoglycan. Genomic background for the generation of multiple PG-M transcripts

We previously showed not only the presence of multiple RNA transcripts of different sizes encoding the core protein of mouse PG-M, but also their tissue-dependent expression. Major causes for the multiple forms were found to be due to alternative usage of the two different chondroitin sulfate attachment domains (alpha and beta). In this study, genomic DNA analysis has revealed that these domains are encoded by two large exons, exon VII (2880 base pairs) and exon VIII (5229 base pairs). The splice sites of these two exons were consistent with the occurrence of alternative splicing without frameshift. Furthermore, the mouse PG-M gene was shown to have four distinct polyadenylation signals and three candidates for the transcription initiation site as well. These genomic structural variations may contribute to the multiplicity of PG-M transcripts. Northern hybridization analysis showed that at least three different transcripts were generated by different usage of the distinct polyadenylation signals.

Expression of PG-M(V3), an alternatively spliced form of PG-M without a chondroitin sulfate attachment in region in mouse and human tissues

We showed previously that the alternative splicing of chondroitin sulfate attachment domains (CS alpha and CS beta) yielded multiforms of the PG-M core protein in mouse. A transcript encoding a new short form of the core protein PG-M(V3) was found in various mouse tissues using polymerase chain reaction. DNA sequences of the polymerase chain reaction products suggested that PG-M(V3) had no chondroitin sulfate attachment domain. PG-M(V3) was also detected in various human tissues. The presence of a transcript for PG-M(V3) was further supported by Northern blot analysis. Southern blot analysis confirmed that multiforms of the PG-M core protein, including PG-M(V3), were derived from a single genomic locus by an alternative splicing mechanism. Because PG-M(V3) has no chondroitin sulfate attachment region, which is the most distinctive portion of a proteoglycan molecule, this form may have a unique function.

Multiple forms of mouse PG-M, a large chondroitin sulfate proteoglycan generated by alternative splicing

We have isolated and sequenced cDNA clones that encode the core protein of PG-M-like proteoglycan produced by cultured mouse aortic endothelial cells (Morita, H., Takeuchi, T., Suzuki, S., Maeda, K., Yamada, K., Eguchi, G., and Kimata, K. (1990) Biochem. J. 265, 61-68). A homology search of the cDNA sequence has suggested that the core protein is a mouse equivalent of chick PG-M(V1), one of the alternatively spliced forms of the PG-M core protein, which may correspond to human versican. Northern blot analysis revealed three mRNA species of 10, 9, and 8 kilobases (kb) in size. The analysis of PG-M mRNA species in embryonic limb buds and adult brain revealed the presence of other mRNA species with different sizes; the one with the largest size (12 kb) was found in embryonic limb buds, and the ones with smaller sizes of 7.5 and 6.5 kb were in adult brain. Sequencing of cDNA clones for the smaller forms in the adult brain showed that they were different from PG-M(V1) in encoding the second chondroitin sulfate attachment domain (CS alpha) alone. Occurrence of the PCR products striding over the junction of the first and second chondroitin sulfate attachment domains suggested that a mRNA of 12 kb in size corresponded to a transcript without the alternative splicing (PG-M(V0)). It is likely, therefore, that multiforms of the PG-M core protein may be generated by alternative usage of either or both of the two different chondroitin sulfate attachment domains (alpha and beta) and that molecular forms of PG-M may vary from tissue to tissue by such an alternative splicing.

Skeletal development and osteoarthritis

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