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

Proteoglycan form and function: A comprehensive nomenclature of proteoglycans

We provide a comprehensive classification of the proteoglycan gene families and respective protein cores. This updated nomenclature is based on three criteria: Cellular and subcellular location, overall gene/protein homology, and the utilization of specific protein modules within their respective protein cores. These three signatures were utilized to design four major classes of proteoglycans with distinct forms and functions: the intracellular, cell-surface, pericellular and extracellular proteoglycans. The proposed nomenclature encompasses forty-three distinct proteoglycan-encoding genes and many alternatively-spliced variants. The biological functions of these four proteoglycan families are critically assessed in development, cancer and angiogenesis, and in various acquired and genetic diseases where their expression is aberrant.

Versican modulates gap junction intercellular communication

Versican is a large chondroitin sulfate proteoglycan and belongs to the family of lecticans. Versican possesses two globular domains, G1 and G3 domain, separated by a CS-attachment region. The CS-attachment region present in the middle region is divided into two spliced domains named CSalpha and beta. Alternative splicing of versican generates at least four versican isoforms named V0, V1, V2, and V3. We have successfully cloned the full-length cDNA of chick versican isoforms V1 and V2 and found that versican isoform V1 induced mesenchymal-epithelial transition in NIH3T3 cells. Mesenchymal-epithelial transition induced by V1 in NIH3T3 cells is characterized by expression of E-cadherin and occludin, two epithelial markers, and reduced expression of fibroblastic marker vimentin (Sheng et al., 2006, Mol Biol Cell. 17, 2009-2020). In the present studies, we found that versican V1 isoform not only induced cell transition, but also increased intercellular communication via gap junction channels composed of connexin proteins. Our results showed that V1 induces plasma membrane localization of connexin 43, resulting in increased cell communication. This was further confirmed by blocking assays. Gap junctions mediated the transfer of small cytoplasmic molecules and the diffusion of second messenger molecules between adjacent cells. The ability of versican in regulating gap junction implied a potential role of versican in coordinating functions.

Deletions in mRNA encoding the chicken leptin receptor gene binding domain

The leptin binding domain of the chicken leptin receptor gene was analyzed for alternative splicing. Polymerase chain reaction (PCR) primers were designed to amplify exons 8-14 of the gene which is known to encode the leptin binding domain. Four cDNA products from reverse transcribed chicken anterior pituitary and basal hypothalamic RNA were generated. One encoded the predicted full length leptin binding domain while the other cDNAs were shorter as a consequence of different deletions in exon 9, and one had a further deletion in exon 10. Two of the deletions in exon 9 had the potential to disrupt the leptin binding domain. Genomic DNA analysis demonstrated that the alternative splicing sites with potential to generate these deletions occurred in the chicken genome. All four cDNAs were amplified from reverse transcribed RNA from basal hypothalami and anterior pituitary glands from four breeds of chicken, demonstrating that the nucleotide deletions were not breed specific. In conclusion, alternative spliced forms of the leptin binding domain in chicken leptin receptor mRNAs occur in the chicken neuroendocrine system with the potential to give rise to alternative transcripts which could modulate the biological action of the ligand for the chicken leptin receptor.

Versican mediates mesenchymal-epithelial transition

Versican is a large extracellular chondroitin sulfate proteoglycan that belongs to the family of lecticans. Alternative splicing of versican generates at least four isoforms named V0, V1, V2, and V3. We show here that ectopic expression of versican V1 isoform induced mesenchymal-epithelial transition (MET) in NIH3T3 fibroblasts, and inhibition of endogenous versican expression abolished the MET in metanephric mesenchyme. MET in NIH3T3 cells was demonstrated by morphological changes and dramatic alterations in both membrane and cytoskeleton architecture. Molecular analysis showed that V1 promoted a "switch" in cadherin expression from N- to E-cadherin, resulting in epithelial specific adhesion junctions. V1 expression reduced vimentin levels and induced expression of occludin, an epithelial-specific marker, resulting in polarization of V1-transfected cells. Furthermore, an MSP (methylation-specific PCR) assay showed that N-cadherin expression was suppressed through methylation of its DNA promoter. Exogenous expression of N-cadherin in V1-transfected cells reversed V1's effect on cell aggregation. Reduction of E-cadherin expression by Snail transfection and siRNA targeting E-cadherin abolished V1-induced morphological alteration. Transfection of an siRNA construct targeting versican also reversed the changed morphology induced by V1 expression. Silencing of endogenous versican prevented MET of metanephric mesenchyme. Taken together, our results demonstrate the involvement of versican in MET: expression of versican is sufficient to induce MET in NIH3T3 fibroblasts and reduction of versican expression decreased MET in metanephric mesenchyme.

Versican in the developing brain: lamina-specific expression in interneuronal subsets and role in presynaptic maturation

Chondroitin sulfate proteoglycans (CSPGs) of the extracellular matrix help stabilize synaptic connections in the postnatal brain and impede regeneration after injury. Here, we show that a CSPG of the lectican family, versican, also promotes presynaptic maturation in the developing brain. In the embryonic chick optic tectum, versican is expressed selectively by subsets of interneurons confined to the retinorecipient laminae, in which retinal axons arborize and form synapses. It is a major receptor for the Vicia villosa B4 lectin (VVA), shown previously to inhibit invasion of the retinorecipient lamina by retinal axons (Inoue and Sanes, 1997). In vitro, versican promotes enlargement of presynaptic varicosities in retinal axons. Depletion of versican in ovo, by RNA interference, results in retinal arbors with smaller than normal varicosities. We propose that versican provides a lamina-specific cue for presynaptic maturation and discuss the related but distinct effects of versican depletion and VVA blockade.

Versican: signaling to transcriptional control pathways

Versican, a chondroitin sulfate proteoglycan, is one of the main components of the extracellular matrix, which provides a loose and hydrated matrix during key events in development and disease. Versican participates in cell adhesion, proliferation, migration, and angiogenesis, and hence plays a central role in tissue morphogenesis and maintenance. In addition, versican contributes to the development of a number of pathologic processes including atherosclerotic vascular diseases, cancer, tendon remodeling, hair follicle cycling, central nervous system injury, and neurite outgrowth. Versican is a complex molecule consisting of modular core protein domains and glycosaminoglycan side chains, and there are various steps of synthesis and processes regulating them. Also, there is differential temporal and spatial expression of versican by multiple cell types and in different developmental and pathological time frames. To fully appreciate the functional roles of versican as it relates to changing patterns of expression in development and disease, an in depth knowledge of versican's biosynthetic processing is necessary. The goal of this review is to evaluate the current status of our knowledge regarding the transcriptional control of versican gene regulation. We will be focusing on the signal transduction pathways, promoter regions, cis-acting elements, and trans-factors that have been characterized.

Versican/PG-M regulates chondrogenesis as an extracellular matrix molecule crucial for mesenchymal condensation

Mesenchymal cell condensation is an essential step for cartilage development. Versican/PG-M, a large chondroitin sulfate proteoglycan, is one of the major molecules expressed in the extracellular matrix during condensation. However, its role, especially as an environment for cells being condensed, has not been elucidated. Here we showed several lines of evidence for essential roles of versican/PG-M in chondrogenic condensation using a new chondrocytic cell line, N1511. Chondrogenic stimuli (treatment with parathyroid hormone, dexamethasone, 10% serum) induced a marked increase in the transcription and protein synthesis of versican/PG-M. Stable antisense clones for versican/PG-M, depending on suppression of the expression of versican/PG-M, showed different capacities for chondrogenesis, as indicated by the expression and deposition of aggrecan, a major chondrocytic cell product. The cells in the early stages of the culture only expressed V0 and V1 forms, having more chondroitin sulfate chains among the four variants of versican/PG-M, and treatment of those cells with chondroitinase ABC suppressed subsequent chondrogenesis. Furthermore, treatment with beta-xyloside, an artificial chain initiator of chondroitin sulfate synthesis to consequently inhibit the synthesis on the core proteins, suppressed chondrogenesis. In addition, forced expression of the variant V3, which has no chondroitin sulfate chain, disrupted the deposition and organization of native versican/PG-M (V0/V1) and other extracellular matrix molecules known to be expressed during the mesenchymal condensation and resulted in the inhibition of subsequent chondrogenesis. These results suggest that versican/PG-M is involved in positively regulating the formation of the mesenchymal matrix and the onset of chondrocyte differentiation through the attached chondroitin sulfate chains.

Differential versican isoforms and aggrecan expression in the chicken embryo aorta

Members of the family of large chondroitin sulfate proteoglycans (CSPGs), such as versican and aggrecan, are involved in early heart development, and in the development and progression of atherosclerosis and restenosis. Given the important roles played by versican and aggrecan in such processes, we sought to determine whether these molecules are present in the aortic wall during the advanced stages of chicken embryo development and the endothelial-mesenchymal transformation (EMT). Immunolabeling of serial cryosections revealed versican immunoreactivity around the cells within the intimal thickening, and the cells organized in lamellar and interlamellar cell layers. In contrast, a weak aggrecan immunoreactivity was limited to the cells arranged into lamellar and interlamellar cell layers. Immunolabeling also demonstrated that V2 is the main versican isoform present at the intimal thickening. According to immunoblotting analysis, the aggrecan content was very low in all stages examined, and two versican isoforms (V0 and V2) were present at day 14 of development. We also investigated whether versican isoforms were present during EMT in vitro. Versican immunoreactivity was detected in patches of endothelial cells; in the detaching and migrating cells, and the extracellular matrix (ECM) deposited by them; and in cells that had acquired mesenchymal characteristics. These data indicate that versican and aggrecan have different spatial and temporal patterns of expression, and they have different functions during remodeling of the aortic wall. Also, the different immunoreactivity and immunolocalization patterns observed for versican both in vivo and in vitro, in addition to being associated with the presence of different versican isoforms, may be related to the predominance of the V2 isoform during intimal thickening formation and EMT.

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.

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.

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.

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.

Proteoglycans in the developing brain: new conceptual insights for old proteins

Proteoglycans are a heterogeneous class of proteins bearing sulfated glycosaminoglycans. Some of the proteoglycans have distinct core protein structures, and others display similarities and thus may be grouped into families such as the syndecans, the glypicans, or the hyalectans (or lecticans). Proteoglycans can be found in almost all tissues being present in the extracellular matrix, on cellular surfaces, or in intracellular granules. In recent years, brain proteoglycans have attracted growing interest due to their highly regulated spatiotemporal expression during nervous system development and maturation. There is increasing evidence that different proteoglycans act as regulators of cell migration, axonal pathfinding, synaptogenesis, and structural plasticity. This review summarizes the most recent data on structures and functions of brain proteoglycans and focuses on new physiological concepts for their potential roles in the developing central nervous system.

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.

Versican/PG-M isoforms in vascular smooth muscle cells

The expression of increased amounts of proteoglycans in the extracellular matrix may play a role in vascular stenosis and lipid retention. The large chondroitin sulfate proteoglycan versican is synthesized by vascular smooth muscle cells (SMCs), accumulates during human atherosclerosis and restenosis, and has been shown to bind LDLs. We recently demonstrated that adult rat aortic SMCs express several versican mRNAs. Four versican splice variants, V0, V1, V2, and V3, have recently been described, which differ dramatically in length. These variants differ in the extent of modification by glycosaminoglycan chains, and V3 may lack glycosaminoglycan chains. In this study, we characterized versican RNAs from rat SMCs by cloning, sequencing, and hybridization with domain-specific probes. DNA sequence was obtained for the V3 isoform, and for a truncated V0 isoform. By hybridization of polyadenylated RNA with domain-specific probes, we determined that the V0, V1, and V3 isoforms are present in vascular SMCs. We confirmed the presence of the V3 isoform in polyadenylated RNA and in RT-PCR products by hybridization with an oligonucleotide that spans the splice junction between the hyaluronan-binding domain and the epidermal growth factor-like domain. In addition, a novel splice variant was cloned by PCR amplification from both rat and human SMC RNA. This appears to be an incompletely spliced variant, retaining the final intron. PCR analysis shows that this intron can be retained in both V1 and V3 isoforms. The predicted translation product of this variant would have a different carboxy-terminus than previously described versican isoforms.

Differential usage of two 5' splice sites in a complex exon generates additional protein sequence complexity in chicken CLIP-170 isoforms

Reverse transcription-polymerase chain reaction amplification and cloning of cDNA encoding the chicken CLIP-170(11) isoform of the Cytoplasmic Linker Protein 170 gene revealed an unusual source of protein sequence variation. In addition to differential combinatorial splicing of two cassette exons to yield four CLIP-170 protein isoforms, we found differential usage of alternative 5'-splice junctions in a single exon. Splicing at the downstream site yields message containing 18 bp of nucleotide sequence that is missing from message spliced at the more 5' site. This 18 bp sequence encodes a segment of 6 amino acids that fills a gap in the alignment of chicken and human CLIP-170 homologue sequences. Differential usage of the 5'-splice junctions in this complex exon appears to be tissue- rather than isoform-specific.

Dynamic expression of a native chondroitin sulfate epitope reveals microheterogeneity of extracellular matrix organization in the embryonic chick heart

TC2 is a novel monoclonal antibody produced by in vitro immunization of splenocytes with a peanut agglutinin-positive fraction from extracts of prechondrogenic micromass cultures of chick limb mesenchyme. ELISA results demonstrated TC2 reactivity with a native epitope on a glycosaminoglycan (GAG) enriched in chondroitin-4-sulfate and with multiple intact proteoglycans, but not with other GAGs tested. TC2 immunohistochemical reactivity was abolished by pretreatment of sections with chondroitinase AC or preadsorption with chondroitin-4-sulfate GAG. Strong TC2 localization occurred throughout the developing heart at stage 9. As looping ensued, a graded reactivity was observed from lowest in the atrium to highest in the conotruncus that correlated well with versican localization. The superior atrioventricular cushion stained preferentially with TC2 as compared to the inferior cushion at stages 16-18. At these later stages TC2 patterns did not agree completely with anti-versican reactivity. By stage 23 there was a marked reduction in TC2 localization in the heart, however, strong reactivity remained at certain sites, including the conotruncus and in subcompartments of both atrioventricular cushions. A heterogeneous distribution of other native chondroitin sulfate glycosaminoglycan epitopes recognized by monoclonal antibodies d1C4 and CS-56 was observed as well. The distribution of the TC2 epitope usually did not overlap with d1C4 or CS-56 localization at the stages examined. Overall, the spatiotemporal characteristics of TC2 reactivity in the developing chick heart appear to correlate with subdomains of the endocardial cushions as well as with trabecular and atrial septal formation.

Matrix proteoglycans: from molecular design to cellular function

The proteoglycan superfamily now contains more than 30 full-time molecules that fulfill a variety of biological functions. Proteoglycans act as tissue organizers, influence cell growth and the maturation of specialized tissues, play a role as biological filters and modulate growth-factor activities, regulate collagen fibrillogenesis and skin tensile strength, affect tumor cell growth and invasion, and influence corneal transparency and neurite outgrowth. Additional roles, derived from studies of mutant animals, indicate that certain proteoglycans are essential to life whereas others might be redundant. The review focuses on the most recent genetic and molecular biological studies of the matrix proteoglycans, broadly defined as proteoglycans secreted into the pericellular matrix. Special emphasis is placed on the molecular organization of the protein core, the utilization of protein modules, the gene structure and transcriptional control, and the functional roles of the various proteoglycans. When possible, proteoglycans have been grouped into distinct gene families and subfamilies offering a simplified nomenclature based on their protein core design. The structure-function relationship of some paradigmatic proteoglycans is discussed in depth and novel aspects of their biology are examined.

Versican V2 is a major extracellular matrix component of the mature bovine brain

We have isolated and characterized the proteoglycan isoforms of versican from bovine brain extracts. Our approach included (i) cDNA cloning and sequencing of the entire open reading frame encoding the bovine versican splice variants; (ii) preparation of antibodies against bovine versican using recombinant core protein fragments and synthetic peptides; (iii) isolation of versican isoforms by ammonium sulfate precipitation followed by anion exchange and hyaluronan affinity chromatography; and (iv) characterization by SDS-polyacrylamide gel electrophoresis and Coomassie Blue staining or immunoblotting. Our results demonstrate that versican V2 is, together with brevican, a major component of the mature brain extracellular matrix. Versicans V0 and V1 are only present in relatively small amounts. Versican V2 migrates after chondroitinase ABC digestion with an apparent molecular mass of about 400 kDa, whereas it barely enters a 4-15% polyacrylamide gel without the enzyme treatment. The 400-kDa product is recognized by antibodies against the glycosaminoglycan-alpha domain and against synthetic NH2- and COOH-terminal peptides. Our preparations contain no major proteolytic products of versican, e.g. hyaluronectin or glial hyaluronate-binding protein. Having biochemical quantities of versican V2 available will allow us to test its putative modulatory role in neuronal cell adhesion and axonal growth.