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

V3: an enigmatic isoform of the proteoglycan versican

V3 is an isoform of the extracellular matrix (ECM) proteoglycan (PG) versican generated through alternative splicing of the versican gene such that the two major exons coding for sequences in the protein core that support chondroitin sulfate (CS) glycosaminoglycan (GAG) chain attachment are excluded. Thus, versican V3 isoform carries no GAGs. A survey of PubMed reveals only 50 publications specifically on V3 versican, so it is a very understudied member of the versican family, partly because to date there are no antibodies that can distinguish V3 from the CS-carrying isoforms of versican, that is, to facilitate functional and mechanistic studies. However, a number of in vitro and in vivo studies have identified the expression of the V3 transcript during different phases of development and in disease, and selective overexpression of V3 has shown dramatic phenotypic effects in "gain and loss of function" studies in experimental models. Thus, we thought it would be useful and instructive to discuss the discovery, characterization, and the putative biological importance of the enigmatic V3 isoform of versican.

The synthesis and secretion of versican isoform V3 by mammalian cells: A role for N-linked glycosylation

Versican is a large extracellular matrix (ECM) chondroitin sulfate (CS) proteoglycan found in most soft tissues, which is encoded by the VCAN gene. At least four major isoforms (V0, V1, V2, and V3) are generated via alternative splicing. The isoforms of versican are expressed and accumulate in various tissues during development and disease, where they contribute to ECM structure, cell growth and migration, and immune regulation, among their many functions. While several studies have identified the mRNA transcript for the V3 isoform in a number of tissues, little is known about the synthesis, secretion, and targeting of the V3 protein. In this study, we used lentiviral generation of doxycycline-inducible rat V3 with a C-terminal tag in stable NIH 3T3 cell lines and demonstrated that V3 is processed through the classical secretory pathway. We further show that N-linked glycosylation is required for efficient secretion and solubility of the protein. By site-directed mutagenesis, we identified amino acids 57 and 330 as the active N-linked glycosylation sites on V3 when expressed in this cell type. Furthermore, exon deletion constructs of V3 revealed that exons 11-13, which code for portions of the carboxy region of the protein (G3 domain), are essential for V3 processing and secretion. Once secreted, the V3 protein associates with hyaluronan along the cell surface and within the surrounding ECM. These results establish critical parameters for the processing, solubility, and targeting of the V3 isoform by mammalian cells and establishes a role for V3 in the organization of hyaluronan.

Overexpression of Galnt3 in chondrocytes resulted in dwarfism due to the increase of mucin-type O-glycans and reduction of glycosaminoglycans

Galnt3, UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 3, transfers N-acetyl-D-galactosamine to serine and threonine residues, initiating mucin type O-glycosylation of proteins. We searched the target genes of Runx2, which is an essential transcription factor for chondrocyte maturation, in chondrocytes and found that Galnt3 expression was up-regulated by Runx2 and severely reduced in Runx2(-/-) cartilaginous skeletons. To investigate the function of Galnt3 in chondrocytes, we generated Galnt3(-/-) mice and chondrocyte-specific Galnt3 transgenic mice under the control of the Col2a1 promoter-enhancer. Galnt3(-/-) mice showed a delay in endochondral ossification and shortened limbs at embryonic day 16.5, suggesting that Galnt3 is involved in chondrocyte maturation. Galnt3 transgenic mice presented dwarfism, the chondrocyte maturation was retarded, the cell cycle in chondrocytes was accelerated, premature chondrocyte apoptosis occurred, and the growth plates were disorganized. The binding of Vicia villosa agglutinin, which recognizes the Tn antigen (GalNAc-O-Ser/Thr), was drastically increased in chondrocytes, and aggrecan (Acan) was highly enriched with Tn antigen. However, safranin O staining, which recognizes glycosaminoglycans (GAGs), and Acan were severely reduced. Chondroitin sulfate was reduced in amount, but the elongation of chondroitin sulfate chains had not been severely disturbed in the isolated GAGs. These findings indicate that overexpression of Galnt3 in chondrocytes caused dwarfism due to the increase of mucin-type O-glycans and the reduction of GAGs, probably through competition with xylosyltransferases, which initiate GAG chains by attaching O-linked xylose to serine residues, suggesting a negative effect of Galnt family proteins on Acan deposition in addition to the positive effect of Galnt3 on chondrocyte maturation.

Versican G3 domain modulates breast cancer cell apoptosis: a mechanism for breast cancer cell response to chemotherapy and EGFR therapy

Overexpression of EGFR and versican has been reported in association with breast cancers. Considered oncogenic, these molecules may be attractive therapeutic targets. Possessing anti-apoptotic and drug resistant properties, overexpression of these molecules is accompanied by selective sensitization to the process of apoptosis. In this study, we exogenously expressed a versican G3 construct in breast cancer cell lines and analyzed the effects of G3 on cell viability in fetal bovine serum free conditioned media and evaluated the effects of apoptotic agent C2-ceramide, and chemotherapeutic agents including Docetaxel, Doxorubicin, and Epirubicin. Versican G3 domain enhanced tumor cell resistance to apoptosis when cultured in serum free medium, Doxorubicin, or Epirubicin by up-regulating pERK and GSK-3β (S9P). However, it could be prevented by selective EGFR inhibitor AG 1478 and selective MEK inhibitor PD 98059. Both AG 1478 and PD 98059 enhanced expression of pSAPK/JNK, while selective JNK inhibitor SP 600125 enhanced expression of GSK-3β (S9P). Versican G3 promoted cell apoptosis induced by C2-ceramide or Docetaxel by enhancing expression of pSAPK/JNK and decreasing expression of GSK-3β (S9P), an observation blocked by AG 1478 or SP 6000125. Inhibition of endogenous versican expression by siRNA or reduction of versican G3's expression by linking G3 with 3'UTR prevented G3 modulated cell apoptosis. The dual roles of G3 in modulating breast cancer cell resistance to chemotherapeutic agents may in part explain a potential mechanism for breast cancer cell resistance to chemotherapy and EGFR therapy. The apoptotic effects of chemotherapeutics depend upon the activation and balance of down stream signals in the EGFR pathway. GSK-3β (S9P) appears to function as a key checkpoint in this balance of apoptosis and anti-apoptosis. Investigation and potential consideration of targeting GSK-3β (S9P) merits further study.

MicroRNA miR-378 regulates nephronectin expression modulating osteoblast differentiation by targeting GalNT-7

MicroRNAs (miRNAs) are small fragments of single-stranded RNA containing 18-24 nucleotides, and are generated from endogenous transcripts. MicroRNAs function in post-transcriptional gene silencing by targeting the 3'-untranslated region (UTR) of mRNAs, resulting in translational repression. We have developed a system to study the role of miRNAs in cell differentiation. We have found that one of the miRNAs tested in our system (miR-378, also called miR-378*) plays a role in modulating nephronectin-mediated differentiation in the osteoblastic cell line, MC3T3-E1. Nephronectin is an extracellular matrix protein, and we have demonstrated that its over-expression enhanced osteoblast differentiation and bone nodule formation. Furthermore, we found that the nephronectin 3'-untranslated region (3'UTR) contains a binding site for miR-378. Stable transfection of MC3T3-E1 cells with miR-378 inhibited cell differentiation. MC3T3-E1 cells stably transfected with nephronectin exhibited higher rates of differentiation and nodule formation as compared with cells transfected with nephronectin containing the 3'UTR in the early stages of development, suggesting that endogenous miR-378 is present and active. However, in the later stages of MC3T3-E1 development, the differentiation rates were opposite, with higher rates of differentiation and nodule formation in the cells over-expressing the 3'UTR of nephronectin. This appeared to be the consequence of competition between nephronectin and UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 7 (GalNAc-T7 or GalNT7) for miR-378 binding, resulting in increased GalNT7 activity, which in turn lead to increased nephronectin glycosylation and product secretion, thereby resulting in a higher rate of osteoblast differentiation.

Characterization of proteolytically digested zebrafish chorion as extracellular matrix

Extracellular matrix (ECM) plays pivotal roles in developments, wound healing, migration, angiogenesis, and other tissue developments. Developing new ECM may give us an innovative approach of controlling the cell and tissue behavior, regenerating damaged tissues, and other biotechnology usages. Here we report new ECM like biomaterials, pFZC, obtained from the fertilized Zebrafish chorion (FZC). pFZC was prepared by digesting the fertilized Zebrafish chorion with pronase. Differentiation of P19 embryonic carcinoma cells (P19 EC) on pFZC demonstrated that the level of the differentiation of P19 EC was comparable to cells on gelatin or fibronectin coated cover slide. Analyses of glycosylations of Zebrafish chorions using various Lectins revealed that during the fertilization, structurally organized glycosylations of Zebrafish chorion were increased and FZC was glycosylated with mannose, fucose, n-acetylglucosamine, and sialic acid. In addition, after FZC proteolytic digestion with pronase, the digested FZC proteins, pFZC, displayed similar glycosylation manner. SDS gel electrophoresis and mass spectroscopy analysis of pFZC revealed that the major component of pFZC is ZP2 protein. Newly identified ECM like biomaterials, pFZC, will provide more broad ways of controlling the cell and tissue environments.

Cartilage in normal and osteoarthritis conditions

The preservation of articular cartilage depends on keeping the cartilage architecture intact. Cartilage strength and function depend on both the properties of the tissue and on their structural parameters. The main structural macromolecules are collagen and proteoglycans (aggrecan). During life, cartilage matrix turnover is mediated by a multitude of complex autocrine and paracrine anabolic and catabolic factors. These act on the chondrocytes and can lead to repair, remodeling or catabolic processes like those that occur in osteoarthritis. Osteoarthritis is characterized by degradation and loss of articular cartilage, subchondral bone remodeling, and, at the clinical stage of the disease, inflammation of the synovial membrane. The alterations in osteoarthritic cartilage are numerous and involve morphologic and metabolic changes in chondrocytes, as well as biochemical and structural alterations in the extracellular matrix macromolecules.

Adaptation of sensory neurons to hyalectin and decorin proteoglycans

Proteoglycans are abundantly expressed in the pathways of developing and regenerating neurons, yet the responses of neurons to specific proteoglycans are not well characterized. We have shown previously that one chondroitin sulfate proteoglycan (CSPG), aggrecan, is potently inhibitory to sensory axon extension in short-term assays and that over time, embryonic neurons adapt to aggrecan-mediated inhibition through the transcriptional upregulation of integrin expression (Condic et al., 1999). Here, we have compared the response of embryonic sensory neurons to structurally distinct CSPGs that belong to either the hyalectin (or lectican) family of large, aggregating proteoglycans or the decorin (or small leucine-rich proteoglycan) family of smaller proteoglycans. Both of these structurally diverse proteoglycan families are expressed in developing embryos and inhibit outgrowth of embryonic sensory neurons in short-term cultures. These results document a previously uncharacterized inhibitory function for the decorin-family proteoglycan biglycan. Interestingly, embryonic neurons adapt to these diverse proteoglycans over time. Adaptation is associated with upregulation of select integrin alpha subunits in a proteoglycan-specific manner. Overexpression of specific integrin alpha subunits improves neuronal regeneration on some but not all decorin-family CSPGs, suggesting that neurons adapt to inhibition mediated by closely related proteoglycans using distinct mechanisms. Our findings indicate that CSPGs with diverse core proteins and distinct numbers of chondroitin sulfate substitution sites mediate a similar response in sensory neurons, suggesting that increased integrin expression may be an effective means of promoting axonal regeneration in the presence of diverse inhibitory proteoglycan species in vivo.

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.

The interaction of versican with its binding partners

Versican belongs to the family of the large aggregating chondroitin sulfate proteoglycans located primarily within the extracellular matrix (ECM). Versican, like other members of its family, has unique N- and C-terminal globular regions, each with multiple motifs. A large glycosaminoglycan-binding region lies between them. This review will begin by outlining these structures, in the context of ECM proteoglycans. The diverse binding partners afforded to versican by virtue of its modular design will then be examined. These include ECM components, such as hyaluronan, type I collagen, tenascin-R, fibulin-1, and -2, fibrillin-1, fibronectin, P- and L-selectins, and chemokines. Versican also binds to the cell surface proteins CD44, integrin beta 1, epidermal growth factor receptor, and P-selectin glycoprotein ligand-1. These multiple interactors play important roles in cell behaviour, and the roles of versican in modulating such processes are discussed.

A unique polypeptide from the C-terminus of the exocellular esterase of Acinetobacter venetianus RAG-1 modulates the emulsifying activity of the polymeric bioemulsifier apoemulsan

An exocellular esterase from the oil-degrading Acinetobacter venetianus RAG-1 was previously shown to enhance the emulsification and emulsion stabilization properties of the amphipathic, aminopolysaccharide bioemulsifier, emulsan [Bach H, Berdichevsky Y, Gutnick D (2003) An exocellular protein from the oil-degrading microbe Acinetobacter venetianus RAG-1 enhances the emulsifying activity of the polymeric bioemulsifier emulsan. Appl Environ Microbiol 69:2608-15]. This enhancement was specific for the RAG-1 esterase and was independent of catalytic activity. In this report, fragments from both the N'- and C'-termini were cloned as fusions to the C-terminus of the maltose-binding protein (MBP) and were tested for enhancement activity in the presence of the deproteinated form of emulsan, apoemulsan. The activity could be localized to the C-terminal third of the protein which exhibited the same activity as the intact enzyme. MBP itself was completely inactive and could be cleaved from the fusion without affecting the subsequent emulsification. However, the enhancement completely depended on the presence of a unique C-terminal 20 amino acid peptide not found in any other protein in the databases. In addition, progressive removal of amino acids from the N-terminus of the active MBP polypeptide resulted in a concomitant loss of activity, indicating that enhancement is also proportional to the size of the peptide fragment. The middle third and the C-terminal third of the enzyme each contained a copy of the conserved Cardin-Weintraub consensus sequence for protein binding to heparin. These sequences were not detected in homologous esterases from a closely related strain, Acinetobacter calcoaceticus BD413.

The roles of versican V1 and V2 isoforms in cell proliferation and apoptosis

Versican is a large chondroitin sulfate proteoglycan belonging to the lectican family. Alternative splicing of versican generates at least four isoforms named V0, V1, V2, and V3. We have shown that the versican V1 isoform not only enhanced cell proliferation, but also modulated cell cycle progression and protected the cells from apoptosis. Futhermore, the V1 isoform was able to not only activate proto-oncogene EGFR expression and modulate its downstream signaling pathway, but also induce p27 degradation and enhance CDK2 kinase activity. As well, the V1 isoform down-regulated the expression of the proapoptotic protein Bad. By contrast, the V2 isoform exhibited opposite biological activities by inhibiting cell proliferation and down-regulated the expression of EGFR and cyclin A. Furthermore, V2 did not contribute apoptotic resistance to the cells. In light of these results, we are reporting opposite functions for the two versican isoforms whose expression is differentially regulated. Our studies suggest that the roles of these two isoforms are associated with the subdomains CSbeta and CSalpha, respectively. These results were confirmed by silencing the expression of versican V1 with small interfering RNA (siRNA), which abolished V1-enhanced cell proliferation and V1-induced reduction of apoptosis.

PG-M/versican binds to P-selectin glycoprotein ligand-1 and mediates leukocyte aggregation

P-selectin glycoprotein ligand-1 (PSGL-1), a glycoprotein expressed on the cell surface of leukocytes, binds to selectins and mediates leukocyte rolling on the vascular endothelium. Here we report that PSGL-1 binds to the C-terminal (G3 domain) of the extracellular proteoglycan PG-M/versican. Cells transfected with PSGL-1 or a shorter form containing the binding site, or cells expressing endogenous PSGL-1 aggregate in the presence of versican or G3 product. The aggregation appears to be induced by G3 multimers that bind to PSGL-1 and form a network. Endogenous versican and/or G3-containing fragments also bind to PSGL-1 in human plasma. Removal of the endogenous G3-containing fragments reduces the effect of plasma on leukocyte aggregation. Finally, the roles of G3-containing fragments in leukocyte aggregation were confirmed in a mouse model. Taken together, our results strongly support a physiologically relevant role for PSGL-1/versican binding and may have implications in the immunoresponse.

Versican V1 isoform induces neuronal differentiation and promotes neurite outgrowth

The chondroitin sulfate proteoglycan versican is one of the major extracellular components in the developing and adult brain. Here, we show that isoforms of versican play different roles in neuronal differentiation and neurite outgrowth. Expression of versican V1 isoform in PC12 cells induced complete differentiation, whereas expression of V2 induced an aborted differentiation accompanied by apoptosis. V1 promoted neurite outgrowth of hippocampal neurons, but V2 failed to do so. V1 transfection enhanced expression of epidermal growth factor receptor and integrins, and facilitated sustained extracellular signal-regulated kinase/MAPK phosphorylation. Blockade of the epidermal growth factor receptor, beta1 integrin, or Src significantly inhibited neuronal differentiation. Finally, we demonstrated that versican V1 isoform also promoted differentiation of neural stem cells into neurons. Our results have implications for understanding how versican regulates neuronal development, function, and repair.

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.

Cleavage of the carboxyl tail from the G3 domain of aggrecan but not versican and identification of the amino acids involved in the degradation

Aggrecan, a major structural proteoglycan in cartilage, contains three globular domains, G1, G2, and G3, as well as sequences for glycosaminoglycan modification. A large number of proteases are implicated in aggrecan cleavage in normal metabolism, aging, and arthritis. These proteases are known to cleave at the IGD, KS, and CS domains. Here we report for the first time evidence of cleavage at a novel site, the carboxyl tail of aggrecan. Results from deletion mutants of the tail indicated that the likely cleavage sites were two consensus sequences, RRLXK and RSPR, present in the aggrecan analogs of many species. This was confirmed by site-directed mutagenesis. A construct containing two G3 domains (G3G3) was also found to cleave between the G3 duplicates. When G3 tail was linked to a glycosaminoglycan-modifying sequence, it was protected from cleavage. Furin inhibitor also reduced the levels of tail cleavage. The carboxyl tails of chicken and human versican were not cleaved, despite the presence of the consensus sequence. Our studies indicate that the basic amino acids present in the tail play an important role in cleavage, and this mechanism is specific to aggrecan.

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.

Structure and function of aggrecan

Aggrecan is the major proteoglycan in the articular cartilage. This molecule is important in the proper functioning of articular cartilage because it provides a hydrated gel structure (via its interaction with hyaluronan and link protein) that endows the cartilage with load-bearing properties. It is also crucial in chondroskeletal morphogenesis during development. Aggrecan is a multimodular molecule expressed by chondrocytes. Its core protein is composed of three globular domains (G1, G2, and G3) and a large extended region (CS) between G2 and G3 for glycosaminoglycan chain attachment. G1 comprises the amino terminus of the core protein. This domain has the same structural motif as link protein. Functionally, the G1 domain interacts with hyaluronan acid and link protein, forming stable ternary complexes in the extracellular matrix. G2 is homologous to the tandem repeats of G1 and of link protein and is involved in product processing. G3 makes up the carboxyl terminus of the core protein. It enhances glycosaminoglycan modification and product secretion. Aggrecan plays an important role in mediating chondrocyte-chondrocyte and chondrocyte-matrix interactions through its ability to bind hyaluronan.