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

Integrin-mediated cell-matrix interaction in physiological and pathological blood vessel formation

Physiological as well as pathological blood vessel formation are fundamentally dependent on cell-matrix interaction. Integrins, a family of major cell adhesion receptors, play a pivotal role in development, maintenance, and remodeling of the vasculature. Cell migration, invasion, and remodeling of the extracellular matrix (ECM) are integrin-regulated processes, and the expression of certain integrins also correlates with tumor progression. Recent advances in the understanding of how integrins are involved in the regulation of blood vessel formation and remodeling during tumor progression are highlighted. The increasing knowledge of integrin function at the molecular level, together with the growing repertoire of integrin inhibitors which allow their selective pharmacological manipulation, makes integrins suited as potential diagnostic markers and therapeutic targets.

Proteoglycan degradation by the ADAMTS family of proteinases

Proteoglycans are key components of extracellular matrices, providing structural support as well as influencing cellular behaviour in physiological and pathological processes. The diversity of proteoglycan function reported in the literature is equally matched by diversity in proteoglycan structure. Members of the ADAMTS (A Disintegrin And Metalloproteinase with ThromboSpondin motifs) family of enzymes degrade proteoglycans and thereby have the potential to alter tissue architecture and regulate cellular function. In this review, we focus on ADAMTS enzymes that degrade the lectican and small leucine-rich repeat families of proteoglycans. We discuss the known ADAMTS cleavage sites and the consequences of cleavage at these sites. We illustrate our discussion with examples from the literature in which ADAMTS proteolysis of proteoglycans makes profound changes to tissue function.

Altered versican cleavage in ADAMTS5 deficient mice; a novel etiology of myxomatous valve disease

In fetal valve maturation the mechanisms by which the relatively homogeneous proteoglycan-rich extracellular matrix (ECM) of endocardial cushions is replaced by a specialized and stratified ECM found in mature valves are not understood. Therefore, we reasoned that uncovering proteases critical for 'remodeling' the proteoglycan rich (extracellular matrix) ECM may elucidate novel mechanisms of valve development. We have determined that mice deficient in ADAMTS5, (A Disintegrin-like And Metalloprotease domain with ThromboSpondin-type 1 motifs) which we demonstrated is expressed predominantly by valvular endocardium during cardiac valve maturation, exhibited enlarged valves. ADAMTS5 deficient valves displayed a reduction in cleavage of its substrate versican, a critical cardiac proteoglycan. In vivo reduction of versican, in Adamts5(-/-) mice, achieved through Vcan heterozygosity, substantially rescued the valve anomalies. An increase in BMP2 immunolocalization, Sox9 expression and mesenchymal cell proliferation were observed in Adamts5(-/-) valve mesenchyme and correlated with expansion of the spongiosa (proteoglycan-rich) region in Adamts5(-/-) valve cusps. Furthermore, these data suggest that ECM remodeling via ADAMTS5 is required for endocardial to mesenchymal signaling in late fetal valve development. Although adult Adamts5(-/-) mice are viable they do not recover from developmental valve anomalies and have myxomatous cardiac valves with 100% penetrance. Since the accumulation of proteoglycans is a hallmark of myxomatous valve disease, based on these data we hypothesize that a lack of versican cleavage during fetal valve development may be a potential etiology of adult myxomatous valve disease.

Versican and CD44 in in vitro valvular interstitial cell injury and repair

BACKGROUND

Versican is one of the key components of the extracellular matrix (ECM) that is expressed during injury, inflammatory, and repair processes. The current study evaluated the relationship between versican and the membrane receptor CD44 during in vitro valvular interstitial cell (VIC) injury and repair.

METHODS

Subconfluent, confluent, and wounded cultures of human VICs were fixed and immunostained to detect versican and the membrane receptor CD44. To examine the relationship between versican and CD44, a blocking antibody to CD44 was added to cultured VICs, and in vitro wound repair along with pericellular versican organization and stress fiber formation were examined.

RESULTS

Immunohistochemistry demonstrated that versican is prominent intracellularly, as well as extracellularly, in actively proliferating VICs. In contrast, versican was only localized to fibrils in the extracellular space in between cells in confluent (quiescent) cultures. Following wounding, versican expression was up-regulated, and it was secreted as ECM at the trailing edge of migrating cells. The staining for CD44 was similarly localized to the trailing edge of migrating VICs in wounded cultures. Treatment of VICs with a CD44-blocking antibody disrupted the organization of versican in the pericellular matrix and inhibited stress fiber formation in these cells. Functionally, blocking CD44 significantly inhibited VIC-mediated contraction of type I collagen gels (35.7%±0.7% vs. 23.3%±1.4% of initial gel area, P<.01).

CONCLUSIONS

Versican is a key component of the provisional wound repair ECM that is expressed following injury to VICs. The receptor CD44 plays an important role in organizing the provisional ECM.

SUMMARY

Our data suggests VICs synthesize and secrete versican following injury. These cells also up-regulate CD44, a receptor that binds versican. Blocking CD44 disrupted the organization of versican and inhibited stress fiber formation. Functionally, blocking CD44 inhibited cell-mediated contraction of a collagen matrix. Collectively, these data suggest versican expression and organization are important to valve cell injury and repair.

Impact of pre-existing elastic matrix on TGFβ1 and HA oligomer-induced regenerative elastin repair by rat aortic smooth muscle cells

Regenerating elastic matrices lost to disease (e.g. in aneurysms) is vital to re-establishing vascular homeostasis but is challenged by the poor elastogenicity of post-neonatal cells. We previously showed that exogenous hyaluronan oligomers (HA-o) and TGFβ1 synergistically enhance tropo- and matrix elastin deposition by healthy adult rat aortic SMCs (RASMCs). Towards treating aortic aneurysms (AAs), which exhibit cause- and site-specific heterogeneity in matrix content/structure and contain proteolytically-injured SMCs, we investigated the impact of pre-existing elastic matrix degeneration on elastogenic induction of injured RASMCs. Elastin-rich RASMC layers at 21 days of culture were treated with 0.15 U/ml (PPE15) and 0.75 U/ml (PPE75) porcine pancreatic elastase to degrade the elastic matrix variably, or left uninjured (control). One set of cultures was harvested at 21 days, before and after injury, to quantify viable cell count, matrix elastin loss. Other injured cell layers were cultured to 42 days with or without factors (0.2 µg/ml HA oligomers, 1 ng/ml TGFβ1). We showed that: (a) the ability of cultures to self-repair and regenerate elastic matrices following proteolysis is limited when elastolysis is severe; (b) HA oligomers and TGFβ1 elastogenically stimulate RASMCs in mildly-injured (i.e. PPE15) cultures to restore both elastic matrix amounts and elastic fibre deposition to levels in healthy cultures; and (c) in severely injured (i.e. PPE75) cultures, the factors stimulate matrix elastin synthesis and crosslinking, although not to control levels. The outcomes underscore the need to enhance elastogenic factor doses based on the severity of elastin loss. This study will help in customizing therapies for elastin regeneration within AAs, based on cause and location.

Versican G3 promotes mouse mammary tumor cell growth, migration, and metastasis by influencing EGF receptor signaling

Increased versican expression in breast tumors is predictive of relapse and has negative impact on survival rates. The C-terminal G3 domain of versican influences local and systemic tumor invasiveness in pre-clinical murine models. However, the mechanism(s) by which G3 influences breast tumor growth and metastasis is not well characterized. Here we evaluated the expression of versican in mouse mammary tumor cell lines observing that 4T1 cells expressed highest levels while 66c14 cells expressed low levels. We exogenously expressed a G3 construct in 66c14 cells and analyzed its effects on cell proliferation, migration, cell cycle progression, and EGFR signaling. Experiments in a syngeneic orthotopic animal model demonstrated that G3 promoted tumor growth and systemic metastasis in vivo. Activation of pERK correlated with high levels of G3 expression. In vitro, G3 enhanced breast cancer cell proliferation and migration by up-regulating EGFR signaling, and enhanced cell motility through chemotactic mechanisms to bone stromal cells, which was prevented by inhibitor AG 1478. G3 expressing cells demonstrated increased CDK2 and GSK-3β (S9P) expression, which were related to cell growth. The activity of G3 on mouse mammary tumor cell growth, migration and its effect on spontaneous metastasis to bone in an orthotopic model was modulated by up-regulating the EGFR-mediated signaling pathway. Taken together, EGFR-signaling appears to be an important pathway in versican G3-mediated breast cancer tumor invasiveness and metastasis.

Identifying the human aggrecanase

It is clear that A Disintegrin And Metalloproteinase with ThromboSpondin motif (ADAMTS)-5 is the major aggrecanase in mouse cartilage, however it is not at all clear which enzyme is the major aggrecanase in human cartilage. Identifying the human aggrecanase is difficult because multiple, independent, molecular processes determine the final level of enzyme activity. As investigators, we have good methods for measuring changes in the expression of ADAMTS mRNA, and good methods for detecting aggrecanase activity, but no methods that distinguish the source of the activity. In between gene expression and enzyme action there are many processes that can potentially enhance or inhibit the final level of activity. In this editorial we discuss how each of these processes affects ADAMTS activity and argue that measuring any one process in isolation has little value in predicting overall ADAMTS activity in vivo.

Genome-wide expression profiling reveals new candidate genes associated with osteoarthritis

INTRODUCTION

Although the extracellular matrix (ECM) is the functional element in articular cartilage and its degradation is central in the pathogenetic process in osteoarthritis (OA), increasing the knowledge about the cellular OA phenotype is essential. The aim of this study is therefore to provide a more complete picture of the cellular and molecular alterations detected in OA cartilage.

MATERIAL AND METHODS

Human articular cartilage biopsies were collected from donors with macroscopical and microscopical signs of OA as well as donors with no previous history of OA and with microscopically intact cartilage. RNA was isolated from the biopsies and subjected to whole genome microarray analysis. Important results from the microarray analysis were verified using real-time PCR and immunohistochemistry.

RESULTS

Our results reveal several new candidate genes not previously associated with OA to display significantly higher expression in OA cartilage than in normal donor cartilage, including genes involved in bone formation (CLEC3B, CDH11, GPNMB, CLEC3A, CHST11, MSX1, MSX2) and genes encoding collagens (COL13A1, COL14A1, COL15A1, COL8A2).

DISCUSSION

This study is the first to report a comprehensive gene expression analysis of human OA cartilage compared to control cartilage from donors lacking macroscopical and microscopical signs of OA using recently developed microarrays containing the whole human genome. Our results could broadly confirm previously published data on many characteristic features of OA as well as adding a panel of genes to the list of genes known to be differentially expressed in OA. Elucidation of the phenotypical alterations occurring in OA chondrocytes is important for the development of effective treatments for OA.

A missense mutation in the aggrecan C-type lectin domain disrupts extracellular matrix interactions and causes dominant familial osteochondritis dissecans

Osteochondritis dissecans is a disorder in which fragments of articular cartilage and subchondral bone dislodge from the joint surface. We analyzed a five-generation family in which affected members had autosomal-dominant familial osteochondritis dissecans. A genome-wide linkage analysis identified aggrecan (ACAN) as a prime candidate gene for the disorder. Sequence analysis of ACAN revealed heterozygosity for a missense mutation (c.6907G > A) in affected individuals, resulting in a p.V2303M amino acid substitution in the aggrecan G3 domain C-type lectin, which mediates interactions with other proteins in the cartilage extracellular matrix. Binding studies with recombinant mutated and wild-type G3 proteins showed loss of fibulin-1, fibulin-2, and tenascin-R interactions for the V2303M protein. Mass spectrometric analyses of aggrecan purified from patient cartilage verified that V2303M aggrecan is produced and present in the tissue. Our results provide a molecular mechanism for the etiology of familial osteochondritis dissecans and show the importance of the aggrecan C-type lectin interactions for cartilage function in vivo.

Transcriptional profiling of bovine intervertebral disc cells: implications for identification of normal and degenerate human intervertebral disc cell phenotypes

INTRODUCTION

Nucleus pulposus (NP) cells have a phenotype similar to articular cartilage (AC) cells. However, the matrix of the NP is clearly different to that of AC suggesting that specific cell phenotypes exist. The aim of this study was to identify novel genes that could be used to distinguish bovine NP cells from AC and annulus fibrosus (AF) cells, and to further determine their expression in normal and degenerate human intervertebral disc (IVD) cells.

METHODS

Microarrays were conducted on bovine AC, AF and NP cells, using Affymetrix Genechip(R) Bovine Genome Arrays. Differential expression levels for a number of genes were confirmed by quantitative real time polymerase chain reaction (qRT-PCR) on bovine, AC, AF and NP cells, as well as separated bovine NP and notochordal (NC) cells. Expression of these novel markers were further tested on normal human AC, AF and NP cells, and degenerate AF and NP cells.

RESULTS

Microarray comparisons between NP/AC&AF and NP/AC identified 34 NP-specific and 49 IVD-specific genes respectively that were differentially expressed > or =100 fold. A subset of these were verified by qRT-PCR and shown to be expressed in bovine NC cells. Eleven genes (SNAP25, KRT8, KRT18, KRT19, CDH2, IBSP, VCAN, TNMD, BASP1, FOXF1 & FBLN1) were also differentially expressed in normal human NP cells, although to a lesser degree. Four genes (SNAP25, KRT8, KRT18 and CDH2) were significantly decreased in degenerate human NP cells, while three genes (VCAN, TNMD and BASP1) were significantly increased in degenerate human AF cells. The IVD negative marker FBLN1 was significantly increased in both degenerate human NP and AF cells.

CONCLUSIONS

This study has identified a number of novel genes that characterise the bovine and human NP and IVD transcriptional profiles, and allows for discrimination between AC, AF and NP cells. Furthermore, the similarity in expression profiles of the separated NP and NC cell populations suggests that these two cell types may be derived from a common lineage. Although interspecies variation, together with changes with IVD degeneration were noted, use of this gene expression signature will benefit tissue engineering studies where defining the NP phenotype is paramount.

Reduced versican cleavage due to Adamts9 haploinsufficiency is associated with cardiac and aortic anomalies

Here, we demonstrate that ADAMTS9, a highly conserved versican-degrading protease, is required for correct cardiovascular development and adult homeostasis. Analysis of Adamts9(+/LacZ) adult mice revealed anomalies in the aortic wall, valvulosinus and valve leaflets. Abnormal myocardial projections and 'spongy' myocardium consistent with non-compaction of the left ventricle were also found in Adamts9(+/LacZ) mice. During development, Adamts9 was expressed in derivatives of the Secondary Heart Field, vascular smooth muscle cells in the arterial wall, mesenchymal cells of the valves, and non-myocardial cells of the ventricles, but expression also continued in the adult heart and ascending aorta. Thus, the adult cardiovascular anomalies found in Adamts9(+/LacZ) hearts could result from subtle developmental alterations in extracellular matrix remodeling or defects in adult homeostasis. The valvular and aortic anomalies of Adamts9(+/LacZ) hearts were associated with accumulation of versican and a decrease in cleaved versican relative to WT littermates. These data suggest a potentially important role for ADAMTS9 cleavage of versican, or other, as yet undefined substrates in development and allostasis of cardiovascular extracellular matrix. In addition, these studies identify ADAMTS9 as a potential candidate gene for congenital cardiac anomalies. Mouse models of ADAMTS9 deficiency may be useful to study myxomatous valve degeneration.

ADAMTS metalloproteases generate active versican fragments that regulate interdigital web regression

We show that combinatorial mouse alleles for the secreted metalloproteases Adamts5, Adamts20 (bt), and Adamts9 result in fully penetrant soft-tissue syndactyly. Interdigital webs in Adamts5(-/-);bt/bt mice had reduced apoptosis and decreased cleavage of the proteoglycan versican; however, the BMP-FGF axis, which regulates interdigital apoptosis was unaffected. BMP4 induced apoptosis, but without concomitant versican proteolysis. Haploinsufficiency of either Vcan or Fbln1, a cofactor for versican processing by ADAMTS5, led to highly penetrant syndactyly in bt mice, suggesting that cleaved versican was essential for web regression. The local application of an aminoterminal versican fragment corresponding to ADAMTS-processed versican, induced cell death in Adamts5(-/-);bt/bt webs. Thus, ADAMTS proteases cooperatively maintain versican proteolysis above a required threshold to create a permissive environment for apoptosis. The data highlight the developmental significance of proteolytic action on the ECM, not only as a clearance mechanism, but also as a means to generate bioactive versican fragments.

Transforming growth factor beta3 regulates the versican variants in the extracellular matrix-rich uterine leiomyomas

Uterine leiomyoma are common, benign tumors that are enriched in extracellular matrix. The tumors are characterized by a disoriented and loosely packed collagen fibril structure similar to other diseases with disrupted Transforming growth factor beta (TGF-beta) signaling. Here we characterized TGF-beta3 signaling and the expression patterns of the critical extracellular matrix component versican in leiomyoma and myometrial tissue and cell culture. We also demonstrate the regulation of the versican variants by TGF-beta3. Using leiomyoma and matched myometrium from 15 patients, messenger RNA (mRNA) from leiomyoma and myometrium was analyzed by semiquantitative real time reverse transcription-polymerase chain reaction (RT-PCR), while protein analysis was done by western blot. Transforming growth factor beta3 transcripts were increased 4-fold in leiomyoma versus matched myometrium. Phosphorylated-TGF-beta RII and phosphorylated-Smad 2/3 complex were greater in leiomyoma as documented by Western blot. The inhibitor Smad7 transcripts were decreased 0.44-fold. The glycosaminoglycan (GAG)-rich versican variants were elevated in leiomyoma versus myometrial tissue: specifically V0 (4.27 +/- 1.12) and V1 (2.01 +/- 0.27). Treatment of leiomyoma and myometrial cells with TGF-beta3 increased GAG-rich versican variant expression 7 to 12 fold. Neutralizing TGF-beta3 antibody decreased the expression of the GAG-rich versican variants 2 to 8 fold in leiomyoma cells. Taken together, the aberrant production of excessive and disorganized extracellular matrix that defines the leiomyoma phenotype involves the activation of the TGF-beta signaling pathway and excessive production of GAG-rich versican variants.

The biological role and regulation of versican levels in cancer

Increased expression of the proteoglycan, versican is strongly associated with poor outcome for many different cancers. Depending on the cancer type, versican is expressed by either the cancer cells themselves or by stromal cells surrounding the tumor. Versican plays diverse roles in cell adhesion, proliferation, migration and angiogenesis, all features of invasion and metastasis. These wide ranging functions have been attributed to the central glycosaminoglycan-binding region of versican, and to the N-(G1) and C-(G3) terminal globular domains which collectively interact with a large number of extracellular matrix and cell surface structural components. Here we review the recently identified mechanisms responsible for the regulation of versican expression and the biological roles that versican plays in cancer invasion and metastasis. The regulation of versican expression may represent one mechanism whereby cancer cells alter their surrounding microenvironment to facilitate the malignant growth and invasion of several tumor types. A greater understanding of the regulation of versican expression may contribute to the development of therapeutic methods to inhibit versican function and tumor invasion.

Topographical variation in the distributions of versican, aggrecan and perlecan in the foetal human spine reflects their diverse functional roles in spinal development

We evaluated the immunohistochemical distribution of three major proteoglycans of cartilage, i.e., aggrecan, versican and perlecan vis-a-vis collagens I and II in the developing human spine of first-trimester foetuses. Aggrecan and perlecan were prominently immunolocalised in the cartilaginous vertebral body rudiments and to a lesser extent within the foetal intervertebral disc. In contrast, versican was only expressed in the developing intervertebral disc interspace. Using domain-specific monoclonal antibodies against the various modules of versican, we discovered the V0 isoform as the predominant form present. Versican immunolocalisations conducted with antibodies directed to epitopes in its N and C termini and GAG-alpha and GAG-beta core protein domains provided evidence that versican in the nucleus pulposus was either synthesised devoid of a G3 domain or this domain was proteolytically removed in situ. The V0 versican isoform was localised with prominent fibrillar components in the annular lamellae of the outer annulus fibrosus. Perlecan was a notable pericellular proteoglycan in the annulus fibrosus and nucleus pulposus but poorly immunolocalised in the marginal tissues of the developing intervertebral disc, apparently delineating the intervertebral disc-vertebral body interface region destined to become the cartilaginous endplate in the mature intervertebral disc. The distribution of collagens I and II in the foetal spine was mutually exclusive with type I present in the outer annulus fibrosus, marginal tissues around the vertebral body rudiment and throughout the developing intervertebral disc, and type II prominent in the vertebral rudiment, absent in the outer annulus fibrosus and diffusely distributed in the inner annulus fibrosus and nucleus pulposus. Collectively, our findings suggest the existence of an intricate and finely balanced interplay between various proteoglycans and collagens and the spinal cell populations which synthesise and assemble these components during spinal development.

N-terminal domains of CCN family 2/connective tissue growth factor bind to aggrecan

CCN2/CTGF (CCN family 2/connective tissue growth factor) is a multi-cellular protein with a broad range of activities. It modulates many cellular functions, including proliferation, migration, adhesion and extracellular matrix production, and it is thus involved in many biological and pathological processes. In particular, CCN2/CTGF is essential for normal skeletal development. To identify CCN2/CTGF-interactive proteins capable of modulating its action in cartilage, we carried out a yeast two-hybrid screening using CCN2/CTGF peptide as a bait and a cDNA library from a chondrocytic cell line, HCS-2/8. In the present paper, we report the identification of aggrecan, which is a major proteoglycan of the extracellular matrix in cartilage, as a CCN2/CTGF-binding protein. Among the four domains of CCN2/CTGF, the IGFBP [IGF (insulin-like growth factor)-binding protein-like] and/or VWC (von Willebrand factor type C) domains had a direct interaction with aggrecan in a yeast two-hybrid assay. The results of a solid-phase-binding assay using aggrecan-coated plates also showed binding to recombinant CCN2/CTGF in a dose-dependent manner. rIGFBP (recombinant IGFBP) and rVWC (recombinant VWC) module peptides had stronger binding to aggrecan compared with rTSP1 (recombinant thrombospondin type 1 repeat) and rCT (recombinant C-terminal cystine knot) module peptides. SPR (surface plasmon resonance) analysis showed the direct interaction between the CCN2/CTGF and aggrecan, and ectopically overexpressed CCN2/CTGF and AgG3 (G3 domain of aggrecan) confirmed their binding In vivo. Indirect immunofluorescence analysis indicated that CCN2/CTGF was extracellularly co-localized with aggrecan on HCS-2/8 cells. The rIGFBP-rVWC peptide effectively enhanced the production and release of aggrecan compared with the rTSP-rCT peptide in chondrocytes. These results indicate that CCN2/CTGF binds to aggrecan through its N-terminal IGFBP and VWC modules, and this binding may be related to the CCN2/CTGF-enhanced production and secretion of aggrecan by chondrocytes.

Versican/PG-M Assembles Hyaluronan into Extracellular Matrix and Inhibits CD44-mediated Signaling toward Premature Senescence in Embryonic Fibroblasts

Versican/PG-M is a large chondroitin sulfate proteoglycan of the extracellular matrix which interacts with hyaluronan at the N-terminal G1 domain, composed of A, B, and B' subdomains. Recently, we generated knock-in mice Cspg2(Delta3/Delta3), whose versican, without the A subdomain, has decreased hyaluronan (HA) binding affinity, thereby exhibiting reduced deposition of versican in the extracellular matrix. Here, we show that the Cspg2(Delta3/Delta3) fibroblasts within 20 passages proliferate more slowly and acquire senescence. Whereas the extracellular matrix of the wild type fibroblasts exhibited a network structure of hyaluronan and versican, that of the Cspg2(Delta3/Delta3) fibroblasts exhibited approximately 35 and approximately 85% deposition of versican and HA, without such a structure. The Cspg2(Delta3/Delta3) fibroblasts showed a substantial increase of ERK1/2 phosphorylation and expression of senescence markers p53, p21, and p16. Treatment of wild type fibroblasts with hyaluronidase and exogenous hyaluronan enhanced ERK1/2 phosphorylation, and treatment with an anti-CD44 antibody that blocks HA-CD44 interaction inhibited the phosphorylation. These results demonstrate that versican is essential for matrix assembly involving hyaluronan and that diminished versican deposition increases free hyaluronan fragments that interact with CD44 and increase phosphorylation of ERK1/2, leading to cellular senescence.

Alteration of chondroitin sulfate composition on proteoglycan produced by knock-in mouse embryonic fibroblasts whose versican lacks the A subdomain

Versican/proteoglycan-mesenchymal (PG-M) is a large chondroitin sulfate (CS) proteoglycan of the extracellular matrix (ECM) that is constitutively expressed in adult tissues such as dermis and blood vessels. It serves as a structural macromolecule of the ECM, while in embryonic tissue it is transiently expressed at high levels and regulates cell adhesion, migration, proliferation, and differentiation. Knock-in mouse embryonic (Cspg2(Delta3/Delta3)) fibroblasts whose versican lack the A subdomain of the G1 domain exhibit low proliferation rates and acquire senescence. It was suspected that chondroitin sulfate on versican core protein would be altered when the A subdomain was disrupted, so fibroblasts were made from homozygous Cspg2(Delta3/Delta3) mouse embryos to investigate the hypothesis. Analysis of the resulting versican deposition demonstrated that the total versican deposited in the Cspg2(Delta3/Delta3) fibroblasts culture was approximately 50% of that of the wild type (WT), while the versican deposited in the ECM of Cspg2(Delta3/Delta3) fibroblasts culture was 35% of that of the WT, demonstrating the lower capacity of mutant (Cspg2(Delta3/Delta3)) versican deposited in the ECM. The analysis of CS expression in the Cspg2(Delta3/Delta3) fibroblasts culture compared with wild-type fibroblasts showed that the composition of the non-sulfate chondroitin sulfate isomer on the versican core protein increased in the cell layer but decreased in the culture medium. Interestingly, chondroitin sulfate E isomer was found in the culture medium. The amount of CS in the Cspg2(Delta3/Delta3) cell layer of fibroblasts with mutant versican was dramatically decreased, contrasted to the amount in the culture medium, which increased. It was concluded that the disruption of the A subdomain of the versican molecule leads to lowering of the amount of versican deposited in the ECM and the alteration of the composition and content of CS on the versican molecule.

MIG-17/ADAMTS controls cell migration by recruiting nidogen to the basement membrane in C. elegans

Mutations in the a disintegrin and metalloprotease with thrombospondin motifs (ADAMTS) family of secreted proteases cause diseases linked to ECM abnormalities. However, the mechanisms by which these enzymes modulate the ECM during development are mostly unexplored. The Caenorhabditis elegans MIG-17/ADAMTS protein is secreted from body wall muscle cells and localizes to the basement membrane (BM) of the developing gonad where it controls directional migration of gonadal leader cells. Here we show that specific amino acid changes in the ECM proteins fibulin-1C (FBL-1C) and type IV collagen (LET-2) result in bypass of the requirement for MIG-17 activity in gonadal leader cell migration in a nidogen (NID-1)-dependent and -independent manner, respectively. The MIG-17, FBL-1C and LET-2 activities are required for proper accumulation of NID-1 at the gonadal BM. However, mutant FBL-1C or LET-2 in the absence of MIG-17 promotes NID-1 localization. Furthermore, overexpression of NID-1 in mig-17 mutants substantially rescues leader cell migration defects. These results suggest that functional interactions among BM molecules are important for MIG-17 control of gonadal leader cell migration. We propose that FBL-1C and LET-2 act downstream of MIG-17-dependent proteolysis to recruit NID-1 and that LET-2 also activates a NID-1-independent pathway, thereby inducing the remodeling of the BM required for directional control of leader cell migration.

Localization of chondroitin sulfate proteoglycan versican in adult brain with special reference to large projection neurons

Versican is a chondroitin sulfate proteoglycan belonging to the lectican family. Versican has two glycosaminoglycan attachment regions, named the GAG alpha and GAG beta domains, which are both regulated by alternative splicing and yield four protein isoforms. We have investigated the expression and localization of versican in the developing and adult brain by using anti-versican GAG alpha and GAG beta antibodies. Western analysis revealed that GAG alpha-reactive isoform was dominant in the adult brain. Immunohistochemical study demonstrated that GAG alpha immunoreactivity was detectable from neonatal periods to adulthood, whereas GAG beta immunoreactivity completely disappeared within 3 weeks of birth. In the adult brain, GAG alpha immunoreactivity was seen in the white matter regions and was also localized in the gray matter including somata and dendrites of cortical and hippocampal pyramidal neurons and cerebellar Purkinje cells. In contrast, GAG alpha immunoreactivity was not localized on parvalbumin-positive interneurons and cerebellar stellate cells. Furthermore, GAG alpha immunoreactivity was not co-localized with perineuronal net markers such as Wisteria floribunda agglutinin lectin and phosphacan. Thus, versican was localized on large projection neurons rather than small interneurons. To confirm the binding mechanism of versican to neurons, hyaluronan and chondroitin sulfates were enzymatically removed from brain sections before the immunolabeling of versican. These treatments had no effect on the labeling pattern of versican, suggesting that other versican-interactive molecules are involved in the binding of versican to neurons.

Deoxyribozyme-mediated knockdown of xylosyltransferase-1 mRNA promotes axon growth in the adult rat spinal cord

In the injured spinal cord, proteoglycans (PGs) within scar tissue obstruct axon growth through their glycosaminoglycan (GAG)-side chains. The formation of GAG-side chains (glycosylation) is catalysed by xylosyltransferase-1 (XT-1). Here, we knocked down XT-1 mRNA using a tailored deoxyribozyme (DNAXTas) and hypothesized that this would decrease the amount of glycosylated PGs and, consequently, promote axon growth in the adult rat spinal cord. A continuous 2-week delivery of DNAXTas near the rostral border of a peripheral nerve graft bridging the transected dorsal columns in the thoracic spinal cord resulted in an 81% decrease in XT-1 mRNA, an average of 1.4-fold reduction in GAG-side chains of chondroitin sulphate or heparan sulphate-PGs and 2.2-fold reduction in neurocan and brevican core proteins in scar tissue. Additionally, compared to control deoxyribozyme, the DNAXTas treatment resulted in a 9-fold increase in length and a 4-fold increase in density of ascending axons growing through the nerve graft and scar tissue present at the rostral spinal cord. Together our data showed that treatment with a deoxyribozyme against XT-1 mRNA decreased the amount of glycosylated PGs and promoted axon growth through scar tissue in the injured spinal cord. The deoxyribozyme approach may become a contributing factor in spinal cord repair strategies.

Deregulation of versican and elastin binding protein in solar elastosis

Several changes in skin appearance including loss of elasticity and wrinkle formation are associated with alterations in the composition of the dermal extracellular matrix. They are induced by intrinsic aging or by environmental factors such as UV light referred to as photoaging. A general characteristic in the histology of photoaged skin is the accumulation of elastotic material suggesting impaired formation and/or massive breakdown of elastic fibres. In order to shed light on some of the underlying mechanisms we tracked two of the major players in elastic fibre formation in different skin conditions: EBP (elastin binding protein), a regulator of elastic fibre assembly and VER (versican), a component of functional elastic fibres as well as non-functional elastotic material. Using quantitative RT-PCR on skin biopsies we found that the expression levels of VER and EBP were unaltered during intrinsic skin aging. Upon acute UV stress however, VER and EBP showed different regulation patterns: VER mRNA increased after 6 h and was further up-regulated until 24 h. The EBP mRNA by contrast was reduced after 6 h but showed massive induction at 24 h after acute UV stress. In chronically sun-exposed skin, VER protein was accumulated similar to elastotic material in the extracellular space, whereas its mRNA level was consistently reduced compared to sun-protected skin. The EBP mRNA by contrast showed slightly increased expression levels in the sun-exposed area compared to its sun-protected counterpart. Based on these data we propose a model which may help to explain parts of the mechanisms leading to the formation of elastotic masses. We further hypothesize that the presence of elastotic material triggers some yet unknown feedback mechanism(s) resulting in altered expression patterns of VER and EBP in chronically sun-exposed skin.

Extracellular matrix of the central nervous system: from neglect to challenge

The basic concept, that specialized extracellular matrices rich in hyaluronan, chondroitin sulfate proteoglycans (aggrecan, versican, neurocan, brevican, phosphacan), link proteins and tenascins (Tn-R, Tn-C) can regulate cellular migration and axonal growth and thus, actively participate in the development and maturation of the nervous system, has in recent years gained rapidly expanding experimental support. The swift assembly and remodeling of these matrices have been associated with axonal guidance functions in the periphery and with the structural stabilization of myelinated fiber tracts and synaptic contacts in the maturating central nervous system. Particular interest has been focused on the putative role of chondroitin sulfate proteoglycans in suppressing central nervous system regeneration after lesions. The axon growth inhibitory properties of several of these chondroitin sulfate proteoglycans in vitro, and the partial recovery of structural plasticity in lesioned animals treated with chondroitin sulfate degrading enzymes in vivo have significantly contributed to the increased awareness of this long time neglected structure.

Characterization and functional analysis of osteoblast-derived fibulins in the human hematopoietic stem cell niche

OBJECTIVE

In the bone marrow stem cell niche, osteoblasts lining the endosteum are of major importance in supporting hematopoietic stem cell maintenance. Our objective was to analyze expression of the fibulins, highly conserved calcium-binding glycoproteins, which are components of the extracellular matrix of human osteoblasts, and to provide insights into their functional interactions with hematopoietic progenitor cells.

MATERIALS AND METHODS

Expression of the fibulins by human osteoblasts was determined by reverse transcription polymerase chain reaction analysis and by immunofluorescence staining and immunoblotting using fibulin-specific antisera. Recombinant fibulins were used in cell proliferation and differentiation assays with human CD34(+) hematopoietic progenitor cells. Adhesive interactions of CD34(+) cells with fibulins were investigated using cell-adhesion assays.

RESULTS

Human osteoblasts strongly express and secrete fibulin-1 and -2. Whereas fibulin-1 is secreted in its intact form, fibulin-2 synthesized by human osteoblasts undergoes rapid proteolytic degradation. The matrix metalloproteinase-2, which is constitutively expressed by the osteoblasts, seems to be responsible for fibulin-2 degradation. Fibulin-1 showed an inhibitory effect on short-term CD34(+) hematopoietic progenitor cell proliferation. Both fibulin-1 and fibulin-2 were able to diminish erythroid and myeloid colony formation. The CD34(+) cell line KG1a strongly attached to fibulin-2, whereas magnetic-activated cell sorted CD34(+) hematopoietic progenitors did not adhere to either fibulin-1 or fibulin-2. On the other hand, fibulin-1 can strongly interfere with CD34(+) cell adhesion to fibronectin.

CONCLUSION

Fibulins seem to be important components of the extracellular matrix of osteoblasts and are likely to negatively influence the proliferation rate of stem cells and the overall adhesive properties of the endosteal stem cell niche.

Basement membrane derived fibulin-1 and fibulin-5 function as angiogenesis inhibitors and suppress tumor growth

The fibulins are a family of secreted glycoproteins that are characterized by repeated epidermal-growth-factor-like domains and a unique C-terminus structure. Fibulins modulate cell morphology, growth, adhesion, and motility. Our initial basement membrane degradome screen using Cathepsin D, a tumor microenvironment-associated protease, contained fragments of fibulin-1 and full length fibulin-5. In this report, we evaluate the antiangiogenic activity of fibulin-1 and fibulin-5. Tumor studies demonstrate that both fibulin-1 and fibulin-5 suppress HT1080 tumor growth. CD31 labeling and TUNEL assay further reveal that fibulin-1 suppression of HT1080 tumor growth is associated with diminished angiogenesis and also enhanced apoptosis of endothelial cells and tumor cells. In contrast, fibulin-5 inhibits tumor angiogenesis with a minimal anti-apoptotic affect. Cathepsin D digestion of fibulin-1 produces a fragment with nearly the same molecular weight as fibulin-5, and this fragment (named Neostatin) inhibits endothelial cell proliferation. Additionally, degradation of basement membrane by cathepsin D liberates both fibulin-1 fragments and fibulin-5, which function to inhibit angiogenesis.

Transgenic mice expressing D469Δ mutated cartilage oligomeric matrix protein (COMP) show growth plate abnormalities and sternal malformations

In humans, mutations in cartilage oligomeric matrix protein (COMP) cause autosomal dominantly inherited skeletal dysplasias. We have generated transgenic mouse lines to study the role of mutant D469Delta COMP in the pathogenesis of pseudoachondroplasia. Biochemical characterization of cartilage tissue demonstrated that transgenic and endogenous COMP subunits were able to form mixed, pentameric molecules in vivo. Mutant COMP was more difficult to extract than the wildtype protein, suggesting an altered anchorage within the matrix. Although both transgenic wildtype and mutant COMP were detected throughout the growth plate, mutant molecules were restricted to the pericellular matrix while wildtype COMP showed a uniform distribution throughout the extracellular matrix. Mice expressing the mutant transgene showed a slight gender specific growth retardation. In mutant animals, the columnar organization in the growth plate was disturbed, proteoglycans were lost and improperly formed collagen fibrils were observed. In some chondrocytes the endoplasmic reticulum was dilated, most probably due to an impaired secretion of mutant COMP similar to that observed in patients. Later in development, the growth plate was irregularly shaped and prematurely invaded by bony tissue. In addition, a fusion of the third and fourth sternebrae was frequently observed.

Extracellular matrix in the trabecular meshwork

The extracellular matrix (ECM) of the trabecular meshwork (TM) is thought to be important in regulating intraocular pressure (IOP) in both normal and glaucomatous eyes. IOP is regulated primarily by a fluid resistance to aqueous humor outflow. However, neither the exact site nor the identity of the normal resistance to aqueous humor outflow has been established. Whether the site and nature of the increased outflow resistance, which is associated with open-angle glaucoma, is the same or different from the normal resistance is also unclear. The ECMs of the TM beams, juxtacanalicular region (JCT) and Schlemm's canal (SC) inner wall are comprised of fibrillar and non-fibrillar collagens, elastin-containing microfibrils, matricellular and structural organizing proteins, glycosaminoglycans (GAGs) and proteoglycans. Both basement membranes and stromal ECM are present in the TM beams and JCT region. Cell adhesion proteins, cell surface ECM receptors and associated binding proteins are also present in the beams, JCT and SC inner wall region. The outflow pathway ECM is relatively dynamic, undergoing constant turnover and remodeling. Regulated changes in enzymes responsible for ECM degradation and biosynthetic replacement are observed. IOP homeostasis, triggered by pressure changes or mechanical stretching of the TM, appears to involve ECM turnover. Several cytokines, growth factors and drugs, which affect the outflow resistance, change ECM component expression, mRNA alternative splicing, cellular cytoskeletal organization or all of these. Changes in ECM associated with open-angle glaucoma have been identified.

The major basement membrane components localize to the chondrocyte pericellular matrix — A cartilage basement membrane equivalent?

In this study, we demonstrate that articular cartilage chondrocytes are surrounded by the defining basement membrane proteins laminin, collagen type IV, nidogen and perlecan, and suggest that these form the functional equivalent of a basement membrane. We found by real-time PCR that mouse chondrocytes express these four cardinal components of basement membranes and demonstrated by immunohistochemistry that the proteins are present in bovine and mouse cartilage tissues and are deposited in a thin pericellular structure. Immunoelectron microscopy confirmed high laminin concentration in the pericellular matrix. In cartilage from newborn mice, basement membrane components are widespread in the territorial and interterritorial matrix, while in mature cartilage of adult mice the basement membrane components are localized mainly to a narrow pericellular zone. With progression into old age, this layer becomes less distinct, especially in areas of obvious mechanical attrition. Interestingly, individual laminin subunits were located in different zones of the cartilage, with laminin alpha1 showing preferential localization around a select population of superficial layer chondrocytes. We propose that the chondrocyte, like several other cell types of mesenchymal origin, is surrounded by the functional equivalent of a basement membrane. This structure is presumably involved in maintaining chondrocyte phenotype and viability and may well allow a new understanding of cartilage development and provide clues to the progression of degenerative joint disorders.

Hyaluronan and chondroitin sulfate proteoglycans in the supramolecular organization of the mammalian vitreous body

The mammalian vitreous gel is a specialized type of highly hydrated extracellular matrix, which is composed of interwoven networks of uronic acid-containing polyanionic macromolecules, (i.e., hyaluronan, versican, and IX collagen) and collagen fibrils. Hyaluronan comprises the vast majority of the uronic acid-containing molecules, which contributes to structure and function of vitreous in at least two ways: its unique biophysical and hydrodynamic properties influence the vitreous homeostasis and biomechanics; it is also a template for assembly of other extracellular macromolecules, for example, versican. The other uronic acid-containing molecules namely versican and IX collagen--two chondroitin sulfate (CS) proteoglycans--occur in the vitreous without significant quantitative variations among different mammalians but with some marked variations on the molecular size and sulfation pattern of their chondroitin sulfate side chains. The contribution of versican and IX collagen (through their protein and their CS side chains) to the supramolecular organization of the vitreous gel is poorly understood. However, versican having the ability to bind hyaluronan via its N-terminal and other binding partners via its C-terminal region can play a crucial role on the structural stability and functionality of the vitreous.

TM14 is a new member of the fibulin family (fibulin-7) that interacts with extracellular matrix molecules and is active for cell binding

We identified a new extracellular protein, TM14, by differential hybridization using mouse tooth germ cDNA microarrays. TM14 cDNA encodes 440 amino acids containing a signal peptide. The protein contains 3 EGF modules at the center, a C-terminal domain homologous to the fibulin module, and a unique Sushi domain at the N terminus. In situ hybridization revealed that TM14 mRNA was expressed by preodontoblasts and odontoblasts in developing teeth. TM14 mRNA was also expressed in cartilage, hair follicles, and extraembryonic tissues of the placenta. Immunostaining revealed that TM14 was localized at the apical pericellular regions of preodontoblasts. When the dentin matrix was fully formed and dentin mineralization occurred, TM14 was present in the predentin matrix and along the dentinal tubules. We found that the recombinant TM14 protein was glycosylated with N-linked oligosaccharides and interacted with heparin, fibronectin, fibulin-1, and dentin sialophosphoprotein. We also found that TM14 preferentially bound dental mesenchyme cells and odontoblasts but not dental epithelial cells or nondental cells such as HeLa, COS7, or NIH3T3 cells. Heparin, EDTA, and anti-integrin beta1 antibody inhibited TM14 binding to dental mesenchyme cells, suggesting that both a heparan sulfate-containing cell surface receptor and an integrin are involved in TM14 cell binding. Our findings indicate that TM14 is a cell adhesion molecule that interacts with extracellular matrix molecules in teeth and suggest that TM14 plays important roles in both the differentiation and maintenance of odontoblasts as well as in dentin formation. Because of its protein characteristics, TM14 can be classified as a new member of the fibulin family: fibulin-7.

Expression of extracellular matrix proteins fibulin-1 and fibulin-2 by human corneal fibroblasts

PURPOSE

The fibulins are a family of extracellular matrix (ECM) molecules that regulate the organ shape along with other growth factors and stromal cells. We report here the in vitro expression of ECM proteins fibulin-1 and fibulin-2 by human corneal fibroblasts. The ability of fibulin-1 to modulate cell motility was investigated.

METHODS

Fibulin-1 and fibulin-2 mRNA and proteins expression were analyzed in primary and immortalized human corneal fibroblasts (CHN) respectively by gene array, RT-PCR, and immunocytochemistry. The motility and adhesion of the cells transfected with fibulin-1 siRNA were analyzed on tissue culture polystyrene coated with Matrigel or ECM secreted by those fibroblasts.

RESULTS

(1) The microarray analysis shows the expression of fibulin-1, fibulin-2, and their binding partners (i.e., fibronectin, nidogen-1, aggrecan, fibrilin-1, endostatin, and laminin alpha-2 chain). Interestingly, a matrix metalloprotease, ADAMTS-1, for which fibulin-1 acts as a cofactor, was also detected in CHN. (2) The synthesis by CHN of fibulin-1 and 2 mRNA and proteins was confirmed respectively by RT-PCR and immunocytochemistry. (3) Transfection of CHN by fibulin-1 siRNA has no effect on cell adhesion but increases cell migration compared with that of the control cells. This observation suggests an important role of fibulin-1 on cell motility.

CONCLUSIONS

The expression of fibulins and that of their binding partners by human corneal fibroblasts indicate the important role of these proteins in the organization of supramolecular ECM structures of cornea. The variation of their expression and the structural changes of fibulins remain to be determined in corneal pathology.

Versican proteolysis mediates myocardial regression during outflow tract development

An important phase of cardiac outflow tract (OFT) formation is the remodeling of the distal region of the common outlet in which the myocardial sleeve is replaced by with smooth muscle. Here we demonstrate that expression of the proteoglycan versican is reduced before the loss of myocardium from the distal cardiac outlet concomitant with an increase in production of the N-terminal cleavage fragment of versican. To test whether versican proteolysis plays a role in OFT remodeling, we determined the effects of adenoviral-mediated expression of a versican isoform devoid of known matrix metalloproteinase cleavage sites (V3) and an N-terminal fragment of versican (G1). V3 expression promoted an increase in thickness of the proximal OFT myocardial layer independent of proliferation. In contrast, the G1 domain caused thinning and interruptions of the OFT myocardium. These in vivo findings were consistent with findings using cultured primary cardiomyocytes showing that the V3 promoted myocardial cell-cell association while the G1 domain caused a loss of myocardial cell-cell association. Taken together, we conclude that intact versican and G1-containing versican cleavage products have opposing effects on myocardial cells and that versican proteolysis may facilitate the loss of distal myocardium during OFT remodeling.

Versican, a major hyaluronan-binding component in the dermis, loses its hyaluronan-binding ability in solar elastosis

Versican interacts with hyaluronan (HA) at its N-terminus and with fibrillin-1 at its C terminus. As versican in the dermis connects microfibrils to the HA-rich matrix for viscoelasticity, dermal diseases may involve destruction of these complexes. A recombinant versican protein, rVN, covering the HA binding region (HABR) of human versican and a polyclonal antibody, 6084, against rVN were prepared and characterized. Blotting analyses of skin extracts with 6084 and biotin-conjugated HA revealed that versican was a major HA-binding component in the dermis. Matrix metalloprotease-12, which is expressed in areas of solar elastosis, degraded versican and abrogated its HA-binding ability. Immunohistochemical analyses revealed that the elastic materials in solar elastosis lesions were negative for 6084, but positive for 2B1, an antibody recognizing the C-terminus of versican, indicating loss of the HABR in the aggregated elastic fibers. This loss of the HA-binding ability of versican followed by HA exclusion may be responsible for the pathological and phenotypical changes observed in solar elastosis.

Phosphoproteins of the chicken eggshell calcified layer

The chicken eggshell matrix is a complex mixture of proteins and proteoglycans. It also contains phosphoproteins that are thought to affect mineralization of the matrix. Several of the matrix phosphoproteins, such as the major component osteopontin, have already been identified as phosphoproteins in other tissues, but the phosphorylation status of the eggshell matrix forms was unknown. The phosphopeptides, obtained after cleavage of the matrix proteins with several different cleavage methods, were enriched by anion-exchange chromatography and reversible binding to titanium oxide and identified by LC-MS(n) or pseudo-MS(n) analysis following neutral loss scanning. Altogether we identified 39 phosphorylated matrix proteins, 22 of which were not known to be phosphorylated before. Eight of the proteins were identified as eggshell matrix components for the first time. Together these proteins contained more than 150 different phosphorylation sites, 103 of which were determined with high confidence. Among the major phosphorylated proteins of the chicken eggshell matrix were osteopontin and the eggshell-specific proteins ovocleidin-17, ovocleidin-116, and ovocalyxin-32.

Differences in Expression of Specific Biomarkers Distinguish Human Beard from Scalp Dermal Papilla Cells

Androgen exposure stimulates the growth of beard hair follicles. The follicle dermal papilla appears to be the site of androgen action; however, the molecular mechanisms that regulate this process are not well understood. In an attempt to identify genes that contribute to the androgen-responsive phenotype, we compared gene expression patterns in unstimulated and androgen-treated cultured human dermal papilla cells isolated from beard (androgen-sensitive) and occipital scalp (androgen-insensitive) hair follicles. Through this analysis, we identified three genes that are expressed at significantly higher levels in beard dermal papilla cells. One of these genes, sfrp-2 has been identified as a dermal papilla signature gene in mouse pelage follicles. Two of these genes, mn1 and atp1beta1, have not been studied in the hair follicle. A fourth, fibulin-1d, was slightly upregulated in beard dermal papilla cells. The differences in the expression of these genes in cultured beard and scalp dermal papilla cells reflected similar differences in microdissected dermal papilla isolated from intact beard and scalp follicles. Our findings introduce potentially novel signaling pathways in dermal papilla cells. In addition, this study supports that cultured dermal papilla cells provide a cell-based model system that is reflective of the biology of in vivo hair follicle cells.

Proteolytic cleavage of versican during cardiac cushion morphogenesis

The proteoglycan versican is essential to the formation of endocardial cushion mesenchyme by epithelial-mesenchymal transformation (EMT). A potentially important factor in the regulation of versican activity during cushion EMT is proteolysis by ADAMTS metalloproteinases. Using antibodies to the DPEAAE neoepitope created by ADAMTS proteolysis of versican, we detected the amino terminal 70-kDa versican cleavage fragment in cardiac cushions. Initially (i.e., 9.5 days post coitum [dpc]), the fragment is associated with endocardial cells undergoing EMT and with newly derived mesenchymal cells. ADAMTS-1 and its cofactor fibulin-1 were also associated with these cells. As cushions become increasingly populated with mesenchymal cells (10.5-12.5 dpc), the fragment remains asymmetrically distributed compared with the pattern of total versican. Highest levels of the fragment are present in regions immediately subjacent to the endocardium characterized as having densely packed, rounded cells, lacking cellular protrusions. With further development (i.e., 12.5-14.5 dpc), the pattern of fragment distribution within cushions broadens to include the ECM surrounding loosely packed mesenchymal cells in the cushion core. Together, the findings reveal that versican proteolysis leading to the production of the 70-kDa fragment is integral to the formation and differentiation of endocardial cushion mesenchyme.

From barriers to bridges: chondroitin sulfate proteoglycans in neuropathology

Emerging studies have revealed new roles for the neural extracellular matrix in neuropathologies. The structure of this matrix is unusual and uniquely enriched in chondroitin sulfate proteoglycans, particularly those of the lectican family. Historically, lecticans have attracted considerable interest in the normal and injured brain for their prominent roles as inhibitors of cellular motility, neurite extension and synaptic plasticity. However, these molecules are structurally heterogeneous, have distinct expression patterns and mediate unique interactions, suggesting that they might have other functions in addition to their traditional role as chemorepulsants. Here, we review recent work demonstrating unique modifications and structural microheterogeneity of the lecticans in the diseased CNS, which might relate to novel roles of these molecules in neuropathologies.

A genome-wide analysis of biomineralization-related proteins in the sea urchin Strongylocentrotus purpuratus

Biomineralization, the biologically controlled formation of mineral deposits, is of widespread importance in biology, medicine, and engineering. Mineralized structures are found in most metazoan phyla and often have supportive, protective, or feeding functions. Among deuterostomes, only echinoderms and vertebrates produce extensive biomineralized structures. Although skeletons appeared independently in these two groups, ancestors of the vertebrates and echinoderms may have utilized similar components of a shared genetic "toolkit" to carry out biomineralization. The present study had two goals. First, we sought to expand our understanding of the proteins involved in biomineralization in the sea urchin, a powerful model system for analyzing the basic cellular and molecular mechanisms that underlie this process. Second, we sought to shed light on the possible evolutionary relationships between biomineralization in echinoderms and vertebrates. We used several computational methods to survey the genome of the purple sea urchin Strongylocentrotus purpuratus for gene products involved in biomineralization. Our analysis has greatly expanded the collection of biomineralization-related proteins. We have found that these proteins are often members of small families encoded by genes that are clustered in the genome. Most of the proteins are sea urchin-specific; that is, they have no apparent homologues in other invertebrate deuterostomes or vertebrates. Similarly, many of the vertebrate proteins that mediate mineral deposition do not have counterparts in the S. purpuratus genome. Our findings therefore reveal substantial differences in the primary sequences of proteins that mediate biomineral formation in echinoderms and vertebrates, possibly reflecting loose constraints on the primary structures of the proteins involved. On the other hand, certain cellular and molecular processes associated with earlier events in skeletogenesis appear similar in echinoderms and vertebrates, leaving open the possibility of deeper evolutionary relationships.

Approaching theProteoglycome: Molecular Interactions of Proteoglycans and Their Functional Output

[Image: see text] Through their diverse core protein modules and glycan/glycosaminoglycan moieties, proteoglycans may engage in numerous cellular and molecular interactions which are dispensable during embryogenesis, are essential for the maintenance of a healthy state and are prone to modulation in pathological conditions. Proteoglycan interactions may involve binding to other structural components of the ECM, to cell surface receptors, to membrane-associated components, and to soluble signaling molecules, which through this interaction may become entrapped in the ECM or sequestered at the cell surface. Understanding of these multiple interplays is therefore of paramount importance and requires a detailed mapping through what we define as the proteoglycome.

Effects of covalently attached chondroitin sulfate on aggrecan cleavage by ADAMTS-4 and MMP-13

Aggrecan is degraded by several aggrecanase-1 (ADAMTS-4) isoforms differing in the number of sulfated glycosaminoglycan (sGAG)-binding motifs. ADAMTS-4 and MMPs cleave aggrecan more efficiently within the chondroitin sulfate (CS)-rich region than the interglobular domain (IGD). We investigated the influence of CS on aggrecan core protein cleavage by ADAMTS-4 (p68) and (p40) as well as MMP-13, which has no recognizable GAG-binding sites. Chondroitinase ABC-treated cartilage aggrecan was cleaved with ADAMTS-4 (p68) less efficiently than CS-substituted aggrecan within the CS-2 domain. Keratanase-treated aggrecan exhibited reduced IGD cleavage, but when both CS and KS were removed, the IGD cleavage was restored. This result suggests that KS in the IGD may compete with CS for ADAMTS-4 (p68) binding. In the absence of KS, however, p68 binding was shifted to the CS-2 domain. CS-deficient full-length recombinant aggrecan (rbAgg) was produced by chondroitinase ABC treatment, or by expression in the xylosyltransferase-deficient CHO-pgsA745 cell line. When digested with the ADAMTS-4 (p68), each of these preparations exhibited reduced CS-2 domain cleavage compared to CS-substituted CHO-K1 cell-derived aggrecan. Additionally, CS-deficient rbAgg showed increased IGD scission prior to cleavage within the CS-2 domain. ADAMTS-4 (p40) readily cleaved both rbAggs within the IGD, but cleaved poorly within the CS-2 domain, indicating little CS dependence. MMP-13, in contrast, cleaved the CS region and the IGD of both CS-substituted and CS-deficient rbAgg equally well. These data indicate that covalently bound CS enhances ADAMTS-4-mediated cleavage within the CS-rich region. MMP-13 also cleaves preferentially within the CS-region, but by an apparently CS-independent mechanism.

Limb chondrogenesis is compromised in the versican deficient hdf mouse

It has been suggested that the matrix proteoglycan, versican, may perform a functional role during early events of limb skeletogenesis largely by virtue of its spatiotemporal expression pattern in precartilage mesenchymal aggregations. The versican-deficient hdf transgenic mouse has provided the first model to explore the implications of a null mature versican on limb chondrogenesis. Due to lethality of hdf homozygous embryos prior to limb cartilage differentiation, high-density micromass cultures were employed to compare the chondrogenic capacity of hdf mutant limb mesenchyme to that of wild-type. In homozygous hdf mesenchyme, aggregation was severely compromised and neither cartilage-characteristic Type II collagen nor alcian blue positive foci were detected during a 6-day period of culture. Three-dimensional culture of hdf mutant mesenchyme, however, showed that in a permissive environment mutant cells also expressed Type II collagen. Results strongly suggest that mature versican proteoglycan is essential for precartilage aggregation and subsequent cartilage differentiation.

Fibulin-4 is a target of autoimmunity predominantly in patients with osteoarthritis

Autoimmunity to chondrocyte-producing proteins has been reported in patients with osteoarthritis (OA) as well as in those with rheumatoid arthritis (RA). To answer whether or not OA-specific autoimmunity exist, we performed screening of chondrocyte-producing autoantigens by two-dimensional electrophoresis and Western blotting with each of 20 OA and 20 RA serum samples. We identified an apparently OA-specific autoantigen spot with a molecular mass of 52 kDa and a Isoelectric point of 4.1 as fibulin-4 by mass fingerprinting. By preparing recombinant proteins of fibulin-4, we determined prevalence of the autoantibodies to fibulin-4 in 92 patients with OA, 67 patients with RA, 40 patients with systemic lupus erythematosus, and 43 patients with systemic scleroderma. As a result, the IgG type anti-fibulin-4 autoantibodies were detected in 23.9% of sera from patients with OA, in 8.9% of sera from patients with RA, in 2.5% of sera from patients with systemic lupus erythematosus, and in 9.3% of sera from patients with systemic scleroderma. Furthermore, we immunized DBA/1J, ICR, BALB/c, and C57BL/6 mice with the recombinant fibulin-4 proteins to investigate arthritogenecity of fibulin-4. As a result, mild synovitis was detected in all of the four strains. In addition, we demonstrated expression of fibulin-4 in chondrocytes at both mRNA and protein levels in vivo and in vitro by RT-PCR, Western blotting, and immunohistochemistry. Taken together, fibulin-4, expressed in chondrocytes and recognized as an autoantigen mainly in OA rather than in RA, may play pathogenic roles in OA.

Identification and characterization of versican/PG-M aggregates in cartilage

Versican/PG-M is a large chondroitin sulfate proteoglycan of the extracellular matrix with a common domain structure to aggrecan and is present in cartilage at low levels. Here, we characterized cartilage versican during development and growth. Immunostaining showed that versican was mainly localized in the interterritorial zone of the articular surface at 2 weeks in mice, whereas aggrecan was in the pericellular zone of prehypertrophic and hypertrophic cells of the growth plate. Although its transcription level rapidly diminished during growth, versican remained in the articular cartilage. Biochemical analysis of normal articular cartilage and aggrecan-null cartilage from cmd (cartilage matrix deficiency)/cmd mice revealed that versican was present as a proteoglycan aggregate with both link protein and hyaluronan. Chondroitin sulfate chains of versican digested with chondroitinase ABC contained 71% nonsulfated and 28% 4-sulfated unsaturated disaccharides, whereas those of aggrecan contained 25% nonsulfated and 70% 4-sulfated. Link protein overexpression in chondrocytic N1511 cells at the early stage of differentiation, in which versican is expressed, enhanced versican deposition in the matrix and prevented subsequent aggrecan deposition. These results suggest that versican is present as an aggregate distinct from the aggrecan aggregate and may play specific roles in the articular surface.

Expression of the cadherin-11 gene is a discriminative factor between articular and growth plate chondrocytes

OBJECTIVE

Calcification of hypertrophic chondrocytes is the final step in the differentiation of growth plates, although the precise mechanism is not known. We have established two growth plate-derived chondrocyte cell lines, MMR14 and MMR17, from p53-/- mice (Nakamata T, Aoyama T, Okamoto T, Hosaka T, Nishijo K, Nakayama T, et al. In vitro demonstration of cell-to-cell interaction in growth plate cartilage using chondrocytes established from p53-/- mice. J Bone Miner Res 2003;18:97-107). Prolonged in vitro culture produced calcified nodules in MMR14, but not in MMR17. Factors responsible for the difference in calcification between the two cell lines may also be involved in the physiological calcification in growth plate.

DESIGN

Gene expression profiles of MMR14 and MMR17 were compared using a cDNA microarray to identify candidate genes involved in the calcification process.

RESULTS

Forty-five genes were identified as upregulated in MMR14, including the cadherin-11 (Cdh-11) gene. The expression of Cdh-11 in MMR14 was detected in cell-cell junctions, while no expression was observed in MMR17. Primary cultured chondrocytes from growth plate (GC) also expressed the Cdh-11, and the staining of Cdh-11 was observed in the late hypertrophic zone of growth plate. Cell aggregation assays showed that chondrocytes required Ca2+ to form nodules, and knockdown of the Cdh-11 gene expression using short interfering RNA inhibited the formation of calcified nodules in MMR14. The introduction of Cdh-11 into MMR17 failed to produce calcified nodules indicating that Cdh-11 is one, but not the sole, factor responsible for the production of calcified nodules.

CONCLUSION

Although the physiological role is still unclear, Cdh-11 is a discriminative factor between articular and growth plate chondrocytes.

The C-type lectin-like domain superfamily

The superfamily of proteins containing C-type lectin-like domains (CTLDs) is a large group of extracellular Metazoan proteins with diverse functions. The CTLD structure has a characteristic double-loop ('loop-in-a-loop') stabilized by two highly conserved disulfide bridges located at the bases of the loops, as well as a set of conserved hydrophobic and polar interactions. The second loop, called the long loop region, is structurally and evolutionarily flexible, and is involved in Ca2+-dependent carbohydrate binding and interaction with other ligands. This loop is completely absent in a subset of CTLDs, which we refer to as compact CTLDs; these include the Link/PTR domain and bacterial CTLDs. CTLD-containing proteins (CTLDcps) were originally classified into seven groups based on their overall domain structure. Analyses of the superfamily representation in several completely sequenced genomes have added 10 new groups to the classification, and shown that it is applicable only to vertebrate CTLDcps; despite the abundance of CTLDcps in the invertebrate genomes studied, the domain architectures of these proteins do not match those of the vertebrate groups. Ca2+-dependent carbohydrate binding is the most common CTLD function in vertebrates, and apparently the ancestral one, as suggested by the many humoral defense CTLDcps characterized in insects and other invertebrates. However, many CTLDs have evolved to specifically recognize protein, lipid and inorganic ligands, including the vertebrate clade-specific snake venoms, and fish antifreeze and bird egg-shell proteins. Recent studies highlight the functional versatility of this protein superfamily and the CTLD scaffold, and suggest further interesting discoveries have yet to be made.

Identification of Proteoglycan‐Binding Proteins

There are many proteins that bind to proteoglycans; they include proteins in extracellular matrices, growth factors or cytokines, plasma proteins, transmembrane proteins, and cytoplasmic proteins as listed in this chapter. Proteins that bind to a proteoglycan have been searched by using a proteoglycan as a ligand. Alternatively, a ligand protein has been used to find a proteoglycan as a binding partner. When the glycosaminoglycan (GAG) portion of a proteoglycan is responsible for the binding, a native proteoglycan is necessary for the analysis of binding. When the protein portion is responsible for the binding, a recombinant core protein without GAG chains may be used for analysis. This chapter describes experimental procedures dealing with two native proteoglycans, versican (PG-M) and syndecan-4 (ryudocan). Versican has been identified as a proteoglycan with binding capability to a growth factor, midkine. Purified syndecan-4 has been used to identify proteins that bind to the proteoglycan.

Calcium-dependent self-association of the C-type lectin domain of versican

Versican is a large (1-2 x 10(6) Da) chondroitin-sulfate proteoglycan that can form large aggregates by means of interaction with hyaluronan and also binds to a series of other extracellular matrix proteins, chemokines and cell-surface molecules. Versican is a multifunctional molecule with roles in cell adhesion, matrix assembly, cell migration and proliferation. Characterization of the binding interactions mediated by the various domains of versican is a first step towards understanding the functions of versican and interacting molecules in the extracellular matrix. In this study we investigated a recombinant construct corresponding to the C-type lectin domain of versican and demonstrated a calcium-dependent self-association of this region by blot overlay and plasmon surface resonance assays. Electron microscopy provided further evidence of the relevance of the binding reaction by demonstrating a mixture of monomers, dimers and complex aggregates of recombinant versican C-type lectin domain. This binding reaction could contribute to the ability of versican to organize formation of the proteoglycan extracellular matrix by inducing binding of individual versican molecules or by modulating binding reactions to other matrix components.