The expression of functional link protein in a baculovirus system: analysis of mutants lacking the A, B and B' domains

Essential hyaluronan structure for binding with hyaluronan-binding protein (HABP) determined by glycotechnological approach

Hyaluronan specifically binds to aggrecan globular domain 1, which is often referred to as just hyaluronan binding protein (HABP), however, the hyaluronan carbohydrate structure recognized by HABP had not been studied in detail. The aim of the present study was to investigate the important structure of hyaluronan for binding to HABP. We prepared hybrid oligosaccharides from hyaluronan and chondroitin, with or without modification of the reducing or non-reducing terminus, as tools to determine the preferred structure of hyaluronan for binding to the HABP by a competitive ELISA-like method. The non-reducing terminal structure was critical, especially, the glucuronic acid (GlcUA) and N-acetylglucosamine (GlcNAc) of the hyaluronan-unit are essential for complete HABP binding activity, and for any HABP binding activity, respectively. It is possible to replace GlcUAβ-1-3GlcNAc of the internal disaccharide units with GlcUAβ-1-3N-acetylgalactosamine (GalNAc), if the chain length is decasaccharide or larger.

The hyaluronan and proteoglycan link proteins: Organizers of the brain extracellular matrix and key molecules for neuronal function and plasticity

The hyaluronan and proteoglycanbinding link protein (Hapln) is a key molecule in the formation and control of hyaluronan-based condensed perineuronal matrix in the adult brain. This review summarizes the recent advances in understanding the role of Haplns in the formation and control of two distinct types of perineuronal matrices, one for "classical" PNN and the other for the specialized extracellular matrix (ECM) at the node of Ranvier in the central nervous system (CNS). We introduce the structural components of each ECM organization including the basic concept of supramolecular structure named "HLT model". We furthermore summarize the developmental and physiological role of perineuronal ECMs from the studies of Haplns and related molecules. Finally, we also discuss the potential mechanism modulating PNNs in the adult CNS. This layer of organized matrices may exert a direct effect via core protein or sugar moiety from the structure or by acting as a binding site for biologically active molecules, which are important for neuronal plasticity and saltatory conduction.

Tissue engineering by molecular disassembly and reassembly: biomimetic retention of mechanically functional aggrecan in hydrogel

In vitro assembly of key functional extracellular matrix constituents for tissue-engineered constructs may provide a tool to modulate the retention of proteoglycan (PG) aggregates, which are crucial to compressive biomechanical properties of connective tissues. This study tested the hypotheses that (1) biomimetic molecular reassembly of PG aggregates (native aggrecan [AGC] with hyaluronan [HA] ± link protein [LP]) affects AGC retention kinetics in hydrogel constructs, (2) the compressive properties of such hydrogel constructs are related to the content of retained AGC, and (3) the reassembly method is compatible with chondrocytes. Addition of HA to AGC in hydrogel constructs increased AGC retention in a dose-dependent manner, and the addition of LP to AGC + HA further enhanced AGC retention. The level of AGC retention, in turn, was associated with increased equilibrium compressive stress of the constructs. Chondrocytes could be included in the process, and maintained expression of the chondrogenic phenotype, secreting type II collagen but little type I collagen. Thus, by altering the assembly of PG aggregates with HA ± LP, which affects AGC retention, it may be possible to achieve the targeted levels of PG components to modulate the mechanical properties of the engineered construct for cartilage as well as other tissues containing PG and PG aggregates.

Engineering of Biomaterials Surfaces by Hyaluronan

This review addresses the area of study that defines the field of surface modification of biomedical materials and devices by hyaluronan (HA), as related to the exploitation of HA biological properties. To provide a comprehensive view of the subject matter, initial sections give a quick introduction to basic information on HA-protein and HA-cell interactions, together with some discussion on the bioactive role of HA in wound healing and related phenomena. This is followed by a description of current theories that correlate HA properties to its molecular structure in aqueous media, underlying how HA molecular details are crucial for its biological interaction and role. Finally, existing approaches to surface modification by HA are reviewed, stressing the need for HA-surface engineering founded on the knowledge and control of the surface-linked HA molecular conformation at the solid/aqueous interface.

Expression and purification of functionally active hyaluronan-binding domains from human cartilage link protein, aggrecan and versican: formation of ternary complexes with defined hyaluronan oligosaccharides

The chondroitin sulfate proteoglycan aggrecan forms link protein-stabilized complexes with hyaluronan (HA), via its N-terminal G1-domain, that provide cartilage with its load bearing properties. Similar aggregates (potentially containing new members of the link protein family), in which other chondroitin sulfate proteoglycans (i.e. versican, brevican, and neurocan) substitute for aggrecan, may contribute to the structural integrity of many other tissues including skin and brain. In this study, cartilage link protein (cLP) and the G1-domains of aggrecan (AG1) and versican (VG1) were expressed in Drosophila S2 cells. The recombinant human proteins were found to have properties similar to those described for the native molecules (e.g. cLP was able to form oligomers, and HA decasaccharides were the minimum size that could compete effectively for their binding to polymeric HA). Gel filtration and protein cross-linking/matrix-assisted laser desorption ionization time-of-flight peptide fingerprinting showed that cLP and AG1 interact in the absence or presence of HA. Conversely, cLP and VG1 did not bind directly to each other in solution yet formed ternary complexes with HA24. N-linked glycosylation of AG1 and VG1 was demonstrated to be unnecessary for either HA binding or the formation of ternary complexes. Surprisingly, the length of HA required to accommodate two G1-domains was found to be significantly larger for aggrecan than versican, which may reflect differences in the conformation of HA stabilized on binding these proteins.

Cartilage link protein interacts with neurocan, which shows hyaluronan binding characteristics different from CD44 and TSG-6

The interaction of neurocan with hyaluronan was qualitatively characterized with alkaline phosphatase fusion proteins secreted by mammalian cells. The wild type neurocan hyaluronan binding domain fused to alkaline phosphatase bound to immobilized hyaluronan under physiological as well as moderately hypertonic conditions, whereas its ability to bind to immobilized chondroitin sulfate dropped rapidly with increasing salt concentration. Strong hyaluronan binding ability was still evident when in both link modules within the hyaluronan binding domain a basic amino acid was mutated, which is well conserved among link modules of hyaluronan binding proteins. A strong enhancement of the binding of neurocan to immobilized hyaluronan was observed after preincubation of the immobilized hyaluronan with cartilage link protein. Moreover, this preincubation mediated also the binding of a fusion protein representing only the immunoglobulin module of neurocan linked to alkaline phosphatase, which showed no binding to immobilized hyaluronan alone. The interaction of the neurocan immunoglobulin module with link protein could also be shown by overlay blot analysis. These observations suggest that the hyaluronan binding characteristics of paired link modules are different from those of single link modules, and that the reported temporal co-expression of cartilage link protein and of neurocan in developing brain implicates the possibility of a cooperative function of these molecules.

Link protein has greater affinity for versican than aggrecan

The function of link protein in stabilizing the interaction between aggrecan and hyaluronan to form aggrecan aggregates, via the binding of link protein to the aggrecan G1 domain and hyaluronan, is well established. However, it is not known whether link protein can function with similar avidity with versican, another member of the large hyaluronan-binding proteoglycan family that also binds to hyaluronan via its G1 domain. To address this issue, we have compared the interaction of the versican and aggrecan G1 domains with link protein and hyaluronan using recombinant proteins expressed in insect cells and BIAcore analysis. The results showed that link protein could significantly improve the binding of both G1 domains to hyaluronan and that its interaction with VG1 is of a higher affinity than that with AG1. These observations suggest that link protein may function as a stabilizer of the interaction, not only between aggrecan and hyaluronan in cartilage, but also between versican and hyaluronan in many tissues.

Experimental immunity to the G1 domain of the proteoglycan versican induces spondylitis and sacroiliitis, of a kind seen in human spondylarthropathies

OBJECTIVE

Experimental immunity to the G1 domain of the cartilage proteoglycan (PG) aggrecan (AG1) leads to the development of spondylitis as well as polyarthritis in BALB/c mice. The PG versican contains a structurally similar G1 domain (VG1). This study was conducted to determine whether immunity to VG1 would elicit similar pathology in these mice.

METHODS

Recombinant natively folded VG1 and AG1 were prepared. BALB/c mice received either a series of 5 injections of human VG1 or AG1, or no protein. Polyarthritis was determined clinically, and spondylitis and sacroiliitis histologically. Immunohistochemistry of rat tissues was used to study the localization of versican. Enzyme-linked immunosorbent assays were employed to study humoral immunity to the recombinant proteins as well as to overlapping synthetic peptides covering all these human G1 domains and mouse homologs. Affinity-purified antibodies to human AG1 and VG1 were isolated from sera of hyperimmunized mice. T lymphocyte proliferation assays were performed using recombinant human proteins. T cell lines reactive with specific immunodominant T cell epitopes in human AG1 and VG1 were isolated. Synthetic peptides encoding sequences in these human proteins and in corresponding mouse proteins were used in these analyses. Guanidinium chloride extracts of mouse spines were also used in Western blots to study antibody cross-reactivity.

RESULTS

Immunity to recombinant VG1 did not result in clinical polyarthritis. There was, however, clear evidence that VG1, like AG1, could induce spondylitis in the lumbar spine and sacroiliitis. Accumulation of mononuclear cells was observed in spinal ligaments adjacent to the intervertebral disc, in the intervertebral disc, and in the sacroiliac joints, the same sites where versican is localized. In contrast to AG1-immunized mice, in which T cells reactive with human AG1 cross-reacted with mouse AG1, there was no evidence in VG1-immunized mice that T cell immunity to human VG1 was cross-reactive with a mouse synthetic peptide that contained the sequence corresponding to the single immunodominant T cell sequence recognized in human VG1. Antibodies to specific sequences in human VG1 did, however, cross-react with human AG1 and with corresponding peptide sequences in mouse versican and aggrecan and with mouse proteins containing VG1 and AG1, present in mouse spine extracts. Similarly, antibodies to human AG1 cross-reacted with human VG1 and with extracted mouse VG1 and AG1 and synthetic peptides containing mouse sequences that corresponded to the reactive human epitopes in AG1 and VG1.

CONCLUSION

These observations suggest that humoral immunity to human VG1 is involved in the induction of experimental spondylitis and sacroiliitis in BALB/c mice. This humoral immunity is cross-reactive with mouse versican and aggrecan but is not associated with polyarthritis, probably because of the lack of cross-reactive T cell immunity and the absence of detectable versican in articular cartilage limbs. Induction of polyarthritis by bovine or human aggrecan requires the involvement of immunity mediated by T lymphocytes that are cross-reactive to a mouse aggrecan epitope. Together these observations suggest that humoral immunity to versican as well as immunity to aggrecan may be of importance in the development of the spinal pathology characteristic of spondylarthropathies.

Distinct interaction of versican/PG-M with hyaluronan and link protein

The proteoglycan aggregate is the major structural component of the cartilage matrix, comprising hyaluronan (HA), link protein (LP), and a large chondroitin sulfate (CS) proteoglycan, aggrecan. Here, we found that another member of aggrecan family, versican, biochemically binds to both HA and LP. Functional analyses of recombinant looped domains (subdomains) A, B, and B' of the N-terminal G1 domain revealed that the B-B' segment of versican is adequate for binding to HA and LP, whereas A and B-B' of aggrecan bound to LP and HA, respectively. BIAcore trade mark analyses showed that the A subdomain of versican G1 enhances HA binding but has a negligible effect on LP binding. Overlay sensorgrams demonstrated that versican G1 or its B-B' segment forms a complex with both HA and LP. We generated a molecular model of the B-B' segment, in which a deletion and an insertion of B' and B are critical for stable structure and HA binding. These results provide important insights into the mechanisms of formation of the proteoglycan aggregate and HA binding of molecules containing the link module.

Chondrodysplasia of gene knockout mice for aggrecan and link protein

The proteoglycan aggregate of the cartilage is composed of aggrecan, link protein, and hyaluronan and forms a unique gel-like moiety that provides resistance to compression in joints and a foundational cartilage structure critical for growth plate formation. Aggrecan, a large chondroitin sulfate proteoglycan, is one of the major structural macromolecules in cartilage and binds both hyaluronan and link protein through its N-terminal domain G1. Link protein, a small glycoprotein, is homologous to the G1 domain of aggrecan. Mouse cartilage matrix deficiency (cmd) is caused by a functional null mutation of the aggrecan gene and is characterized by perinatal lethal dwarfism and craniofacial abnormalities. Link protein knockout mice show chondrodysplasia similar to but milder than cmd mice, suggesting a supporting role of link protein for the aggregate structure. Analysis of these mice revealed that the proteoglycan aggregate plays an important role in cartilage development and maintenance of cartilage tissue and may provide a clue to the identification of human genetic disorders caused by mutations in these genes.

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.

Mapping the hyaluronan-binding site on the link module from human tumor necrosis factor-stimulated gene-6 by site-directed mutagenesis

Link modules are hyaluronan-binding domains found in extracellular proteins involved in matrix assembly, development, and immune cell migration. Previously we have expressed the Link module from the inflammation-associated protein tumor necrosis factor-stimulated gene-6 (TSG-6) and determined its tertiary structure in solution. Here we generated 21 Link module mutants, and these were analyzed by nuclear magnetic resonance spectroscopy and a hyaluronan-binding assay. The individual mutation of five amino acids, which form a cluster on one face of the Link module, caused large reductions in functional activity but did not affect the Link module fold. This ligand-binding site in TSG-6 is similar to that determined previously for the hyaluronan receptor, CD44, suggesting that the location of the interaction surfaces may also be conserved in other Link module-containing proteins. Analysis of the sequences of TSG-6 and CD44 indicates that the molecular details of their association with hyaluronan are likely to be significantly different. This comparison identifies key sequence positions that may be important in mediating hyaluronan binding, across the Link module superfamily. The use of multiple sequence alignment and molecular modeling allowed the prediction of functional residues in link protein, and this approach can be extended to all members of the superfamily.

The roles of matrix molecules in mediating chondrocyte aggregation, attachment, and spreading

The most abundant macromolecules in cartilage are hyaluronan, collagen, aggrecan, and link protein, which are believed to play roles in maintaining a unique three-dimensional network for a functional joint. This study was designed to investigate the roles of the major extracellular molecules in mediating chondrocyte-matrix interactions. We employed specific approaches to remove components individually or in combination: hyaluronan was digested with hyaluronidase; type II collagen was digested with collagenase; aggrecan expression was inhibited with antisense and beta-xyloside approaches; and link protein expression was inhibited with antisense oligonucleotides. Digestion of hyaluronan induced chondrocyte attachment to tissue culture plates, collagen-coated plates, and fibroblast-like chondrocyte cultures, and induced chondrocyte aggregation. Treated chondrocytes exhibited a fibroblast-like morphology, and the effects of hyaluronidase were dose-dependent. Conversely, the effect of collagenase on chondrocyte adhesion and aggregation was far less pronounced. Treatment with Arg-Gly-Asp peptide inhibited chondrocyte-collagen interaction. Chondrocyte attachment was enhanced by antisense oligonucleotides complementary to aggrecan and link protein and by beta-xyloside treatment. Nevertheless, hyaluronan seems to predominate over the other molecules in mediating chondrocyte-matrix interactions.

Localization and characterization of the hyaluronan-binding site on the link module from human TSG-6

BACKGROUND

The interactions of hyaluronan (HA) with proteins are important in extracellular matrix integrity and leukocyte migration and are usually mediated by a domain termed a Link module. Although the tertiary structure of a Link module has been determined, the molecular basis of HA-protein interactions remains poorly understood.

RESULTS

Isothermal titration calorimetry was used to characterize the interaction of the Link module from human TSG-6 (Link_TSG6) with HA oligosaccharides of defined length (HA(4)-HA(16)). All oligomers bound (except HA(4)) with K(d) values ranging from 0.2-0.5 microM at 25 degrees C. The reaction is exothermic with a favourable entropy and the thermodynamic profile is similar to those of other glycosaminoglycan-protein interactions. The HA(8) recognition site on Link_TSG6 was localized by comparing nuclear magnetic resonance (NMR) spectra from a 1:1 complex with free protein. Residues perturbed on HA binding include both amino acids that are likely to be directly involved in the interaction (i.e., Lys11, Tyr59, Asn67, Phe70, Lys72 and Tyr78) and those affected by a ligand-induced conformational change in the beta4/beta5 loop. The sidechain of Asn67 becomes more rigid in the complex suggesting that it is in close proximity to the binding site.

CONCLUSIONS

In TSG-6 a single Link module is sufficient for a high-affinity interaction with HA. The HA-binding surface on Link_TSG6 is found in a similar position to that suggested previously for CD44, indicating that its location might be conserved across the Link module superfamily. Here we find no evidence for the involvement of linear sequence motifs in HA binding.

TSG-6 interacts with hyaluronan and aggrecan in a pH-dependent manner via a common functional element: implications for its regulation in inflamed cartilage

Cartilage matrix is stabilised by the interactions of proteins with hyaluronan (HA). We compare the pH dependences of HA binding by aggrecan, link protein and TSG-6. Aggrecan and link protein exhibit maximal binding across a wide pH range (6.0-8.0). TSG-6, a protein that is only produced during inflammation, binds maximally at about pH 6.0 but shows a dramatic loss of function with increasing pH. TSG-6 also interacts with aggrecan, with a similar pH dependence, and this can be inhibited by HA. Thus, a common binding surface on TSG-6 may be involved in HA and aggrecan binding. We propose that TSG-6 is involved in matrix dissociation and that this is regulated by pH gradients in cartilage.

Identification of hyaluronan-binding domains of aggrecan

Aggrecan, a large cartilage proteoglycan, interacts with hyaluronan (HA), to form aggregates which function to resist compression in joints. The N-terminal region of aggrecan contains two structurally related globular domains, G1 and G2 separated by IGD domain. The G1 domain consists of three subdomains, A, B, and B', structural features characteristic to many other HA-binding proteoglycans. Here, we studied the interaction of aggrecan domains with HA using recombinant proteins expressed in 293 cells, an embryonal kidney cell line. Deglycosylation of the recombinant aggrecan fragment reduced the HA binding activity. We found that both the B and B' subdomains were required for HA binding and that a single module of A, B, or B' was unable to bind HA. The A subdomain increased the HA binding activity of the B-B' region. The G2 domain had no HA binding activity confirming previous reports. Studies of HA-binding properties using a BIAcoreTM biosensor system revealed that the KD of recombinant aggrecan fragment (AgW) consisting of G1, IGD, and G2 was 0.226 microM, whereas the KD of another HA-binding protein, native bovine link protein, is 0.089 microM. In contrast, AgMut11 which lacked subdomain A showed little HA binding activity. AgMut12 consisting of only B-B' had a 3.4-fold lower affinity and AgMut13 containing A-B-B' was 1.5-fold lower than AgW. These results suggest that carbohydrates are essential for high level aggrecan binding to HA and that the A subdomain of aggrecan functions in a cooperative manner with subdomains B and B'.

TSG-6: an IL-1/TNF-inducible protein with anti-inflammatory activity

The pro-inflammatory cytokines IL-1 and TNF-alpha are primary mediators of the acute phase response, the complex reaction of the mammalian organism to infection and injury. Among the genes activated by TNF-alpha and IL-1 in a variety of cells is TNF-stimulated gene 6 (TSG-6). The TSG-6 cDNA encodes a secreted 35 kDa glycoprotein which is abundant in synovial fluids of patients with various forms of arthritis and detectable in serum of patients with different inflammatory or autoimmune disorders. TSG-6 protein consists of two structural domains: a hyaluronan-binding link module, the characteristic domain of the hyaladherin family of proteins, and a C-terminal CUB domain, present in a variety of diverse proteins. TSG-6 forms a stable complex with components of the plasma protein inter-alpha-inhibitor (I[alpha]I), a Kunitz-type serine protease inhibitor. TSG-6 and I(alpha)I synergize to inhibit plasmin, a serine protease involved in the activation of matrix metalloproteinases which are part of the proteolytic cascade associated with inflammation. Recombinant human TSG-6 protein exerts a potent anti-inflammatory effect in a murine model of acute inflammation. Modulation of the proteolytic network associated with inflammatory processes may be a mechanism whereby TSG-6, in cooperation with I(alpha)I, inhibits inflammation. Activation of the TSG-6 gene by pro-inflammatory cytokines, presence of TSG-6 protein in inflammatory lesions and its anti-inflammatory effect suggest a role for TSG-6 in a negative feed-back control of the inflammatory response.

Solution structure of the link module: a hyaluronan-binding domain involved in extracellular matrix stability and cell migration

Link modules are hyaluronan-binding domains found in proteins involved in the assembly of extracellular matrix, cell adhesion, and migration. The solution structure of the Link module from human TSG-6 was determined and found to consist of two alpha helices and two antiparallel beta sheets arranged around a large hydrophobic core. This defines the consensus fold for the Link module superfamily, which includes CD44, cartilage link protein, and aggrecan. The TSG-6 Link module was shown to interact with hyaluronan, and a putative binding surface was identified on the structure. A structural database search revealed close similarity between the Link module and the C-type lectin domain, with the predicted hyaluronan-binding site at an analogous position to the carbohydrate-binding pocket in E-selectin.

A blood group-related polymorphism of CD44 abolishes a hyaluronan-binding consensus sequence without preventing hyaluronan binding

CD44 is a widely expressed integral membrane protein that acts as a receptor for hyaluronan (HA) and is proposed to be important to cell-extracellular matrix interaction. The Indian (In) blood group antigens reside on CD44, and most individuals express the Inb antigen. Homozygosity for the Ina allele occurs as a rare event and is associated with production of alloantibody to the common Inb antigen after transfusion or pregnancy. The present study demonstrates that a single point mutation (G252 --> C) causes an Arg46 --> Pro substitution, which is responsible for the Inb/Ina polymorphism. Additional mutations were found in In(a+b-) cDNA but were not necessary to the antigenic phenotype as determined in site-directed mutagenesis studies. In studies using CD44 chimeric constructs, Arg46 has previously been shown to be crucial for maintenance of HA-binding ability to a CD44 peptide. However, the present study demonstrates that the Arg46 --> Pro substitution does not reduce HA binding to the intact CD44 protein, which contains two proposed extracellular HA-binding motifs. Down-regulation of HA binding to In(a+b-) CD44 by anti-CD44 monoclonal antibody (mAb) ligands, however, was weakened, although all mAbs tested bound In(a+b-) and In(a-b+) CD44 equally well. Competitive inhibition studies using human anti-Inb also showed that some mAbs that inhibit HA binding to CD44 may do so by interacting with a domain separate from, but affecting the structure of, the Inb epitope.