Cartilage proteoglycan aggregates. The link protein and proteoglycan amino-terminal globular domains have similar structures

Hyaladherins May be Implicated in Alcohol-Induced Susceptibility to Bacterial Pneumonia

Although the epidemiology of bacterial pneumonia and excessive alcohol use is well established, the mechanisms by which alcohol induces risk of pneumonia are less clear. Patterns of alcohol misuse, termed alcohol use disorders (AUD), affect about 15 million people in the United States. Compared to otherwise healthy individuals, AUD increase the risk of respiratory infections and acute respiratory distress syndrome (ARDS) by 2-4-fold. Levels and fragmentation of hyaluronic acid (HA), an extracellular glycosaminoglycan of variable molecular weight, are increased in chronic respiratory diseases, including ARDS. HA is largely involved in immune-assisted wound repair and cell migration. Levels of fragmented, low molecular weight HA are increased during inflammation and decrease concomitant with leukocyte levels following injury. In chronic respiratory diseases, levels of fragmented HA and leukocytes remain elevated, inflammation persists, and respiratory infections are not cleared efficiently, suggesting a possible pathological mechanism for prolonged bacterial pneumonia. However, the role of HA in alcohol-induced immune dysfunction is largely unknown. This mini literature review provides insights into understanding the role of HA signaling in host immune defense following excessive alcohol use. Potential therapeutic strategies to mitigate alcohol-induced immune suppression in bacterial pneumonia and HA dysregulation are also discussed.

Perineuronal Nets and Metal Cation Concentrations in the Microenvironments of Fast-Spiking, Parvalbumin-Expressing GABAergic Interneurons: Relevance to Neurodevelopment and Neurodevelopmental Disorders

Because of their abilities to catalyze generation of toxic free radical species, free concentrations of the redox reactive metals iron and copper are highly regulated. Importantly, desired neurobiological effects of these redox reactive metal cations occur within very narrow ranges of their local concentrations. For example, synaptic release of free copper acts locally to modulate NMDA receptor-mediated neurotransmission. Moreover, within the developing brain, iron is critical to hippocampal maturation and the differentiation of parvalbumin-expressing neurons, whose soma and dendrites are surrounded by perineuronal nets (PNNs). The PNNs are a specialized component of brain extracellular matrix, whose polyanionic character supports the fast-spiking electrophysiological properties of these parvalbumin-expressing GABAergic interneurons. In addition to binding cations and creation of the Donnan equilibrium that support the fast-spiking properties of this subset of interneurons, the complex architecture of PNNs also binds metal cations, which may serve a protective function against oxidative damage, especially of these fast-spiking neurons. Data suggest that pathological disturbance of the population of fast-spiking, parvalbumin-expressing GABAergic inhibitory interneurons occur in at least some clinical presentations, which leads to disruption of the synchronous oscillatory output of assemblies of pyramidal neurons. Increased expression of the GluN2A NMDA receptor subunit on parvalbumin-expressing interneurons is linked to functional maturation of both these neurons and the perineuronal nets that surround them. Disruption of GluN2A expression shows increased susceptibility to oxidative stress, reflected in redox dysregulation and delayed maturation of PNNs. This may be especially relevant to neurodevelopmental disorders, including autism spectrum disorder. Conceivably, binding of metal redox reactive cations by the perineuronal net helps to maintain safe local concentrations, and also serves as a reservoir buffering against second-to-second fluctuations in their concentrations outside of a narrow physiological range.

Demystifying the extracellular matrix and its proteolytic remodeling in the brain: structural and functional insights

The extracellular matrix (ECM) plays diverse roles in several physiological and pathological conditions. In the brain, the ECM is unique both in its composition and in functions. Furthermore, almost all the cells in the central nervous system contribute to different aspects of this intricate structure. Brain ECM, enriched with proteoglycans and other small proteins, aggregate into distinct structures around neurons and oligodendrocytes. These special structures have cardinal functions in the normal functioning of the brain, such as learning, memory, and synapse regulation. In this review, we have compiled the current knowledge about the structure and function of important ECM molecules in the brain and their proteolytic remodeling by matrix metalloproteinases and other enzymes, highlighting the special structures they form. In particular, the proteoglycans in brain ECM, which are essential for several vital functions, are emphasized in detail.

Modificazioni morfo-funzionali della cartilagine nella senescenza e nell'osteoartrosi

La cartilagine articolare è un tessuto connettivo avascolare, aneurale che ricopre le superfici articolari. La funzione di assorbimento delle sollecitazioni meccaniche, a protezione dell'osso subcondrale, rende la supeficie articolare idonea a sostenere il carico.

Le funzioni inerenti le modalità di assorbimento della sollecitazione meccanica, che fanno sì che la deformazione sia reversibile, dipendono in larga parte dalle caratteristiche della cartilagine, intesa come struttura altamente organizzata. Nell'osteoartrosi umana e nei suoi modelli animali l'alterazione strutturale dei proteoglicani cartilaginei rappresenta l'evento centrale.

Vengono discusse, alla luce delle acquisizioni più recenti, le implicazioni sulle proprieta fisico-chimiche e morfo-strutturali della cartilagine articolare riguardanti le caratteristiche di base dei proteoglicani, la struttura dei collageni, l'organizzazione della matrice extracellulare e le sue modificazioni nella senescenza ed in corso di osteoartrosi con le relative conseguenze sulle proprietà biomeccaniche del disco intervertebrale. Le conoscenze relative alle alterazioni della struttura proteoglicanica e lo sviluppo di nuovi metodi di determinazione dei markers biochimici del danno cartilagineo potrebbero migliorare la comprensione delle relazioni fra senescenza ed osteoartrosi, nonchè il riconoscimento delle modificazioni più precoci e la valutazione della risposta terapeutica.

Hyaluronic Acid-Based Biocompatible Supramolecular Assembly for Sustained Release of Antiretroviral Drug

Human immunodeficiency virus (HIV) infection and its associated diseases continue to increase despite the progress in our understanding of HIV biology and the availability of a number of antiretroviral drugs. Adherence is a significant factor in the success of HIV therapy and current HIV treatment regimens require a combination of antiviral drugs to be taken at least daily for the remainder of a patient's life. A drug delivery system that allows sustained drug delivery could reduce the medical burden and costs associated with medication nonadherence. Here, we describe a novel supramolecular assembly or matrix that contains an anionic polymer hyaluronic acid, cationic polymer poly-l-lysine, and anionic oligosaccharide sulfobutylether-beta-cyclodextrin. HIV reverse transcriptase inhibitors Zidovudine and Lamivudine were successfully encapsulated into the polymer assembly in a noncovalent manner. The physicochemical properties and antiviral activity of the polymer assemblies were studied. The results of this study suggest that the supramolecular assemblies loaded with HIV drugs exert potent antiviral activity and allow sustained drug release. A novel drug delivery formulation such as the one described here could facilitate our efforts to reduce the morbidity and mortality associated with HIV infections and could be utilized in the design of therapeutic approaches for other diseases.

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.

The extractability of extracellular matrix components as a marker of cartilage remodeling in laryngeal squamous cell carcinoma

Sequential extraction was applied to investigate the proteoglycan (PG) organization in healthy laryngeal cartilage (HLC) and laryngeal cartilage squamous cell carcinoma (LCSCC). Highly stable aggrecan aggregates, extracted from both HLC and LCSCC with strong dissociative reagents, i.e., 4 M guanidine HCl (GdnHCl), represented 53% and 7%, respectively, of total extracted macromolecules. Less stable complexes/aggregates, extracted with mild dissociative reagents (1 and 2 M GdnHCl), represented 40% and 61% of total extracted PGs from healthy and cancerous cartilage, respectively. Interestingly, a relative high proportion (32%) of uronic acid (UA)-containing macromolecules were removed from the cancerous cartilage using associative extracting solutions (PBS and 0.5 M GdnHCl), which obviously represented molecules freely extractable from the tissue. In contrast, the corresponding proportion in HLC was impressively low (about 7%). The major proportion of these molecules was chondroitin sulfate-containing PGs (CSPGs), which identified mainly as aggrecan. Differential digestion of the sequential extracts with chondroitinase ABC and chondroitinase AC II demonstrated the presence of dermatan sulfate-containing PGs (DSPGs) in both HLC and LCSCC, being mainly present in the 1 M GdnHCl extract, and identified as decorin. All cancerous extracts were found to be rich in 4-sulfated disaccharides, mostly participating in DS structures. In conclusion, the applied procedure permitted the elucidation of the changes in the cartilage status, regarding the stability and identity of its proteoglycan aggregates/complexes, in both HLC and LCSCC.

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.

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

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

Two distinct populations of tumor necrosis factor-stimulated gene-6 protein in the extracellular matrix of expanded mouse cumulus cell-oocyte complexes

After the luteinizing hormone surge, the cumulus cell-oocyte complexes (COCs) in the preovulatory follicles produce a viscoelastic extracellular matrix, a process that requires the synthesis of hyaluronan as well as the incorporation of some components of the inter-alpha-trypsin inhibitor (IalphaI) family. In this study we report, that a hyaluronan-binding protein, the translated product of tumor necrosis factor-stimulated gene-6 (TSG-6), is also specifically accumulated in this matrix. TSG-6 mRNA expression is quickly upregulated and peaks at approximately 1500 copies/cell 4 h after the ovulatory stimuli as assessed by quantitative reverse transcription-polymerase chain reaction. Immunohistochemistry reveals the colocalization of the TSG-6 protein and hyaluronan around the cumulus and granulosa cells. The TSG-6 protein exists in two distinct populations in the COC matrix as demonstrated by Western-blot analysis. One population is a monomer that is anchored to the matrix by a noncovalent interaction. The second population is a covalent complex with either of the heavy chains of IalphaI and is bound to hyaluronan through a strong interaction that is resistant to denaturing conditions. The specific incorporation of the TSG-6 protein into the COC matrix suggests a structural role for this molecule.

Domain organization, genomic structure, evolution, and regulation of expression of the aggrecan gene family

Proteoglycans are complex macromolecules, consisting of a polypeptide backbone to which are covalently attached one or more glycosaminoglycan chains. Molecular cloning has allowed identification of the genes encoding the core proteins of various proteoglycans, leading to a better understanding of the diversity of proteoglycan structure and function, as well as to the evolution of a classification of proteoglycans on the basis of emerging gene families that encode the different core proteins. One such family includes several proteoglycans that have been grouped with aggrecan, the large aggregating chondroitin sulfate proteoglycan of cartilage, based on a high number of sequence similarities within the N- and C-terminal domains. Thus far these proteoglycans include versican, neurocan, and brevican. It is now apparent that these proteins, as a group, are truly a gene family with shared structural motifs on the protein and nucleotide (mRNA) levels, and with nearly identical genomic organizations. Clearly a common ancestral origin is indicated for the members of the aggrecan family of proteoglycans. However, differing patterns of amplification and divergence have also occurred within certain exons across species and family members, leading to the class-characteristic protein motifs in the central carbohydrate-rich region exclusively. Thus the overall domain organization strongly suggests that sequence conservation in the terminal globular domains underlies common functions, whereas differences in the central portions of the genes account for functional specialization among the members of this gene family.

Immune responses to cartilage link protein and the G1 domain of proteoglycan aggrecan in patients with osteoarthritis

OBJECTIVE

To determine whether patients with osteoarthritis (OA) express cellular immunity to cartilage link protein (LP) and the G1 globular domain of proteoglycan (PG) aggrecan, and whether immunity to the G1 domain is influenced by the removal of keratan sulfate (KS).

METHODS

LP and the G1 globular domain of PG were isolated from human and/or bovine cartilage and used in proliferation assays with peripheral blood lymphocytes (PBL) from 42 patients with OA and 40 healthy control subjects.

RESULTS

Patients with OA expressed a higher prevalence of cellular immunity to human cartilage LP (42.4%) compared with the control group (13.3%). The prevalence of immune reactivity to bovine LP in patients with OA was lower (35.7%) compared with the immunity to human LP, but remained similar in the control group (13.8%). PBL from patients with OA exhibited low reactivity to the native G1 domain of bovine PG. However, removal of KS chains from the G1 globular domain resulted in increased cellular immune responses to the G1 domain in OA patients (45.8%) compared with the control group (7.7%).

CONCLUSION

These results indicate the presence of immunity to cartilage-derived LP and the G1 globular domain of PG aggrecan in patients with OA and the inhibitory effect of KS chains on the G1 domain on the expression of this immunity in OA patients. This immune reactivity is commonly observed in patients with inflammatory joint disease and can experimentally induce arthritis. Thus, it may be involved in the pathogenesis of OA.

Induction of arthritis in BALB/c mice by cartilage link protein: involvement of distinct regions recognized by T and B lymphocytes

Both type II collagen and the proteoglycan aggrecan are capable of inducing an erosive inflammatory polyarthritis in mice. In this study we provide the first demonstration that link protein (LP), purified from bovine cartilage, can produce a persistent, erosive, inflammatory polyarthritis when injected repeatedly intraperitoneally into BALB/c mice. We discovered a single T-cell epitope, located within residues 266 to 290 of bovine LP (NDGAQIAKVGQIFAAWKLLGYDRCD), which is recognized by bovine LP-specific T lymphocytes. We also identified three immunogenic regions in bovine LP that contain epitopes recognized by antibodies in hyperimmunized sera. One of these B-cell regions is found in the most species-variable domain of LP (residues 1 to 36), whereas the other epitopes are located in the most conserved regions (residues 186 to 230 and 286 to 310). The latter two regions contain an AGWLSDGSVQYP motif shared by the G1 globulin domain of aggrecan core protein, versican, neurocan, glial hyaluronan-binding protein, and the hyaluronan receptor CD44. Our data reveal that the induction of arthritis is associated with antibody reactivities to B-cell epitopes located at residues 1 to 19. Together, these observations show that another cartilage protein, LP, like type II collagen and the proteoglycan aggrecan, is capable of inducing an erosive inflammatory arthritis in mice and that the immunity to LP involves recognition of both T- and B-cell epitopes. This immunity may be of importance in the pathogenesis of inflammatory joint diseases, such as juvenile rheumatoid arthritis, in which cellular immunity to LP has been demonstrated.

Coding sequence, exon-intron structure and chromosomal localization of murine TNF-stimulated gene 6 that is specifically expressed by expanding cumulus cell-oocyte complexes

Tumor necrosis factor stimulated gene-6 (TSG-6) has been previously shown to be induced in vitro in several cell types by proinflammatory cytokines, and in vivo in pathological conditions such as rheumatoid arthritis. In this study, we report the complete coding sequence for the mouse TSG-6 protein, and the exon intron structure and the chromosomal localization of the gene. We have identified a 1605 nt cDNA sequence from mouse cumulus cell oocyte complexes (COCs) induced to expand in vivo. The sequence contains an open reading frame of 825 nt that codes for the 275 amino acid TSG-6 protein. The gene contains six exons separated by 1.1-5.8 kb introns and has been localized to the murine chromosome 2 by linkage analysis. Comparative reverse transcription-polymerase chain reaction studies have revealed that TSG-6 mRNA is specifically expressed after COC expansion induced in vivo, identifying the first non-pathological process in which TSG-6 may play an important role. Since TSG-6 binds to hyaluronan and interacts with inter-alpha-trypsin inhibitor (IalphaI), molecules that are essential for matrix formation by COCs, this protein may have a structural role in the matrix or may enhance the antiproteolytic effect of IalphaI to protect the matrix from degradation.

CD44: structure, function, and association with the malignant process

CD44 is a ubiquitous multistructural and multifunctional cells surface adhesion molecule involved in cell-cell and cell-matrix interactions. Twenty exons are involved in the genomic organization of this molecule. The first five and the last 5 exons are constant, whereas the 10 exons located between these regions are subjected to alternative splicing, resulting in the generation of a variable region. Differential utilization of the 10 variable region exons, as well as variations in N-glycosylation, O-glycosylation, and glycosaminoglycanation (by heparan sulfate or chondroitin sulfate), generate multiple isoforms (at least 20 are known) of different molecular sizes (85-230 kDa). The smallest CD44 molecule (85-95 kDa), which lacks the entire variable region, is standard CD44 (CD44s). As it is expressed mainly on cells of lymphohematopoietic origin, CD44s is also known as hematopoietic CD44 (CD44H). CD44s is a single-chain molecule composed of a distal extracellular domain (containing, the ligand-binding sites), a membrane-proximal region, a transmembrane-spanning domain, and a cytoplasmic tail. The molecular sequence (with the exception of the membrane-proximal region) displays high interspecies homology. After immunological activation, T lymphocytes and other leukocytes transiently upregulate CD44 isoforms expressing variant exons (designated CD44v). A CD44 isform containing the last 3 exon products of the variable region (CD44V8-10, also known as epithelial CD44 or CD44E), is preferentially expressed on epithelial cells. The longest CD44 isoform expressing in tandem eight exons of the variable region (CD44V3-10) was detected in keratinocytes. Hyaluronic acid (HA), an important component of the extracellular matrix (ECM), is the principal, but by no means the only, ligand of CD44. Other CD44 ligands include the ECM components collagen, fibronectin, laminin, and chondroitin sulfate. Mucosal addressin, serglycin, osteopontin, and the class II invariant chain (Ii) are additional, ECM-unrelated, ligands of the molecule. In many, but not in all cases, CD44 does not bind HA unless it is stimulated by phorbol esters, activated by agonistic anti-CD44 antibody, or deglycosylated (e.g., by tunicamycin). CD44 is a multifunctional receptor involved in cell-cell and cell-ECM interactions, cell traffic, lymph node homing, presentation of chemokines and growth factors to traveling cells, and transmission of growth signals. CD44 also participates in the uptake and intracellular degradation of HA, as well as in transmission of signals mediating hematopoiesis and apoptosis. Many cancer cell types as well as their metastases express high levels of CD44. Whereas some tumors, such as gliomas, exclusively express standard CD44, other neoplasms, including gastrointestinal cancer, bladder cancer, uterine cervical cancer, breast cancer and non-Hodgkin's lymphomas, also express CD44 variants. Hence CD44, particularly its variants, may be used as diagnostic or prognostic markers of at least some human malignant diseases. Furthermore, it has been shown in animal models that injection of reagents interfering with CD44-ligand interaction (e.g., CD44s- or CD44v-specific antibodies) inhibit local tumor growth and metastatic spread. These findings suggest that CD44 may confer a growth advantage on some neoplastic cells and, therefore, could be used as a target for cancer therapy. It is hoped that identification of CD44 variants expressed on cancer but not on normal cells will lead to the development of anti-CD44 reagents restricted to the neoplastic growth.

Biotinylated hyaluronan: a versatile and highly sensitive probe capable of detecting nanogram levels of hyaluronan binding proteins (hyaladherins) on electroblots by a novel affinity detection procedure

Hyaluronan influences cellular proliferation and migration in developing, regenerating and remodelling tissues and in tissues undergoing malignant tumour-cell invasion. The widespread occurrence of hyaluronan-binding proteins indicates that the recognition of hyaluronan is important to tissue organisation and the control of cellular behaviour. A number of extracellular matrix and cellular proteins, which have been termed the hyaladherins, have specific affinities for hyaluronan. These include cartilage link-protein, hyaluronectin, neurocan, versican and aggrecan, which all bind to HA within the extracellular matrix. Cellular receptors for hyaluronan such as CD44 and RHAMM (receptor for hyaluronate-mediated motility) have also been identified. In the present study biotinylated hyaluronan (bHA) was prepared by reacting adipic dihydrazide with a 170 kDa hyaluronan sample using the bifunctional reagent 1-ethyl-3-[3-(dimethylamino) propyl] carbodiimide. The resultant free amine moeity of the hydrazido-hyaluronan was then reacted with biotin succinimidyl ester (sulfo-NHS-biotin) to prepare the bHA. After 4-20% gradient sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and electroblotting to nitrocellulose membranes, bHA and avidin alkaline phosphatase conjugate could be used in conjunction with nitroblue tetrazolium/5-bromo-4-chloro-3-indolyl phosphate substrates to specifically visualise with high sensitivity (> or = 2 ng), bovine nasal cartilage link-protein, aggrecan hyaluronan binding region, and human fibroblast hyaluronan receptors such as CD-44. Conventional Western blotting using specific monoclonal antibodies to these proteins was also used to confirm the identities of these proteins.

N- and O-linked keratan sulfate on the hyaluronan binding region of aggrecan from mature and immature bovine cartilage

In the hyaluronan binding region (HABR) peptide of aggrecan, there is a marked increase in the level of keratan sulfate (KS) during aging. To determine the sites of KS attachment, KS-containing peptides were prepared from HABRs from immature and mature bovine articular cartilage by digestion with trypsin or papain followed by carbohydrate analysis and peptide sequencing. KS is attached to Thr42 within loop A in mature, but not in immature, HABR. Within loop B KS is N-linked to Asn220 in both HABRs, but in the immature HABR the chains are shorter. Asn314 in loop B' of mature HABR is substituted either with a KS chain or with an oligosaccharide of the complex type. In immature HABR this site does not carry KS. In the interglobular domain, 2 threonine residues within the sequence TIQTVT are substituted in both calf and steer, and in steer further substitution occurs within the sequence NITEGEA, which contains a major catabolic cleavage site (Sandy, J., Neame, P.J., Boynton, R., and Flannery, C.R. (1991) J. Biol. Chem. 266, 8683-8685). The extreme polydispersity of mature HABR was investigated by preparing four subfractions of increasing molecular size which had essentially the same protein core, i.e. Val1-Arg367 or Val1-Arg375. The smaller species lacked the KS chains attached to loop A. These results show that KS substitution occurs within each of the disulfide-bonded loops of the HABR, that the KS may be either N- or O-linked, and that variations in the addition of KS are responsible for the polydispersity of mature HABR.

Regulation of proteoglycan expression in fibrotic liver and cultured fat-storing cells

Considerable progress has been made in recent years with the molecular dissection of proteoglycans in normal and fibrotic human and rat liver. Proteoglycans constitute a major fraction of extracellular, pericellular and intracellular glycoconjugates. In former times, proteoglycans were classified nearly exclusively on the basis of the composition of their carbohydrate chain (glycosaminoglycan, GAG) attached to the core protein. Accordingly, three main types are discerned in liver, which are in order of decreasing concentrations heparan sulfate (HS, more than 60% of total GAG), dermatan sulfate and chondroitin-4,6-sulfate isomers. Keratan sulfate has not been detected in rat and human liver. Recently, proteoglycans have been characterized by sequencing and cloning of the core proteins to which a number of specific glycosaminoglycan side chains are covalently linked. Accordingly, decorin and biglycan have been identified as major chondroitin sulfate/dermatan sulfate proteoglycans in the extracellular space. In addition, evidence was obtained recently for the expression of aggrecan and lumican, both keratan sulfate bearing proteoglycans, and of syndecan in liver. Using in situ hybridization techniques the temporal and spatial pattern of expression of biglycan, decorin and aggrecan has been assessed. These studies together with Northern blot hybridizations performed with isolated parenchymal and nonparenchymal liver cells confirm that fat-storing cells (Ito cells, perisinusoidal lipocytes), are the most important, principal cellular site of proteoglycan production in diseased liver. The level of expression is regulated by a number of cytokines among which TGF beta, TNF alpha and TGF alpha play significant roles. The effects of these cytokines on proteoglycan expression are dependent on the stage of phenotypic transition of fat storing cells to the activated myofibroblast. Taken together, these data point to the potentially significant role which proteoglycans might fulfil in the regulation of cellular functions and in the maintenance of the supramolecular organization of the extracellular matrix in normal and in diseased liver during the process of fibrogenesis.

Length variation in the keratan sulfate domain of mammalian aggrecan

The keratan sulfate domain of aggrecan consists of a series of tandemly repeating hexapeptides which have the consensus sequence Glu-Glu/Lys-Pro-Phe-Pro-Ser, where the serine side-chains presumably provide sites for the attachment of keratan sulfate (KS) chains. The number of hexapeptide repeats varies between species, ranging from four in rat (Doege et al., 1987) and mouse (Walcz et al., 1992) to 13 in human (Doege et al., 1991) and 23 in bovine aggrecan (Antonsson et al., 1989). Chicken aggrecan (Chandrasekaran and Tanzer, 1992) does not contain a KS domain with a recognizable hexapeptide motif. The extent of this variation among mammalian and avian species is not known, and there is currently no explanation to predict how differences in the size of the KS domain would affect aggrecan function. We used polymerase chain reaction (PCR) to amplify the portion of the human, canine and porcine aggrecan gene that codes for the KS domain. We sequenced the amplified products in each case. Human aggrecan, with 13 hexapeptide repeats (Doege et al., 1987), was used as reference and found to be essentially identical to published data. The canine and porcine KS domains consisted of six and ten hexapeptide repeats respectively. The same PCR protocol was used to amplify the KS domain from genomic DNA of eight other mammalian species. Comparison of the size of these amplified products, as determined by agarose gel electrophoresis, with those for which sequence data are available allowed us to estimate the number of repeats in the KS domain. In almost half the species examined, the KS domain consisted of 13 hexapeptide repeats.

The cartilage proteoglycan aggregate: assembly through combined protein-carbohydrate and protein-protein interactions

In vitro reassembled aggregates of cartilage proteoglycan (aggrecan) were studied by glycerol spraying/rotary shadowing electron microscopy and compared to the corresponding native (i.e. never dissociated) structures. In both cases a tightly packed central filament structure was observed consisting of the hyaluronate binding region (HABR) of the proteoglycan, link protein (LP) and hyaluronate (HA). This differs from earlier results where a discontinuous central filament structure was seen after spreading proteoglycan aggregates at a water/air interphase. Binding of isolated HABR to HA is random but upon addition of link protein a clustering of the HA-binding proteins is observed, indicating a cooperativity. In a fully saturated aggregate the HA is covered by a continuous protein shell consisting of HABR and LP. When added in amounts below saturation HABR and LP bind to the HA in clusters which are interrupted by free strands of HA. The proteoglycan aggregate is thus an example for a structure where a polysaccharide forms a template for a supramolecular assembly largely stabilized by protein-protein interactions.

The link proteins

Aggregates of chondroitin-keratan sulfate proteoglycan (aggrecan) and hyaluronic acid (hyaluronan) are the major space-filling components of cartilage. A glycoprotein, link protein (LP; 40-48 kDa) stabilizes the aggregate by binding to both hyaluronic acid and aggrecan. In the absence of LP, aggregates are smaller (as estimated by rotary shadowing of electron micrographs) and less stable (they dissociate at pH 5) than they are in the presence of LP. The proteoglycan aggregate, including LP, is dissociated in the presence of chaotropes such as 4 M guanidine hydrochloride. On removal of the chaotrope, the complex will reassociate. This forms the basis of the isolation of LP from cartilage and has been described in detail elsewhere. Tryptic digestion of the proteoglycan aggregates results in a high molecular weight product that consists of hyaluronic acid to which is bound LP and the N-terminal globular domain of aggrecan (hyaluronic acid binding region; HABR) in a 1:1 stoichiometry. The amino acid sequences of LP and HABR are surprisingly similar. The amino acid sequence can be divided into three domains; an N-terminal domain that falls into the immunoglobulin super-family and two C-terminal domains that are similar to each other. The DNA structure echoes this similarity, in that the major domains are reflected in three separate exons in both LP and HABR. The two C-terminal domains are largely responsible for the association with HA and are related to two recently described hyaluronate-binding proteins, CD44 and TSG-6. A variety of approaches, including analysis of the forms of LP in vivo, rotary shadowing and analysis of the sequence in the immunoglobulin-like domain, have shed considerable light on the structure-function relationships of LP. This review describes the structure and function of LP in detail, focusing on what can be inferred from the similarity of LP, HABR and related molecules such as immunoglobulins and lymphocyte HA-receptors.

The link proteins

Aggregates of chondroitin-keratan sulfate proteoglycan (aggrecan) and hyaluronic acid (hyaluronan) are the major space-filling components of cartilage. A glycoprotein, link protein (LP; 40-48 kDa) stabilizes the aggregate by binding to both hyaluronic acid and aggrecan. In the absence of LP, aggregates are smaller (as estimated by rotary shadowing of electron micrographs) and less stable (they dissociate at pH 5) than they are in the presence of LP. The proteoglycan aggregate, including LP, is dissociated in the presence of chaotropes such as 4 M guanidine hydrochloride. On removal of the chaotrope, the complex will reassociate. This forms the basis of the isolation of LP from cartilage and has been described in detail elsewhere. Tryptic digestion of the proteoglycan aggregates results in a high molecular weight product that consists of hyaluronic acid to which is bound LP and the N-terminal globular domain of aggrecan (hyaluronic acid binding region; HABR) in a 1:1 stoichiometry. The amino acid sequences of LP and HABR are surprisingly similar. The amino acid sequence can be divided into three domains; an N-terminal domain that falls into the immunoglobulin super-family and two C-terminal domains that are similar to each other. The DNA structure echoes this similarity, in that the major domains are reflected in three separate exons in both LP and HABR. The two C-terminal domains are largely responsible for the association with HA and are related to two recently described hyaluronate-binding proteins, CD44 and TSG-6. A variety of approaches, including analysis of the forms of LP found in vivo, rotary shadowing and analysis of the sequence in the immunoglobulin-like domain, have shed considerable light on the structure-function relationships of LP. This review describes the structure and function of LP in detail, focusing on what can be inferred from the similarity of LP, HABR and related molecules such as immunoglobulins and lymphocyte HA-receptors.

CD44 and its interaction with extracellular matrix

It is now generally accepted that CD44 is a cell adhesion receptor and that hyaluronan is one of its ligands. Like many cell adhesion receptors, CD44 is broadly distributed, and its ligand, hyaluronan, is a common component of extracellular matrices and extracellular fluids. Yet a great variety of responses has been reported to result from CD44 ligation. These include cell adhesion, cell migration, induction (or at least support) of hematopoietic differentiation, effects on other cell adhesion mechanisms, and interaction with cell activation signals. This diversity of responses indicates that downstream events following ligand binding by CD44 may vary depending on the cell type expressing CD44 and on the environment of that cell. CD44 is expressed on cells in the early stages of hematopoiesis and has been shown to participate in at least some aspects of the hematopoietic process. In mature lymphocytes, CD44 is upregulated in response to antigenic stimuli and may participate in the effector stage of immunological responses. Along with other adhesion receptors that show alterations in expression after activation, CD44 probably contributes to differences in the recirculation patterns of different lymphocyte subpopulations. CD44 ligand-binding function on lymphocytes is strictly regulated, such that most CD44-expressing cells do not constitutively bind ligand. Ligand-binding function may be activated as a result of differentiation, inside-out signaling, and/or extracellular stimuli. This regulation, which in some situations can be rapid and transient, potentially provides exquisite specificity to what would otherwise be a common interaction. CD44 is not a single molecule, but a diverse family of molecules generated by alternate splicing of multiple exons of a single gene and by different posttranslational modifications in different cell types. It is not yet clear how these modifications influence ligand-binding function. The significance of the multiple isoforms of CD44 is not understood, but association of some isoforms with malignancies has been observed. And in at least some experimental systems, a contribution of CD44 isoforms to metastatic behavior has been demonstrated.

Differential expression of keratan sulphate proteoglycans fibromodulin, lumican and aggrecan in normal and fibrotic rat liver

In this study we investigated in rat liver the expression of genes coding for the core proteins of fibromodulin, lumican and aggrecan. By means of Northern analysis and in situ hybridization we present evidence for their differential transcription during liver fibrogenesis. Whereas no fibromodulin expression could been detected, both lumican and aggrecan transcripts were found displaying different time-courses of expression during the fibrogenic process. Based on studies performed in non-hepatic tissues, these proteoglycans are considered to have keratan sulphate glycosaminoglycan side chains. The expression of the respective core protein genes in liver is unexpected since published data have shown neither keratan sulphate nor its synthesis de novo in this tissue. The results also point to a putative role of aggrecan in the modulation of the inflammatory process in the liver.

Immunological analysis of proteoglycan structural changes in the early stage of experimental osteoarthritic canine cartilage lesions

Specific modifications of the proteoglycan (PG) structure of osteoarthritic (OA) dog cartilage in relation to endogenous metalloprotease activity were examined using murine anti-proteoglycan monoclonal antibodies (MoAbs). OA lesions were induced over a period of 8 weeks in crossbred dogs (Pond-Nuki model). The articular cartilage was removed and homogenized in a Tris buffer, pH 7.5, and then divided into four groups: direct PG extraction, no addition, presence of 1 mM p-aminophenyl mercuric acetate (APMA), and presence of 1 mM APMA and 10 mM o-phenanthroline, incubated for 42 h at 37 degrees C followed by PG extraction. MoAbs reactive with PG protein and carbohydrate epitopes included 1C6, 3B3, 5D4, D1B2, and m4D6. The results showed marked alterations induced by APMA activation of the endogenous metalloproteases. PG changes were apparent at at least three sites: one was either in the hyaluronic acid-binding region or between the hyaluronic-binding region and the G2 globular domain, another was between the keratan-sulfate-rich domain and the chondroitin sulfate-attachment domain, and a third was in the chondroitin sulfate-attachment domain. Constitutive metalloprotease activity resulted in less marked PG alterations with preservation of functional PG aggregation to hyaluronan.

Ultrastructural localization of hyaluronan in myelin sheaths of the rat central and rat and human peripheral nervous systems using hyaluronan-binding protein-gold and link protein-gold

Neural tissue of central (rat spinal cord) and peripheral origin (rat sciatic nerve, nerve fascicles of rat skin and iris and of human conjunctiva) was processed by osmium tetroxide/microwave fixation and embedded in epoxy resin. Hyaluronan-binding proteins and link proteins coupled to 15-20-nm gold particles were used as markers in a one-step post-embedding procedure for identifying hyaluronan (hyaluronic acid) at the ultrastructural level. All myelin sheaths in both rat and human material were found to be intensely labelled. The specificity of the hyaluronan-binding probes was demonstrated by the total loss of labelling following treatment of sections with hyaluronidase or by preincubating either the probes with hyaluronan oligosaccharides or the sections with unlabelled hyaluronan-binding protein. The identified hyaluronan appears to be located extracellularly, but is precise role here remains to be elucidated.

Cleavage of proteoglycan aggregate by leucocyte elastase

The partial degradation of proteoglycan aggregate by human leucocyte elastase yielded products that banded with Mr 190,000, 140,000, 88,000, and 71,000 when analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide electrophoresis. Analysis of these bands revealed that the 190,000- and 140,000-Da bands contained chondroitin and keratan sulfate stubs and had N-terminal amino acid sequences corresponding to a sequence starting at residue 398 of the core protein of rat or human aggrecan. With increased time of digestion, the staining intensities of the 190,000-, 140,000-, and 88,000-Da bands decreased relative to the 71,000-Da band. Analysis of the 88,000- and 71,000-Da bands showed that they contained peptides substituted only with keratan sulfate stubs and that each band contained two peptides with different N-terminal sequences. One of these corresponded to a sequence that started at residue 398 of rat or human aggrecan and the other to the N-terminal sequence of bovine aggrecan. Under conditions of complete digestion, bands of 71,000 and 56,000 Da which contained only keratan sulfate stubs were observed on SDS-polyacrylamide electrophoresis. The 71,000-Da band was shown to have a single sequence similar to that starting at residue 398 of human and rat aggrecan and thus represents the globular domain 2 (G2) of the core protein of aggrecan. The 56,000-Da band was shown to have a sequence similar to that of the N-terminal sequence of bovine aggrecan indicating that this peptide corresponds to the globular domain 1 (G1) of the molecule. These results suggest that leucocyte elastase cleaves the core protein of aggrecan between valine 397 and isoleucine 398, which are located in the interglobular domain linking the G1 and G2 domains of the core protein of aggrecan. Further digestion of the proteoglycan aggregate with elastase resulted in the cleavage of the core protein within the chondroitin sulfate attachment domains.

Evidence of proteoglycan/proteoglycan interactions within aggregates

Nonaggregated proteoglycan monomers, digested fragments of the monomers, as well as link proteins have been shown to self-associate. These associations have not been shown to occur on the aggregate. However, previous reports, using the Kleinschmidt technique of monolayer electron microscopy, have noted proteoglycan subunits on the aggregate that appear to interact, either as branched proteoglycans or as proteoglycan subunits that appear to share the same attachment site on the hyaluronic acid chain. Branching and shared attachments were noted in all aggregates analyzed in this study. Increasing the average space between proteoglycan subunits on the reconstituted aggregate resulted in a significant decrease in branched proteoglycans, indicating either a weak association occurring on the aggregate, or an artifact created by a three-dimensional structure being reduced to a two-dimensional monolayer image. The shared attachments were independent of both the presence of link proteins and changes in spacing between proteoglycans, suggesting a proteoglycan-proteoglycan interaction occurring before aggregation. The interactions were not influenced by proteoglycan concentration at the time of aggregation. Link proteins, however, did increase the number of proteoglycans on the aggregate that could be cross-linked with a bifunctional reagent, suggesting that link proteins facilitate proteoglycan-proteoglycan interactions.

Characterization of the extracellular matrix of the primate temporomandibular joint

The distribution of type I and II collagens, fibronectin and the fibronectin-integrin receptor, tenascin, laminin, link protein, and cartilage-specific glycosaminoglycans was examined in the primate temporomandibular joint complex using an immunohistochemical approach. In general, type I collagen, fibronectin, and the fibronectin-integrin receptor were found to co-distribute throughout the joint complex. Immunostaining for these proteins was notably intense in the prechondroblastic and mineralization zones of the articular cartilages of the joint. Tenascin was identified in several structures of the joint, including the articular cartilages, where intense staining was observed in the prechondroblastic and cartilagenous zones. Laminin was detected only in the adventitia of blood vessels located in the attachment tissues of the disc and joint synovium. Cartilage-specific glycosaminoglycans and type II collagen were observed in the cartilagenous zones of the articular cartilages of the mandibular condyle and temporal bone. In addition, immunostaining for cartilage-specific glycosaminoglycans also was detected throughout the extracellular matrix surrounding "chondrocyte-like" cells located in the joint disc. Despite the localization of cartilage-specific glycosaminoglycans in the disc, type II collagen was not detected in this structure. It is suggested that a fibronectin-integrin receptor mechanism may be involved in the regulation of growth of the articular cartilages of the temporomandibular joint.

A matrix protein of Mr 55,000 that accumulates in human articular cartilage with age

Adult human articular cartilage contains a protein of Mr 55,000 which is deficient in newborn cartilage. In the adult the molecule represents one of the most abundant non-collagenous, non-proteoglycan molecules in 4 M guanidinium chloride extracts of the tissue. The molecular size of the protein on SDS-PAGE remains constant under reducing and non-reducing conditions, suggesting that it does not exist as a disulphide-bonded multimer, nor do intramolecular disulphide bonds greatly influence its conformation. The protein has the ability to interact with some immunoglobulin preparations making its detection possible by Western blotting with some non-specific antisera. Labeling with [3H]leucine in organ culture indicates that protein of this size is being made by the chondrocytes. However, during purification the newly synthesized molecules do not behave as the resident protein on ion-exchange chromatography, suggesting that the protein may accumulate with age rather than being a major synthetic product of the adult chondrocytes. Amino terminal protein sequence analysis indicates that the N-terminus of the protein is blocked. Sequences derived from peptides generated with cyanogen bromide do not show homology with previously characterized proteins. Molecules of a similar size and composition have been described in bovine cartilage.

Proteoglycans of articular cartilage: changes in aging and in joint disease

In human osteoarthritis and animal models of degenerative joint disease, damage to the structure of cartilage proteoglycan is a central event. Loss of proteoglycan from the matrix alters the physicochemical properties of the tissue, but the pathological process and biochemical mechanisms that lead to this loss are poorly understood. This review examines the present state of knowledge regarding proteoglycan structure and the changes that occur in aging and osteoarthritis. It also discusses how these studies will influence the development of new methods for measuring cartilage breakdown.

Studies on the interaction of newly secreted proteoglycan subunits with hyaluronate in human articular cartilage

Newly secreted proteoglycans from adult human cartilage do not interact well with hyaluronate, but attain this ability with time in the extracellular matrix. The conversion process occurs in all types of cartilagenous matrix, as newborn cartilage cultures, chondrosarcoma cultures and adult chondrocyte cultures each secreted proteoglycan subunits which exhibited the delayed aggregation phenomenon. However, the rate of conversion is probably dependent upon the structure of the surrounding matrix and the cell type. In vitro, link protein appears to enhance an initial change in the hyaluronate-binding region of the newly secreted proteoglycan subunits to allows stronger interaction with hyaluronate. In a second step, which is pH- and temperature-dependent, the change becomes irreversible. Thus, in addition to its role in stabilizing the interaction of mature proteoglycan subunits with hyaluronate, link protein may also aid in promoting the conversion of the newly synthesized proteoglycan subunit to a form that is capable of strong interaction with hyaluronate.

Monoclonal antibodies reacting with tryptic hyaluronic acid-binding region and link protein fragments of bovine nasal cartilage proteoglycan

BALB/c mice were immunized with isolated trypsin-produced hyaluronic acid-binding region (HABR) or HABR/link protein complex from bovine nasal cartilage proteoglycan (PG) aggregates conjugated to keyhole limpet hemocyanin. The monoclonal antibodies (Mabs) raised were characterized by solid-phase ELISA inhibition and SDS-polyacrylamide gel electrophoresis immunoblotting. Nine Mabs react with intact PG monomer, HABR and HABR/link complex. Two of the anti-HABR Mabs appear to be directed to epitopes in or near the HA-binding site of PG monomer; one of these epitopes is insensitive to reduction and alkylation and pronase treatment and is likely to consist of carbohydrate. The remaining anti-HABR Mabs react with PG aggregate, are non-reactive with pronase-treated PG monomer and vary in their reactivity with reduced and alkylated PG monomer. Three Mabs react with link protein-related epitopes. One of these Mabs reacts only with the tryptic link protein fragment, the others also react with PG aggregates and the two native link proteins. Immunological studies of cartilage PG should be facilitated by the availability of Mabs specific for these functionally significant components in native PG aggregates.

In situ cross-linking of cartilage proteoglycans

Although the in vitro interactions between purified cartilage matrix components have been studied extensively, little is known about these interactions in situ. In this study, cartilage was treated with a cross-linking reagent with a span of 1.2 nm between its reactive terminal groups in order to preserve the native relationships between closely associated matrix components throughout extraction, purification, and preparation for electron microscopy. After in situ cross-linking, electron microscopy and gel chromatography revealed that about one-half of the guanidine hydrochloride extractable proteoglycans were polymeric, usually with two to five proteoglycan subunits in each polymer. Cross-linking consistently involved the thin segments of the proteoglycan subunits. Some of the proteoglycan polymers were capable of binding hyaluronic acid and were parts of aggregates under associative conditions. SDS-polyacrylamide gel electrophoresis revealed that link proteins were present within the polymers, and studies in which purified proteoglycans were cross-linked in vitro confirmed that the link proteins increased the proportion of polymeric proteoglycans. These findings suggest that individual proteoglycans within cartilage have intimate associations with other proteoglycans that are mediated by link proteins.

A competitive assay of lipoprotein: proteoglycan interaction using a 96-well microtitration plate

A method for the microassay in vitro of lipoprotein: proteoglycan interactions is described. The wells of a plastic 96-well microtitration plate are coated with low density lipoprotein. A limiting quantity of biotin-conjugated proteoglycan is allowed to bind to each coated well, and the amount of the latter retained in wells is estimated spectrophotometrically through subsequent binding of alkaline phosphatase-conjugated avidin. Many of the incubation parameters (e.g., time, pH, salt concentration, divalent cations), which influence the extent of binding of biotin-conjugated proteoglycan, have been studied and optimized. The effect upon binding of introducing different levels of proteoglycans or lipoproteins at the interaction step can be measured readily. Thus, the orders of increasing relative binding affinities were found to be high density lipoprotein less than Lipoprotein (a) less than low density lipoprotein; rat chondrosarcoma proteoglycan less than bovine nasal cartilage proteoglycan less than human aorta proteoglycan; chondroitin 4-sulfate less than chondroitin 6-sulfate less than dermatan sulfate for lipoproteins, proteoglycans, and glycosaminoglycans, respectively.