Proteoglycans from bovine nasal cartilage. Immunochemical studies of link protein

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.

Maintenance of ZPA signaling in cultured mouse limb bud cells

The positional signal localized to the posterior (zone of polarizing activity or ZPA) region of the vertebrate limb is transiently expressed during development and a decline in ZPA signaling is accelerated when posterior cells are dissociated and cultured in vitro. The evidence that cultured posterior cells display a precocious decline in ZPA signaling when compared to in vivo studies suggests that a factor present in the limb bud maintains or stabilizes ZPA signaling during limb outgrowth and that this maintenance factor is lost and/or exhausted in in vitro studies. We have developed a new culture technique, ‘microdissociation’, which preserves extracellular components that we have found to be necessary for ZPA signal maintenance. Our data suggest that the limb bud ectoderm produces a maintenance activity that becomes stored in the extracellular matrix where it acts on limb bud cells to stabilize the activity of the ZPA signal. Using our initial characterization of this maintenance activity, we have identified a growth factor, FGF-2 (bFGF), that can replace all of the ZPA signaling maintenance activity observed in microdissociate cultures. The existence of various members of the FGF family in the developing limb strongly argues a role for FGF in stabilizing ZPA signaling in vivo.

The immunologic detection and characterization of cartilage proteoglycan degradation products in synovial fluids of patients with arthritis

Antibodies were used in radioimmunoassays with gel chromatography to detect the hyaluronic acid-binding region, core protein, and keratan sulfate of human cartilage proteoglycan in the synovial fluids of patients with rheumatoid arthritis, juvenile rheumatoid arthritis, and osteoarthritis. All fluids contained proteoglycan that was mainly included on Sepharose CL-4B; this result indicates cleavage of proteoglycan (which is normally excluded). The hyaluronic acid-binding region was the smallest and most commonly detected fragment. It was relatively free of keratan sulfate and core protein, and it could sometimes bind to hyaluronic acid. Other larger fragments containing core protein and/or keratan sulfate were detected in every fluid.

Microdetermination of proteoglycans and glycosaminoglycans in the presence of guanidine hydrochloride

This report describes a microprocedure that may be used for direct measurement of proteoglycans and glycosaminoglycans, after chromatographic elution with chaotropic reagents. The assay is based on the ability of the sulfated glycosaminoglycans to bind to the cationic dye, dimethylmethylene blue, in solution. Inclusion of guanidinium chloride (0.24 M) in the assay resulted in a stable dye-proteoglycan interaction, but eliminated the interference of other anionic macromolecules such as DNA. The assay is rapid, sensitive, and reproducible and therefore useful for processing several samples. Finally, the procedure can be used for quantitative determination of several types of proteoglycans and glycosaminoglycans.

Glycosaminoglycan antigens in peripheral nerve. Studies with antibodies from a patient with neuropathy and monoclonal gammopathy

An immunoblot staining procedure was developed for the detection of antibody binding to glycosaminoglycans (GAGs). The method was used to study the binding of a human monoclonal antibody (M-protein) from a patient with peripheral neuropathy, previously found to react with proteoglycans (PGs). GAGs prepared from human peripheral nerve were separated by electrophoresis on cellulose acetate membranes, transferred onto nitrocellulose sheets, and immunostained with the M-protein. The M-protein bound to a single GAG band with intermediate mobility which eluted with 1.25 N NaCl on ion-exchange chromatography and was chondroitinase sensitive. The M-protein appeared to bind to chondroitin sulfate containing proteoglycans in peripheral nerve.

Microinjury to the synovial membrane may cause disaggregation of proteoglycans in rabbit knee joint articular cartilage

Proteoglycans (PGs) isolated from articular cartilage (AC) of mature rabbits subjected to two or more consecutive intraarticular (IA) injections of sterile saline 24 h apart showed an aggregation defect in the presence of excess hyaluronic acid (HA). Although the PG contents of experimental and control cartilages were indistinguishable, a higher proportion of PGs were extractable from the 3 X IA tissues, as assessed by uronic acid analysis. Proteoglycans from experimental and control cartilages when examined by Sepharose CL-2B chromatography showed two subunit populations, the smaller (KAV = 0.70) containing more ketatan sulphate than the larger (KAV = 0.31). Cultures of AC from IA joints released more 35SO4-labelled PGs into the media over 72 h than control tissues and consisted mainly of PG degradation products although 20% could aggregate in the presence of HA. Examination of PG aggregation 2 weeks after 2 X IA or 3 X IA injections showed that the defect initiated was still present; however, cartilage of immature rabbits was not affected by the 2 X IA procedure.

Biochemical and morphologic studies of cartilage from the adult human sacroiliac joint

The cartilage of the adult sacroiliac joint contains type II collagen and a high concentration of glycosaminoglycan. Furthermore, large aggregating proteoglycans and link proteins were extracted from the sacral cartilage. In these respects the sacroiliac cartilage is similar to that in the peripheral joints. However, while the organization of the collagen in the sacral cartilage is typical of an articular cartilage, the organization of the ilial cartilage is very different, and throughout its depth it possesses narrow collagen fibrils arranged parallel to the articular surface.

A new delineation of the congeries of osteoarthritis

New concepts of osteoarthritis (OA) are reviewed as preliminary to establishing a new classification of OA, defining sub-groups of OA based on combined biochemical and clinical data. The biochemical data are derived from assays of serum proteins--transferrin, ceruloplasmin, albumin, a1-acid glycoprotein, a1-antitrypsin, immunoglobulins A, G, and M. Standard clinical techniques are followed to provide clinical data, including medical history, physical examination, x-ray studies and routine biochemical tests. Genetic polymorphism is proposed as an explanation for subsets of OA. Extensive family history data is important in establishing connections.

Immunity to homologous type III collagen after partial meniscectomy and sham surgery in rabbits

Cellular immunity to rabbit types I, II, and III collagen was detected after both partial medial meniscectomy and sham operation in rabbits when an in vitro 3H-thymidine incorporation assay was used. The blastogenic responses were commonly directed towards peptides derived from collagens by cyanogen bromide cleavage used to mimic proteinase degradation products. The responses to type III collagen peptides were by far the strongest and were seen in most rabbits that were operated on. We suggest that the responses to types I and II collagen were caused by a cross-reaction with type III collagen peptides. This conclusion was supported by the observation that spleen cells from rabbits directly immunized with homologous type III collagen peptides in Freund's complete adjuvant responded strongly to the immunizing peptides and also cross-reacted with types I and II collagen peptides. Immunity to cartilage proteoglycans was observed primarily in rabbits that had undergone meniscectomy and had severe cartilage degeneration. These results indicated that immunity to collagens will develop merely as a result of joint surgery, whereas immunity to proteoglycans is largely dependent upon an osteoarthritic lesion.

Preparation and characterization of an antiserum against purified proteoglycan link proteins from avian cartilage

Link proteins have been purified from avian xyphoid process. Cartilage was extracted in 4.0 M guanidine hydrochloride and a link fraction (A1D5) was obtained by sequential cesium chloride centrifugation. Link proteins were separated from low buoyant density proteoglycans by chromatography on Sephacryl S-200 and polyacrylamide gel electrophoresis in sodium dodecyl sulfate. The presence of contaminating proteoglycans at various purification steps was monitored in an enzyme-linked immunosorbent assay using antiserum against avian cartilage proteoglycan monomer (anti A1D1-1400 Vo). Antiserum generated against this purified link preparation (anti link[SDS]) was characterized for its ability to bind link proteins, proteoglycan monomer, and aggregate. The serum was specific only for link proteins when tested by an enzyme-linked immunosorbent assay. Some reactivity against proteoglycan monomer was observed in a Farr-type assay.

Age-related changes in the antigenicity of human articular cartilage proteoglycans

Rabbits were immunized with adult or foetal human articular cartilage proteoglycans and cell-mediated and antibody immunity assessed in vitro. Cellular immunity to proteoglycans of different ages was measured by the stimulation of 3H-thymidine incorporation into cultured spleen cells. Antibodies to proteoglycans in the rabbit sera were detected with an enzyme-linked immunosorbent assay. The ability of adult and foetal proteoglycans to preabsorb antibody activity in this assay was compared. The results from both the cellular and antibody studies support the same conclusion, that adult proteoglycans express all foetal antigenic determinants and have additional determinants not seen on foetal proteoglycans. The reactivity of rabbit spleen cells to proteoglycans of different ages indicated that these determinants appear during maturation when the major changes in proteoglycan structure occur. The results did not appear to be due to differences in chondroitin sulphate content of the preparations, since the same observations were made with chondroitinase ABC-digested proteoglycans. The observations might be explained by other differences in glycosylation between the preparations or by postulating the synthesis during maturation of an additional proteoglycan species with a different core-protein. Such changes in the antigenicity of articular cartilage proteoglycans with age could account for the development of autoimmunity to proteoglycans in inflammatory joint diseases and may play a role in their pathogenesis.