The human pharmacokinetics of oral ingestion of glucosamine and chondroitin sulfate taken separately or in combination

OBJECTIVE

As part of the National Institutes of Health (NIH)-sponsored Glucosamine/Chondroitin sulfate Arthritis Intervention Trial (GAIT) our objective here was to examine (1) the pharmacokinetics (PK) of glucosamine (GlcN) and chondroitin sulfate (CS) when taken separately or in combination as a single dose in normal individuals (n=29) and (2) the PK of GlcN and CS when taken as a single dose after 3 months daily dosing with GlcN, CS or GlcN+CS, in patients with symptomatic knee pain (n=28).

METHODS

The concentration of GlcN in the circulation was determined by established fluorophore-assisted carbohydrate electrophoresis (FACE) methods. The hydrodynamic size and disaccharide composition of CS chains in the circulation and dosage samples was determined by Superose 6 chromatography and FACE.

RESULTS

We show that circulating levels of CS in human plasma are about 20 microg/ml. Most significantly, the endogenous concentration and CS disaccharide composition were not detectably altered by ingestion of CS, when the CS was taken alone or in combination with GlcN. On the other hand, the Cmax (single-dose study) and AUC values (multiple-dose study) for ingested GlcN were significantly reduced by combination dosing with CS, relative to GlcN dosing alone.

CONCLUSIONS

We conclude that pain relief perceived following ingestion of CS probably does not depend on simultaneous or prior intake of GlcN. Further, such effects on joint pain, if present, probably do not result from ingested CS reaching the joint space but may result from changes in cellular activities in the gut lining or in the liver, where concentrations of ingested CS, or its breakdown products, could be substantially elevated following oral ingestion. Moreover, since combined dosing of GlcN with CS was found to reduce the plasma levels seen with GlcN dosing alone, any improved pain relief by combination dosing cannot be explained by higher circulating concentrations of GlcN.

Co-culture of mechanically injured cartilage with joint capsule tissue alters chondrocyte expression patterns and increases ADAMTS5 production

We studied changes in chondrocyte gene expression, aggrecan degradation, and aggrecanase production and activity in normal and mechanically injured cartilage co-cultured with joint capsule tissue. Chondrocyte expression of 21 genes was measured at 1, 2, 4, 6, 12, and 24h after treatment; clustering analysis enabled identification of co-expression profiles. Aggrecan fragments retained in cartilage and released to medium and loss of cartilage sGAG were quantified. Increased expression of MMP-13 and ADAMTS4 clustered with effects of co-culture, while increased expression of ADAMTS5, MMP-3, TGF-beta, c-fos, c-jun clustered with cartilage injury. ADAMTS5 protein within cartilage (immunohistochemistry) increased following injury and with co-culture. Cartilage sGAG decreased over 16-days, most severely following injury plus co-culture. Cartilage aggrecan was cleaved at aggrecanase sites in the interglobular and C-terminal domains, resulting in loss of the G3 domain, especially after injury plus co-culture. Together, these results support the hypothesis that interactions between injured cartilage and other joint tissues are important in matrix catabolism after joint injury.

Keratan sulphate in the interglobular domain has a microstructure that is distinct from keratan sulphate elsewhere on pig aggrecan

The microstructure of keratan sulphate purified from the interglobular domain, the keratan sulphate-rich region and total aggrecan was compared using fluorophore-assisted-carbohydrate-electrophoresis. Keratan sulphate in the interglobular domain was substantially less sulphated than keratan sulphate elsewhere on aggrecan, based on the ratio of unsulphated: monosulphated disaccharides generated by endo-beta-galactosidase digestion, and the ratio of monosulphated: disulphated disaccharides generated by keratanase II digestion. The ratio of unsulphated: monosulphated: disulphated disaccharides was 1:4:5 for keratan sulphate from total aggrecan and the keratan sulphate-rich region, but only 1:0.9:0.8 for the interglobular domain. These results show that keratan sulphate in the interglobular domain of pig aggrecan has a microstructure that is distinct from keratan sulphate in the keratan sulphate-rich region.

Altered fine structures of corneal and skeletal keratan sulfate and chondroitin/dermatan sulfate in macular corneal dystrophy

The content and fine structure of keratan and chondroitin/dermatan sulfate in normal human corneas and corneas affected by macular corneal dystrophies (MCD) types I and II were examined by fluorophore-assisted carbohydrate electrophoresis. Normal tissues (n = 11) contained 15 microg of keratan sulfate and 8 microg of chondroitin/dermatan sulfate per mg dry weight. Keratan sulfates consisted of approximately 4% unsulfated, 42% monosulfated, and 54% disulfated disaccharides with number of average chain lengths of approximately 14 disaccharides. Chondroitin/dermatan sulfates were significantly longer, approximately 40 disaccharides per chain, and consisted of approximately 64% unsulfated, 28% 4-sulfated, and 8% 6-sulfated disaccharides. The fine structural parameters were altered in all diseased tissues. Keratan sulfate chain size was reduced to 3-4 disaccharides; chain sulfation was absent in MCD type I corneas and cartilages, and sulfation of both GlcNAc and Gal was significantly reduced in MCD type II. Chondroitin/dermatan sulfate chain sizes were also decreased in all diseased corneas to approximately 15 disaccharides, and the contents of 4- and 6-sulfated disaccharides were proportionally increased. Tissue concentrations (nanomole of chains per mg dry weight) of all glycosaminoglycan types were affected in the disease types. Keratan sulfate chain concentrations were reduced by approximately 24 and approximately 75% in type I corneas and cartilages, respectively, and by approximately 50% in type II corneas. Conversely, chondroitin/dermatan sulfate chain concentrations were increased by 60-70% in types I and II corneas. Such changes imply a modified tissue content of individual proteoglycans and/or an altered efficiency of chain substitution on the core proteins. Together with the finding that hyaluronan, not normally present in healthy adult corneas, was also detected in both disease subtypes, the data support the conclusion that a wide range of keratocyte-specific proteoglycan and glycosaminoglycan remodeling processes are activated during degeneration of the stromal matrix in the macular corneal dystrophies.

Keratan sulfate disaccharide composition determined by FACE analysis of keratanase II and endo-beta-galactosidase digestion products

Many tissues contain glycoproteins and proteoglycans, which are substituted with N-or O-linked keratan sulfate, a glycosaminoglycan in which the lactosamine (-galbeta1,4glcNAc-) disaccharide backbone is variably modified by sulfation, fucosylation, and sialylation. We report here a rapid, sensitive, and quantitative procedure for obtaining a complete disaccharide compositional analyses for keratan sulfates after FACE separation of products generated by hydrolysis of the glycosaminoglycans with B. fragillis keratanase II and E. freundii endo-beta-galactosidase. Seven digestion end products are separable in a single electrophoretic step using Monosaccharide composition gels. These are: the unsulfated disaccharide, glcNAcbeta1,3gal, the fucosylated trisaccharide, galbeta1,2[fucalpha1,3]glcNAc6S, the mono- and disulfated disaccharides, galbeta1,4glcNAc6S or gal6Sbeta1,4glcNAc6S from the chain interior, and the sialylated mono- and disulfated trisaccharides neuAalpha2,3galbeta1,4glcNAc6S or neuAalpha2,3gal6Sbeta1,4glcNAc6S from the nonreducing terminus. FACE analyses also revealed the presence of a contaminant beta-galactosidase activity in keratanase II enzyme preparations which cleaves the disaccharide, galbeta1,4glcNAc6S to its constituent monosaccharides, gal and glcNAc6S. It was particularly prominent at enzyme concentrations > 2 mU per nmole substrate glcNH(2) or after prolonged digestion times (> 12 h), and was not inhibitable by thiogalactosides or N-acetyl-lactosamine. As these monosaccharide products would not be detectable using the commonly described analytical methods for KS hydrolase products, such as (1)H-NMR and HPLC analyses, our data illustrate that the FACE procedure represents an improved approach for accurate compositional microanalyses of corneal and skeletal keratan sulfates, especially applicable to experimentation involving small amounts (1-2 microg) of this glycosaminoglycan.

Proteoglycans and glycosaminoglycan fine structure in the mouse tail tendon fascicle

The isolated mouse tail tendon fascicle, a functional and homogenous volume of tendon extracellular matrix, was utilized as an experimental system to examine the structure function relationships in tendon. Our previous work using this model system demonstrated relationships between mean collagen fibril diameter and fascicle mechanical properties in isolated tail tendon fascicles from three different groups of mice (3-week and 8-week control and 8-week Mov13 transgenic) K.A. Derwin, L.J. Soslowsky, J. Biomech. Eng. 121 (1999) 598-604. These groups of mice were chosen to obtain tendon tissues with varying collagen fibril structure and/or biochemistry, such that relationships with material properties could be investigated. To further investigate the molecular details of matrix composition and organization underlying tendon function, we report now on the preparation, characterization, and quantitation of fascicle PGs (proteoglycans) from these three groups. The chondroitin sulfate/dermatan sulfate (CS/DS)-substituted PGs, biglycan and decorin, which are the abundant proteoglycans of whole tendons, were also shown to be the predominant PGs in isolated fascicles. Furthermore, similar to the postnatal maturation changes in matrix composition previously reported for whole tendons, isolated fascicles from 8-week mice had lower CS/DS PG contents (both decorin and biglycan) and a higher collagen content than 3-week mice. In addition, CS/DS chains substituted on PGs from 8-week fascicles were shorter (based on a number average) and richer in disulfated disaccharide residues than chains from 3-week mice. Fascicles from 8-week Mov13 transgenic mice were found to contain similar amounts of total collagen and total CS/DS PG as age-matched controls, and CS/DS chain lengths and sulfation also appeared normal. However, both decorin and biglycan in Mov13 tissue migrated slightly faster on sodium dodecyl sulfate polyacrylamide gel electorphoresis (SDS-PAGE) than the corresponding species from 8-week control, and biglycan from the 8-week Mov 13 fascicles appeared to migrate as a more polydisperse band, suggesting the presence of a unique PG population in the transgenic tissue. These observations, together with our biomechanical data [Derwin and Soslowsky, 1999] suggest that compensatory pathways of extracellular matrix assembly and maturation may exist, and that tissue mechanical properties may not be simply determined by the contents of individual matrix components or collagen fibril size.

Binding of a large chondroitin sulfate/dermatan sulfate proteoglycan, versican, to L-selectin, P-selectin, and CD44

Here we show that a large chondroitin sulfate proteoglycan, versican, derived from a renal adenocarcinoma cell line ACHN, binds L-selectin, P-selectin, and CD44. The binding was mediated by the interaction of the chondroitin sulfate (CS) chain of versican with the carbohydrate-binding domain of L- and P-selectin and CD44. The binding of versican to L- and P-selectin was inhibited by CS B, CS E, and heparan sulfate (HS) but not by any other glycosaminoglycans tested. On the other hand, the binding to CD44 was inhibited by hyaluronic acid, chondroitin (CH), CS A, CS B, CS C, CS D, and CS E but not by HS or keratan sulfate. A cross-blocking study indicated that L- and P-selectin recognize close or overlapping sites on versican, whereas CD44 recognizes separate sites. We also show that soluble L- and P-selectin directly bind to immobilized CS B, CS E, and HS and that soluble CD44 directly binds to immobilized hyaluronic acid, CH, and all the CS chains examined. Consistent with these results, structural analysis showed that versican is modified with at least CS B and CS C. Thus, proteoglycans sufficiently modified with the appropriate glycosaminoglycans should be able to bind L-selectin, P-selectin, and/or CD44.

Proteoglycan metabolism during repair of the ruptured medial collateral ligament in skeletally mature rabbits

The metabolism of the chondroitin/dermatan sulfate (CS/DS) proteoglycans (PGs) decorin and biglycan is markedly altered during short-term (3-6 weeks) and long-term (40 weeks-2 years) repair of surgically ruptured medial collateral ligaments from mature rabbits. A PG-rich extracellular matrix accumulates in injury gaps by 3 weeks postsurgery and extends into tissue regions containing the original ligaments, and elevated PG levels remain apparent up to 2 years postinjury. CS/DS PGs were prepared from such ligaments and identified after SDS-polyacrylamide gel electrophoresis by Alcian blue staining or immunoblotting. In normal ligaments, decorin is the most abundant proteoglycan (accounting for approximately 80% of the total); the remainder is biglycan and a large PG, possibly versican. In repairing ligaments, decorin is barely detected, but instead a large proteoglycan and abundant amounts of biglycan accumulate. Biglycan is present in two forms in repairing ligaments, and they can be separated on SDS-PAGE into 200- and 140-kDa forms. The slower migrating species is absent in normal ligaments and may represent a different glycoform (containing either a single or two short chondroitin/dermatan sulfate chains) of biglycan. Alteration in PG expression and posttranslational processing during medial collateral ligament repair are similar to those reported for repair and scar formation of other connective tissues. The accumulation of biglycan observed here may interfere with proper collagen network remodeling and may lead to persistent inflammatory and matrix turnover processes, thus preventing restoration of a long-term functional ligament tissue.

Sulphation heterogeneity in the trisaccharide (GalNAcSbeta1, 4GlcAbeta1,3GalNAcS) isolated from the non-reducing terminal of human aggrecan chondroitin sulphate

We report here the isolation and sulphation isomer analyses of trisaccharides GalNAcS(beta1,4)GlcA(beta1,3)GalNAcS (in which S indicates sulphate) derived from the non-reducing termini of aggrecan chondroitin sulphate. Rat chondrosarcoma and human aggrecans were digested for 1 h at 37 degrees C with 30 micro-units of endo-chondroitinase ABC per microgram of chondroitin sulphate, and trisaccharides were isolated from the digests by ToyoPearl HW40S gel-filtration chromatography. Four trisaccharide species were identified; their sulphation isomer compositions, as determined by digestion with chondroitinase ACII and fluorescence-based ion-exchange HPLC, were GalNAc4Sbeta1,4GlcAbeta1,3GalNAc4S, GalNAc4Sbeta1,4GlcAbeta1,3GalNAc6S, GalNAc4,6Sbeta1,4GlcAbeta1, 3GalNAc4S and GalNAc4,6Sbeta1,4GlcAbeta1,3GalNAc6S. The abundances of such sequences in chondroitin sulphate on aggrecan from normal (foetal to 72 years of age) and from osteoarthritic human knee cartilages were also established. The results showed that non-reducing terminal GalNAc4S or GalNAc4,6S can be linked to either a 4-sulphated or a 6-sulphated disaccharide, suggesting that the sulphation of the last disaccharide might not have a direct effect on the specificity of chondroitin sulphate terminal GalNAc sulphotransferases. Furthermore, for each aggrecan preparation examined, the 4S-to-6S ratio of all chain interior disaccharides was equivalent to that in the last repeating disaccharides at the non-reducing terminus, suggesting that neither chondroitin 4-sulphotransferase nor chondroitin 6-sulphotransferase shows preferential activity near the chain terminus.

Changes in sulfation patterns of chondroitin sulfate in equine articular cartilage and synovial fluid in response to aging and osteoarthritis

OBJECTIVES

To determine effects of aging on sulfation of chondroitin sulfate (CS) in articular cartilage and synovial fluid from normal equine middle carpal joints, and to determine whether CS compositional analysis can be used to assess alterations in proteoglycan turnover in degenerative cartilage obtained from horses with carpal osteochondral fractures.

SAMPLE POPULATION

Carpal articular cartilage and synovial fluid from 44 cadavers with normal joints and from 16 Thoroughbred racehorses during routine carpal arthroscopic surgery.

PROCEDURE

After papain/chondroitinase digestion of cartilage, CS disaccharides (unsulfated disaccharide delta Di0S, and monosulfated disaccharides delta Di4S and delta Di6S) were quantified by capillary zone electrophoresis. The CS was purified from synovial fluid chondroitinase digested, and analyzed. The CS nonreducing terminal residues, N-acetylgalctosamine (galNAc) or glucuronic acid adjacent to a 4-sulfated or 6-sulfated galNAc, were quantified.

RESULTS

In cartilage, the delta Di6S-to-delta Di4S ratio increased with age; in degenerative cartilage, this ratio was not significantly different from the normal value. Percentage of delta Di0S decreased with age and was significantly higher in degenerative than in normal cartilage. The galNAc4S and galNAc4,6S represented > or = 96% of the terminal residues. There was a significant decrease in 6-sulfation of the terminal residues in degenerative cartilage.

CONCLUSIONS

6-Sulfation of internal and terminal CS residues increased with age. Cartilage degeneration in racehorses was accompanied by deposition of CS chains with altered sulfation patterns, in normal and diseased joints of horses > 2 years old, synovial fluid CS was not indicative of cartilage CS and may represent turnover products of a subpopulation of proteoglycan within the matrix.

Glycosaminoglycan sulfation in human osteoarthritis. Disease-related alterations at the non-reducing termini of chondroitin and dermatan sulfate

Chondroitin lyase products of aggrecan and small proteoglycans from normal and osteoarthritic cartilages were analyzed for chain internal Deltadisaccharides and terminal mono- or disaccharides. Chondroitin and dermatan sulfate chains from arthritic cartilages were of essentially normal size and internal sulfation but had significantly altered sulfation of the terminal residues. Whereas in normal cartilage, approximately 60% of terminal GalNAc4S was 4, 6-disulfated, it was reduced to approximately 30% in osteoarthritic cartilage. This is most likely due to a lower terminal GalNAc4, 6S-disulfotransferase activity and reveals that metabolic changes in osteoarthritis can affect this distinct sulfation step during chondroitin and dermatan sulfate synthesis. GlcAbeta1,3GalNAc6S-, the mimotope for antibody 3B3(-), was present on approximately 8 and approximately 10% of chains from normal and osteoarthritic cartilages, respectively. 3B3(-) assayed by immunodot blot was within the normal range for most osteoarthritic samples, with only 5 of 24 displaying elevated reactivity. This resulted not from a higher content of mimotope, but possibly from other structural changes in the proteoglycan that increase mimotope reactivity. In summary, chemical determination of sulfation isomers at the non-reducing termini of chondroitin and dermatan sulfate provides a reliable assay for monitoring proteoglycan metabolism not only during normal growth of cartilage but also during remodeling of cartilage in osteoarthritis.

Age-related changes in the structure of the keratan sulphate chains attached to fibromodulin isolated from articular cartilage

Bovine articular cartilage fibromodulin has been isolated from animals aged 3 months to 8 years, and the attached keratan sulphate (KS) chains digested with keratanase II. The oligosaccharides generated have been reduced, examined by high-pH anion-exchange chromatography and their structures identified by comparison with standards. It has been shown that in fibromodulin from young articular cartilage, the KS chains do not possess either non-reducing terminal (alpha2-6)-linked N-acetylneuraminic acid or fucose (alpha1-3)-linked to sulphated N-acetylglucosamine residues. However, an age-related increase has been observed in the abundance of both (alpha2-6)-linked N-acetylneuraminic acid and (alpha1-3)-linked fucose, neither of which is found in KS isolated from non-articular cartilage, irrespective of the age of the source. Interestingly, the KS chain length remains constant as a function of age, which possibly relates to a role in collagen fibril assembly. In addition, no significant age-related changes were identified in levels of galactose sulphation.

Inhibition of cartilage degradation and changes in physical properties induced by IL-1beta and retinoic acid using matrix metalloproteinase inhibitors

Bovine cartilage explants were treated with 100 ng/ml recombinant human interleukin-1beta (IL-1beta) or 1 microM all-trans retinoic acid (RA) and changes in biochemical, biomechanical, and physicochemical properties were assessed. Additionally, samples cultured with IL-1beta or RA were treated with 4 microM recombinant human tissue inhibitor of metalloproteinases-1 (TIMP-1) or a synthetic metalloproteinase inhibitor (L-758,354) to inhibit this degradation. Treatment with IL-1beta or RA each resulted in >90% GAG loss after 8 days in culture. Addition of TIMP or L-758,354 to the culture media inhibited IL-1beta-induced loss of tissue GAG by 40 and 65%, respectively, and inhibited RA-induced GAG loss by 35 and 65%, respectively. Analysis of degradation products in the culture media using a G1 antibody indicated that IL-1beta- and RA-treated plugs released 68-kDa fragments of aggrecan, corresponding to a segment of the aggrecan core protein from the G1 domain to the C-terminus NITEGE, consistent with "aggrecanase" activity. Release of the G1 fragment was inhibited by treatment with L-758,354. Both IL-1beta and RA induced significant loss of hyaluronan from cartilage explants after 8 days of exposure and HA loss was also inhibited by addition of L-756,354 to the culture media. IL-1beta, but not RA, induced a significant increase in swelling ratio (wet weight in 0.01 M NaCl normalized to wet weight in DMEM) after 8 days in culture, consistent with degradation of the collagen network, and the increase in tissue swelling was inhibited by treatment with TIMP-1 or L-758,354. Exposure to IL-1beta or RA resulted in significant changes in cartilage physical properties including streaming potential, equilibrium modulus, hydraulic permeability, and electrokinetic coupling coefficient after 8 days in culture, and these changes were inhibited by 40-90% by exposure to TIMP and by 50-90% by exposure to L-758,354. Measurement of dynamic streaming potential showed that changes due to treatment with IL-1beta alone were highly dependent in compression frequency, with dramatic changes seen at high frequency prior to changes in mechanical properties, and little initial change seen at low frequency. Streaming potential and equilibrium modulus of explants treated with RA decreased to 10% of their initial values after 8 days in culture, but decreased to only 40 and 90%, respectively, when treated with RA plus TIMP-1.

Chemical and immunological assay of the nonreducing terminal residues of chondroitin sulfate from human aggrecan

Samples of aggrecan chondroitin sulfate, isolated from normal human knee cartilages of individuals from fetal to 72 years of age, were digested with chondroitin lyases. The products were analyzed by fluorescence-based anion exchange high performance liquid chromatography to separate and quantitate nonreducing terminal structures, in addition to internal unsaturated disaccharide products. The predominant terminal structures were the monosaccharides, GalNAc4S and GalNAc4,6S as they were present on 85-90% of all chains. The remaining chains terminated with the disaccharides GlcAbeta1,3GalNAc4S and GlcAbeta1,3GalNAc6S. Marked changes in the relative abundance of these terminals were identified in the transition from growth cartilage to adult articular cartilage. First, terminal GalNAc residues were almost exclusively 4-sulfated in aggrecan from fetal through 15 years of age, but were approximately 50% 4,6-disulfated in aggrecans from adults (22-72 years of age). Second, the terminal disaccharide GlcAbeta1,3GalNAc4S was on approximately 7% of chains on aggrecan from fetal through 15 years of age, but on only approximately 3% of chains on adult aggrecan. In contrast, the proportion of chains terminating in GlcAbeta1,3GalNAc6S, approximately 9%, was unchanged from fetal to 72 years of age. This terminal disaccharide is proposed to be recognized by the widely used monoclonal antibody 3B3. However, chemical quantitation of the structure together with solid phase 3B3(-) immunoassay of fetal and adult aggrecans showed that the content of the terminal disaccharide does not necessarily correlate with immunoreactivity of the proteoglycan, as chain density and presentation on the solid phase are critical factors for recognition of chain terminals by 3B3. The quantitative results obtained from chemical analyses of all nonreducing termini of aggrecan chondroitin sulfate chains revealed important changes in chain termination that occur when cellular activities are altered as adult articular cartilage is formed after removal of growth cartilage. These findings are discussed in relation to specific enzymatic steps that generate the nonreducing termini of chains in the biosynthesis pathway of chondroitin sulfate proteoglycans and their modulation in tissue development and pathology.

Ion exchange HPLC microanalysis of chondroitin sulfate: quantitative derivatization of chondroitin lyase digestion products with 2-aminopyridine

Sulfated glycosaminoglycans such as chondroitin sulfate are composed of three structural domains, a linkage oligosaccharide, connecting the chain to the core protein, a variably sulfated disaccharide repeat structure within the chain and a nonreducing terminal, and these domains may confer specific functions on particular chain populations. We report here a new and highly sensitive method for the detection and quantitation of all nonreducing terminal residues and internal disaccharides obtained by chondroitinase ABC or ACII digestion of aggrecan chondroitin sulfate. The procedure involves a quantitative reductive animation of the reducing ends of sulfated mono- and disaccharide chondroitinase products with 2-aminopyridine and boranedimethylamine. All derivatized saccharides can be separated and quantitated by fluorescence in a single chromatographic step on an AS4A anion exchange column, eluted with a gradient (0-500 nM) of sodium trifluoroacetate. The reproducibility and stability of the derivatisation, together with the sensitivity of the chromatography system, allowed for routine quantitation in the range of 3-500 pmol of reducing group (corresponding to about 1.5-250 ng of disaccharide or 0.75-125 ng of monosaccharide). Moreover, the fluorescence yield (fluorescence area units per pmol of reducing group) was virtually identical for all saccharides analyzed. Application of this method to an analysis of aggrecan purified from calf epiphyseal cartilage and from rat chondrosarcoma chondrocyte cultures allowed a precise identification and quantitation of the internal disaccharides and the nonreducing terminal structures, together with an estimation of the number average molecular weight of CS chains in these aggrecan preparations.

Proteoglycans of human rotator cuff tendons

Rotator cuff and biceps tendons that appeared grossly normal were procured from adult cadavers without a history of shoulder problems. These tendons were analyzed for the amount and type of glycosaminoglycan, type of proteoglycan, and histology. When compared with the distal/tensional region of biceps tendon, the glycosaminoglycan content of supraspinatus, infraspinatus, and subscapularis tendons was 2.5-fold higher and the glycosaminoglycan content of the proximal/compressed region of biceps tendon was 3-fold higher. The ratio of hyaluronic acid to chondroitin sulfate/dermatan sulfate in all three cuff tendons was approximately 1. Rotator cuff tendons contained large proteoglycan similar to aggrecan, as demonstrated by sodium dodecyl sulfate-polyacrylamide gel migration elution from Sepharose CL-4B, and content of both chondroitin sulfate and keratan sulfate chains. Both decorin and biglycan were also present, as demonstrated by migration in sodium dodecyl sulfate-polyacrylamide gels and core protein immunoreactivity. In contrast decorin was the only proteoglycan prominent in distal/tensional regions of biceps tendon. Histological analysis showed layers of loosely organized alcian blue-stained material running between the longitudinal collagen fiber bundles. The proteoglycan content of rotator cuff tendons was similar to fibrocartilage in tendons that have been subjected to compressive loads in situ. This suggests that cells of normal adult rotator cuff tendons have adapted to loads distinct from pure tension. However, the histological organization did not resemble mature fibrocartilage. The increased amount of proteoglycan in rotator cuff tendons may serve to separate and lubricate collagen bundles as they move relative to each other during normal shoulder motion.

Compression of cartilage results in differential effects on biosynthetic pathways for aggrecan, link protein, and hyaluronan

The differential effects of static compression and recovery from compression on biosynthesis and biosynthetic pathways of aggrecan, link protein, and hyaluronan were assessed. During compression, biosynthesis of aggrecan and link protein were inhibited to approximately 25 and approximately 40%, respectively, of free-swelling control levels. In marked contrast, hyaluronan synthesis was unaffected by static compression. After release from 12-h 50% static compression, aggrecan synthesis remained inhibited for up to 2.5 days; however, link protein synthesis completely recovered to free-swelling control levels within 8 h after release. Hyaluronan synthesis remained at control levels after release of compression. During compression, aggrecan core protein pool size was decreased, whereas the rate of processing into the proteoglycan form remained essentially the same as in free swelling control tissue. Four hours after release from compression, aggrecan core protein pool size remained small and the rate of intracellular processing of aggrecan had become slower than that of free swelling control tissue. Due to the altered core-protein processing kinetics, fewer but longer chondroitin sulfate chains were added to the core proteins. Sulfation was not markedly altered. The differential effects of static compression and release on the biosynthesis of aggrecan, link protein, and hyaluronan are similar to the changes in the biosynthetic pathways that are affected in response to IL-1 treatment, suggesting that the response to static compression is not a general inhibition of cellular activity, but appears to be part of a specific transduction mechanism.

Glycosaminoglycan addition to proteoglycans by articular chondrocytes--evidence for core protein-specific pathways

The intracellular compartmentalization of enzyme activities involved in the elongation and sulfation of glycosaminoglycans on aggrecan, decorin, and fibromodulin was investigated using brefeldin A, a compound with known inhibitory action on normal vesicular transport and secretion of macromolecules. Treatment of bovine chondrocyte cultures with the compound resulted in greater than 98% inhibition of Na35SO4 incorporation into macromolecules, whereas [3H]leucine or [3H]glucosamine continued at 60-70% of the levels measured in control cultures. The release of newly synthesized products into the medium was also decreased markedly by brefeldin A to 7 and 2% of control levels for [3H]leucine- and [3H]glucosamine-labeled macromolecules, respectively. Analysis of [3H]-glucosamine-labeled products in these cultures showed that synthesis of sulfated glycosaminoglycans (chondroitin/dermatan sulfate and keratan sulfate) was inhibited in response to brefeldin A, whereas hyaluronan synthesis was essentially unaffected. Significant amounts of elongated chondroitin continued to be synthesized in the presence of brefeldin A. Immunoprecipitation of [3H]leucine-labeled decorin, aggrecan, and fibromodulin from cells showed that aggrecan and fibromodulin were not substituted with glycosamino-glycans, whereas all decorin molecules synthesized under these conditions were substituted with chondroitin. The results suggest that in articular chondrocytes, elongation of the glycosaminoglycan chains on decorin, but not their sulfation, occurs in a Golgi compartment unaffected by disruption of vesicular core protein transport. This is in contrast to glycosaminoglycan elongation and sulfation on aggrecan and fibromodulin, where both processes apparently occur in the trans-Golgi network, which becomes inaccessible to these core proteins in the presence of brefeldin A. The results further suggest that in brefeldin A-treated cells decorin is contained in a discrete ER-Golgi compartment separated from aggrecan; this compartment is accessible to p-nitrophenyl-beta-D-xylosides, since beta-xylosides become elongated with chondroitin even in the presence of brefeldin A.

Large and small proteoglycans of osteoarthritic and rheumatoid articular cartilage

OBJECTIVE

To identify characteristic changes in large aggregating (aggrecan) and small proteoglycan (PG) populations in articular cartilages during osteoarthritis (OA) and rheumatoid arthritis (RA).

METHODS

Aggrecan populations in guanidine extracts of femoral condylar cartilages of 46 OA and 8 RA patients who underwent total knee arthroplasty, as well as of 2 fetuses and 6 normal adults, were separated in agarose-polyacrylamide composite gels. Small PGs (biglycan, decorin, and fibromodulin) in the same extracts were analyzed in 12% polyacrylamide gels. Gels were stained or electrophoretically transferred and probed with antibodies to aggrecan epitopes and to small PGs. Epitope contents of the samples were also compared by inhibition radioimmunoassay.

RESULTS

There were significant differences found among normal and diseased samples in their electrophoretic mobilities, band distributions, and antibody staining. OA and especially RA samples were heavily degraded, lacked certain aggrecan populations, and contained fewer keratan sulfate and chondroitin-6-sulfate epitopes compared with normal samples. Levels of chondroitin-4-sulfate and "fetal-type" epitopes were elevated in the OA samples compared with the normal ones. More core proteins of small PGs were found in diseased than in normal cartilages, but they were more heterogeneous in size and glycosaminoglycan substitution.

CONCLUSION

There is extensive degradation of both large and small PGs in diseased cartilages, but a repair process does exist, especially in OA cartilages. Chondrocytes of diseased cartilages are able to synthesize fetal-type aggrecans. Small PGs are glycosylated differently in diseased cartilages than in normal ones.

Structure of chondroitin sulfate on aggrecan isolated from bovine tibial and costochondral growth plates

The structure of chondroitin sulfate on aggrecan isolated from the rib and proximal tibial growth plates of bovine fetuses was investigated, and the previously reported increase in the hydrodynamic size of chondroitin sulfate chains between the reserve and hypertrophic zones of the rib was confirmed in the tibial growth plate. Superose 6 gel chromatography, calibrated for chondroitin sulfate chain length by monosaccharide analysis, showed that the average molecular mass of chondroitin sulfate in the reserve and maturing zones of both growth plates was 21,600 and 30,400, respectively. Determination by capillary zone electrophoresis of the disaccharide composition of chains following chondroitinase digestion showed that delta Di-0S, delta Di-4S, and delta Di-6S together accounted for more than 98% of the disaccharides in the digests from all zones of both growth plates; delta disulfated and delta trisulfated disaccharides were not detected. Furthermore, this analysis revealed a gradient in chondroitin sulfate composition from the reserve to the hypertrophic zone, characterized by a marked increase in the content of delta Di-6S (from about 32% to about 52%) and a marked decrease in the content of delta Di-4S (from about 53% to about 35%). Moreover, this altered pattern of sulfation was detected on chains of all sizes in the hypertrophic zone, suggesting that a proportion of the reserve zone aggrecan might be removed and replaced with aggrecan rich in chondroitin-6-sulfate synthesized during the proliferative and maturation stages of the resident chondrocytes. These data are discussed in relation to the biosynthetic mechanisms that control chondroitin sulfate chain length and sulfation on aggrecan and their modification during chondrocyte proliferation, maturation, and hypertrophy in the growth plate.

Immunology of chondroitin/dermatan sulfate

Variable substitutions and locations of the sulfate esters along the backbone of chondroitin/dermatan sulfate chains, combined with their carbohydrate structures, present topographies to immune systems which can be recognized as antigenic. This has led to the development of a number of monoclonal antibodies which recognize distinct epitopes in the native structures of these glycosaminoglycan chains. In some studies, the original chondroitin/dermatan sulfate proteoglycan was digested with chondroitinase enzymes before being used as an immunogen. in this case, the linkage oligosaccharides remaining bound to the core protein contain a modified (4,5-unsaturated) hexuronic acid derivative at their non-reducing ends as a result of the eliminase mechanism of the enzyme. This 'haptenic' structure is highly antigenic and has led to the development of a number of monoclonal antibodies which recognize this structure as part of their epitopes. Examples of the use of some of these monoclonal antibodies for localization of proteoglycan structures in tissue sections and on transblots are described. The precise structures are known for only a few of the native epitopes recognized by these monoclonal antibodies. Recent analytical methods have been developed for determining structures of chondroitin sulfate oligosaccharides. An example of the use of these methods to analyze the structures of the non-reducing termini of chondroitin/dermatan sulfate chains is discussed. The results show their potential value for quantifying the native epitope recognized by a monoclonal antibody, designated 3B3, which recognizes chains terminated by glucuronic acid-N-acetylgalactosamine-6-sulfate. Such methods should be useful for determining the epitope structures for other monoclonal antibodies in this class.

Mechanical regulation of cartilage biosynthetic behavior: physical stimuli

The biosynthetic response of calf cartilage disk explants to small-amplitude dynamic compression was studied in radially unconfined compression over a wide range of frequencies. The relative importance of oscillatory fluid flow, hydrostatic pressure, streaming potential, and cell deformation in modulating chondrocyte metabolism was explored by quantifying the frequency dependence and the spatial (radial) distribution of the biosynthetic response within the 3-mm-diameter explant disks. At frequencies greater than 0.001 Hz (cycle/s), dynamic compression increased biosynthesis of proteoglycans and proteins. While compression at frequencies between 0.002 and 0.01 Hz caused a stimulation of biosynthesis that was distributed throughout the disk, compression at 0.1 Hz caused a stimulation that was confined mainly to the outer radial periphery. These distributions were compared to previous estimates of the radial distribution of physical forces and flows within the matrix. The results suggest that the stimulation of chondrocyte biosynthesis by dynamic mechanical compression at amplitudes up to 10% (stresses up to 0.5 MPa) is related to changes in fluid flow and/or cell shape rather than changes in hydrostatic pressure. Since static compression to the original cut thickness caused a slight decrease in biosynthesis in the center of the disks, we also studied the possible role of limited diffusive transport in the marked inhibition of synthesis seen during large displacement static compression. Experiments in which the surface area-to-volume ratio of disks or the concentration of labeling substrate or serum were varied provided no evidence that limited diffusive transport was responsible for the inhibition of biosynthesis by large displacement static compression. Recovery of biosynthesis from static compression and histological analyses of compressed tissue suggested that there was no significant cell damage even during 12 h of 50% static compression.

Biosynthetic mechanisms for the addition of polylactosamine to chondrocyte fibromodulin

The cartilage matrix glycoprotein fibromodulin contains four N-linked glycosylation sites which act as acceptors for the addition of sulfated polylactosamine (keratan sulfate). In the present study we examined the biosynthetic processing of these N-linked oligosaccharides for subsequent addition of polylactosamine. Chondrocytes were treated with castanospermine, 1-(+)deoxymannojirimycin, and swainsonine, radiolabeled with [3,4,5-3H]leucine, [2-3H]mannose, or [6-3H]glucosamine, and newly synthesized fibromodulin was immunoprecipitated for analysis. Castanospermine and 1-(+)deoxymannojirimycin inhibited polylactosamine addition, whereas swainsonine was not effective. This indicated that the linkage regions must be processed to GlcNAc(Man)5(GlcNAc)2Asn but do not require further modification to GlcNAc(Man)3(GlcNAc)2Asn. In both control and swainsonine-treated cells one or two N-linked oligosaccharides per molecule were modified with polylactosamine containing 4-6 repeating disaccharide units. Moreover, a single short chain was added either to the C-3 or the C-6 branch in control cultures, whereas only the C-3 branch was substituted in the presence of swainsonine. Analysis of endo-beta-galactosidase and keratanase II digestion products of the polylactosamine chains synthesized in both culture conditions showed that only about 25% of the hexosamine residues and less than 5% of the adjacent galactose residues were substituted with sulfate. These findings are discussed in relation to the regulation of fibromodulin glycosylation and the likely influence of polylactosamine structure on the extracellular interactions and turnover of fibromodulin.

A cartilage explant system for studies on aggrecan structure, biosynthesis and catabolism in discrete zones of the mammalian growth plate

The structure, biosynthesis and catabolism of aggrecan has been studied in the bovine fetal rib growth plate. Comparative analyses were made on six 1-mm transverse slices which represent the resting zone (slice 6), proliferative zone (slices 5 and 4), upper hypertrophic zone (slice 3), middle hypertrophic zone (slice 2) and lower hypertrophic zone (slice 1). Aggrecan was abundant and exhibited very high aggregability in all zones. The aggrecan monomer was similar in structure in the resting and proliferative zones but showed a marked increase in hydrodynamic size in the lower hypertrophic zone; this was apparently due to an increase in the size of substituent glycosaminoglycans and an increase in core protein size as indicated by peptide analysis for G3 domain abundance. Biosynthetic studies with [35S]-sulfate showed the rate of synthesis per cell to be highest in the upper hypertrophic zone, and the structure of the newly synthesised molecules to be similar to the resident population in all zones. During explant culture in basal medium both aggregating and non-aggregating forms of aggrecan were released slowly from all zones. Addition of 10 nM retinoic acid to explants stimulated the release of both these forms of aggrecan whereas higher concentrations of retinoic acid (100 nM and 1000 nM) preferentially stimulated the release of the degraded forms. In this regard hypertrophic cells were the most responsive and resting cells were the least responsive. Analysis of the degraded fragments by polyacrylamide gel electrophoresis and by N-terminal sequencing indicated that aggrecan catabolism in all zones of the growth plate is due to the action of aggrecanase, a novel cartilage proteinase which is also active in normal and osteoarthritic articular cartilages (Sandy et al., 1992). These observations are discussed in terms of the role of aggrecan in the extensive matrix remodelling which accompanies chondrocyte hypertrophy in the growth plate.

Effects of compression on the loss of newly synthesized proteoglycans and proteins from cartilage explants

The effects of mechanical compression of calf cartilage explants on the catabolism and loss into the medium of proteoglycans and proteins radiolabeled with [35S]sulfate and [3H]proline were examined. A single 2- or 12-h compression of 3-mm diameter cartilage disks from a thickness of 1.25 to 0.50 mm, or slow cyclic compression (2 h on/2 h off) from 1.25 mm to 1.00, 0.75, or 0.50 mm for 24 h led to transient alterations and/or sustained increases in loss of radiolabeled macromolecules. The effects of imposing or removing loads were consistent with several compression-induced physical mediators including fluid flow, diffusion, and matrix disruption. Cyclic compression induced convective fluid flow and enhanced the loss of 35S- and 3H-labeled macromolecules from tissue into medium. In contrast, prolonged static compression induced matrix consolidation and appeared to hinder the diffusional transport and loss of 35S- and 3H-labeled macromolecules. Since high amplitude cyclic compression led to a sustained increase in the rate of loss of 3H- and 35S-labeled macromolecules that was accompanied by an increase in the rate of loss of [3H]hydroxyproline residues and an increase in tissue hydration, such compression may have caused disruption of the collagen meshwork. The 35S-labeled proteoglycans lost during such cyclic compression were of smaller average size than those from controls, but contained a similarly low proportion (approximately 15%) that could form aggregates with excess hyaluronate and link protein. The size distribution and aggregability of the remaining tissue proteoglycans and 35S-labeled proteoglycans were not markedly affected. The loss of tissue proteoglycan paralleled the loss of 35S-labeled macromolecules. This study provides a framework for elucidating the biophysical mechanisms involved in the redistribution, catabolism, and loss of macromolecules during cartilage compression.

Identification of the keratan sulfate attachment sites on bovine fibromodulin

The small keratan sulfate-substituted proteoglycan (fibromodulin) from articular cartilage was shown to contain keratan sulfate linked to the core protein through N-glycosidic linkages to residues Asn-109, Asn-147, Asn-182, and Asn-272. Biosynthetic experiments with articular chondrocytes in the presence of tunicamycin, an inhibitor of N-linked oligosaccharide synthesis, demonstrated a specific inhibition of [35S]SO4 incorporation into fibromodulin. Under the same conditions no effect on the addition of keratan sulfate to the large aggregating proteoglycan was detected. Fibromodulin substituted with keratan sulfate was purified from bovine articular cartilage extracts by density gradient centrifugation, ion-exchange chromatography, and gel-permeation chromatography. Isolation of glycosylated peptides from tryptic digests of fibromodulin by ion-exchange chromatography and reversed-phase high performance liquid chromatography revealed four separate hexosamine-rich species, that were also immunoreactive with monoclonal antibody 5D4. Sequence analysis of these glycopeptides gave blank cycles at positions which corresponded to Asn followed by X-Ser/Thr in the sequence derived from cDNA (Oldberg, A., Antonsson, P., Lindblom, K., and Heinegard, D. (1989) EMBO J. 8, 2601-2604). Hence, all four Asn residues in the leucine-rich region of the fibromodulin core protein can serve as acceptor sites for keratan sulfate addition.

Effects of tissue compression on the hyaluronate-binding properties of newly synthesized proteoglycans in cartilage explants

The effects of tissue compression on the hyaluronate-binding properties of newly synthesized proteoglycans in calf cartilage explants were examined. Pulse-chase experiments showed that conversion of low-affinity monomers to the high-affinity form (that is, to a form capable of forming aggregates with 1.6% hyaluronate on Sephacryl S-1000) occurred with a t1/2 of about 5.7 h in free-swelling discs at pH 7.45. Static compression during chase (in pH 7.45 medium) slowed the conversion, as did incubation in acidic medium (without compression). Both effects were dose-dependent. For example, the t1/2 for conversion was increased to about 11 h by either (1) compression from a thickness of 1.25 mm to 0.5 mm or (2) medium acidification from pH 7.45 to 6.99. Oscillatory compression of 2% amplitude at 0.001, 0.01, or 0.1 cycles/s during chase did not, however, affect the conversion. Changes in the hyaluronate-binding affinity of [35S]proteoglycans in these experiments were accompanied by no marked change in the high percentage (approximately 80%) of monomers which could form aggregates with excess hyaluronate and link protein. Since static tissue compression would result in an increased matrix proteoglycan concentration and thereby a lower intra-tissue pH [Gray, Pizzanelli, Grodzinsky & Lee (1988) J. Orthop. Res. 6, 777-792], it seems likely that matrix pH may influence proteoglycan aggregate assembly by an effect on the hyaluronate-binding affinity of proteoglycan monomer. Such a pH mechanism might have a physiological role, promoting proteoglycan deposition in regions of low proteoglycan concentration.

Synthesis of small proteoglycans substituted with keratan sulfate by rabbit articular chondrocytes

35S-Labeled proteoglycans produced by chondrocytes from immature and mature rabbits were fractionated on associative CsCl gradients. In all cultures, greater than 85% of the incorporated radioactivity was present in the A1 fraction (rho 1.60) as chondroitin sulfate/keratin sulfate-substituted aggregating proteoglycan monomer; the remainder was present in small proteoglycans in the A2, A3, and A4 fractions of low buoyant densities (rho 1.53, 1.45, 1.37, respectively). Detailed glycosaminoglycan analysis of the A2, A3, and A4 fractions showed dermatan sulfate-rich species were present throughout. However, in both immature and mature cultures, 30-45% of the glycosaminoglycans in the A3/A4 combined fractions were present as keratan sulfate, as shown by insensitivity to digestion with chondroitinase ABC, specific digestion with endo-beta-galactosidase, and reactivity with antibody 5D4. Immature and mature chondrocytes synthesized very similar amounts of the low buoyant density keratan sulfate proteoglycan on a per cell basis. Moreover, 51 and 37% of the total keratan sulfate produced by immature and mature chondrocytes, respectively, were present in the low buoyant density proteoglycan. Pulse-chase experiments indicated that the low buoyant density keratan sulfate was not derived from the large aggregating proteoglycan by proteolysis in the extracellular space. The small keratan sulfate proteoglycans appear to be present as a species distinct from the small dermatan sulfate proteoglycans in these cultures in that they can be separated on Q-Sepharose chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The apparent size (40-60 kDa), composition, and heterogeneity of the keratan sulfate proteoglycans suggest that they may be related to the small keratan sulfate proteoglycans of cornea.

Studies on the hyaluronate binding properties of newly synthesized proteoglycans purified from articular chondrocyte cultures

Primary cultures of rabbit articular chondrocytes have been maintained for 10 days and labeled with [35S]sulfate, [3H]leucine, and [35S]cysteine in pulse-chase protocols to study the structure and hyaluronate binding properties of newly synthesized proteoglycan monomers. Radiolabeled monomers were purified from medium and cell-layer fractions by dissociative CsCl gradient centrifugation with bovine carrier monomer, and analyzed for hyaluronate binding affinity on Sepharose CL-2B in 0.5 M Na acetate, 0.1% Triton X-100, pH 6.8. Detergent was necessary to prevent self-association of newly synthesized monomers during chromatography. Monomers secreted during a 30-min pulse labeling with [35S]sulfate had a low affinity relative to carrier. Those molecules released into the medium during the first 12 h of chase (about 40% of the total) remained in the low affinity form whereas those retained by the cell layer rapidly acquired high affinity. In cultures where more than 90% of the preformed cell-layer proteoglycan was removed by hyaluronidase digestion before radiolabeling the newly synthesized low affinity monomers also rapidly acquired high affinity if retained in the cell layer. Cultures labeled with amino acid precursors were used to establish the purity of monomer preparations and to isolate core proteins for study. Leucine- or cysteine-labeled core proteins derived from either low or high affinity monomer preparations migrated as a single major species on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with electrophoretic mobility very similar to that of core protein derived from extracted proteoglycan monomer. Purified low affinity monomers were converted to the high affinity form by treatment at pH 8.6; however, this change was prevented by guanidinium-HCl at concentrations above 0.8 M. Conversion to high affinity was also achieved by incubation of monomers in aggregate with hyaluronic acid (HA) at pH 6.8 followed by dissociative reisolation of monomer. At both pH 6.8 and 8.6 the conversion process was slow, requiring up to 48 h for the maximum increase in affinity. It is suggested that the slow increase in HA binding affinity seen during extracellular processing of proteoglycans in cartilage and chondrocyte cultures is the result of an irreversible structural change in the HA binding domain following the binding of monomer to hyaluronate. The available evidence suggests that this change involves the formation or rearrangement of disulfide bonds.

Biosynthetic response of cartilage explants to dynamic compression

The biosynthetic response of calf articular cartilage explants to dynamic compression was examined over a wide range of amplitudes, waveforms, and frequencies. Glycosaminoglycan synthesis was assessed by 35S-sulfate incorporation, and amino acid uptake and protein synthesis were assessed by 3H-proline incorporation. Two culture chambers were designed to allow uniaxial radially unconfined compression and mechanical testing of cartilage disks: one chamber was used inside a standard incubator; the other was used with a mechanical spectrometer and allowed load and displacement to be monitored during compression. Dynamic stiffness measurements of 3-mm diameter disks identified a characteristic frequency [0.001 Hz (cycles/sec)] that separated low- and high-frequency regimes in which different flow and deformation phenomena predominated; e.g., at 0.0001-0.0001 Hz, significant fluid was exuded from cartilage disks, whereas at 0.01-1 Hz, hydrostratic pressure increased within disks. At the higher frequencies, oscillatory strains of only approximately 1-5% stimulated 3H-proline and 35S-sulfate incorporation by approximately 20-40%. In contrast, at the lower frequencies (a) compressions of less than 5% had no effect, consistent with the dosimetry of biosynthetic inhibition by static compression (approximately 25% compression caused a approximately 20% inhibition of radiolabel incorporation), and (b) higher amplitudes (cycling between disk thicknesses of 1.25 and 0.88-1.00 mm) stimulated 3S-sulfate incorporation by approximately 20-40%, consistent with the kinetics of response to a single 2-h compression and release. None of the compression protocols was associated with detectable alterations in (e.g., compression-induced depletion of) total glycosaminoglycan content. This study provides a framework for identifying both the physical and biological mechanisms by which dynamic compression can modulate chondrocyte biosynthesis. In addition, the culture and compression methodology potentially allows in vitro evaluation of clinical strategies of continuous passive motion therapy to stimulate cartilage remodeling.

Biosynthesis of cartilage proteoglycan and link protein by articular chondrocytes from immature and mature rabbits

Chondrocytes from immature and mature rabbits have been compared in biosynthetic studies with [3H] leucine and [35S]sulfate as precursors. The time course of incorporation of [3H]leucine into general protein, proteoglycan monomer core protein, and link protein and of [35S]sulfate into proteoglycan monomer has been examined. Proteoglycan monomer was isolated from the high buoyant density (p greater than 1.60) fractions of dissociative CsCl gradients and link protein by immunoprecipitation with antibody 8A4 followed by gel electrophoresis. Results based on the period of linear isotope incorporation showed that mature cells synthesize protein at about 40% of the rate of immature cells and both proteoglycan and link protein at about 20% of the rate of immature cells. The labeling rates obtained suggest that immature cells synthesize an approximate 1:1 molar ratio of link protein to proteoglycan monomer, and for mature cells this ratio is about 0.8:1. While cell layer retention of newly synthesized proteoglycan was markedly lower in mature relative to immature cell cultures, link protein retention was high in both immature and mature cultures; this finding provides an explanation for our previous observation (Plaas, A. H. K., and Sandy, J. D. (1984) Biochem, J. 220, 337-340) that link-free monomer accumulates in the medium of mature but not immature cultures. The link protein synthesized by both ages of cells and isolated from cell layer or medium was a single major species of apparent molecular mass 48-51 kDa. The results suggest that mature chondrocytes are less efficient than immature chondrocytes in the coordinated assembly of link-stabilized proteoglycan aggregates in this culture system.

Structural studies on proteoglycan catabolism in rabbit articular cartilage explant cultures

Mature rabbit articular cartilage cultures have been used to study the catabolism of aggregating proteoglycan monomers in normal cartilage. During the first 4 days of culture, about 40% of monomers are degraded and lose the ability to bind to hyaluronate. The non-aggregating products (NAgg-PG) have been isolated and compared structurally and immunologically to aggregating monomers (Agg-PG) purified from fresh tissue. The results show that: (1) NAgg-PG are smaller, more heterogeneous in size and have a lower protein/glycosaminoglycan ratio than Agg-PG. (2) NAgg-PG and Agg-PG have a very similar chondroitin sulfate/keratan sulfate ratio. (3) NAgg-PG have 25-50% lower disulfide content than Agg-PG. (4) NAgg-PG have only about 20% of the reactivity of Agg-PG towards a monoclonal antibody (12-20/1-C-6) specific for the hyaluronate binding region of the core protein. These results provide further evidence that proteoglycan catabolism in cartilage explants involves proteolysis of core protein resulting in separation of the hyaluronate binding region from the glycosaminoglycan-rich regions.

The biosynthetic response of the mature chondrocyte in early osteoarthritis

Metabolic studies in early experimental osteoarthritis (OA) have shown that the rate of proteoglycan synthesis in the diseased articular cartilage may be markedly enhanced relative to normal; elevated rates of synthesis were however accompanied by increased release of new molecules from the tissue so that the response was apparently non-reparative. Described here are experiments with immature and mature chondrocytes in culture which show that aging of chondrocytes in vivo is accompanied by a marked fall in the capacity of these cells to synthesize link protein and to assemble a proteoglycan-rich matrix. It is suggested that poor deposition of proteoglycan by mature chondrocytes in OA may result from insufficient synthesis of link protein for stabilization of aggregates.

The affinity of newly synthesized proteoglycan for hyaluronic acid can be enhanced by exposure to mild alkali

The affinities for hyaluronic acid of newly synthesized proteoglycan from post-confluent rabbit chondrocyte cultures and purified bovine proteoglycan monomer were compared. In mixtures prepared at pH 6.8 the newly synthesized proteoglycan had the lower affinity; however, in mixtures incubated at pH 8.5 for 24 h before addition of hyaluronic acid, the newly synthesized proteoglycan exhibited a markedly higher affinity than the bovine monomer. The results suggest that proteoglycan secreted without associated link protein [Plaas, Sandy & Muir (1983) Biochem. J. 214, 855-864] has a low affinity for hyaluronate and that this may be increased during subsequent extracellular processing.

Age-related changes in the kinetics of release of proteoglycans from normal rabbit cartilage explants

The release of proteoglycans from explant cultures of articular cartilage from immature and mature rabbits has been studied with the following results. At both ages the tissue proteoglycan was released in two phases: an initial extensive release (day 0 to 3) and a period of slow release (day 4 to 15). The percentage released in the initial phase was, however, significantly greater for mature (55%) than immature (38%) explants. At both ages the newly synthesized proteoglycans (in vivo labeled) were also released in two kinetic pools. Thus, graphical analysis of release data readily resolved the disappearance curves into two linear components with in vitro half-lives of 1 day and 22 days. Again, the percentage in the short half-life pool was much greater for mature (70%) than immature (40%) explants. At both ages the initial release was largely chondrocyte-mediated since freeze-thawing the tissue before culture markedly reduced proteoglycan release. At both ages the released proteoglycans were smaller than equivalent preparations of extracted proteoglycans and they were much less capable of forming aggregates with hyaluronate. The results show that there are age-dependent changes in rabbit articular cartilage that increase the proportion of proteoglycans, both total and newly synthesized, that are susceptible to rapid chondrocyte-mediated catabolism in explant cultures.

A sensitive assay for active link protein from cartilage

A new assay for the activity of cartilage link protein is described. The method is based on the finding [Plaas, Sandy & Muir (1983) Biochem. J. 214, 855-864] that addition of link protein to [35S]sulphate-labelled proteoglycan aggregates from rabbit chondrocyte cultures resulted in the formation of link-stabilized aggregates. The percentage aggregate was found to be related linearly to the amount of purified bovine link protein added in the 20-120 ng range. The assay was used to monitor loss of link-protein activity during heat denaturation and to measure binding of link protein by purified proteoglycan monomer.

Age-related decrease in the link-stability of proteoglycan aggregates formed by articular chondrocytes

Chondrocytes were isolated from the articular cartilage of rabbits aged between 6 and 50 weeks and labelled with [35S]sulphate after 48 h in monolayer culture. The percentage of the total proteoglycan monomers synthesized by each culture that were present as link-stabilized aggregates was shown to be about 83% at 6, 9 and 12 weeks, 73% at 15 weeks, 48% at 30 weeks and 32% at 50 weeks. The proliferative activity of the cells in culture also decreased markedly with the age of the donor. The results suggest that aging of chondrocytes in vivo is accompanied by a decrease in their capacity for link-protein synthesis.

Proteoglycan aggregate formation by articular chondrocytes. Decrease in link-protein synthesis during culture

The synthesis of link-stabilized proteoglycan aggregates by rabbit articular chondrocytes was investigated by [35S]sulphate labelling of primary monolayer cultures maintained for up to 21 days. (1) At all culture times the cells secreted a high-molecular-weight cartilage-type proteoglycan monomer of which 75%-80% formed aggregates with hyaluronic acid. (2) At 2 days of culture all of the aggregates were in link-stabilized form, but by 21 days only 5% were link-stabilized, as shown by displacement of monomers from the aggregate by hyaluronic acid oligosaccharides. (3) The addition of purified link protein to 21-day culture medium increased the proportion of link-stable aggregate from 5% to 70%. (4) Analysis of [3H]serine-labelled proteoglycan aggregates in the medium showed a marked decrease with culture time in the ratio of 3H-labelled link protein to 3H-labelled core protein present. The results suggest that the secretion of proteoglycan monomers and link protein by articular chondrocytes changes independently during prolonged monolayer culture.