Characterization of the proteoglycans recovered under nondissociative conditions from normal articular cartilage of rabbits and dogs

Review of the biomechanics and patterns of limb fractures

The pattern of a limb fracture can be determined by the material property of the bone and the characteristics of the deforming force. In this review we outline the composition and material properties of cortical and cancellous bone, and articular cartilage. We defi ne the biomechanics of fractures and describe the various fracture patterns that are seen clinically.

In vivo MRI of cartilage pathogenesis in surgical models of osteoarthritis

OBJECTIVE

To examine in vivo time-course changes in macromolecular composition of articular cartilage in two surgical models of osteoarthritis (goat: meniscal transection and cartilage incision; rabbit: medial meniscectomy).

DESIGN

Collagen integrity and proteoglycan (PG) content were evaluated in both models by magnetization transfer (MT) and contrast-enhanced MRI, respectively. The MT rate k(m) for the exchange process between the bulk water and water bound to collagen was determined as a marker of the collagen network. Local changes in cartilage fixed charge density, i.e., where PGs are depleted, were derived from T(1) relaxation maps as obtained after an infusion of Gd(DTPA)(2-), a paramagnetic agent.

RESULTS

In the goat model, the MT rate constant k(m) was significantly higher at 2 weeks post surgery, a possible sign of cartilage swelling, then decreased below baseline values, most likely indicative of disruption in the collagen framework. Meanwhile, post-Gd(DTPA)(2-) MRI acquisition indicated a significant and sustained loss of PGs. The rabbit model produced milder lesions. Although the difference was non-significant, k(m) steadily decreased in response to the surgical insult while kinetics of Gd(DTPA)(2-) uptake, after reaching a peak level at 6 weeks, were back to normal values after 12 weeks.

CONCLUSION

In the goat model, joint instability and cartilage damage was a permanent trigger for cartilage degeneration producing MRI changes. However, biomechanical stress due to partial medial meniscectomy in knees of mature rabbits produced only mild, focal lesions and PG depletion that was partially reversible. This proof-of-concept study identified MT and T(1) parameters as useful surrogate markers in animal models of osteoarthritis.

Link protein can retard the degradation of hyaluronan in proteoglycan aggregates

OBJECTIVE

Loss of articular cartilage and intervertebral disc function in arthritis or disc degeneration is associated with degradation of the proteoglycan (PG) aggregates by either proteolysis of aggrecan or hyaluronan (HA) degradation. The aim of this work was to determine whether degradation of HA in PG aggregate degradation is influenced by link protein (LP) stabilization of the PG aggregates.

METHODS

Aggrecan and LP were prepared from fetal bovine epiphyseal cartilage, and PG aggregates were formed in the presence or absence of LP. The PG aggregates were exposed to hyaluronidase or free radicals to promote HA degradation. Degradation of HA, aggrecan and LP were assessed by gel filtration chromatography and polyacrylamide gel electrophoresis.

RESULTS

High concentrations of hyaluronidase cleaved both PG aggregates between each aggrecan molecule, whereas low concentrations gave much less cleavage of the LP-stabilized aggregate. High free radical concentrations gave extensive cleavage of all components of both PG aggregates, whereas low concentrations are more selective for HA damage and to a much lesser extent in the LP-stabilized aggregates. Thus the presence of LP caused a diminution in the capacity of both catabolic agents to degrade HA as long as levels of the degradative agents were not excessive.

CONCLUSION

In addition to stabilizing the PG aggregates towards dissociation, LP may also help protect the PG aggregates from degradation under conditions where tissue catabolism is promoted.

Distribution of small proteoglycans and glycosaminoglycans in humerus-related articular cartilage of chickens

The expression of components present in the cartilaginous extracellular matrix is related to development, gender, and genotype, as well as to the biomechanical properties of each type of cartilage. In the present study, we analyzed small proteoglycans and glycosaminoglycans present in different cartilages of the chicken wing after extraction with guanidine hydrochloride or papain. Quantitative analysis of glycosaminoglycans showed a larger amount in humeral cartilage (around 200 mg/g tissue) than in articular cartilage of the radius and ulna, with 138 and 80 mg/g tissue, respectively. Non-collagenous proteins isolated were predominantly from cartilage in the proximal regions of the humerus and radius. D4 fractions obtained by ultracentrifugation were separated by DEAE-Sephacel and Octyl-Sepharose chromatography and analyzed by SDS-PAGE. Two bands of 57 and 70-90 kDa were observed for all samples treated with beta-mercaptoethanol. Immunoblotting of these proteins was positive for the small proteoglycans fibromodulin and decorin, respectively. Apparently, the 57-kDa protein is present in macromolecular complexes of 160 and 200 kDa. Chondroitin sulfate was detected in all regions. HPLC analysis of the products formed by chondroitinase AC and ABC digestion mainly revealed beta-D-glucuronic acid and N-acetyl beta-D-galactosamine residues. The 4-sulfation/6-sulfation ratio was close to 3, except for the proximal cartilage of the radius (2.5). These results suggest functional differences between the scapula-humerus, humerus-ulna, and humerus-radius joints of the chicken wing. This study contributes to the understanding of the physiology of cartilage and joints of birds under different types of mechanical stress.

Treatment with calcitonin prevents the net loss of collagen, hyaluronan and proteoglycan aggregates from cartilage in the early stages of canine experimental osteoarthritis

OBJECTIVE

To evaluate the effect of calcitonin (CT) on the histology and biochemistry of articular cartilage from unstable operated and nonoperated knee in a canine model of experimental osteoarthritis (OA).

METHODS

Eighteen dogs underwent anterior cruciate ligament transection (ACLT) of the right knee and were randomly distributed into three groups of six dogs each. From day-1 after surgery until sacrifice 84 days post-ACLT, each dog received a daily nasal spray that delivered the placebo, 100 units of CT or 400 units of CT. Histologic lesions were scored. Hyaluronan (HA) and antigenic keratan sulfate (AgKS) were quantified by enzyme-linked immunosorbent assays (ELISAs), whereas aggrecan molecules extracted under nondissociative conditions were characterized by velocity gradient centrifugation.

RESULTS

All canine cruciate-deficient knees developed OA. At a daily dose of 400 units, CT had no effect on the size of osteophytes but significantly reduced the severity of cartilage histologic lesions in unstable knees. CT also enhanced the HA content as well as the size distribution and relative abundance of fast-sedimenting aggrecan aggregates in cartilage from both operated and nonoperated knees. On the other hand, in the CT-treated group, the cartilage content of AgKS increased in operated joints, but not in nonoperated joints.

CONCLUSIONS

Because CT delivered as a nasal spray markedly reduced the severity of most OA changes, both at the histological and biochemical level, this form of therapy may have benefits for humans who have recently experienced a traumatic knee injury, and as well as for dogs who spontaneously rupture their ACL.

Centrifugal and biochemical comparison of proteoglycan aggregates from articular cartilage in experimental joint disuse and joint instability

Two models involving altered joint loading were compared with regard to their effects on the biochemical composition and proteoglycan aggregate structure of articular cartilage. Disuse atrophy was created in greyhound dogs by nonrigid immobilization of the right knee in 90 degrees of flexion, and joint instability was created by transection of the anterior cruciate ligament. Similarities and differences between the two experimental groups at two different time periods were examined to investigate why joint instability induces progressive and irreversible changes to the articular cartilage, whereas joint disuse induces changes that may be reversible when the joint is remobilized. The following studies were performed on the cartilage from all experimental and control groups: (a) compositional analyses to determine water, uronate, and hydroxyproline contents; (b) high performance liquid chromatography for detection of hyaluronan and chondroitin sulfates; and (c) centrifugation analyses of nondissociatively extracted and purified proteoglycans to isolate and quantify the populations of monomers and slow and fast-sedimenting families of aggregates. In general, all cartilage was found to have a decreased ratio of proteoglycan to collagen after 4 weeks of disuse, and this ratio returned to control values at 8 weeks. In contrast, cartilage had an elevated ratio of proteoglycan to collagen as well as increased hydration at 12 weeks after transection of the anterior cruciate ligament. The most striking contrast between the two models was the finding of an approximately 80% decrease in the content of hyaluronan at both time periods after transection of the anterior cruciate ligament, with no evidence of a change after disuse. The results of centrifugation analyses indicated a significant decrease in the quantity of proteoglycan aggregates in both models. However, this decrease was associated primarily with a loss of slow-sedimenting aggregates after disuse and a loss of both slow and fast-sedimenting aggregates after transection of the anterior cruciate ligament. Furthermore, the population of fast-sedimenting aggregates was depleted to a greater extent than that of the slow-sedimenting aggregates. The preservation of fast-sedimenting aggregates as well as hyaluronan after periods of joint disuse but not joint instability suggests a possible mechanism for the reversibility of cartilage changes. Although the proteoglycan aggregates were depleted after disuse atrophy, it is possible that an aggregate-depleted matrix could recover when normal proteoglycan synthesis is resumed. In contrast, although synthesis may be maintained or elevated after transection of the anterior cruciate ligament, the matrix may not be repopulated with aggregates because there is an insufficient amount of hyaluronan.

Structural differences between two populations of articular cartilage proteoglycan aggregates

To determine if articular cartilage contains structurally distinct populations of proteoglycan aggregates, we extracted and purified proteoglycans from canine knee cartilage under associative conditions. Equilibrium density gradient centrifugation separated three proteoglycan populations, on the basis of differences in sedimentation velocity, into groups of 21, 106, and 270 S. Electron microscopic examination showed that the 21 S samples contained free aggrecan molecules and clusters of aggrecan molecules, with a mean of five aggrecan molecules per cluster. The 106 and 270 S samples contained proteoglycan aggregates consisting of central hyaluronan filaments with multiple attached aggrecan molecules. The two populations of aggregates did not differ in mean aggrecan length or in the spacing of aggrecan molecules along the hyaluronan filaments, but the slower sedimenting aggregates (106 S) had significantly shorter hyaluronan filaments as measured by electron microscopy (mean hyaluronan length, 400 compared with 1,162 nm) and one-third as many aggrecan molecules per aggregate (mean number of aggrecan molecules per aggregate, 15 compared with 44). This study shows that articular cartilage contains aggrecan clusters and two structurally distinct populations of proteoglycan aggregates. The differences between the two types of aggregate, in particular the number of aggrecan molecules per aggregate, may reflect differences in their assembly, stability, or turnover and give them different mechanical and biological properties.

Isolation and purification of proteoglycans

Purification of a protein typically involves development of a quantitative assay to track protein integrity (e.g. enzyme activity) during subsequent isolation steps. The generalized procedure involves choosing the source of the protein, defining extraction conditions, developing bulk purification methods followed by refined, more selective methods. The purification of proteoglycans is often complicated by a) limited source quantities, b) necessity of chaotrophic solvents for efficient extraction, c) their large molecular size and d) lack of defined functions to enable purity (i.e. activity, conformation) to be assessed. Because the usual goal of proteoglycan purification is physical characterization (intact molecular weight, core protein and glycosaminoglycan class and size), the problems of a suitable assay and/or native conformation are avoided. The 'assay' for tracking proteoglycan isolation typically utilizes uronic acid content or radiolabel incorporation as a marker. Once extracted from their cellular/extracellular environment, proteoglycans can be isolated by density gradient centrifugation and/or column chromatography techniques. Recent advances in the composition of chromatographic supports have enabled the application of ion-exchange, gel permeation, hydrophobic interaction and affinity chromatography resins using efficient high-pressure liquid chromatography to proteoglycan purification.

Isolation and purification of proteoglycans

Purification of a protein typically involves development of a quantitative assay to track protein integrity (e.g. enzyme activity) during subsequent isolation steps. The generalized procedure involves choosing the source of the protein, defining extraction conditions, developing bulk purification methods followed by refined, more selective methods. The purification of proteoglycans is often complicated by a) limited source quantities, b) necessity of chaotropic solvents for efficient extraction, c) their large molecular size and d) lack of defined functions to enable purity (i.e. activity, conformation) to be assessed. Because the usual goal of proteoglycan purification is physical characterization (intact molecular weight, core protein and glycosaminoglycan class and size), the problems of a suitable assay and/or native conformation are avoided. The 'assay' for tracking proteoglycan isolation typically utilizes uronic acid content or radiolabel incorporation as a marker. Once extracted from their cellular/extracellular environment, proteoglycans can be isolated by density gradient centrifugation and/or column chromatography techniques. Recent advances in the composition of chromatographic supports have enabled the application of ion-exchange, gel permeation, hydrophobic interaction and affinity chromatography resins using efficient high-pressure liquid chromatography to proteoglycan purification.

Proteoglycans nondissociatively extracted from different zones of canine normal articular cartilage: variations in the sedimentation profile of aggregates with degree of physiological stress

Proteoglycans were extracted and purified without dissociation (a-A1 preparations) from superficial and deeper layers of high weight-bearing (HWA) and low weight-bearing (LWA) areas of dog normal articular cartilage. These proteoglycans were then characterized by velocity gradient centrifugation. In each of the 4 different topographical regions, the weight average sedimentation coefficients related strongly with total hexuronate content of the tissue. In the superficial layers, almost all aggregates had low sedimentation coefficients: the aggregates were smaller and less abundant in LWA than in HWA. The deeper layers contained an additional population of faster sedimenting aggregates which appeared smaller and less abundant in LWA than in HWA. Quantification and functional characterization of aggregates as well as in vitro aggregating studies showed that the topographical differences in size and content of aggregates were related to differences in content of hyaluronate and link protein in the a-A1 preparations. Superficial a-A1 specimens contained twice as much hyaluronate as deeper a-A1 preparations and their hyaluronate content increased with degree of physiological stress. Deeper a-A1 specimens from weight-bearing areas did not differ in their hyaluronate content but experiments assessing the saturation with link protein of these different a-A1 preparations suggested that specimens from HWA contained more active link than those from LWA. In contrast, the capacity of aggregation of a-A1D1D1 proteoglycan monomers as well as the molecular weight (Mr = 5 x 10(5) and aggregating capacity of hyluronate molecules appeared very similar in all a-A1 preparations from areas of articular cartilage. It is hypothesized that the synthesis of the three constituents necessary for aggregate formation (i.e. proteoglycan monomers as well as hyaluronate and link protein molecules) increases with degree of physiological load and that aggregation helps to maintain within cartilage the high concentration of proteoglycans that are essential for its biomechanical functions. The reported topographical variations in the distribution of proteoglycan aggregates reflect probably a maximal adaptation of the physiologic and biomechanical properties of the matrix to meet the high stress levels experienced by the articular cartilage in vivo.

The effect of maturation and aging on the structure and content of link proteins in rabbit articular cartilage

We have examined extracts of articular cartilage from rabbits aged 3-100 weeks for evidence of age-related changes in the structure and content of link protein (LP) in this tissue, with the following findings: (a) Two major molecular weight forms of LP were seen on SDS-PAGE (41 and 48 kDa) and the proportion of these changed markedly with age. The 48 kDa species was predominant in young animals (representing about 78% of the total LP at 5 weeks) whereas the 41 kDa species increased in amount with age (representing 35% of the total LP at 100 weeks). A minor form of about 43 kDa, representing less than 20% of the total, was present only during the growth phase. A small amount of fragmented link protein (less than 5% of the total) of about 25-30 kDa was present in samples from mature and aged rabbits only. (b) The quantitation of LP in guanidinium: HCl extracts of cartilage, by radioimmunoassay with monoclonal antibody 8-A-4, was markedly influenced by the conditions of preparation and pretreatment of samples. Assays of dialyzed guanidine extracts following treatment at 80 degrees C for 15 min in 0.025% (w/v) SDS indicated that immature and mature cartilage contains about 50 and 180 micrograms of LP/g of tissue, respectively. On the other hand, assays following treatment at 100 degrees C for 20 min in 0.1% (w/v) SDS suggested that rabbit cartilage contains about 300 micrograms of LP/g of tissue at all ages; finally, assay of CsCl purified proteoglycan samples under these conditions indicated a content of about 500 micrograms of LP/g at all ages. (c) Calculations based on the analysis of proteoglycan preparations for aggregating monomer and link protein suggest that a LP:aggregating monomer molar ratio of about 0.9 is maintained in the articular cartilage throughout maturation and aging in the rabbit.

Chondrons from articular cartilage (II): Analysis of the glycosaminoglycans in the cellular microenvironment of isolated canine chondrons

A chondron rich preparation was isolated from mature canine tibial cartilage using low-speed homogenization techniques. Proteoglycans were extracted from this preparation by exhaustive treatment with 4M guanidine-HCl. A significant proportion of the total proteoglycan, measured as uronic acid, was resistant to extraction and represented 27.9% in intact cartilage chips and 18.6% in the chondron fraction. Histochemical examination of chondrons confirmed that extraction resistant proteoglycans remained within the capsule of the chondron after 4M guanidine-HCl treatment. Electrophoretic analysis of the glycosaminoglycans extracted from intact cartilage chips and the chondron fraction showed approximately equivalent amounts of chondroitin sulphate (79.3%), keratan sulphate (16.3%) and hyaluronic acid (4.3%) present. In contrast, the extraction resistant residue in the chondron fraction was significantly enriched for hyaluronic acid (10.5%, p less than 0.05) but was depleted of chondroitin sulphate (70.9%, p less than 0.05). The major chondroitin sulphate isomer in the resistant fraction was chondroitin 6-sulphate while in the soluble fraction, the quantities of the two isomers were approximately equivalent. Comparison with previously published data suggests a role for minor collagens in the retention of proteoglycans in the cellular microenvironment.

Centrifugal characterization of proteoglycans from various depth layers and weight-bearing areas of normal and abnormal human articular cartilage

Ultracentrifugal polydispersity differential [g(S)] distributions were determined for the proteoglycans of various postmortem human articular cartilage samples extracted from six lateral patellar grooves in nondissociative conditions after mild collagenase digestion of the tissue. The samples consisted of 53 slices (250 microns thick), from normal, mildly fibrillated, and extensively ulcerated knee joints. When statistically analyzed in various subgroupings, the obtained average sedimentation coefficients and polydispersity profiles supported the following conclusions: (a) loss of proteoglycan aggregation and sedimentability is confirmed to be a primary sign of cartilage matrix degradation; (b) higher S values for proteoglycans of the high weight (HW)-bearing areas and lower values for those of the low weight (LW)-bearing areas were a typical finding in normal cartilage samples; (c) inversion of this pattern was indicative of matrix degradation, suggesting that the HW regions are more affected than the LW-bearing areas; (d) the average S value distribution across cartilage thickness tended to resemble the corresponding proteoglycan content versus distance from articular surface; and (e) the deepest cartilage layer had, in most cases, the smallest amount of aggregates while the highest average sedimentability was observed at the middle zone of the normal samples. In the discussion, a role of proteoglycan aggregation for providing a means to "pack" more proteoglycans within the collagen meshwork and to control the generation of osmotic pressure gradients is suggested.

Changes in the sedimentation profile of proteoglycan aggregates in early experimental canine osteoarthritis

Osteoarthritis was induced in 12 normal dogs by severing of the anterior cruciate ligament of the right knees, the left knees serving as sham operated controls. The animals were killed at 7 and 14 weeks postsurgery. The total hexuronate, and thus proteoglycan, content of the articular cartilage of operated knees remained unaltered during the period of study. After pretreatment with a highly purified collagenase and in the presence of selected protease inhibitors, a higher proportion of the tissue hexuronate could be extracted from the different topographical areas of osteoarthritic joints under non dissociative conditions (70-75% versus 55-65% for control knees). The nondissociatively recovered osteoarthritic proteoglycans (a-A1 preparations) displayed progressive and consistent changes in their sedimentation profile. First, the size of the fast sedimenting or more saturated aggregates appeared to be reduced in the different regions of osteoarthritic joints at 7 weeks postoperatively. The disappearance of the faster sedimenting mode as well as a dramatic increase in the proportion of monomers were only detected in the topographical zones exhibiting the most severe surface damage and histologic abnormalities at 14 weeks postsurgery. The proteoglycan molecules present as "free" or "nonaggregated" monomers in a-A1 preparations recovered from normal and osteoarthritic cartilage at different time periods after surgery were separated from their corresponding aggregates by rate zonal centrifugation in isokinetic cesium sulfate gradient. Although they were severely depleted in keratan sulfate, the purified "free" and "aggregated" osteoarthritic monomers appeared to be normal in terms of aggregating capacity and size distribution, and were therefore not degraded. This progressive changes in size distribution of proteoglycan aggregates in the early stages of experimental canine osteoarthritis could contribute significantly to the biochemical and biomechanical alterations of osteoarthritic cartilage.

Progressive depletion of hyaluronic acid in early experimental osteoarthritis in dogs

The hyaluronic acid (HA) content of articular cartilage was studied in early experimental osteoarthritis (OA) in 16 normal dogs. The anterior cruciate ligament in the right knees of the dogs was transected; their left knees served as sham operated controls. The animals were killed at 7 and 14 weeks postsurgery. Although their total hexuronate, and thus proteoglycan, content remained unaltered during the period of study, the different weight-bearing areas of the OA knees displayed a progressive and significant decrease in HA content. We found no differences in the molecular weight and in vitro aggregating capacity of the HA molecules from OA cartilage versus those from control cartilage. This early relative depletion of HA could contribute significantly to the biochemical alterations of OA cartilage. Furthermore, it appears to be a good parameter for the differentiation of changes related to OA and changes related to aging.

Quantification and characterization of hyaluronic acid in different topographical areas of normal articular cartilage from dogs

Normal articular cartilage from adult dogs was analyzed for hyaluronate, hexuronate and hydroxyproline. The low weight-bearing areas of both tibial plateaus and femoral condyles displayed a higher collagen content and a lower proteoglycan content than the regions of maximum contact. Both superficial and deeper layers contained more hyaluronate in areas of maximum than of minimum contact. On the other hand, in each weight-bearing area, the proportion of hyaluronate relative to total proteoglycan content appeared twice as much in the superficial layers than in their corresponding underlying zones. The molecular weight and in vitro aggregating capacity of the hyaluronate molecules were however quite similar in the different topographical areas of the articular tissue.