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

Osteoarthritis of the ankle: bridging concepts in basic science with clinical care

Trends in science are beginning to suggest that cartilage degeneration may be related to a chronic imbalance in extracellular matrix metabolism. In cartilage, a combination of biomechanical, biochemical, and matrix-related signaling pathways regulates the equilibrium between cartilage anabolism and catabolism. A potential limitation of many current treatments of osteoarthritis is that they may not comprehensively restore regulation of a balance between cartilage anabolism and catabolism.

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.

Glycation induced crosslinking of link proteins, in vivo and in vitro

OBJECTIVE

Reducing sugars have the ability to crosslink proteins through creation of advanced glycosylated end products (AGE). In this study, we determined the ability of AGE to induce crosslinking of link proteins and aggrecan proteoglycans.

METHODS

Aggrecan proteoglycans and link proteins were purified from adult human articular cartilage and from young bovine nasal cartilage for in vivo and in vitro studies, respectively. In vitro studies concerned incubation of aggrecan aggregates or link proteins with ribose under physiological conditions. After 30 days, aggregates were centrifuged dissociatively to obtain aggrecan monomers and link proteins. Aggrecan monomers were analyzed by immunoblot assay. Incubated link proteins were analyzed by SDS-PAGE and Sephacryl-200 column chromatography.

RESULTS

After extensive purification, adult human cartilage aggrecan continued to show the presence of link protein antigens by immunoblot analysis. Immunoblot analysis of purified aggrecan derived from ribose-treated aggregates also showed the presence of link protein antigens. Ribose treatment of link protein lead to polymerization that was confirmed by Sephacryl-200.

CONCLUSIONS

These studies suggest that human link proteins tend to become crosslinked to aggrecan in adult cartilage. A likely cause of the crosslinking is formation of AGE due to reducing sugars.

Microscopic and histochemical manifestations of hyaline cartilage dynamics

Structure and function of hyaline cartilages has been the focus of many correlative studies for over a hundred years. Much of what is known regarding dynamics and function of cartilage constituents has been derived or inferred from biochemical and electron microscopic investigations. Here we show that in conjunction with ultrastructural, and high-magnification transmission light and polarization microscopy, the well-developed histochemical methods are indispensable for the analysis of cartilage dynamics. Microscopically demonstrable aspects of cartilage dynamics include, but are not limited to, formation of the intracellular liquid crystals, phase transitions of the extracellular matrix and tubular connections between chondrocytes. The role of the interchondrocytic liquid crystals is considered in terms of the tensegrity hypothesis and non-apoptotic cell death. Phase transitions of the extracellular matrix are discussed in terms of self-alignment of chondrons, matrix guidance pathways and cartilage growth in the absence of mitosis. The possible role of nonenzymatic glycation reactions in cartilage dynamics is also reviewed.

Winner of the 1996 Cabaud Award. The effect of lifelong exercise on canine articular cartilage

The effect of long-term exercise on canine knees was studied to determine whether an increased level of lifelong weightbearing exercise causes degeneration, or changes that may lead to degeneration, of articular cartilage. Eleven dogs were exercised on a treadmill at 3 km/hr for 75 minutes 5 days a week for 527 weeks while carrying jackets weighing 130% of their body weight. Ten control dogs were allowed unrestricted activity in cages for the 550 weeks. At the completion of the study all knee joints were inspected for evidence of joint injury and degeneration. Articular cartilage surfaces from the medial tibial plateau were examined by light microscopy, the cartilage thickness was measured, and the intrinsic material properties were determined by mechanical testing. No joints had ligament or meniscal injuries, cartilage erosions, or osteophytes. Light microscopy did not demonstrate cartilage fibrillation or differences in safranin O staining of the tibial articular cartilages between the two groups. Furthermore, the tibial articular cartilage thickness and mechanical properties did not differ between the two groups. These results show that a lifetime of regular weightbearing exercise in dogs with normal joints did not cause alterations in the structure and mechanical properties of articular cartilage that might lead to joint degeneration.

Effect of link protein concentration on articular cartilage proteoglycan aggregation

Previous work has shown that alterations in proteoglycan aggregates are among the first changes detected with aging, disuse, and degeneration of articular cartilage, yet the cause or causes of these alterations remain unknown. To determine if differences in link protein concentration can explain alterations in the assembly, size, and stability of articular cartilage proteoglycan aggregates, we isolated proteoglycan monomer (aggrecan) and link protein from adult bovine articular cartilage and then assembled proteoglycan aggregates from aggrecan and 0.8% hyaluronan relative to aggrecan weight, in the presence of 0, 2, 4, 6, 8, 10, 15, and 20% concentrations of link protein relative to aggrecan weight. We determined the amount, sedimentation coefficient, and stability of the aggregates by analytical ultracentrifugation and measured their dimensions by electron microscopy with use of the monolayer technique. Increased aggregate size, as determined by ultracentrifugation, was directly correlated with an increased number of aggrecans per aggregate and with increased hyaluronan length, as determined by electron microscopy. The concentration of link protein significantly influenced aggregation: concentrations of 6-8% produced maximum aggregation, aggregate stability, and uniformity of aggrecan spacing; concentrations greater than 10% led to the formation of superaggregates (aggregates with sedimentation velocities greater than 100 S that may result from linking two or more hyaluronan filaments) but decreased aggregate stability; and concentrations of less than 4% link protein significantly decreased aggregation, the size and stability of aggregates, and the regularity of aggrecan spacing. The latter observations suggest that a decline in the concentration of link protein could decrease the organization and stability of the articular cartilage matrix.

Mechanical behavior of articular cartilage in shear is altered by transection of the anterior cruciate ligament

The flow-independent viscoelastic and equilibrium behaviors of canine articular cartilage were examined with time after transection of the anterior cruciate ligament. The equilibrium, transient, and dynamic shear behaviors of cartilage were studied in biaxial compression-torsion testing at two time periods after transection of the anterior cruciate ligament and at two sites on the femoral condyle, in order to test for differences between sites of frequent and less frequent contact. Water content also was measured in cartilage at sites corresponding to the areas of mechanical testing. Transection of the anterior cruciate ligament produced significant decreases in all measured moduli of articular cartilage tested in equilibrium and dynamic shear and in equilibrium compression; the values for these moduli were 61, 56, and 77% of the control values, respectively, beginning at 6 weeks following transection of the anterior cruciate ligament. There was evidence of increased energy dissipation of cartilage in shear, with a 13 and 35% increase in tan delta at 6 and 12 weeks after transection of the anterior cruciate ligament, respectively. Changes in the viscoelastic relaxation function of cartilage in shear also were evident at 12 weeks after surgery. In all tissue, there was a significant increase in hydration of approximately 4% at 6 or 12 weeks after surgery. There was little difference between the material parameters for areas considered to be in frequent and less frequent contact, with the exception of hydration, which was greater for areas of less frequent contact.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyaluronan in canine arthropathies

Soluble hyaluronan (HA), which has been considered as a marker for joint disease in man, was measured in serum and synovial fluid (SF) from dogs with osteoarthritis (OA), rheumatoid arthritis (RA), and rupture of the cranial cruciate ligament (CCL) and from normal dogs (control). Dogs with OA and RA had significantly increased serum HA (P < 0.001) and decreased synovial fluid HA (P < 0.001), as did dogs with CCL rupture (serum, P < 0.05; synovial fluid, P < 0.005). In OA, HA was lower in the SF from the affected joint than in that from the clinically normal (inactive) contralateral joint; no such difference was seen in dogs with CCL rupture. Dogs with liver disease (portocaval shunts, viral infectious hepatitis, metastatic neoplasm and disease secondary to diabetes mellitus) had increased serum HA concentrations (P < 0.001). There was a significant overlap of HA values in the diseased and normal dogs. Therefore, it is unlikely that the measurement of this cartilage breakdown product would be of value for diagnosis or prognosis in canine arthropathies.

Age-related changes in cartilage proteoglycans: quantitative electron microscopic studies

Biochemical and biophysical studies have shown that the composition and sedimentation velocity of cartilage proteoglycans change with age, but these investigations cannot demonstrate the alterations in molecular structure responsible for these changes. Development of quantitative electron microscopic methods has made it possible to define the age-related structural changes in aggregating proteoglycans and to correlate the alterations in their structure with changes in tissue composition and morphology. Electron microscopic measurement of human and animal hyaline cartilage proteoglycans has shown that with increasing age the length of the chondroitin sulfate-rich region of aggregating proteoglycan monomers (aggrecan molecules) decreases, the variability in aggrecan length increases, the density of aggrecan keratan sulfate chains increases, the number of monomers per aggregate decreases, and the proportion of monomers that aggregate declines. Proteoglycans from the nucleus pulposus of the intervertebral disc show similar but more dramatic age-related alterations. At birth, nucleus pulposus aggrecan molecules are smaller and more variable in length than those found in articular cartilage. Within the first year of human life, the populations of aggregates and large aggrecan molecules analogous to those found in articular cartilage decline until few if any of these molecules remain in the central disc tissues of skeletally mature individuals. The mechanisms of the age-related changes in cartilage proteoglycans have not been fully explained, but measurement of proteoglycans synthesized by chondrocytes of different ages suggests that alterations in synthesis produce at least some of the age-related changes in aggrecan molecules. Degradation of aggrecan chondroitin sulfate-rich regions in the matrix probably also contributes to the structural changes seen by electron microscopy. Age-related changes in proteoglycan aggregation may be due to alterations in link protein function or inhibition of aggregation of newly synthesized aggrecan molecules by accumulation of degraded aggrecan molecules.

Differential levels of synovial fluid aggrecan aggregate components in experimental osteoarthritis and joint disuse

The levels of proteoglycan aggregate components (link protein, keratan sulfate epitope, and total sulfated glycosaminoglycan) were determined in the synovial fluid lavages of dogs with experimental osteoarthritis or disuse atrophy. A model of experimental osteoarthritis was created by transection of the anterior cruciate ligament of the right knee; studies were carried out 6 and 12 weeks after surgery. Joint disuse was studied at 4 and 8 weeks after initiation of the disuse. Recovery after disuse also was studied in joints that had 3 weeks of remobilization after 4 or 8 weeks of disuse. Synovial fluid lavages from the right knee joints of untreated animals were used as controls. The concentrations of keratan sulfate epitope, sulfated glycosaminoglycan, and link protein in the synovial fluid lavages at 6 and 12 weeks after transection of the anterior cruciate were elevated compared with the control values. Similar analysis of the fluid after disuse showed that the levels of keratan sulfate epitope and sulfated glycosaminoglycan were increased compared with the control levels and the levels after transection. However, the concentration of link protein in the fluid after disuse was not significantly different from the control level. The levels of keratan sulfate epitope and sulfated glycosaminoglycan in the synovial fluid lavages after disuse with recovery were high, but the levels of link protein remained low. The results indicate that the catabolism of proteoglycan aggregates in articular cartilage during early osteoarthritis and disuse is different. The determination of keratan sulfate epitope in synovial fluid lavages appears to provide a relatively general indication of proteoglycan catabolism, whereas increased levels of link protein may be more indicative of cartilage degeneration.

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.

Mechanical properties of canine articular cartilage are significantly altered following transection of the anterior cruciate ligament

The compressive, tensile, and swelling properties of articular cartilage were studied at two time periods following transection of the anterior cruciate ligament in the knee of greyhound dogs. An experimental protocol was designed to quantify the essential equilibrium and biphasic material properties of cartilage in tension, compression, and shear, as well as the parameters of isometric swelling behavior. All properties were measured at several sites to elicit differences between sites of frequent and less frequent contact. Hydration was determined at each site and was compared with the material properties of cartilage from corresponding sites. There were extensive changes in all compressive, tensile, and swelling properties of cartilage after transection of the anterior cruciate ligament. Twelve weeks after surgery, the intrinsic moduli were reduced significantly in compression (approximately 24% of control values), tension (approximately 64%), and shear (approximately 24%), and the hydraulic permeability was elevated significantly (approximately 48%). Significant increases in hydration (approximately 9%) also were observed, as well as a strong correlation of hydration with hydraulic permeability. The pattern of these changes was not found to differ with site in the joint, but significant differences were observed in the magnitude of change for cartilage from the femoral groove and the femoral condyle. The pattern and extent of changes in the material properties following transection of the anterior cruciate ligament indicate that altered loading of the joint severely compromises the overall mechanical behavior of articular cartilage. The observed loss of matrix stiffness in compression, tension, and shear is associated with increases in the deformation of the solid matrix, a diminished ability to resist swelling, and the increase in hydration observed in this study. The increased swelling and elevated water content were related directly to the increase in hydraulic permeability; this suggests an associated loss of fluid pressurization as the load support mechanism in the degenerated cartilage. Without a successful mechanism for repair, damage to the solid matrix may progress and lead to further degenerative changes in the biochemistry, morphology, and mechanical behavior of articular cartilage.

Determination of kinetic changes of aggrecan-hyaluronan interactions in solution from its rheological properties

The kinetics of interactions between aggregating cartilage proteoglycan (aggrecan) and hyaluronan was examined through their rheological flow behavior using a cone on plate viscometer. The mixing of the two types of molecules was carried out directly on the plate of the viscometer, and aggregation process was monitored through the changes of the sample's steady-shear viscosity and/or dynamic shear modulus as a function of time. The effect of flow conditions on the aggrecan-hyaluronan interaction rates was examined by subjecting samples to steady-shearing motions at specified shear rates, and to oscillatory shear motions of specified frequencies and amplitudes. The characteristics of the kinetics of interaction between aggrecan and hyaluronan molecules depended not only on the flow conditions under which proteoglycan aggregation took place, but also on the concentration of the components in the solution. At high shear rates (> 10 s-1), viscosity of the mixture solution increased monotonically, starting near the viscosity of the aggrecan solution, and reaching the viscosity of the aggregate solution in approximately 35 min. Surprisingly, under slow shearing motions (< 10 s-1), the viscosities of the mixture solutions exceeded those of control aggregate solutions at identical hyaluronan: aggrecan ratios and concentrations. In addition, the aggregation under oscillatory motions took place near physiologic frequency (10 rad s-1) although the rate of aggregation process was much slower than under steady-shearing motion (> 100 min). However, the high-frequency oscillatory shearing (62.8 rad s-1) tended to impede aggregation resulting in a reduction of dynamic modulus over time.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Altered structure-function relationships for articular cartilage in human osteoarthritis and an experimental canine model

A review of the structure-function relationships for normal articular cartilage is provided. This provides the foundation for understanding the roles played by collagen, proteoglycan and water in determining the material properties of the tissue. A summary of biomechanical and compositional changes in human osteoarthritic cartilage is also presented. Finally, the results from our recent interdisciplinary study on an experimental osteoarthritis model is described, and new hypotheses are proposed on the initiating factors responsible for the increase of tissue hydration. At present, it appears that microstructural alterations, rather than compositional changes, of the collagen-proteoglycan solid matrix are responsible for the early increase of hydration and the deterioration of biomechanical properties of articular cartilage.

The effect of periodontal disease on sulphated glycosylaminoglycan distribution in the sheep periodontium

The glycosylaminoglycan (GAG) distribution in the soft periodontal tissues of the sheep was investigated topographically in healthy tissue and the changes associated with periodontal disease determined and correlated with increasing disease severity. Total sulphated GAG content was determined spectrophotometrically and the proportions of individual GAGS measured after separation by cellulose acetate electrophoresis. In healthy tissue, total sulphated GAG distribution was found to mirror that described previously for collagen, being highest in the gum-pad region and adjacent to alveolar bone. This changed markedly in relation to periodontal disease. Early disease-related changes in GAGs were first detected adjacent to alveolar bone. In severely diseases tissue, an overall increase in sulphated GAG, particularly in the gum-pad region, was present. Increasing disease severity was accompanied by a decrease in dermatan sulphate and a concomitant increase in the proportion of chondroitin-4-sulphate. These changes may be due to the combined effects of chronic inflammation and alterations to the mechanical loading of the tissue owing to loss of attachment.