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

Aggrecan and Hyaluronan: The Infamous Cartilage Polyelectrolytes - Then and Now

Cartilages are unique in the family of connective tissues in that they contain a high concentration of the glycosaminoglycans, chondroitin sulfate and keratan sulfate attached to the core protein of the proteoglycan, aggrecan. Multiple aggrecan molecules are organized in the extracellular matrix via a domain-specific molecular interaction with hyaluronan and a link protein, and these high molecular weight aggregates are immobilized within the collagen and glycoprotein network. The high negative charge density of glycosaminoglycans provides hydrophilicity, high osmotic swelling pressure and conformational flexibility, which together function to absorb fluctuations in biomechanical stresses on cartilage during movement of an articular joint. We have summarized information on the history and current knowledge obtained by biochemical and genetic approaches, on cell-mediated regulation of aggrecan metabolism and its role in skeletal development, growth as well as during the development of joint disease. In addition, we describe the pathways for hyaluronan metabolism, with particular focus on the role as a "metabolic rheostat" during chondrocyte responses in cartilage remodeling in growth and disease.Future advances in effective therapeutic targeting of cartilage loss during osteoarthritic diseases of the joint as an organ as well as in cartilage tissue engineering would benefit from 'big data' approaches and bioinformatics, to uncover novel feed-forward and feed-back mechanisms for regulating transcription and translation of genes and their integration into cell-specific pathways.

IL-37 diminishes proteoglycan loss in human OA cartilage: donor-specific link between IL-37 and MMP-3

OBJECTIVE

A hallmark of osteoarthritis (OA) is degradation of articular cartilage proteoglycans. In isolated human OA chondrocytes, the anti-inflammatory cytokine Interleukin-37 (IL-37) lowers the expression of the proteolytic MMP and ADAMTS enzymes, which mediate this degradation. Therefore, we investigated if IL-37 protects against proteoglycan loss in freshly obtained human OA explants.

MATERIAL AND METHODS

Human OA cartilage explants were incubated with IL-37. Release of sulphated proteoglycans (sGAGs) was measured with the dimethylmethylene-blue assay. Production and degradation of newly synthesized proteoglycans was measured using ³⁵S-sulphate. Proteoglycan and proteolytic enzyme expression were analyzed by qPCR and Western Blot. Proteolytic activity was determined by measuring MMP- and ADAMTS-generated aggrecan neo-epitopes with ELISA and by using MMP-3-, MMP-13- or ADAMTS-5-inhibitors.

RESULTS

Over time, a linear release of sGAGs from OA cartilage was measured. IL-37 reduced this release by 87 μg/ml (24%) 95%CI [21.04-141.4]. IL-37 did not affect ³⁵S-sulphate incorporation or proteoglycan gene expression. In contrast, IL-37 reduced loss of ³⁵S-sulphate labeled GAGs and reduced MMP-3 protein expression, indicating that IL-37 inhibits proteoglycan degradation. Remarkably, we observed two groups of patients; one group in which MMP-3-inhibition lowered sGAG release, and one group in which ADAMTS5-inhibition had this effect. Remarkably, IL-37 was only functional in the group of patients that responded to MMP-3-inhibition.

CONCLUSION

We identified a relationship between IL-37 and reduced sGAG loss in OA cartilage. Most likely, this effect is mediated by inhibition of MMP-3 expression. These results suggest that IL-37 could be applied as therapy in a subgroup of OA patients, in which cartilage degradation is mediated by MMP-3.

Role of the Inflammation-Autophagy-Senescence Integrative Network in Osteoarthritis

Osteoarthritis is the most common musculoskeletal disease causing chronic disability in adults. Studying cartilage aging, chondrocyte senescence, inflammation, and autophagy mechanisms have identified promising targets and pathways with clinical translatability potential. In this review, we highlight the most recent mechanistic and therapeutic preclinical models of aging with particular relevance in the context of articular cartilage and OA. Evidence supporting the role of metabolism, nuclear receptors and transcription factors, cell senescence, and circadian rhythms in the development of musculoskeletal system degeneration assure further translational efforts. This information might be useful not only to propose hypothesis and advanced models to study the molecular mechanisms underlying joint degeneration, but also to translate our knowledge into novel disease-modifying therapies for OA.

Modeling IL-1 induced degradation of articular cartilage

In this study, we develop a computational model to simulate the in vitro biochemical degradation of articular cartilage explants sourced from the femoropatellar grooves of bovine calves. Cartilage explants were incubated in culture medium with and without the inflammatory cytokine IL-1α. The spatio-temporal evolution of the cartilage explant's extracellular matrix components is modelled. Key variables in the model include chondrocytes, aggrecan, collagen, aggrecanase, collagenase and IL-1α. The model is first calibrated for aggrecan homeostasis of cartilage in vivo, then for data on (explant) controls, and finally for data on the IL-1α driven proteolysis of aggrecan and collagen over a 4-week period. The model was found to fit the experimental data best when: (i) chondrocytes continue to synthesize aggrecan during the cytokine challenge, (ii) a one to two day delay is introduced between the addition of IL-1α to the culture medium and subsequent aggrecanolysis, (iii) collagen degradation does not commence until the total concentration of aggrecan (i.e. both intact and degraded aggrecan) at any specific location within the explant becomes ≤ 1.5 mg/ml and (iv) degraded aggrecan formed due to the IL-1α induced proteolysis of intact aggrecan protects the collagen network while collagen degrades in a two-step process which, together, significantly modulate the collagen network degradation. Under simulated in vivo conditions, the model predicts increased aggrecan turnover rates in the presence of synovial IL-1α, consistent with experimental observations. Such models may help to infer the course of events in vivo following traumatic joint injury, and may also prove useful in quantitatively evaluating the efficiency of various therapeutic molecules that could be employed to avoid or modify the course of cartilage disease states.

Post-translational aging of proteins in osteoarthritic cartilage and synovial fluid as measured by isomerized aspartate

Introduction

Aging proteins undergo non-enzymatic post-translational modification, including isomerization and racemization. We hypothesized that cartilage with many long-lived components could accumulate non-enzymatically modified amino acids in the form of isomerized aspartate and that its liberation due to osteoarthritis (OA)-related cartilage degradation could reflect OA severity.

Methods

Articular cartilage and synovial fluid were obtained from 14 randomly selected total knee arthroplasty cases (56 to 79 years old) and non-arthritis cartilage from 8 trauma cases (51 to 83 years old). Paired lesional cartilage and non-lesioned OA cartilage were graded histologically using a modified Mankin system. Paired cartilage and synovial fluids were assayed for isomerized aspartate, phosphate-buffered saline/EDTA (ethylenediaminetetraacetic acid) extractable glycosaminoglycans, and total protein. Macroscopically normal non-lesioned OA cartilage was separated into superficial and deep regions when cartilage thickness was at least 3 mm (n = 6).

Results

Normalized to cartilage wet weight, normal cartilage and deep non-lesioned OA cartilage contained significantly (P < 0.05) more isomerized aspartate than superficial non-lesioned OA cartilage and lesioned cartilage. Synovial fluid isomerized aspartate correlated positively (R² = 0.53, P = 0.02) and glycosaminoglycans correlated negatively (R² = 0.42, P = 0.04) with histological OA lesion severity. Neither synovial fluid isomerized aspartate nor glycosaminoglycans nor total protein correlated with histological scores of non-lesioned areas.

Conclusions

We show for the first time that human cartilage and synovial fluid contain measurable quantities of an isomerized amino acid and that synovial fluid concentrations of isomerized aspartate reflected severity of histological OA. Further assessment is warranted to identify the cartilage proteins containing this modification and to assess the functional consequences and biomarker applications of this analyte in OA.

Modulation of depth-dependent properties in tissue-engineered cartilage with a semi-permeable membrane and perfusion: a continuum model of matrix metabolism and transport

The functional properties of cartilaginous tissues are determined predominantly by the content, distribution, and organization of proteoglycan and collagen in the extracellular matrix. Extracellular matrix accumulates in tissue-engineered cartilage constructs by metabolism and transport of matrix molecules, processes that are modulated by physical and chemical factors. Constructs incubated under free-swelling conditions with freely permeable or highly permeable membranes exhibit symmetric surface regions of soft tissue. The variation in tissue properties with depth from the surfaces suggests the hypothesis that the transport processes mediated by the boundary conditions govern the distribution of proteoglycan in such constructs. A continuum model (DiMicco and Sah in Transport Porus Med 50:57-73, 2003) was extended to test the effects of membrane permeability and perfusion on proteoglycan accumulation in tissue- engineered cartilage. The concentrations of soluble, bound, and degraded proteoglycan were analyzed as functions of time, space, and non-dimensional parameters for several experimental configurations. The results of the model suggest that the boundary condition at the membrane surface and the rate of perfusion, described by non-dimensional parameters, are important determinants of the pattern of proteoglycan accumulation. With perfusion, the proteoglycan profile is skewed, and decreases or increases in magnitude depending on the level of flow-based stimulation. Utilization of a semi-permeable membrane with or without unidirectional flow may lead to tissues with depth-increasing proteoglycan content, resembling native articular cartilage.

Aged bovine chondrocytes display a diminished capacity to produce a collagen-rich, mechanically functional cartilage extracellular matrix

Most fundamental studies in cartilage tissue engineering investigate the ability of chondrocytes from young animals to produce cartilaginous matrix under various conditions, while current clinical applications such as autologous chondrocyte implantation, use chondrocytes from donors that are decades past skeletal maturity. Previous investigations have suggested that several characteristics of primary chondrocytes are age-dependent but none have quantified cell proliferation, proteoglycan synthesis and accumulation, collagen synthesis and accumulation, compressive and tensile mechanical properties in order to examine the effects of donor age on all of these parameters. We enzymatically isolated primary bovine chondrocytes from fetal, young and aged animals and cultured these cells in agarose gels to assess the above-mentioned properties. We found that fetal and young (but still skeletally mature i.e. 18-month-old bovine) chondrocytes behaved similarly, while aged chondrocytes (5- to 7-year-old bovine) displayed diminished proliferation ( approximately 2x less), a slightly reduced proteoglycan accumulation per cell ( approximately 20%), and significantly less collagen accumulation per cell ( approximately 55%) compared to the younger cells. Histological observations and mechanical properties supported these findings, where a particularly significant reduction in tensile stiffness produced by aged chondrocytes compared to younger cells was observed. Our findings suggest that donor age is an important factor in determining the outcome and potential success when tissue-engineered cartilage is produced from articular chondrocytes. More specifically, primary chondrocytes from aged donors may not possess sufficient capacity to produce the extracellular matrix that is required for a mechanically resilient tissue.

The local matrix distribution and the functional development of tissue engineered cartilage, a finite element study

Assessment of the functionality of tissue engineered cartilage constructs is hampered by the lack of correlation between global measurements of extra cellular matrix constituents and the global mechanical properties. Based on patterns of matrix deposition around individual cells, it has been hypothesized previously, that mechanical functionality arises when contact occurs between zones of matrix associated with individual cells. The objective of this study is to determine whether the local distribution of newly synthesized extracellular matrix components contributes to the evolution of the mechanical properties of tissue engineered cartilage constructs. A computational homogenization approach was adopted, based on the concept of a periodic representative volume element. Local transport and immobilization of newly synthesized matrix components were described. Mechanical properties were taken dependent on the local matrix concentration and subsequently the global aggregate modulus and hydraulic permeability were derived. The transport parameters were varied to assess the effect of the evolving matrix distribution during culture. The results indicate that the overall stiffness and permeability are to a large extent insensitive to differences in local matrix distribution. This emphasizes the need for caution in the visual interpretation of tissue functionality from histology and underlines the importance of complementary measurements of the matrix's intrinsic molecular organization.

Variations in matrix composition and GAG fine structure among scaffolds for cartilage tissue engineering

OBJECTIVE

To compare matrix composition and glycosaminoglycan (GAG) fine structure among five scaffolds commonly used for in vitro chondrocyte culture and cartilage tissue engineering.

DESIGN

Bovine articular chondrocytes were seeded into agarose, alginate, collagen I, fibrin and polyglycolic acid (PGA) constructs and cultured for 20 or 40 days. In addition to construct DNA and sulfated GAG (sGAG) contents, the delta-disaccharide compositions of the chondroitin/dermatan sulfate GAGs were determined for each scaffold group via fluorophore-assisted carbohydrate electrophoresis (FACE).

RESULTS

Significant differences were found in cell proliferation and extracellular matrix accumulation among the five scaffold groups. Significant cell proliferation was observed for all scaffold types but occurred later (20-40 days) in PGA constructs compared to the other groups (0-20 days). By 40 days, agarose constructs had the highest sGAG to DNA ratio, while alginate and collagen I had the lowest levels. Quantitative differences in the Delta-disaccharide composition of the GAGs accumulated in the different scaffolds were also found, with the most striking variations in unsulfated and disulfated delta-disaccharides. Agarose constructs had the highest fraction of disulfated residues and the lowest fraction of unsulfated residues, with a 6-sulfated/4-sulfated disaccharide ratio most similar to that of native articular cartilage.

CONCLUSIONS

The similarities and differences among scaffolds in proteoglycan accumulation and GAG composition suggest that the scaffold material directly or indirectly influences chondrocyte proteoglycan metabolism and may have an influence on the quality of tissue engineered cartilage.

Chondroprotective agents: glucosamine and chondroitin

The near universal finding of the safety of glucosamine and chondroitin combined with some compelling evidence of their efficacy should spur further research into their mechanism of action, optimal dosing, long-term effects on disease modification, and clinical applicability. When recommending a supplement to patients, the clinician should take into account the purity of the ingredients, reputation of the manufacturer, and the molecular weight of chondroitin supplied.

Age dependence of biochemical and biomechanical properties of tissue-engineered human septal cartilage

The aim of this study was to determine whether the biomechanical and biochemical properties of tissue-engineered human septal cartilage vary with donor age and in vitro culture time. Chondrocytes were isolated from human septal cartilage of patients from 15 to 60 year old and maintained in primary monolayer culture for 14 days. Cells were seeded onto 0.5% PLA coated PGA disks and kept in stationary three-dimensional culture for either 1 day or 3 weeks. Specimens were then implanted subcutaneously into athymic nude mice and harvested after either 4 or 8 weeks. Upon harvest, the equilibrium confined compression modulus was measured as to quantify mechanical properties, and the glycosaminoglycan, hydroxyproline, and DNA contents were determined as measures of tissue proteoglycans, collagen, and cell density. This study demonstrated that native nasal cartilage showed distinct changes in these parameters with age, but cartilage engineered using the cells of these specimens showed no significant dependence on the age of the donor. There was little difference in quality of cartilage between samples cultured for 3 weeks in vitro and those implanted directly after seeding. Together, the results of this study suggest that the process of extracellular matrix assembly by chondrocytes on three-dimensional scaffolds may be independent of in vivo conditions experienced by the tissue prior to harvest.

Growth responses of cartilage to static and dynamic compression

During skeletal development, growth, and maturation, gradual changes in the material properties and physical dimensions of cartilage occur under the influence of mechanical loading. The objective of the current study was to compare glycosaminoglyean biosynthesis and cell proliferation in fetal, calf, and adult bovine cartilage explants, isolated from defined depths from the articular surface, in response to controlled compressive loads. Mechanical testing confirmed that for all cartilage samples subjected to load, there was a marked time-averaged (static) compression, whereas the addition of dynamic load at a frequency of 0.01 Hz induced dynamic strain with amplitude and phase shift characteristics typical of stimuli that previously were found to be associated with stimulation of glycosaminoglycan synthesis. In metabolic studies, the application of static loading (84 kPa) for 24 hours inhibited glycosaminoglycan and deoxyribonucleic acid synthesis in all cultured cartilage samples. The superposition of dynamic loading (200 kPa, 0.01 Hz) induced a 20% stimulation of glycosaminoglycan biosynthesis in calf cartilage from the middle-deep zones over statically-loaded samples and an additional approximate 50% suppression of deoxyribonucleic acid synthesis in fetal and calf cartilage from the articular surface. These results indicate that synthesis of glycosaminoglycan and deoxyribonucleic acid, two distinct indices of cartilage growth, are regulated independently by mechanical loading and that cartilage responds differently to static and dynamic loading at different stages of maturation.

Use of glucosamine and chondroitin sulfate in the management of osteoarthritis

The goals of osteoarthritis therapy are to decrease pain and to maintain or improve joint function. The pharmacologic treatment of this condition has included the use of aspirin, acetaminophen, and nonsteroidal anti-inflammatory drugs. More recently, numerous studies have investigated the potential role of chondroprotective agents in repairing articular cartilage and decelerating the degenerative process. The reports of limited clinical experience with two of these agents, glucosamine and chondroitin sulfate, as well as the accompanying publicity in the popular media, have generated controversy. Advocates of these alternative modalities cite reports of progressive and gradual decline of joint pain and tenderness, improved mobility, sustained improvement after drug withdrawal, and a lack of significant toxicity associated with short-term use of these agents. Critics point out that in the great majority of the relevant clinical trials, sample sizes were small and follow-up was short-term.

Articular cartilage proteoglycan metabolism in avian degenerative joint disease: effects of strain selection and body weight

The effects of strain selection and body weight on proteoglycan metabolism and the onset of degenerative joint disease (DJD) were investigated in avian articular cartilage. Samples from the hock joint (proximal tarsometatarsus, PTM; distal tibiotarsus, DTT) of rapidly growing broiler fowl, fed either ad libitum or on a restricted-diet, were compared with those from a slow growing, light and non-selected strain (J-line). Synthesis and degradation of proteoglycans were investigated by radioactive pulse-chase studies, determination of total sulphated glycosaminoglycans and electrophoretic analysis. By gross morphology, degenerative changes in articular cartilage occurred solely in the DTT from ad libitum-fed broiler fowl, after 13 weeks. Differences in proteoglycan metabolism were also observed, most markedly in the DTT, where the rate of proteoglycan synthesis in the ad libitum-fed group was less than in age-matched J-line cartilage, and the proportions of both newly synthesised and resident proteoglycans released into the culture medium were greater. Results with the feed-restricted group were intermediate between ad libitum-fed and J-line. Electrophoretic analysis of proteoglycans in the culture media showed evidence of degradation solely in the ad libitum-fed group, with earliest onset in the DTT. The results indicate that proteoglycan metabolism in avian articular cartilage is similar to that in mammalian cartilage during the development of DJD, and that the onset of cartilage degeneration is linked with excessive load bearing.

Metabolic kinetics of proteoglycans by embryonic chick sternal cartilage in culture

Explant cultures of embryonic chick sternum have been widely studied, but the kinetics of biosynthesis of proteoglycans by this tissue in culture has not been characterized. Caudal cartilaginous portions of 16-day-old embryonic chick sterna were cultured for 8 days. Histological examination showed that the fresh cartilage contained morphologically homogenous chondrocytes, which were embedded in a uniform extracellular matrix. After culture for 8 days, the histological appearance of the explant remained unchanged but the tissue increased in size with time as indicated by a progressive increase in DNA content and in the content of glycosaminoglycan and collagen. Rates of degradation and release from the tissue of proteoglycans labeled in ovo with 35S were first order during culture, as were the unlabeled proteoglycans. Proteoglycan synthesis was high during the first 2 days of culture, and this then gradually decreased from this high level during the following 2 days. Synthesis was then maintained at a constant level for the remainder of the culture period. After culture for 2 and 7 days, the proteoglycans synthesized by the explants were identical to the preexisting proteoglycans in hydrodynamic size, glycosaminoglycan chain size, and ability to form aggregates. These findings suggest that the embryonic chick sterna maintained a stable cartilage phenotype during the extended culture periods. The initial rapid rate of matrix turnover was probably attributable to an adaptation of the tissue to ex ovo culture conditions and the subsequent maintenance of cellular activities at a lower level indicated the establishment of a steady-state rate of metabolism.

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.

Chondrocyte transplantation to articular cartilage explants in vitro

The transplantation of chondrocytes has shown promise for augmenting the repair of defects in articular cartilage. This in vitro study examined the efficiency of the transplantation of bovine chondrocytes onto articular cartilage disks and the ability of the transplanted chondrocytes to subsequently synthesize and deposit proteoglycan. The radiolabeling of chondrocyte cultures with [3H]thymidine, followed by 4 days of chase incubation, resulted in the incorporation of 98% of the radiolabel into DNA (as assessed by susceptibility to DNase). At the end of the culture period, the [3H]DNA was stable, with a half-life of radioactivity loss into the medium of 73 days. With use of radiolabeled chondrocytes for quantitation, the efficiency of transplantation onto a cartilage substrate was 93 +/- 4% for seeding densities of as much as 650,000 cells per cm2 and a seeding duration of 1 hour. These findings were confirmed both by tracking cells stained with 5-chlormethylfluorescein diacetate and by quantitating DNA. During the 16 hours after seeding onto a cartilage substrate (in which the endogenous cells had been lysed by lyophilization), the transplanted cells synthesized sulfated proteoglycan in direct proportion to the number of cells seeded. Most (83%) of the newly synthesized proteoglycan was released into the medium rather than retained within the layer of transplanted cells and the recipient cartilage substrate. Comparative studies with lyophilized-rehydrated or live cartilage as the recipient substrate indicated a similar efficiency of chondrocyte seeding and proteoglycan synthesis by the seeded chondrocytes. The transplanted cells retained the chondrocyte phenotype, as judged by a high proportion of the [35S]macromolecules being in the form of aggrecan that was capable of aggregating with hyaluronan and link protein, as well as by immunostaining within and around the transplanted cells for type-II, but not type-I, collagen. These results indicate that the number of chondrocytes transplanted onto a cut cartilage surface greatly affects the level of matrix synthesis; this in turn may affect repair.

Pulsed electromagnetic fields preserve proteoglycan composition of extracellular matrix in embryonic chick sternal cartilage

The influence of pulsed electromagnetic fields (PEMF) on proteoglycan composition in cartilage extracellular matrix has been investigated. Day 16 embryonic chick sternal cartilage was explanted to culture and exposed for 3 h per day for 2 days to a repetitive single-pulse PEMF with frequency of 15 Hz and peak magnetic field of 1.25 G. PEMF treatment did not affect cell proliferation, as indicated by [3H]thymidine incorporation, but significantly stimulated the retention of glycosaminoglycans in the explants and reduced the release of glycosaminoglycans into the media. Determination of incorporation of [35S]sulfate and [3H]N-acetylglucosamine into proteoglycans in vitro and breakdown of in ovo labelled [35S]sulfated proteoglycans in vitro showed that PEMF treatment significantly suppressed the synthesis of proteoglycans and the degradation of both newly synthesized and pre-existing proteoglycans. Sepharose CL-2B chromatography demonstrated that PEMF did not affect either the size distribution of newly synthesized and pre-existing [35S]sulfated proteoglycans or their ability to aggregate with hyaluronate. Sepharose CL-6B chromatography followed by cellulose acetate electrophoresis revealed that the chain length and degree of sulfation of [35S]sulfated glycosaminoglycans were identical in control and PEMF-treated cultures. It is concluded that PEMF treatment preserved extracellular matrix integrity of cultured cartilage explants by down-regulating proteoglycan synthesis and degradation in a co-ordinated manner without affecting their gross structural nature.

Characterization of cartilage metabolic response to static and dynamic stress using a mechanical explant test system

A new mechanical explant test system was used to study the metabolic response (via proteoglycan biosynthesis) of mature, weight-bearing canine articular cartilage subjected to static and dynamic compressive stresses. Stresses ranging from 0.5 to 24 MPa were applied sinusoidally at 1 Hz for intervals of 2-24 h. The explants were loaded in unconfined compression and compared to age-matched unloaded explants. Both static and dynamic compressive stress significantly decreased proteoglycan biosynthesis (range 25-85%) for all loading time intervals. The inhibition was proportional to the applied stress but was independent of loading time. After rehydration upon load removal, the measured water content of the loaded explants was not different from the unloaded explants for all test variables. Autoradiographic and electron microscopic analysis of loaded explants showed viable chondrocytes throughout the matrix. Our results suggest that the decreased metabolic response of cyclically loaded explants may be dominated by the static component (RMS) of the dynamic load. Furthermore, the observed decreased metabolism may be more representative of the in situ tissue response than that of unloaded explants, in which we found an increasing rate of metabolism for up to 6 days after explant removal.

Pulsed electromagnetic fields influence hyaline cartilage extracellular matrix composition without affecting molecular structure

Pulsed electromagnetic fields (PEMF) influence the extracellular matrix metabolism of a diverse range of skeletal tissues. This study focuses upon the effect of PEMF on the composition and molecular structure of cartilage proteoglycans. Sixteen-day-old embryonic chick sterna were explanted to culture and exposed to a PEMF for 3 h/day for 48 h. PEMF treatment did not affect the DNA content of explants but stimulated elevation of glycosaminoglycan content in the explant and conserved the tissue's histological integrity. The glycosaminoglycans in sterna exposed to PEMF were indistinguishable from those in controls in their composition of chondroitin sulfate resulting from chondroitinase ABC digestion. Specific examination with [35S]-sulfate labels showed that PEMF treatment significantly suppressed both the degradation of pre-existing glycosaminoglycans biosynthetically labeled in ovo and the synthesis of new [35S]-sulfated glycosaminoglycans. The average size and aggregating ability of pre-existing and newly synthesized [35S]-sulfated proteoglycans extracted with 4 M guanidinium chloride from PEMF-treated cartilage explants were identical to controls. The chain length and degree of sulfation of [35S]-sulfated glycosaminoglycans also were identical in control and PEMF-treated cultures. PEMF treatment also reduced the amount of both unlabeled glycosaminoglycans and labeled pre-existing and newly synthesized [35S]-sulfated glycosaminoglycans recovered from the nutrient media. [35S]-Sulfated proteoglycans released to the media of both control and PEMF-treated cultures were mostly degradation products although their glycosaminoglycan chain size was unchanged. These results demonstrate that exposure of embryonic chick cartilage explants to PEMF for 3 h/day maintains a balanced proteoglycan composition by down-regulating its turnover without affecting either molecular structure or function.

Kinetics of proteoglycan turnover in bovine articular cartilage explants

The turnover of proteoglycans was studied in explant cultures of mature bovine articular cartilage. The aim of the study was to compare the in vitro turnover rates of newly synthesized proteoglycans and endogenous proteoglycans. Cartilage was maintained in the presence of various serum concentrations in order to determine the conditions of steady-state proteoglycan metabolism. The steady state was achieved in medium containing 20% fetal calf serum. The proteoglycan synthesis rate and the half-life of labeled proteoglycans in steady-state cultures were used to calculate the size of the metabolic pool of newly synthesized proteoglycans in steady state. This metabolic pool was shown to be equal to the total amount of proteoglycans in the matrix. It is concluded that all of the proteoglycans in the matrix have the same half-life in vitro. Taking another approach, aggrecan was isolated from the cartilage and the medium of steady-state cultures prelabeled with [35S]sulfate. The specific activity of the glycosaminoglycans from cartilage aggrecan were compared with that of glycosaminoglycans from medium aggrecan. These proved to be the same throughout the culture period. This shows that newly synthesized aggrecan and endogenous aggrecan have the same turnover rate in vitro. The significance of explant culture systems for the study of proteoglycan turnover is discussed.

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.

Double-blind multicenter trial on a new medium molecular weight glycosaminoglycan. Current therapeutic effects and perspectives for clinical use

The ability of glycosaminoglycans to bind to a wide number of biologically active macromolecules has already been investigated. Recent clinical trials on the possible therapeutic benefits of glycosaminoglycans must be placed in perspective, even if they appear to be particularly encouraging, especially as regards the glycosaminoglycan effects on certain coagulation factors. A multicenter, medium-term, double-blind, crossover trial was performed by several Italian Lipid Clinics to determine whether administration of a medium molecular weight glycosaminoglycan (Sulodexide) has a significant clinical effect. Patients affected by peripheral vascular disease and/or hyperlipidemia (type IIa, IIb and IV) were submitted to a 4-week wash-out period, followed by parenteral Sulodexide (S) or placebo (P) administration for 2 weeks, another 2 week wash-out period, parenteral crossover drug or P administration for 2 weeks and, finally, oral S administration for 6 months. Sulodexide lowered plasma viscosity and plasma fibrinogen in all patients. There was also a drop in triglycerides together with a rise in apo A-I and HDL-C in type IV hyperlipoproteinemics, whereas there was no significant effect on total or LDL-plasma cholesterol in type IIa and IIb patients. Moreover, there was a percent increase in peak flow and rest flow in the lower limbs of peripheral vascular disease patients. No side effects or intolerance phenomena were detected. The results indicate that Sulodexide administration may be useful in long-term treatment of patients with peripheral vascular disease and a concomitant increase in plasma triglycerides and/or fibrinogen and/or viscosity.

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.

Acquisition of hyaluronate-binding affinity in vivo by newly synthesized cartilage proteoglycans

We have studied the hyaluronate-binding properties of aggregating cartilage proteoglycans synthesized in vivo by immature (6-week), mature (25-week) and aged (75-week) rabbits. Precursor isotope (35SO4) was given by intra-articular injection and articular cartilage was removed from rabbits after periods ranging from 1.5 h to 168 h. Proteoglycans were extracted with 4 M-guanidinium/HCl and monomers were isolated by CsCl gradient centrifugation under dissociative conditions. The percentages of both radiolabelled and total tissue monomers with a high affinity for hyaluronate [that is, capable of forming aggregates on Sepharose CL-2B in the presence of 0.8% (w/w) hyaluronate] were then determined. For all samples about 30% of the tissue monomers were high-affinity; however, less than 5% of the radiolabelled monomers were high-affinity at 1.5 h after injection, and this figure increased gradually with time in vivo. The increase was rapid in immature rabbits, such that after 24 h, about 30% of the radiolabelled monomers were high-affinity; on the other hand for mature and aged rabbits the increase was markedly slower such that 30% high-affinity was attained only after about 72 h. The results show that aggregating cartilage proteoglycans are secreted in vivo in a 'precursor' form with a low affinity for hyaluronate, and suggest that conversion of these monomers to a form with a higher binding affinity occurs with a half-time of about 12 h in immature cartilages but greater than 24 h in mature cartilages. The possible relationship of these findings to the process of proteoglycan aggregation in vivo is discussed.

Fluorometric assay of DNA in cartilage explants using Hoechst 33258

A simple two-step fluorometric assay of DNA in cartilage explants, utilizing the bisbenzimidazole dye Hoechst 33258, is described. Cartilage explants were prepared for assay by digestion with papain. Aliquots of the digest were mixed with dye solution, and the fluorescence emission measured. The enhancement in fluorescence of dye was specific for DNA, as demonstrated by 97% sensitivity to DNase and resistance to RNase. In addition, little or no interference was caused by non-DNA tissue components, since DNA caused an equal enhancement in fluorescence independent of the presence of papain-digested cartilage. By performing the assay on isolated chondrocytes, the cellular content of DNA was computed to be 7.7 pg per chondrocyte. The assay was stable for at least 2 h and sensitive to as little as 6 ng of DNA or equivalently less than 1000 cells. This procedure offers advantages over other established DNA assays of cartilage and may be especially useful in metabolic studies of cartilage explants.

Canine osteoarthritis: effects of endogenous neutral metalloproteoglycanases on articular cartilage proteoglycans

The aim of this study was to analyze the mechanisms by which neutral metalloproteoglycanases (NMPE) degrade proteoglycans (PGs) in the cartilage of an experimental model of osteoarthritis (OA). We demonstrated that chondrocytes in osteoarthritic cartilage synthesize PGs with the same functional characteristics as those found in normal cartilage. Osteoarthritic cartilage contains NMPE in both active and latent forms. Both forms can degrade newly synthesized and endogenous PG macromolecules, as indicated by the reduced hydrodynamic size found in the two PG populations of osteoarthritic cartilage. PG monomers, derived from the included fraction of Sepharose CL2B chromatography, were unable to form aggregates with hyaluronic acid. Reduction and alkylation showed that PG monomers from osteoarthritic cartilage had a small hydrodynamic size, especially after activation with amino-phenylmercuric acetate. No significant differences were observed in the size of the chondroitin sulfate chain when normal cartilage was compared with its osteoarthritic equivalent. These results suggest that the proteolytic degradation of cartilage matrix PGs by NMPE occurs at both the hyaluronate-binding region and at the chondroitin sulfate-rich region of the core protein.

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.