An in vitro model of ageing of human articular cartilage sulphated-proteoglycans

Synthesis of low buoyant density proteoglycans by human chondrocytes in culture

Human chondrocyte strains were derived from explant outgrowth of nonarthritic or osteoarthritic human cartilage. Chondrocytes radiolabeled with [35SO4] or [35S]-methionine were used to measure the biosynthesis of proteoglycans recovered from the most buoyant fraction (A4) of a CsCl density gradient centrifugation performed under associative conditions. The proteoglycans isolated from the A4 fraction (rho < 1.47 g/ml) were hydrodynamically small and contained both large and small glycosaminoglycan chains. When assessed by SDS/PAGE using 3-16% gradient gels, two subpopulations of small proteoglycans (smPG) were identified. The larger of the two species (smPG-I) migrated slower than the 200 kDa marker protein; when reassessed on 3-5% acrylamide gels, its apparent molecular mass was larger than the 480 kDa and 440 kDa alpha and beta heavy chains of dynein. We estimated the apparent molecular size of this smPG to be approximately 520 kDa. The smaller smPG (smPG-II) had an apparent average molecular mass of 180 kDa (range 170-210 kDa) after 3-16% SDS/PAGE. Three monoclonal antibodies, 1C6, 5D4, and S103L, reactive with the hyaluronic acid binding region of the aggregating proteoglycan core protein, keratan sulfate, and a core protein domain in the chondroitin sulfate attachment region, respectively, reacted with a single protein (apparent molecular mass, 180 kDa) that was similar in size to smPG-II.

Decrease in the basal levels of cytosolic free calcium in chondrocytes during aging in culture: possible role as differentiation-signal

Cell- and matrix-related parameters, which characterize the aging and differentiation process of cartilage in vivo, were measured in cultured chick epiphyseal chondrocytes during maintenance in a suspension culture for 34 days. A gradual decrease in the rates of proliferation and an increase in the size of the cells were observed. Ultrastructural examination revealed increased vacuolization and appearance of glycogen-storing pools. The rate of proteoglycan synthesis gradually increased. Age-related changes in the composition of the proteoglycan consisted of an increase in the ratio of keratan sulfate/chondroitin sulfate. The results indicate that the process of aging in culture resembles maturation and differentiation of cartilage tissue in vivo. The levels of cytosolic free calcium ions ([Ca2+]i) were measured in fura-2-loaded cells during the course of aging in culture. A gradual decrease in [Ca2+]i was observed. In 5-day cultures, a value of 184 nM [Ca2+]i was measured; this value decreased to 61 nM in 34-day cultures. On the basis of the present data and the previous results, which showed that cartilage-derived growth factors caused a decrease in [Ca2+]i, concomitantly with enhancing differentiation, whereas factors which elevated [Ca2+]i caused an increase in proliferation and a decrease in proteoglycan synthesis, we suggest a model for control of chondrocyte differentiation and aging. The model suggests that the rate of differentiation may be paced by changes in steady-state levels of [Ca2+]i.

Qualitative changes in human osteoarthritic hip cartilage proteoglycan synthesis during long-term explant culture

Cartilage explants from 14 human osteoarthritic (OA) femoral heads synthesize 4 subpopulations of proteoglycan (PG) based on hydrodynamic size on Sepharose CL-2B (Kav: I, 0.05; II, 0.28; III, 0.68; IV, 0.9-1.0). A detailed analysis of newly synthesized PG monomer from each PG subpopulation was made during protracted (20 days vs. 1 day) explant culture of the cartilage specimens. Subpopulations I and II--high-density PG each eluted off Sepharose CL-2B as a unimodal peak, Kav, 0.2-0.25. High-density PG from subpopulation II appeared as a broad polydisperse symmetrical peak. Subpopulation IV eluted as 2 peaks; a minor peak of large size, and a major peak, Kav, 0.9. Large pore composite polyacrylamide gel electrophoresis (CAPAGE) of intact PG monomer (fraction D1D1) resolved at least 2 discrete PG subpopulations as constituents of both subpopulations I and II. Subpopulations III and IV PG monomer consisted of several heterogeneous subpopulations. The size of the high-density PG from subpopulations I and II decreased with time-in-culture. No significant differences were found in the average glycosaminoglycan (GAG) chain size (Kav, 0.65 on Sepharose CL-6B) or in susceptibility to chondroitinase ABC or AC-II (both 80%) in subpopulations I, II and III high density PG as a function of time-in-culture. The average GAG chain length of subpopulation IV high density PG (Kav, 0.75-0.9) was significantly shorter than the high-density PG of other subpopulations. Variations in the average GAG length and chondroitinase susceptibility did not appear to underly the smaller size of subpopulations I and II high-density PG with culture age. By contrast, keratanase susceptibility of subpopulations I and II high-density PG increased as a function of culture time.

Slowing down aging of cultured embryonal chick chondrocytes by maintenance under lowered oxygen tension

Cultured epiphyseal-chondrocytes from embryonic chick may serve as a useful in vitro model to study aging processes in cartilage. The accelerated aging process in cultured chondrocytes is completed within a month and is manifested by typical changes in both cellular and extracellular compartments. Under common maintenance conditions, cells show a gradual loss of replicative capacity, increase in the rate of proteoglycan synthesis and age-dependent changes in the structure and composition of proteoglycan. An environmental factor--reduced oxygen tension--was found to slow down aging processes and preserve the young features of chondrocytes for a longer duration in culture. Cultures maintained under lower oxygen tension had higher proliferation rate, smaller cell size, lower rate of proteoglycan synthesis, and lower content of keratan sulfate side chains in the proteoglycan. In addition higher concentrations of free cytosolic calcium [Ca2+]in as compared to control cultures, was found. It is suggested that the increased proliferation rate and the decrease in proteoglycan synthesis caused by low oxygen tension may be signalled by the higher [Ca2+]in in these cells.

Changes in proteoglycans of human osteoarthritic cartilage maintained in explant culture: implications for understanding repair in osteoarthritis

The maintenance of human osteoarthritic femoral head cartilage in long-term explant culture has been used to assess changes in newly synthesized and endogenous proteoglycans. Minced cartilage was radiolabeled with [3H]-leucine for 24 h. and the high density proteoglycans digested with tosyl-phenylalanine chloromethyl ketone-trypsin. Chondroitin-sulfate-rich peptides were separated from chondroitin-sulfate-poor peptides by DEAE-cellulose chromatography and hexuronic acid, protein, and galactosamine and glucosamine molar ratios determined. The incorporation of [3H]-leucine was highest in peptides enriched in chondroitin sulfate. when day 1 cultures were compared to those maintained for 20 days, several prominent changes were seen including, a reduction in the galactosamine to glucosamine ratio. Taken together with other data which showed a reduction in the hydrodynamic size of these proteoglycans with time in culture, these results showed that changes in the existing extracellular matrix pertinent to the osteoarthritic process can be assessed by maintaining cartilage in long-term organ-explant culture. A reduced hydrodynamic size of newly synthesized proteoglycans is consistent with a loss of chondroitin-sulfate and an increase in keratan sulfate in the high density proteoglycans.