AGGRECAN SYNTHESIZED BY MATURE BOVINE CHONDROCYTES SUSPENDED IN ALGINATE - IDENTIFICATION OF 2 DISTINCT METABOLIC MATRIX POOLS

Proteoglycans synthesized by chondrocytes in alginate beads are found in two compartments: the cell-associated matrix and the further remove d matrix (Hauselmann, H. J,, Aydelotte M, B,, Schumacher B, L,, Kuettn er K, E,, Gitelis, S, Ii,, and Thonar, E, J,-M, A (1992) Matrix 12, 11 6-129), To study the metabolism of aggrecan in these two compartments, mature bovine articular chondrocytes in alginate beads were pulsed wi th [S-35]sulfate for 30 min or 16 h on day 7 of culture and then chase d in isotope-free medium for up to 21 days, At different times, the tw o matrix pools were separately isolated, and the S-35-proteoglycans qu antified, purified, and characterized, Radiolabeled aggrecan molecules exhibited a very long average half life in the beads (t(1/2) = 95 day s), In contrast, small non-aggregating proteoglycans, which made up ap proximately 4% of the S-35-proteoglycans synthesized, were rapidly los t from the beads (t(1/2) = <24 h), Approximately half the S-35-aggreca n subunits, representing mostly molecules which showed a delay in abil ity to form aggregates in the presence of exogenous hyaluronan and lin k protein, spent only a short time (t(1/2) = 4 h) in the cell-associat ed matrix before moving into the further removed matrix, They exhibite d a much longer average half life in the beads than S-35-aggrecan mole cules which became resident of the cell-associated matrix (t(1/2) = >9 5 days versus 15 days), Radiolabeled aggrecan subunits in the two matr ix compartments had a similar average hydrodynamic size and polydisper sity; importantly, the size of these molecules did not change during t he chase period, Catabolism of S-35-aggrecan in the cell-associated ma trix was the only significant contributor to the appearance in the med ium of partially degraded S-35-aggrecan which had lost the ability to bind to hyaluronan, These results strongly suggest aggrecan molecules which reside in the pericellular and territorial matrix compartments i n close proximity to the chondrocytes have a much faster rate of turno ver than their counterpart in the interterritorial areas further remov ed from the cells,