PROTEOGLYCAN TURNOVER IN THE SCLERA OF NORMAL AND EXPERIMENTALLY MYOPIC CHICK EYES

PURPOSE. The turnover of chick scleral proteoglycans from control and form-vision deprived (myopic) eyes was compared in vivo and in explant cultures to determine whether proteoglycan degradation is altered dur ing the development of myopia and to characterize the mechanism of pro teoglycan turnover in the sclera. METHODS. Seven-day-old chicks were r adiolabeled via an intraperitoneal injection of (SO4)-S-35, and monocu lar form deprivation was induced 48 hours later. After 1, 2, and 3 wee ks of form deprivation, birds were killed, and the amount of (SO4)-S-3 5-proteoglycans remaining in different scleral regions was measured in control and deprived eyes. Posterior sclera were also radiolabeled in organ culture containing (SO4)-S-35, and radiolabeled scleral proteog lycans were chased into unlabeled medium for 0 to 11 days. (SO4)-S-35- labeled proteoglycans within the scleral matrix and those released int o the medium were characterized by Sepharose CL-2B chromatography and western blot analysis. RESULTS. The biological half-life of scleral pr oteoglycans was significantly shorter within the posterior pole of for m-deprived eyes (t(1/2) = 7.212 days) compared with the same region of control eyes (t(1/2) = 9.619 days; P < 0.001), whereas no differences in turnover rates were seen in the anterior sclera or equatorial scle ra. When posterior scleral punches were placed in organ culture, (SO4) -S-35-labeled proteoglycan turnover rates were similar for control and form-deprived eyes. Chromatographic and western blot analyses indicat ed that approximate to 80% of the total (SO4)-S-35 within the posterio r sclera is incorporated into the aggrecan. Western blot analyses of a ggrecan core protein released into the medium by control and form-depr ived scleral punches indicated that the core protein was degraded into a series of smaller fragments of M-r = 102 to 220 kDa. A specific ant iserum (anti-FVDIPEN) detected the presence of a 50-kDa C-terminal agg recan fragment released into the medium, which was generated by the ac tion of the matrix metalloproteinase gelatinase A and/or stromelysin. CONCLUSIONS. The turnover rate of (SO4)-S-35-labeled scleral proteogly cans is vision dependent and is accelerated in the posterior sclera of chick eyes during the development of experimental myopia. The loss of proteoglycans from the scleral matrix involves proteolytic cleavage a t various sites along the aggrecan core protein through the action, at least in part, of gelatinase A and/or stromelysin.