Identification of choroidal ovotransferrin as a potential ocular growth regulator

Abstract

Purpose. In an effort to identify choroidal factors potentially involved in the regulation of ocular growth, proteins released into culture medium of organ-cultured choroids were compared between control eyes and eyes recovering from form deprivation myopia. Methods. The choroids were obtained from the posterior poles of control and recovering chick eyes, and placed into organ culture containing 35 S-methionine/ 35 S-cysteine. Culture medium was collected after 24 hours and proteins were separated and identified by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), fluorography, immunoprecipitation, western blot analysis and by amino acid sequencing. Choroidal proteins were tested for their effect on scleral proteoglycan synthesis by measuring 35 SO 4 incorporation into scleral glycosaminoglycans (GAG) in vitro. Choroidal thickness and axial elongation were measured in control and recovering eyes using high frequency A-scan ultrasound. Results. The synthesis of an 80 kD protein was greatly increased in the choroids of recovering eyes compared with those of control eyes. Amino acid sequencing and immunoprecipitation indicated that the newly synthesized 80 kD protein was ovotransferrin (transferrin, conalbumin). Ovotransferrin release into the culture medium by isolated recovering choroids was asssociated with a decrease in the rate of axial elongation in recovering eyes. When tested in vitro, ovotransferrin (500 ng/µl) inhibited scleral proteoglycan synthesis in the sclera by 62% in a dose-dependent manner. Conclusions. Chick choroids of recovering eyes synthesize and release ovotransferrin during the recovery from form deprivation myopia. Ovotransferrin significantly inhibited proteoglycan synthesis by the sclera, indicating that ovotransferrin may play a role in slowing the rate of vitreous chamber elongation and facilitating the recovery from induced myopia.