EXPRESSION, SECRETION, AND AGE-RELATED DOWN-REGULATION OF PIGMENT EPITHELIUM-DERIVED FACTOR, A SERPIN WITH NEUROTROPHIC ACTIVITY

Retinal pigment epithelial (RPE) cells form a functional complex with photoreceptor neurons of the retina, interacting through the interphot oreceptor matrix (IPM). We now provide evidence that the gene for pigm ent epithelium-derived factor (PEDF), a protein possessing neurotrophi c and neuronal-survival activities, is highly expressed by both fetal and young adult RPE cells. PEDF mRNA is present in RPE cells of the hu man eye at 17 weeks of gestation, demonstrating its potential for acti on in vivo during early retinal development. The PEDF protein is secre ted in vivo where it constitutes a part of the fetal and adult IPM sur rounding photoreceptor outer segments. A polyclonal PEDF antibody reco gnizes at least four isoforms of secreted human and bovine PEDF by two dimensional gel analysis, and detects a similar 50 kDa protein in the IPM of several other vertebrate species. Within soluble extracts of R PE cells, however, where little, if any, of the 50 kDa species can be detected, an immunoreactive 36 kDa protein is observed by Western blot analysis. By immunofluorescence, PEDF is localized intracellularly in association with the nucleus, presumptive secretory granules, and cyt oskeletal elements of cultured RPE cells with PEDF and actin antibodie s colocalizing to the same cytoskeletal structures. During initial sta ges of attachment, PEDF and actin also concentrate at the tips of pseu dopods extended by the cultured RPE cells. However, with successive pa ssages, synthesis, and secretion of the PEDF protein as well as transc ription of its mRNA decrease and are lost by about 10 passages. In par allel, cultured RPE cells lose their proliferative potential and chang e from an epithelial-like morphology in early passages to a more fibro blast-like appearance by about the 10th passage. PEDF is thus apparent ly present intracellularly and extracellularly in both fetal and early adult periods where it could be involved in cellular differentiation and survival and with its loss, in the onset of senescence.