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Expression, secretion, and age-related downregulation of pigment epithelium-derived factor, a serpin with
J Tombran-Tink1, S M Shivaram, G J Chader
1Laboratory of Retinal Cell and Molecular Biology, National Eye Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Insights
Pigment epithelium-derived factor (PEDF) is expressed by retinal cells and secreted into the interphotoreceptor matrix, potentially aiding neuronal survival. Its loss correlates with RPE cell senescence.
Area of Science:
- Ophthalmology
- Cell Biology
- Neuroscience
Background:
- Retinal pigment epithelial (RPE) cells and photoreceptor neurons interact via the interphotoreceptor matrix (IPM).
- Pigment epithelium-derived factor (PEDF) exhibits neurotrophic and neuronal-survival activities.
Purpose of the Study:
- To investigate the expression, localization, and potential role of PEDF in RPE cells during retinal development and senescence.
Main Methods:
- Analysis of PEDF mRNA and protein expression in fetal and adult human RPE cells.
- Immunofluorescence and Western blot analysis to determine PEDF localization and isoforms.
- Culturing RPE cells to assess changes in PEDF expression and cell morphology over passages.
Main Results:
- PEDF mRNA and protein are highly expressed in fetal and young adult RPE cells and secreted into the IPM.
- Secreted PEDF exists as multiple isoforms (e.g., 50 kDa), while intracellular PEDF is predominantly 36 kDa.
- PEDF expression and secretion decrease with RPE cell passaging, correlating with loss of proliferative potential and onset of senescence.
Conclusions:
- PEDF is present intracellularly and extracellularly in developing and adult retinas, suggesting a role in RPE cell differentiation and survival.
- The decline of PEDF expression in cultured RPE cells is linked to cellular senescence.
Abstract:
Retinal pigment epithelial (RPE) cells form a functional complex with photoreceptor neurons of the retina, interacting through the interphotoreceptor matrix (IPM). We now provide evidence that the gene for pigment epithelium-derived factor (PEDF), a protein possessing neurotrophic and neuronal-survival activities, is highly expressed by both fetal and young adult RPE cells. PEDF mRNA is present in RPE cells of the human eye at 17 weeks of gestation, demonstrating its potential for action in vivo during early retinal development. The PEDF protein is secreted in vivo where it constitutes a part of the fetal and adult IPM surrounding photoreceptor outer segments. A polyclonal PEDF antibody recognizes 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 RPE 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 cytoskeletal elements of cultured RPE cells with PEDF and actin antibodies colocalizing to the same cytoskeletal structures. During initial stages of attachment, PEDF and actin also concentrate at the tips of pseudopods extended by the cultured RPE cells. However, with successive passages, synthesis, and secretion of the PEDF protein as well as transcription of its mRNA decrease and are lost by about 10 passages. In parallel, cultured RPE cells lose their proliferative potential and change from an epithelial-like morphology in early passages to a more fibroblast-like appearance by about the 10th passage. PEDF is thus apparently 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.