Work in this research group is aimed at elucidating the mechanism of action of pigment epithelium-derived factor (PEDF). PEDF, an extracellular glycoprotein of the interphotoreceptor matrix, vitreous and aqueous, has interesting neurotrophic activities. It promotes neuronal differentiation of retinoblastoma cells, the survival of cerebellar granule cells, neurite-outgrowth and survival of developing spinal motor neurons. It can delay the death of photoreceptors in mouse models of inherited retinal degenerations. By sequence homology, PEDF is included in the serpin superfamily, but it has no known inhibitory activity against serine proteases. Using classical radioligand binding assays, we showed that retinoblastoma and cerebellar granule cells have high-affinity cell-surface receptors for PEDF. A biologically active peptide derived from PEDF spanning positions 44-121, 44-mer, is a competitor of the full length PEDF for the binding to cell-surface receptors, and represents the receptor binding region of PEDF. PEDF also binds to high- affinity receptors on membranes of bovine neural retina. Peptide 44-mer competes for this binding with similar kinetics as that of unlabeled PEDF, while cytochrome c, a control protein, does not. We continued the studies on the interactions of PEDF with components of extracellular matrixes. PEDF binds specifically to hyaluronan, a major glycosaminoglycan component of the interphotoreceptor matrix and vitreous. This binding is mediated via ionic interactions. Heparin and heparan sulfate are natural glycosaminoglycans secreted by retinoblastoma cells to the media. Binding of PEDF to retinoblastoma cell membranes increases when supplemented with conditioned media. Removal of heparan sulfates from cell cultures decreases the binding of PEDF to the cell-surface receptors on retinoblastoma cells. - neurotrophic factor, serpin, receptor-binding, glycosaminoglycan, human retinoblastoma cells, bovine retina, retinal degenerations
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