Work in this research group is aimed at elucidating the mechanism of action of pigment epithelium-derived factor (PEDF). PEDF is an extracellular glycoprotein identified in the native eye, which has interesting neurotrophic activities. By sequence homology, PEDF is included in the serpin superfamily, but it does not have a demonstrable inhibitory activity against serine proteases. During the past year we studied the interactions of PEDF with components associated with the surface of cells, such as human retinoblastoma cells, bovine retina and rat cerebellar granule neurons. We found that PEDF has binding-affinity for glycosaminoglycans isolated from the culturing media of retinoblastoma cells, and that these enhance the binding of PEDF to membrane extracts of retinoblastoma cells and bovine retina. We also found that the PEDF protein binds specifically to a single class of high affinity receptors on human retinoblastoma cells, the bovine neural retina, and rat cerebellar granule neurons (in collaboration with Dr. J.P. Schwartz). In addition, cells from each of these sources contain an integral membrane protein with binding affinity for PEDF with similar chromatographic characteristics among them. Further characterization of the PEDF-binding protein is ongoing. We continued studying the structure-function relationship of PEDF. In collaboration with Dr. L.A. Perez, we prepared several recombinant PEDF proteins with altered amino acid positions corresponding to the proposed glycosaminoglycan binding site, and at it N-glycosylation site. In collaboration with Drs. L. Montuenga and A. Martinez, we observed that immunocytochemistry of the rat retina with a polyclonal antiserum to human PEDF shows that the most significant immunostaining is in the interphotoreceptor matrix (IPM). In the developing murine retina, PEDF-immunoreactivity was significantly and specifically detected in the IPM as early as post-natal day (PD)2, and the level of immunoreactivity was found increasing at later time points (PD5, PD10). In collaboration with Dr. C. Gravel, we evaluated the effect of intravitreal injection of PEDF protein on photoreceptors of animal models for Retinitis Pigmentosa. We found that PEDF can prevent apoptosis of photoreceptors and the decrease of the column height of the photoreceptor cell layer associated with photoreceptor degeneration in the rd and rds mouse models.
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