Photoreceptor Pathology Associated With Drusen

Abstract

Abstract: : Purpose: Drusen are extracellular deposits that form between the RPE and Bruch's membrane. Numerous studies have correlated drusen with visual deficits in early age–related macular degeneration (AMD), however little is known about how drusen adversely affect retinal neurons, especially photoreceptors, to cause vision loss. Prior studies have documented a reduction in the overall number of photoreceptor cells in the maculas of eyes with heavy drusen loads in early AMD. We recently showed that photoreceptors situated directly over drusen display morphological and immunohistochemical signs of degeneration. The purpose of this study was to further characterize the changes in photoreceptors that are associated with drusen formation. Methods: Human donor eyes were shipped from the Oregon Lions Eye Bank fixed in 4% paraformaldehyde or fresh frozen on dry ice. Fixed tissue was processed for (i) paraffin embedding, nissl staining, and subsequent cell counting, (ii) agarose embedding followed by immunohistochemistry and confocal microscopy, or (iii) resin embedding followed by transmission electron microscopy. Normal photoreceptors and photoreceptors over drusen were isolated from frozen tissue sections using laser capture microdissection and processed for quantitative RT–PCR. Results: Significantly fewer cells are observed in the ONL directly over drusen when compared to nearby "normal" ONL. Cell counts demonstrate that photoreceptor cell numbers also decrease as far as 100um lateral to individual drusen. Electron microscopy and immunohistochemical analyses using synaptic terminal markers demonstrate decreased synaptic terminal density over, and also adjacent to, drusen. Of the synaptic terminals remaining over drusen, many are situated within the ONL, and not the outer plexiform layer, apparently due to retraction toward the cell body. Quantitative–PCR demonstrates that photoreceptors remaining over drusen decrease the transcription of signal transduction and synapse–related genes while upregulating the transcription of stress–response genes. Conclusions: Photoreceptors overlying drusen exhibit signs of dedifferentiation and deconstruction, including the retraction of synaptic terminals, the shortening of outer segments, a decrease in the expression of signal transduction and synapse associated genes, and an increase in the expression of stress response genes. Many of these changes are also observed to occur in the ONL distant from drusen, suggesting that the adverse influence of drusen extend well beyond their physical borders.

Keywords: age-related macular degeneration • drusen • photoreceptors