Synaptic pathology, altered gene expression, and degeneration in photoreceptors impacted by drusen

Drusen are risk factors for age-related macular degeneration and have been shown to negatively impact cells of the RPE and retina. In this study, the effects of drusen on the synaptic machinery of retinal photoreceptors are investigated.

Human donor eye tissue containing retina, RPE, and choroid was processed for confocal immunofluorescence microscopy, laser capture microdissection, and light and electron microscopy. Tissue sections were immunostained with a panel of antibodies to synapse-associated proteins. Populations of photoreceptors over drusen and normal populations of photoreceptors were microdissected from fresh frozen tissue, RNA was purified, and quantitative PCR was performed to compare relative levels of gene expression.

The number of photoreceptor synaptic terminals is reduced in regions of the outer plexiform layer over drusen, synaptic proteins are mislocalized in photoreceptor cells, and synaptic terminals are often observed within the outer nuclear layer. Photoreceptors over drusen also increase expression of the stress response proteins apolipoprotein E and alphaB-crystallin. Abnormal immunolabeling patterns are not restricted to photoreceptors directly over drusen but are also observed in cells flanking drusen. Gene expression analysis confirms reductions in the expression of genes coding for synapse-associated proteins and signal transduction proteins and increases in the expression of apolipoprotein E and alphaB-crystallingene transcripts. Ultrastructural analysis of photoreceptor synaptic terminals over drusen reveals significant abnormalities, and cell counts show a reduction in photoreceptor density directly over, and lateral to, drusen of all sizes.

Photoreceptors overlying and flanking drusen exhibit morphologic and biochemical signs of degeneration. The expression of synapse-associated proteins decreases in photoreceptor synaptic terminals, whereas the expression of stress-response proteins increases. Reductions in photoreceptor cell densities over, and flanking, drusen suggest that these degenerative effects eventually result in the death of photoreceptors.