Purpose: We investigated the interplay between photoreceptors expressing mutant ELOVL4 (responsible for Stargardt-like disease, STGD3) and RPE in the initial stages of retinal degeneration.
Methods: Using electron microscopy and electroretinogram (ERG), we assessed RPE and photoreceptor ultrastructure and function in transgenic ELOVL4 (TG1-2 line; TG) and wild-type (WT) littermates. Experiments were done at P30, 1 month before photoreceptor loss in TG and at P90, a time point with approximately 30% rod loss. To further elucidate the mechanism underlying our ultrastructural and functional results, we undertook Western blotting and immunohistochemistry of key proteins involved in phagocytosis of outer segments by RPE cells.
Results: Firstly, we showed that in TG mouse photoreceptors, endogenous ELOVL4 protein is not mislocalized in the presence of the mutated ELOVL4 protein. Secondly, we found evidence of RPE toxicity at P30, preceding any photoreceptor loss. Pathology in RPE cells was exacerbated at P90. Furthermore, higher proportions of phagosomes remained at the apical side of RPE cells. Subretinal lysosomal deposits were immunopositive for phagocytic proteins. Ultrastructural analysis of photoreceptor (rod) outer segments showed disrupted surface morphology consisting of disc spacing irregularities. Finally, rods and RPE exhibited signs of dysfunction as measured by the ERG a-wave leading edge (P30) and c-wave (P90), respectively.
Conclusions: The presence of human mutant ELOVL4 in transgenic mouse photoreceptors leads to early outer segment disc pathology and RPE cytotoxicity. Defective processing of these abnormal discs by RPE cells ultimately may be responsible for outer segment truncation, photoreceptor death, and vision loss.