Iron, ferritin, transferrin, and transferrin receptor in the adult rat retina

PURPOSE. The retina and other tissues need iron to survive. However, the no rmal iron metabolism in rodent retinas had not been characterized. This stu dy was intended to investigate iron and iron homeostasis protein (ferritin, transferrin [Tf] and transferrin receptor [Tf-R]) distribution in 20- to 5 5-day-old rat retinas. METHODS. Iron was revealed on retinal sections directly by proton-induced x -ray emission (PIXE) and indirectly by electron microscopy (EM). Ferritin, Tf, and Tf-R proteins were localized by immunohistochemistry. Transferrin e xpression was localized by in situ hybridization (ISH). Transferrin and fer ritin proteins and mRNA were analyzed by Western blot analysis and reverse transcription-polymerase chain reaction (RT-PCR), respectively. RESULTS. Iron is widely and unevenly distributed throughout the adult rat r etina. The highest concentration was observed by PIXE in the choroid and th e retinal pigmented epithelial cell (RPE) layer, and in inner segments of p hotoreceptors (IS). Outer segments of photoreceptors (OS) also contain iron . EM studies suggested the presence of iron inclusions inside the photorece ptor discs. Choroid, RPE, and IS showed a strong immunoreactivity for ferri tin. Transferrin accumulated mainly in the IS and OS areas and in RPE cells but can also be detected slightly in retinal capillaries. Western blot ana lysis for Tf and ferritin confirmed their presence in the adult neural reti na. By RT-PCR, H- and L-chains of ferritin and Tf mRNAs were expressed in n eural retina, but the main sites of Tf synthesis observed by ISH were the R PE and choroid cell layers. Tf-R immunoreactivity was detected in the gangl ion cell layer, inner nuclear layer, outer plexiform layer, IS, RPE, and ch oroid. These results were similar for all stages studied. CONCLUSIONS. For the first time, the present study characterized both iron and iron homeostasis proteins in rodent retinas. In the outer retina, iron and ferritin shared the same distribution patterns. In contrast, Tf, mainly synthesized by RPE cells and detected in OS and IS areas, probably helps t o transport iron to photoreceptors through their Tf-R. This is a likely pat hway for filling iron needs in the outer retina.