Data suggests that avian corneal epithelial (CE) cells have evolved a novel mechanism for preventing UV-induced oxidative damage. This involves having the iron-sequestering molecule ferritin in a nuclear localization rather than the cytoplasmic location it has in other cell types. This nuclear localization of ferritin involves a tissue-specific nuclear transport molecule termed ferritoid. Recently it has been shown that ferritoid not only serves as the nuclear transporter for ferritin, but once within the nucleus, ferritoid retains its association with ferritin, where together they form a stable ferritin-ferritoid complex(es). This complex has structural and functional properties that make it unique among eukaryotic ferritins, including: (1) its size - which is approximately half that of a "typical", cytoplasmic ferritin, (2) its intrinsic low content of iron - which may make it exceptionally effective in sequestering iron and thus preventing iron-mediated oxidative damage, and (3) its ability to bind to DNA - where it could be most effective in preventing damage by sequestering DNA-associated iron and through physical association with the DNA.
One aim of the proposed studies is to examine further the structural and functional properties of the nuclear ferritin-ferritoid complex(es) of CE cells. These analyses will involve determining whether the complex(es) are a singular molecular type or whether there are multiple types of complexes. These analyses will utilize column chromatography, analytical ultracentrifugation, and electron microscopy. Also, as one proposed mechanism of protection afforded by the complex(es) involves abrogating the deleterious effects of free iron, analyses will also be performed on their uptake of iron. Another aim of the studies will be to examine UV protection by ferritin-ferritoid complexes. Certain of these studies will employ a whole corneal organ culture system in which the CE cells maintain their normal, stratified arrangement and recapitulate the events observed for ferritin and ferritoid production during normal development. This organ culture system also allows the manipulation of synthesis of ferritin and ferritoid in CE cells by using the iron chelator deferoxamine - which reversibly blocks the synthesis of both components. Other studies will be performed using human CE cells. The studies will evaluate UV protection by the endogenous heteropolymeric nuclear ferritin-ferritoid complexes and by homopolymeric complexes of ferritin and ferritoid. Lastly, the possibility will be examined that nuclear ferritin may have an additional function(s) in protecting cells from UV damage and death - which involves affecting cell signaling. The proposed experiments include analysis of developmental changes in signaling activity (before and after the developmental acquisition of nuclear ferritin) and gain- and loss-of-function experiments. These manipulations will involve UV-B irradiation followed by evaluation of cell damage and death.
Ultraviolet (UV) light constitutes a major environmental insult to all exposed tissues of the body - as this source of radiation can damage a wide variety of macromolecules, including DNA. However, corneal epithelial cells seem to be refractory to such damage, suggesting that these cells have evolved defense mechanisms that prevent such damage to their DNA. By determining how this protection is afforded, it may be possible to convey this to other cell types.
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