Distribution of ferritin chains in canine lenses with and without age-related nuclear cataracts

It was determined in an earlier study that ferritin-heavy (H) and -light (L) chains in lens fiber cells are modified in comparison to those in lens epithelial cells. The purpose of the present study was to determine whether changes in ferritin chain characteristics are developmental, age-related, or associated with cataractogenesis, by analyzing the distribution of modified chains throughout the lens fiber mass.

After removing the capsule, noncataractous and cataractous lenses were separated into six layers of fiber cells. The content of ferritin H and L chains in each layer was determined by western blotting with chain-specific antibodies. The level of ferritin complex (450 kDa protein made up of assembled L and H chains) was determined using the enzyme-linked immunosorbent assay. The ability of ferritin complex to bind iron was assessed by in vitro labeling with (59)Fe.

Fiber cell ferritin L chains were 30 kDa (modified from the normal 19 kDa), and were present at the highest level in the outermost layers of both cataractous and non-cataractous lenses. The amount of modified L chains decreased gradually in the inner layers of the fiber mass, and was undetectable in the inner two layers of cataractous lenses. The ferritin H chains were also modified to 12 kDa (perhaps truncated from the normal 21 kDa size) in both cataractous and non-cataractous lenses. Similar levels of this modified H chain were found throughout the normal lens. Interestingly, in cataractous lenses, the modified H chains were found in decreasing amounts towards the interior of the lens, and were undetectable in the nucleus. However, in these cataractous lenses, the normal-sized ferritin H chains (21 kDA) appear in small quantities in the outer fiber layers, and increase in quantity and size (to 29 kDa) in the inner layers. This observation was best seen and demonstrated in advanced cataracts. Ferritin, which can bind iron, was found mainly in the outer layers of the lens fiber mass of normal lenses, but was more evenly distributed in fiber layers from cataractous lenses.

Both ferritin H and L chains were modified in lens fiber cells from normal and cataractous canine lenses. These modifications were not age-related, and most likely occur during the differentiation of epithelial cells to fiber cells, since only normal-sized chains have been found in lens epithelial cells. In addition, there was a specific and distinct distribution of these modified chains throughout the lens fiber mass. The most striking differences between normal and cataractous lenses fiber cells were the appearance of normal-sized ferritin H chains and the relatively even distribution of iron binding capacity throughout the fiber mass of the cataractous lenses. These differences may reflect a response of the lens to increased oxidative stress during cataractogenesis.