Retinal regional differences in photoreceptor cell death and regeneration in light-lesioned albino zebrafish

Teleost fish regenerate retinal cells from a population of inner nuclear layer (INL) stem cells. To characterize photoreceptor regeneration in zebrafish (Danio rerio), adult albino fish were subjected to constant intense light to cause photoreceptor cell death. Retinal morphometry was performed on histological sections of control and light-lesioned albino retinas to compare the extent of light damage in the ventral, central and dorsal retinal regions. In addition, opsin immunohistochemistry and TUNEL were used to compare photoreceptor cell death in these different retinal areas, while PCNA immunolabeling quantified the cell proliferation that precedes the photoreceptor regeneration. Transgenic albino; Tg(alpha I-tubulin:egfp) zebrafish were also exposed to the intense light in order to examine regeneration-related gene expression changes. The light-lesioned retinas are characterized by extensive rod and cone photoreceptor cell death in the central and dorsal regions. In contrast, many of the rods and cones survive in the ventral retina. The highest levels of INL cell proliferation, which occurs subsequent to photoreceptor death, correspond to the retinal regions that suffer the greatest levels of photoreceptor damage. In the ventral retina, where photoreceptor cell death is minimal, cell proliferation is confined to the ONL. In addition, EGFP expression from the alpha 1-tubulin promoter is increased in Muller glial cells in the light-damaged central and dorsal retina, while transgene expression in the ventral retina is restricted to small, round INL cells. Furthermore, expression of the HuC/D neuronal antigen is detected in a subpopulation of the Muller cells in the lightdamaged superior retinal region. These data demonstrate that adult albino zebrafish display retinal regional differences in photoreceptor cell death and in the regeneration-related INL cell proliferation response. The high levels of INL cell proliferation and alpha 1-tubulin:egfp transgene expression in the Muller cells may be graded in response to the degree of photoreceptor cell death. This suggests that the levels of photoreceptor damage may directly influence cell responses in the underlying retinal layers. (c) 2005 Elsevier Ltd. All rights reserved.