Photic history modifies susceptibility to retinal damage in albino trout.

Albino vertebrates exposed to intense light typically lose photoreceptors via apoptosis, and thus serve as useful models of retinal degeneration. In contrast, albino rainbow trout exposed to intense light maintain populations of rod and cone nuclei despite substantial damage to rod outer segments (ROS). The aim of this study was to differentiate between two hypotheses that could account for this divergent result: (1) trout rod nuclei remain intact during light damage, or (2) rod nuclei die but are replaced by cell proliferation. A further aim was to examine whether photic history modulates retinal damage, as in rodents. Albino and normally pigmented trout were moved from defined photic regimes into full daylight, while some were not moved to serve as protected controls. ROS were always maintained in pigmented fish and in albinos protected from full daylight. In albinos exposed to full daylight, ROS were removed over most of the central retina, whereas rod nuclei were maintained in the outer nuclear layer over 10 days. Pyknotic and TUNEL-labeled rod nuclei were abundant in affected albinos at all time-points tested. Rod death occurred without a decrease in the number of rod nuclei, confirming that proliferation must be replacing cells. Indeed a transient increase in proliferation was observed in retinal progenitors of albinos receiving 5 days of damaging light. This proliferative response was decreased with further damage. Cones remained intact even in areas where rod nuclei had degenerated. Pretreatment with light of moderate versus low intensity light affected the cell death and proliferative responses, and the ectopic localization of rod opsin. We conclude that apoptotic demise of rods, but not cones, occurred during light damage in retinas of albino trout and proliferative responses have a limited a capacity to replace lost rods.

Chromatin immunoprecipitation assay on the rainbow trout opsin proximal promoters illustrates binding of NF-kappaB and c-jun to the SWS1 promoter in the retina.

Misexpression of opsins has been linked to apoptosis of photoreceptor cells in the vertebrate retina. Salmonid fish lose their ultraviolet-sensitive (UVS) cones through post-natal developmental apoptosis mediated by thyroid hormone (TH). In order to identify genetic mechanisms that may play a role in the loss of UVS cones, the transcriptional regulation of the SWS1 opsin in the rainbow trout (Oncorhynchus mykiss) was investigated. The Transfac database was interrogated with promoter sequence acquired by genome-walking PCR using MatInspector V2.2 to identify putative transcription factor (TF) binding sites. Putative binding sites for AP-1 (c-jun) and NF-kappaB were found in the SWS1 opsin promoter and were chosen for further investigation due to their high MatInspector scores, their established role in photoreceptor apoptosis, and their relative exclusion from other opsin promoters. NF-kappaB and c-jun proteins were visualized in rainbow trout retinal tissue with immunohistochemistry and c-jun was identified in rainbow trout retinal protein homogenate by immunoblot. A chromatin immunoprecipitation-polymerase chain reaction technique was employed to examine the in vivo interaction of c-jun and NF-kappaB proteins with their proposed binding sites in the opsin promoters. This analysis demonstrated that NF-kappaB and c-jun bind to the SWS1 opsin promoter, but not to the other rod and cone opsin promoters tested. Given the role of NF-kappaB and c-jun during photoreceptor apoptosis, the influence of their activity through TH and their selective binding to the SWS1 opsin promoter in rainbow trout, these TFs represent good candidates of mechanisms underlying UVS cone degeneration in salmonids.