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A. C. Morris, T. Scholz, J. M. Fadool; Molecular Characterization of the Teleost Rod Progenitor Cell. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4477.
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© ARVO (1962-2015); The Authors (2016-present)
An understanding of the molecular mechanisms that regulate neural stem cell proliferation and differentiation would be beneficial to research aimed at stimulating photoreceptor replacement following retinal degeneration. The eyes of teleost fish grow throughout life; new rods are continuously generated from a pool of rod progenitor cells scattered across the outer nuclear layer (ONL). We have characterized the XOPS-mCFP transgenic zebrafish line, which displays rapid and exclusive rod photoreceptor degeneration. This degeneration induces a substantial increase in rod progenitor proliferation but does not cause any secondary effects to the cone photoreceptors. We have utilized this line to study the developmental maturation of the rod progenitor proliferative response, the affect of rod degeneration on Muller glial cells, and to evaluate expression of developmentally important genes in the rod progenitor population.
Rod degeneration and cone survival in the zebrafish XOPS-mCFP transgenic line has been described previously (Morris et al., 2005). Wild type and mCFP transgenic zebrafish were incubated with 0.5% BrdU for 2 to 16 hours. Retinal cryosections from wild-type and transgenic animals were immunolabeled with an antibody to BrdU to detect proliferating cells, as well as antibodies to retinal Muller cells and to the transcription factor Nr2e3, followed by labeling with fluorescently-conjugated secondary antibodies. Sections were observed by fluorescence microscopy. Blastula transplants were conducted at the 1000-cell stage.
Rod progenitor proliferation was observed in XOPS-mCFP animals at seven days post fertilization (dpf), roughly three days before ONL mitotic activity was detectable in wild type animals. There was no corresponding increase in Muller cell or inner nuclear layer (INL) progenitor proliferation in XOPS-mCFP animals. Mitotic progenitor cells in the ONL colabeled for both BrdU and Nr2e3. Genetic mosaic analysis showed that rod progenitor proliferation was not inhibited by the presence of healthy, mature rods.
Rod progenitor cells are present, but normally mitotically inactive, from the larval stage in zebrafish. Although they respond to rod degeneration by upregulating their proliferation, the presence of healthy rods does not inhibit their activity. Rod degeneration alone is not sufficient to provoke a proliferative response in the INL, nor does it induce activation of Muller cells. Finally, rod progenitors appear to be committed to a photoreceptor cell fate, and express Nr2e3 before leaving the cell cycle.
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