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E.C. Oh, D. Chung, T. Rex, E. Strettoi, H. Khanna, N. Khan, M.A. Raven, B.E. Reese, J. Bennett, A. Swaroop; Nrl is a Master Regulator of Rod Photoreceptor Determination . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4191.
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Photoreceptor progenitor cells lacking the bZIP transcription factor Nrl acquire an S–cone like fate at the expense of rod photoreceptors. Our goal is to dissect the precise temporal and spatial requirements needed to specify photoreceptor cell fate in the vertebrate retina.
We have generated transgenic mice that express Nrl under the control of a Crx and S–opsin promoter. The phenotype of each transgenic line was analyzed at various developmental time–points by establishing microarray profiles and examining retinal morphology and immunohistochemistry in a wildtype, Nrl–/–, Crx–/–, or rd7 background. In utero subretinal adeno–associated viral delivery of Nrl into the Nrl–/– background was performed to assess the window when Nrl can instruct progenitors/precursors to acquire rod fate. Electroretinogram (ERG) studies to evaluate retinal function and biochemical assays (mobility shift experiments, transient transfections and ChIP) were performed to assess Nrl function.
Ectopic expression of Nrl under the control of the Crx promoter in photoreceptor progenitors results in a complete functional transformation of cone to rod photoreceptors in both wild–type and Nrl–/– background. A total loss of cone markers (cone opsin and arrestin) as well as other signatures for cone somata (cytochrome oxidase) and cone pedicles (piccolo) were observed in the mature retina. Early cone markers were absent in the developing retina, demonstrating an early induction of the rod lineage from photoreceptor progenitors. ERGs indicate a full recovery of rod function with the transgene expressed in the Nrl–/– background and a complete suppression of cone amplitudes in both wild–type and Nrl–/– backgrounds. Delivery of AAV–Nrl to E14 Nrl–/– fetuses resulted in efficient transduction and partial restoration of visual function and measurable rhodopsin in adult mice. Using biochemical studies, we show Nrl can bind to multiple cone promoter elements and can function as a novel suppressor of cone development.
Nrl functions as an instructive factor that transactivates rod specific genes, while potentially suppressing the differentiation of cone photoreceptors.
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