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A. K. Hennig, G.-H. Peng, H. Aaron, H. Wang, S. Chen; The CREB-Binding Protein (Cbp) Is a Crx Coactivator Essential for Photoreceptor Function. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5401. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Cbp, a ubiquitously expressed transcription coactivator, is critical for many biological functions. In the retina, Cbp is recruited to photoreceptor gene promoters/enhancers during transcriptional activation by the cone-rod homeobox protein Crx. This study was designed to determine the importance of Cbp in Crx-mediated photoreceptor gene expression, development and maintenance using loss-of-function approaches.
Cre-loxP technology was used to knock out Cbp in developing photoreceptors. Mice carrying floxed alleles of Cbp were crossed with mice expressing Cre-recombinase driven by a mouse Crx promoter. The flox/Cre lines were also bred to Crx-/- mice to investigate whether there is a genetic interaction between Cbp and Crx. The resulting conditional knockout (CKO) mice and their littermate controls were characterized for biochemical, morphological, and physiological measures. Expression of Cbp, Crx and their target opsin genes was analyzed by quantitative RT-PCR and immunohistochemistry (IHC). Photoreceptor function was assessed by electroretinography of dark- and light-adapted mice of various ages over an 8-log range of stimulus intensities.
Conditional knockout of both Cbp alleles decreased Cbp immunoreactivity in photoreceptors and some inner nuclear layer cells. Rhodopsin and cone opsin mRNA levels were decreased 20-30% relative to littermate controls, which correlated with decreases in opsin IHC staining of retinal sections. Some Cre-positive animals also showed decreases in photoreceptor outer segment length and outer nuclear layer thickness, accompanied by rare whorl-like outer retinal defects. These morphological and biochemical changes correlated with decreases in ERG responsiveness at 6 weeks of age. Furthermore, knockout of a single allele of Cbp decreased opsin mRNA expression in Crx+/-, but not Crx+/+ mice, suggesting a genetic interaction between Crx and Cbp.
Cbp is a component of the Crx-regulatory pathway required for optimal opsin expression. Loss of Cbp could affect photoreceptor integrity and function. Further studies on Cbp’s role in photoreceptor development and survival at various ages may provide insights into Crx’s mechanism of action, and whether Cbp contributes to Crx-linked retinal degenerative diseases.
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