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Alexander Aslanidis, Marcus Karlstetter, Yana Walczak, Anika Lückoff, Eva Scheiffert, Nadine Bremicker, Thomas Langmann; Sterile alpha motif containing 7 (Samd7) is a novel Crx- and Nrl-regulated transcriptional repressor in the retina. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3731.
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Inherited retinal diseases are mainly caused by mutations in photoreceptor-specific genes. The majority of these genes is regulated by Cone rod homeobox (Crx) and Neural retina leucine zipper (Nrl). Using genome-wide Crx and Nrl chromatin immunoprecipitation data, we have identified a novel sterile alpha motif (SAM) domain containing protein, Samd7, as a Crx and Nrl target. The aim of this study was to characterize the expression and function of Samd7 in the mouse retina.
Tissue-specific and developmental expression of Samd7 was assessed using qRT-PCR. In vitro electroporation of fluorescent reporters into explanted P0 mouse retinas was used to characterize cis-regulatory regions of the Samd7 gene. Samd7 protein levels and localization were determined by immunohistochemistry in wild type and Crx/Nrl knockdown retinas. In vitro transfection studies were carried out to identify the subcellular localization of Samd7. A repressor function of Samd7 on Crx-regulated genes was studied using luciferase co-transfection assays.
Samd7 mRNA expression is confined to the retina. Crx binds to two specific regions in the promoter (Crx-bound region 1, CBR1) and first intron (CBR2) of the Samd7 gene. CBR2 showed high reporter activity in electroporated retinas, which was dependent on Crx and Nrl. Site-directed mutagenesis identified two Crx binding sequences, that are crucial for regulatory activity. Immunohistochemistry revealed Samd7 expression in the outer nuclear layer, which was significantly decreased in Crx and Nrl knockdown retinas. Subcellular Samd7 localization in HEK293 and COS7 cells was confined to the nucleus. In accordance, Samd7 exhibited transcriptional repressor activity when co-transfected with Crx-regulated promoters including Rs1, Samd7 itself and an artificial 5xCrxtk-luc reporter construct.
In this study, we have identified and characterized Samd7, a novel retina-specific member of the SAM domain family. Our analyses implicate that Samd7 could act as a transcriptional repressor involved in fine-tuning of Crx-regulated photoreceptor gene expression.
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