Excess cone cell proliferation due to lack of a functional NR2E3 causes retinal dysplasia and degeneration in rd7/rd7 mice

Abstract

The rd7 mouse is a model for hereditary retinal degeneration characterized clinically by retinal spotting throughout the fundus and late onset retinal degeneration, and histologically by retinal dysplasia manifesting as folds and whorls in the photoreceptor layer. This study demonstrates that the rd7 phenotype results from a splicing error created by a genomic deletion of an intron and part of an exon. Hematoxylin/eosin staining of rd7 tissue shows that the whorls in the outer nuclear layer of the retina do not appear during embryonic development but manifest by postnatal day 12.5 (P12.5). Furthermore, in situ hybridization data indicates that the Nr2e3 message is first present at barely discernable levels at embryonic day 18.5, becomes abundant by P2.5, and reaches maximal adult levels by P10.5. Results from these experiments indicate that Nr2e3 message is expressed prior to the development of S-cones. This data coincides with studies in humans showing that mutations in Nr2e3 result in a unique type of retinal degeneration known as enhanced S-cone syndrome, where patients have a 30-fold increase in S-cone sensitivity compared to normal. Immunohistochemical staining of cone cells demonstrates that rd7 retinas have an increased number of cone cells compared to wild-type retinas. Thus, Nr2e3 may function by regulating genes involved in cone cell proliferation, and mutations in this gene lead to retinal dysplasia and degeneration by disrupting normal photoreceptor cell topography as well as cell-cell interactions.

Regulation of photoreceptor gene expression by Crx-associated transcription factor network

“…These findings, however, do not necessarily rule out the second possibility that Nr2e3 has a function in limiting proliferation of early S-cone precursors (Haider et al, 2001;Yanagi et al, 2002). As seen in zebrafish retina , using more sensitive immunostaining assays with GFP-labeled cones, Nr2e3 was recently found to be expressed in developing and mature M/S-cones (Haider et al, 2006).…”

“…The rd7 mouse retina contains whorls and rosettes in the photoreceptor layer at early ages, followed by slow photoreceptor degeneration. Similar to ESCS in humans, the rd7 retina has excess cones that express mostly S-cone opsin (Haider et al, 2001). Rod and cone function measured by electroretinography (ERG) is near normal in young adults but significantly declines in older mice as a result of degeneration of both rods and cones (Akhmedov et al, 2000;Ueno et al, 2005;Haider et al, 2006).…”

“…Nr2e3 is expressed in retinal photoreceptor cells beginning around E18 in the mouse, peaking during rod differentiation and persisting into adulthood at a decreased level (Kobayashi et al, 1999;Takezawa et al, 2007). Expression appears primarily localized to the outer nuclear layer (Kobayashi et al, 1999;Haider et al, 2000;Haider et al, 2001). This persistent expression, mainly in rods in mammals (Bumsted O'Brien et al, 2004;Chen et al, 2005;, suggests that Nr2e3 plays a major role in rod differentiation and maintenance.…”

“…Genetic analysis revealed that these mice carry a homozygous 380-bp deletion in the coding region of the Nr2e3 cDNA (Akhmedov et al, 2000;Haider et al, 2001), due to mRNA splicing defects resulting from an L1-retrotransposon insertion (Chen et al, 2006). This leads to a frame-shift with a premature termination.…”

FIZ1 is expressed during photoreceptor maturation, and synergizes with NRL and CRX at rod-specific promoters in vitro

“…(Akagi, et al, 2004, Brown, et al, 1998, Brown, et al, 2001, Hojo, et al, 2000, Kanekar, et al, 1997, Marquardt and Gruss, 2002, Moore, et al, 2002, Morrow, et al, 1999, Yan and Wang, 1998 Several transcription factors are clearly essential for photoreceptor development, and their mutations cause retinal degenerations: NRL, CRX, Otx2, Trβ2 (thyroid hormone receptor β2), and NR2E3. (Bessant, et al, 1999, DeAngelis, et al, 2002, Freund, et al, 1997, Haider, et al, 2000, Haider, et al, 2001, Martinez-Gimeno, et al, 2001, Ng, et al, 2001, Nishida, et al, 2003, Swain, et al, 1997 However, our knowledge of the interactions of these cell-specific proteins with each other and upstream signal transduction proteins remains sparse. We do not know how this pool of cell-specific transcription factors interacts with the pool of ubiquitous proteins, which include the general transcription machinery and epigenetic regulators of chromatin/DNA structure.…”