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Temporal and spatial expression patterns of the CRX transcription factor and its downstream targets. Critical
L C Bibb1, J K Holt, E E Tarttelin
1Section of Cell and Molecular Biology, Imperial College School of Medicine, Sir Alexander Fleming Building, Exhibition Road, London SW7 2AZ, UK.
Abstract:
Cone--rod homeobox (CRX), a paired-like homeobox transcription factor, plays a major role in photoreceptor development and maintenance of the retina. Fifteen different mutations in the CRX gene have been identified as a cause of blinding retinal dystrophy. As a step towards characterizing the underlying pathophysiology of disease, temporal and spatial gene expression patterns during human and mouse eye development were investigated for CRX and for downstream retinally expressed genes, postulated to be transactivated by CRX. We found that human CRX was expressed at 10.5 weeks post-conception (p.c.). This was significantly later than observed in mouse development. Immunocytochemistry in human retina showed that CRX protein was not detected until >4 weeks later at 15 weeks p.c., implying that it would be unable to transactivate PDEB, IRBP and arrestin, which were all expressed before 15 weeks. These data therefore eliminate CRX as the major transcriptional activator of these three genes from a wide group of retinal genes that can be transactivated by CRX in vitro. Additionally, PDEB was expressed 2 weeks before CRX whereas murine Pdeb was expressed after Crx, highlighting a potential difference for the role of PDEB in human eye development. Previous data had shown CRX expression in the adult human retina to be photoreceptor-specific; however, we demonstrate that this gene is also expressed in the inner nuclear layer (INL) of the human and mouse retina by in situ hybridization and immunocytochemistry. INL localization of murine Crx was confirmed in rd/rd,cl mice, as in this mouse model the photoreceptors are absent. We have found important differences in the temporal expression of this gene in human and mouse retina, although spatial expression of the CRX gene appears to be conserved. In addition, downstream targets of CRX in vitro might not represent in vivo function during development. These data support concerns about the extent to which we can extrapolate from rodent models regarding embryonic development and disease pathophysiology.
Insights
Cone-rod homeobox (CRX) gene expression differs between human and mouse development. CRX protein appears later in humans, suggesting it
Area of Science:
- Ophthalmology
- Developmental Biology
- Genetics
Background:
- Cone-rod homeobox (CRX) is crucial for photoreceptor development and retinal maintenance.
- Mutations in the CRX gene cause blinding retinal dystrophies.
- Understanding CRX's role in development is key to disease pathophysiology.
Purpose of the Study:
- To investigate temporal and spatial gene expression patterns of CRX and its downstream targets during human and mouse eye development.
- To characterize the role of CRX in the early stages of retinal development.
- To identify differences in CRX function between human and mouse models.
Main Methods:
- In situ hybridization and immunocytochemistry were used to analyze CRX expression in human and mouse retinas.
- Temporal expression patterns were compared from early embryonic stages to postnatal development.
- Expression of downstream genes (PDEB, IRBP, arrestin) was analyzed in relation to CRX expression.
Main Results:
- Human CRX expression begins later (10.5 weeks post-conception) compared to mouse.
- CRX protein is detected even later in humans (15 weeks p.c.), after key retinal genes are already expressed.
- CRX is also expressed in the inner nuclear layer (INL) of both human and mouse retinas, not just photoreceptors.
- Differences in temporal expression of CRX and PDEB between humans and mice were observed.
Conclusions:
- CRX is unlikely to be the primary activator for PDEB, IRBP, and arrestin during early human retinal development.
- Significant temporal differences exist in CRX expression between human and mouse eye development.
- Spatial expression of CRX appears conserved, but temporal differences challenge direct extrapolation from rodent models for disease pathophysiology.