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K. E. Bollinger, G. H. Grossman, G. J. T. Pauer, S. A. Hagstrom; Gene Expression Profiling in Tulp1-/- Mice. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3059.
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Mutations in the human gene TULP1 cause autosomal recessive retinitis pigmentosa. Tulp1 knockout mice develop early-onset, progressive photoreceptor degeneration. Tulp1 is specifically expressed in photoreceptor cells and previous data indicated that it was capable of functioning as a transcriptional activator. We hypothesized that the photoreceptor degeneration in tulp1-/- mice results from aberrant gene expression in these cells. We therefore compared the gene expression profile in the retinas of tulp1-/- mice to their wild-type (WT) littermates.
RNA was isolated from the retinas of tulp1-/- and WT littermate mice at P15. The RNA was used to make biotinylated cRNA for gene chip hybridization. cRNAs were hybridized to Mouse Genome 430 2.0 arrays (Affymetrix) containing ~39,000 transcripts. Data were confirmed using quantitative real-time RT-PCR and immunohistochemistry.
We identified 97 transcripts that were significantly altered in the tulp1-/- retinas in comparison to WT. Many of the mRNAs that were down-regulated in the mutant retinas corresponded to proteins with known functions in protein trafficking, cargo binding and dynein-based movement. We focused our studies on guanylate cyclase activating protein type 2 (GCAP2), a retina-specific protein that was greatly decreased in the tulp1-/- retinas. Using quantitative RT-PCR we confirmed that GCAP2 was reduced 15 fold in the tulp1-/- retinas. Interestingly, the residual GCAP2 protein that was expressed in the mutant retinas was severely mislocalized. In WT mice, GCAP2 is found almost exclusively within the photoreceptor outer segments. However, in the tulp1-/- retinas, the residual GCAP2 was present within rod and cone inner segments, cell bodies, and connecting cilia. We also found that a number of other outer segment marker proteins, including GCAP1 and Rhodopsin, were mislocalized in the tulp1-/- retinas.
Lack of Tulp1 significantly alters the gene expression profile of the retina. Our results indicate a novel function for Tulp1 in transporting proteins within the photoreceptor cell and motivate further studies to determine its specific role in retinal degeneration.
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