The studies described here are designed to elucidate the molecular mechanisms underlying the production of a specific class of neurons - cone photoreceptors of the retina. The factors controlling the development of this specialized cell type are not well understood. Using microarray analysis of a mouse mutant that produces excess cones, our lab has identified a number of genes upregulated concommitantly with this cellular effect. Two of these genes are the cone transducin subunits: Gnat2, a G-protein alpha subunit, and Gnb3, a G-protein beta subunit. In the adult vertebrate retina, Gnat2 is expressed exclusively in cone photoreceptors, while Gnb3 is found in bipolar cells and cones. Here they function to transduce light stimulus in a signal transduction cascade downstream of opsins, which are G-protein coupled receptors. Preliminary evidence presented in this proposal shows that Gnat2 and Gnb3 are both expressed specifically in the early photoreceptor layer in both mice and chickens as assayed by RNA in situ hybridizations. This expression is precocious as compared to the other transduction pathway components, most of which are not expressed until much later in development. This proposal will test whether these G-protein subunits function early in development to specify the cone fate or allow for the proper differentiation of cone photoreceptors. Loss-of-function and gain-of-function studies will be used in the chicken retina to test the necessity and/or sufficiency of these genes in the production of cones. A candidate G-protein coupled receptor family that could function upstream of Gnat2 and Gnb3 early in development is the Frizzled family of receptors. Gnat2 has been shown to couple to Frizzled2 receptor in response to WntSa, a secreted ligand for Frizzled receptors. Experiments will be designed to extend previous findings that Wnt signaling is involved in cone development.
The final aim will be to transcriptionally profile the early cone photoreceptor lineage to identify other candidate genes that function to generate fully differentiated cones. We envision these studies providing new insights into how the primary photoreceptor used in human vision, the cone, is derived during development. One of the candidate photoreceptor-genesis genes that will be tested (Gnat2) has been identified as a genetic cause of achromatopsia (no color vision) and this proposal will address the mechanism for this disease. At least 20 to 25 million people are afflicted with genetic diseases that cause the degeneration of photoreceptors and a decrease in their vision. A basic understanding of the normal mechanisms that generate photoreceptors will provide the basis for replacing these cells with new functional photoreceptors. ? ? ?

National Institute of Health (NIH)
National Eye Institute (NEI)
Postdoctoral Individual National Research Service Award (F32)
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Special Emphasis Panel (ZRG1-F05-J (20))
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Mariani, Andrew P
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Harvard University
Schools of Medicine
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Emerson, Mark M; Cepko, Constance L (2011) Identification of a retina-specific Otx2 enhancer element active in immature developing photoreceptors. Dev Biol 360:241-55
Billings, Nathan A; Emerson, Mark M; Cepko, Constance L (2010) Analysis of thyroid response element activity during retinal development. PLoS One 5:e13739
Samson, Maria; Emerson, Mark M; Cepko, Constance L (2009) Robust marking of photoreceptor cells and pinealocytes with several reporters under control of the Crx gene. Dev Dyn 238:3218-25