Thyroid hormone is a critical factor for the development of the nervous system. Our study of the thyroid hormone receptor b gene (Thrb) identified a key role for a thyroid hormone receptor, TRb2, in the differentiation of retinal cone photoreceptors, the light-sensitive cells that mediate vision in bright light and also mediate color vision. These findings indicated that the retina is an unexpectedly sensitive target of thyroid hormone. Color vision depends upon the differential expression in cone photoreceptors of opsin photopigments for response to different regions of the visible light spectrum. Most mammalian species are dichromatic and express opsins for sensitivity to medium-longer (M, green) or short (S, blue) wavelengths of light. The mechanisms that pattern opsins are central for color vision but the underlying controls remain largely elusive. We previously showed that deletion of TRb2 results in loss of M opsin and therefore identified TRb2 as a key factor for the diversification of M and S cone sub-populations in mice. This finding raises intriguing questions about a functional link between the endocrine and visual systems. Previously, studies of human thyroid disorders largely overlooked the possibility of defects in color vision or in photoreceptor function. However, evidence now indicates that mutations in the human THRB gene are associated with retinal and photoreceptor defects. This project investigates how TRb2 regulates cone differentiation, function and survival. The project addresses: 1. Factors that cooperate with TRb2 in the development of cone photoreceptors. We have shown that the type 2 and type 3 deiodinase enzymes that activate or inactivate thyroid hormone, respectively, modify cone photoreceptor development and survival. Previous studies indicated that type 3 deiodinase protects cones from excessive exposure to ligand, thus preventing loss of cones by apoptosis. Recent evidence indicates that type 2 deiodinase also modifies retinal cone survival and function in mouse models. These studies investigate the importance of mechanisms that augment as well as constrain thyroid hormone action in the differentiation of specific cell types during development. 2. Investigation of genes that are regulated by TRb2 during cone differentiation. We have developed new mouse models that allows marking and isolation of cones during development. Previously, detailed analyses of cones have not been possible because of the technical limitations that cones are scarce (represent only 3% of retinal cells in mice) and beause of the lack of a specific marker for isolating immature cones at early stages. Isolated cones are amenable to analysis by next generation sequencing and our approach has yielded high quality transcriptome data for cones. Ongoing studies comparing the transcriptome of TRb2-deficient and control cones, will reveal candidate target genes for TRb2. 3. Investigation of direct target genes for TRb2. We are pursuing this goal using a model that allows high affinity purification of TRb2 with associated chromatin from retina. Next generation sequencing analysis has identified genomic binding sites for TRb2 and allows investigation of mechanisms of gene regulation by TRb2 in cone differentiation. Our study offers new insights into the genetic and hormonal controls that promote the differentiation and survival of cone photoreceptors. Understanding these processes is expected to advance our knowledge of how dysfunction of these processes may result in developmental or degenerative diseases of the retina.
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