Thyroid hormone is well-known as an important 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 color vision. The findings indicate that, unexpectedly, 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. Humans share this system but have trichromatic function because of the presence of third opsin gene for sensitivity to long wave (L, red) light. The mechanisms that pattern opsins are central for color vision but the underlying controls remain largely elusive. Our study showed that deletion of TRb2 results in loss of M opsin and therefore revealed that TRb2 is critical for the diversification of M and S cone sub-populations in mice. This unexpectedly critical 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. This project investigates how TRb2 regulates cone differentiation and long term survival. Progress: 1. Candidate factors that cooperate with TRb2 in the development of cone photoreceptors include the type 2 and type 3 deiodinase enzymes that activate or inactivate thyroid hormone, respectively, in tissues at key stages in development. Previous studies indicated that type 3 deiodinase protects cones from excessive exposure to ligand, thus preventing loss of cones by apoptosis. We are also investigating the role of type 2 deiodinase in retinal development. 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 are using a new model that allows marking and isolation of cones throughout development. Previously, detailed analyses of cones have not been possible because of the technical limitations that cones are scarce (and 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 genes that are direct targets of TRb2. We continue to pursue this goal using a new model that allows high affinity purification of TRb2 with associated chromatin from retina. Next generation sequencing analysis identifies genomic binding sites that suggest novel target genes for TRb2. In summary, our study offers new insights into the genetic and hormonal controls that promote the differentiation of cone photoreceptors. Understanding these processes underlying the differentiation and survival of cones is also 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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