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 color vision and also vision in bright light or daytime conditions. Our findings indicated that the retina is an unexpectedly sensitive target of thyroid hormone. Color vision depends upon cone populations that express opsin photopigments for response to different regions of the light spectrum. Most mammalian species are dichromatic and express opsins for sensitivity to medium-longer (M, green) or short (S, blue) wavelengths of light. Apart from opsins, little is known of transcriptome distinctions between cone sub-types, nor of the mechanisms that generate cone diversity. We previously showed that deletion of TRb2 results in loss of M opsin and extended expression of S opsin in all cones. This finding established that TRb2 is critical for the diversification of cone sub-populations in mice. These results suggest an intriguing link between the endocrine and visual systems. Previous studies of human thyroid disorders largely overlooked the possibility of defects in color vision or retinal function. However, emerging evidence indicates that mutations in the human THRB gene are associated with varying degrees of impairment of photoreceptor function. This project investigates how TRb2 regulates cone differentiation, function and survival in model systems: 1. Factors that cooperate with TRb2 in the differentiation 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 aim to ascertain the importance of mechanisms that augment as well as constrain thyroid hormone action in the differentiation of specific cell types. 2. Investigation of downstream gene networks that are regulated by TRb2 during cone differentiation. We have developed new mouse genetic models that allow marking and isolation of cones during development. Previously, such detailed analyses of cones had not been possible because of technical limitations presented by the scarcity of cones, which represent only 3% of retinal cells in mice, and beause of 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 yields high quality transcriptome data for cones. To address the fundamental question of what distinguishes cone sub-populations, our studies use single cell genomics techniques to investigate the transcriptome of cone sub-populations. 3. Elucidate a gene regulation program for cone diversity. Given the central role of TRb2 in determining opsin identities in cones, we are pursuing a genomics approach to determine how the wider cone transcriptome is altered following deletion of TRb2 with the goal of identifying the downstream gene networks that promote cone diversity. These studes employ a novel genetic model that allows high affinity purification of TRb2 with associated chromatin from retina. Next generation sequencing can identify genomic binding sites for TRb2 to allow investigation of mechanisms of gene regulation by TRb2 in cone differentiation. This study offers new insights into the genetic and hormonal controls that promote the differentiation and survival of cone photoreceptors. Further collaborative studies with Dr. David Cobrinik (Children's Hospital Los Angeles) indicate a role for TRb2 in the behavior of retinoblastoma cells, which are thought to arise from cone-like precursor cells in retina. Gaining a deeper understanding of the role of TRb2 in cone differentiation and function is expected to advance our knowledge of how dysfunction of these processes may result in developmental or degenerative diseases of the retina.
Showing the most recent 10 out of 11 publications
