Between rods and cones: The role of NR2E3 in retinal development.

Abstract

NR2E3 is a photoreceptor-specific nuclear receptor, mutations in which have been identified in human patients with the disease enhanced S-cone syndrome. In humans and in the <italic>rd7</italic> mouse, loss of functional NR2E3 results in increased numbers of S-opsin containing cones and early-onset rod degeneration. <italic>Nr2e3</italic> transcripts are not detected in the neural retina leucine zipper knock-out (<italic>Nrl<super>-/-</super></italic>) mice, which exhibit excess functional S-cones and a rod-less retina. However, the biological function of NR2E3 in photoreceptor development and the mechanism of increased S-cones observed in the absence of NR2E3 had not been elucidated when this study was undertaken. I began my investigations to test the hypothesis that NR2E3 regulates the expression of genes critical for photoreceptor differentiation and function. To examine developmental expression pattern, I generated an anti-NR2E3 polyclonal antibody and showed that the NR2E3 protein is preferentially expressed in the developing and mature rod photoreceptors, but not in the cones. I then demonstrated that NR2E3 interacts with NRL, CRX and NR1D1 (another orphan nuclear receptor) and synergistically activates the promoters of several rod-specific genes. Moreover, NR2E3, together with NRL, abolishes CRX-mediated transactivation of the <italic>S-opsin</italic> promoter. By tagging the new-born rods with green fluorescence protein (GFP), I established that in the <italic>rd7</italic> mouse S-cone genes are expressed in the photoreceptors fated to be rods, providing a possible mechanism of enhanced S-cones observed upon the loss of Nr2e3. My studies also showed that ectopic expression of Nr2e3 in photoreceptor precursors of the <italic>Nrl<super>-/-</super></italic> mice induces rod-like morphology with concurrent expression of rod-specific genes while the expression of cone-specific genes is repressed. However, these rod-like cells do not respond to visual stimuli probably due to the lack of rod transducin. These dual transcriptional regulatory functions of Nr2e3 depend on the timing and level of its expression, but not on the presence of Nrl and/or Crx. I also performed gene profiling of GFP+ photoreceptors from wild-type and mutant mouse retinas. These studies have yielded novel insights into possible gene regulatory networks involved in photoreceptor development. In conclusion, my research investigations have established NR2E3 as a key transcriptional regulator of rod versus cone photoreceptor specification during mammalian retinal development.