In mammalian development, the formation of a tissue and its specialized cell types is often guided by a genetic hierarchy involving the successive activation of key developmental control genes. These genes often encode transcription factors that prompt an immature, non-specialized cell mass to become a mature, functioning organ composed of appropriate proportions of specific cell types. Nuclear receptors are ligand-regulated transcription factors that respond to hormones or other ligands. Orphan receptors are a special group of nuclear receptors that lack known physiological ligands but play critical functions in tissue development. The retinoid-related orphan nuclear receptor b gene (Rorb), is prominently expressed in the brain and retina. It has recently been reported that mutations in the human RORB gene are associated with cases of intellectual disability and seizure susceptibility, thus implicating the RORB gene with an important role in the human nervous system. However, the functions of the RORB gene and how defects in the gene result in disease are incompletely understood. The goal of this project is to elucidate the tissue-specific functions of the Rorb gene in development in a mammalian model species and to indicate how the dysfunction of the gene causes disease. The study of the Rorb gene offers the opportunity to elucidate novel functions for an orphan nuclear receptor in a defined neurodevelopmental system. Progress: 1. The role of Rorb in neurodevelopment. We have investigated in detail the expression pattern of the Rorb gene in the nervous system and continue to extend these studies in sensory systems and the brain. In the retina, undifferentiated progenitor cells divide and generate a range of specific cell types including photoreceptors, different types of interneurons and ganglion cells. The expression pattern of the Rorb gene suggests functions in the divergent cell fate choice between rod and cone photoreceptors and in the early steps in the generation of horizontal and amacrine interneurons. We have studied the role of the Rorb gene in directing the differentiation of cone and rod photoreceptors. In dichromatic mammals, cones express opsin photopigments that are sensitive to short (S, blue) or medium-longer (M, green) wavelengths of light whereas rods mediate detection of dim light. Our studies have established that Rorb also directs the generation of two classes of retinal interneurons: (i) horizontal cells that modify synaptic transmission between photoreceptors and bipolar neurons, and (ii) amacrine cells that modify transmission between bipolar neurons and ganglion cells. 2. The Rorb gene is remarkable for the variety of different functions it controls in retinal development. To investigate how a single gene can control distinct neurodevelopmental functions, we are pursuing studies to determine the roles of two N-terminal isoform products encoded by the Rorb gene. These Rorb1 and Rorb2 isoforms are differentially expressed in different tissues and cell types of the nervous system. The consequences of targeted deletions of the RORb1 and RORb2 isoforms in the mouse, a mammalian model species,are being determined to indicate how this developmental control gene is capable of regulating different cell-specific functions. Our studies investigate how these differentially expressed isoforms provide additional level of versatility in directing cell-specific functions in neuronal development. We are pursuing further studies to investigate critical downstream target genes that underlie the functions of these orphan receptor isoforms, using a range of tissue and cell-isolation procedures and next generation sequencing approaches.
