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 may 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 epilepsy, thus implicating this gene with an important role in the human nervous system. However, the functions of this orphan receptor gene and how defects in the gene result in disease are poorly understood. The goal of this project is to elucidate the tissue-specific functions of the Rorb gene in development in a mammalian model and to indicate how dysfunction of this 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 Rorb gene in neurodevelopment. We have investigated in detail the expression and function of the Rorb gene in the nervous system and have extended these studies to different sensory systems and the brain in the mouse, a mammalian model species. In the retina, undifferentiated progenitor cells generate a range of specific cell types including photoreceptors and different types of interneurons and ganglion cells. The expression pattern of the Rorb gene suggests functions in the cell fate choice between rod and cone photoreceptors and in the early steps in the generation of horizontal and amacrine interneurons. We have established a key role for Rorb in the differentiation of rods, the photoreceptors that mediate vision in dim light. The Rorb gene is essential for the induction of the key rod-determining gene, Nrl. In the absence of Rorb, there is a nearly complete loss of rods and an excess of cone-like cells, consistent with the Rorb acting in the same pathway as the Nrl gene. 2. The Rorb gene is remarkable for the variety of functions it controls in retinal development. To investigate how a single gene can control diverse developmental functions, we have determined the roles of two N-terminal products encoded by the Rorb gene. These Rorb1 and Rorb2 isoforms are differentially expressed in the nervous system. Targeted deletions of RORb1 and RORb2 have indicated that each isoform controls the differentiation of distinct cell types. Our studies have established that Rorb1 directs the generation of two classes of retinal interneurons, horizontal cells and amacrine cells, that modify visual information as it is relayed from the photoreceptor layer through the inner nuclear layer of the retina to the ganglion cells that form the optic nerve. 3. The Rorb gene is also a valuable marker for cell types in other regions of the brain, including the superior colliculus, a region that receives different inputs and responds by influencing head and eye movements. Studies in collaboration with Dr. In-Jung Kim (Yale University), showed that the Rorb1 isoform marks a layer of neurons in the superficial region of the superior colliculus, which has been useful in defining these neuronal sub-types. We continue to build on these findings by pursuing 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.
