In mammalian development, the formation of a tissue and its specialized cell types is often guided by the activation of a hierarchy of key developmental control genes. Such genes often encode transcription factors that prompt undifferentiated progenitor cells to form a mature, functioning organ composed of a variety of cell types. Nuclear receptors are ligand-regulated transcription factors that respond to hormones or other ligands. Orphan receptors are a sub-group of nuclear receptors that lack known physiological ligands but play a critical role in development and cellular differentiation. The retinoid-related orphan nuclear receptor b gene (Rorb), is expressed in the retina, spinal cord, certain brain regions and in a few other tissues. It has recently been reported that mutations in the human RORB gene are associated with 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. This project aims to elucidate the tissue-specific functions of the Rorb gene in development in mammalian models and to reveal how dysfunction of the gene causes disease. This study of the Rorb gene offers the opportunity to elucidate novel functions for an orphan receptor in a defined developmental system. Our studies address: 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 sensory systems and the central nervous system in mouse models. In the retina, undifferentiated progenitor cells generate a range of cell types including photoreceptors and different types of interneurons and ganglion cells. The expression pattern of the Rorb gene suggests it has key functions in the cell fate choice of rod and cone photoreceptors and 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 this gene controls 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 and selected other tissues. Targeted deletions of RORb1 and RORb2 have indicated that each isoform controls the differentiation of distinct cell types. Our studies established that RORb1 directs the generation of two classes of retinal interneurons, horizontal cells and amacrine cells, neuronal types 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 tissues, including specific brain regions, including the superior colliculus, a region of the visual pathway that receives different sensory inputs and which influences head and eye movements. Collaborative studies 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. Further collaboration with Dr David Monroe (Mayo Clinic) have revealed a role for the Rorb gene in osteoprotection, suggesting a novel role for this gene in a tissue outside the nervous system. We are building on these findings to pursue studies to investigate the downstream target genes that presumably underlie the transcriptional functions of this orphan receptor in cell differentiation and neurodevelopment, using a range of tissue and cell-isolation procedures and next generation sequencing approaches.
