In mammalian development, the formation of a tissue and its specialized cell types is often guided by the activation 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 a few other tissues. It has been reported that mutations in the human RORB gene are associated with intellectual disability, epilepsy and other impairments, 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 aims to reveal novel functions for an orphan receptor in defined developmental systems. Our studies address: 1. Function of the Rorb gene in neurodevelopment. We have investigated the expression and function of the Rorb gene in the central nervous system and sensory systems using mouse genetic models. In the retina, undifferentiated progenitor cells generate a range of cell types including photoreceptors, interneurons and ganglion cells that relay visual information to the brain. The expression pattern of the Rorb gene suggests it has key functions in the determination of specific retinal cell lineages, including rod and cone photoreceptors as well as horizontal and amacrine interneurons. We have established a key role for Rorb in the differentiation of rods, the photoreceptors that mediate vision at night or in dim light conditions. The Rorb gene is essential for the induction of a key rod-determining gene, Nrl. In the absence of Rorb, there is a nearly complete loss of rods and an excess of primitive 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 functions, we have determined the roles of two N-terminal products encoded by the Rorb gene. These RORb1 and RORb2 receptor isoforms are differentially expressed in the nervous system and in certain other tissues. Targeted deletions of RORb1 and RORb2 indicate that each isoform controls the differentiation of distinct cell types. We established that RORb1 is critical for the generation of two classes of retinal interneurons, horizontal cells and amacrine cells, neuronal types that process visual information as it is relayed from the photoreceptor layer through the inner nuclear layer of the retina to the the optic nerve. 3. The Rorb gene is also a valuable marker for cell types in other tissues, and in brain regions, including the superior colliculus, a component 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) has revealed a role for Rorb in osteoprotection, suggesting a novel role for this gene in additional tissues outside the nervous system. We are building on these findings to pursue investigation of the downstream target genes that presumably underlie the transcriptional functions of this orphan receptor in cell differentiation and development. We address these aims using a range of genetic approaches, cell-isolation procedures and next generation sequencing analyses.
