Rorß regulates selective axon-target innervation in the mammalian midbrain.

Developmental control of long-range neuronal connections in the mammalian midbrain remains unclear. We explored the mechanisms regulating target selection of the developing superior colliculus (SC). The SC is a midbrain center that directs orienting behaviors and defense responses. We discovered that a transcription factor, Rorß, controls establishment of axonal projections from the SC to two thalamic nuclei: the dorsal lateral geniculate nucleus (dLGN) and the lateral posterior nucleus (LP). A genetic strategy used to visualize SC circuits revealed that in control animals Rorß+ neurons abundantly innervate the dLGN but barely innervate the LP. The opposite phenotype was observed in global and conditional Rorb mutants: projections to the dLGN were strongly decreased, and projections to the LP were increased. Furthermore, overexpression of Rorb in the wild type showed increased projections to the dLGN and decreased projections to the LP. In summary, we identified Rorß as a key developmental mediator of colliculo-thalamic innervation. Such regulation could represent a general mechanism orchestrating long-range neuronal connections in the mammalian brain.

Retinoid-Related Orphan Receptor ß and Transcriptional Control of Neuronal Differentiation.

The ability to generate neuronal diversity is central to the function of the nervous system. Here we discuss the key neurodevelopmental roles of retinoid-related orphan receptor ß (RORß) encoded by the Rorb (Nr1f2) gene. Recent studies have reported loss of function of the human RORB gene in cases of familial epilepsy and intellectual disability. Principal sites of expression of the Rorb gene in model species include sensory organs, the spinal cord, and brain regions that process sensory and circadian information. Genetic analyses in mice have indicated functions in circadian behavior, vision, and, at the cellular level, the differentiation of specific neuronal cell types. Studies in the retina and sensory areas of the cerebral cortex suggest that this orphan nuclear receptor acts at decisive steps in transcriptional hierarchies that determine neuronal diversity.

A gene regulatory network controls the binary fate decision of rod and bipolar cells in the vertebrate retina.

Gene regulatory networks (GRNs) regulate critical events during development. In complex tissues, such as the mammalian central nervous system (CNS), networks likely provide the complex regulatory interactions needed to direct the specification of the many CNS cell types. Here, we dissect a GRN that regulates a binary fate decision between two siblings in the murine retina, the rod photoreceptor and bipolar interneuron. The GRN centers on Blimp1, one of the transcription factors (TFs) that regulates the rod versus bipolar cell fate decision. We identified a cis-regulatory module (CRM), B108, that mimics Blimp1 expression. Deletion of genomic B108 by CRISPR/Cas9 in vivo using electroporation abolished the function of Blimp1. Otx2 and RORß were found to regulate Blimp1 expression via B108, and Blimp1 and Otx2 were shown to form a negative feedback loop that regulates the level of Otx2, which regulates the production of the correct ratio of rods and bipolar cells.

Identification of Rorß targets in cultured osteoblasts and in human bone.

Control of osteoblastic bone formation involves the cumulative action of numerous transcription factors, including both activating and repressive functions that are important during specific stages of differentiation. The nuclear receptor retinoic acid receptor-related orphan receptor ß (Rorß) has been recently shown to suppress the osteogenic phenotype in cultured osteoblasts, and is highly upregulated in bone marrow-derived osteogenic precursors isolated from aged osteoporotic mice, suggesting Rorß is an important regulator of osteoblast function. However the specific gene expression patterns elicited by Rorß are unknown. Using microarray analysis, we identified 281 genes regulated by Rorß in an MC3T3-E1 mouse osteoblast cell model (MC3T3-Rorß-GFP). Pathway analysis revealed alterations in genes involved in MAPK signaling, genes involved in extracellular matrix (ECM) regulation, and cytokine-receptor interactions. Whereas the identified Rorß-regulated ECM genes normally decline during osteoblastic differentiation, they were highly upregulated in this non-mineralizing MC3T3-Rorß-GFP model system, suggesting that Rorß may exert its anti-osteogenic effects through ECM disruption. Consistent with these in vitro findings, the expression of both RORß and a subset of RORß-regulated genes were increased in bone biopsies from postmenopausal women (73±7 years old) compared to premenopausal women (30±5 years old), suggesting a role for RORß in human age-related bone loss. Collectively, these data demonstrate that Rorß regulates known osteogenic pathways, and may represent a novel therapeutic target for age-associated bone loss.