Genetic Underpinnings of ipRGC Diversity - PROJECT SUMMARY Light is a profoundly important regulator of physiology and behavior across a wide range of organisms. Light information is relayed via diverse retinal ganglion cell types to approximately 50 distinct targets in the brain. The melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs) represent 6 of the ~40-50 retinal ganglion cell types present in the mouse retina. M1-M6 ipRGCs are defined by a distinct complement of subtype-defining morphological, physiological, and transcriptional characteristics. However, how this cellular diversity is achieved is largely unknown. This proposal will investigate the molecular underpinnings of ipRGC diversity. ipRGCs also represent an excellent microcosm for studying diversity in the larger RGC population because 1) ipRGCs can be specifically manipulated with multiple, existing genetic tools, 2) transcriptional profiling has revealed gene expression programs that designate ipRGC subtypes, and 3) the diversity of ipRGC subtype morphology, physiology, central projections, and roles in behavior are a broad, and representative subsample of subtype-defining features of RGCs. Thus, ipRGCs are an approachable, representative, and genetically malleable population in which to study cellular diversity. In the experiments outlined below, we will exploit the advantages of the ipRGC system by combining targeted, single cell measurements of ipRGC morphology and physiology with powerful, large-scale transcriptomic approaches in new genetic models to assess how the full range of ipRGC subtypes are established. Brn3b (Pouf42) is a transcription factor involved in RGC specification, and its expression is maintained in ipRGC subtypes into adult stages, where it is poised to influence developmental and adult gene expression programs that give rise to the subtype-defining properties of ipRGCs. This proposal will test the hypothesis that Brn3b actively shapes the morphological, physiological, and transcriptional identity of ipRGC subtypes. In Aim 1 we will use new mouse lines to manipulate Brn3b expression in ipRGCs during development and adulthood to determine the consequences of increasing or reducing Brn3b levels on the morphophysiological features of ipRGCs. In Aim 2 we will determine the roles of Brn3b in shaping gene expression patterns across retinal development and within individual ipRGC subtypes. Excitingly, our preliminary data indicate that Brn3b expression levels in ipRGC subtypes not only correlate with multiple ipRGC features, but also actively regulate defining features of ipRGC subtypes, suggesting that Brn3b is repurposed throughout development and adulthood to fine-tune ipRGC circuit structure and function. These studies will generate a blueprint for understanding how the patterning of transcriptional programs establishes diverse cellular identities in the retina and nervous system.
