Retinitis pigmentosa (RP) and allied inherited retinal dystrophies are a leading cause of blindness for which there is currently no cure. One potential treatment for retinal degenerative diseases is cell-based transplantation therapy, whereby progenitor cells are transplanted into the diseased eye to replace the lost photoreceptors. While this is an exciting possibility, several challenges to the implementation of transplantation therapies must be overcome, including the inefficient integration of retinal progenitor cells (RPCs) into the recipient retina, and the difficulty of obtaining sufficient numbers of photoreceptor precursors for clinical application. Protocols for the in vitro culture of RPCs must be developed that will produce large numbers of photoreceptor precursors and will promote their survival and differentiation once transplanted. Defining the transcriptional networks that promote specification of photoreceptor precursors is a necessary next step for the evolution of therapeutic strategies. One of the long-term goals of our laboratory is to contribute to these efforts by studying photoreceptor development and regeneration in the zebrafish. The zebrafish is especially useful for studying photoreceptor biology, because its retina contains numerous cone subtypes in addition to rods. Furthermore, unlike mammals, the zebrafish retina is able to regenerate neurons in response to experimental damage. We have recently identified the transcriptional repressor Insm1a as a candidate regulator of photoreceptor differentiation and regeneration. The experiments described in this proposal will define the role of Insm1a during retinal neurogenesis through the application of genetic and molecular genetics approaches. Our specific aims are as follows: Specific Aim I: Determine the role of Insm1a in regulating photoreceptor differentiation during retinal development at the cellular level, using both loss- and gain-of-function experiments to place Insm1 within the genetic hierarchy of known photoreceptor development genes and to determine whether it is required for retinal progenitor cell cycle exit; Specific Aim II: Identify the molecular targets of Insm1 regulation in the retina, using a combination of gene expression profiling, digital gene expression analysis, in vitro reporter assays and in vivo chromatin immunoprecipitation. Completion of our proposal will bridge important gaps in our understanding of the molecular mechanisms of vertebrate photoreceptor differentiation, and reveal underlying principles relevant to the development of approaches for the effective treatment of human retinal disease.