Light is an important regulator of circadian biology and behavior. Light-triggered visual functions are orchestrated by multiple channels conveyed and/or conducted by specific retinal ganglion cell (RGC) types. Despite significant progress in our understanding of the morphologies and functions of the twenty-plus RGC types, the genetic and molecular basis underlying the formation and survival of the diverse RGC types remain poorly understood. This grant application focuses on the roles of transcriptional factors Tbr2 and Tbr1 in regulating specific types of retinal ganglion cells (RGCs). Our current understanding is that Tbr2 participates in specifying the fate and maintaining the survival of ipRGCs, and some Tbr2+ RGCs serve as reservoir of ipRGCs. Additionally, Tbr1 marks 2 distinct subsets of RGCs, including OFF direction-selective JAM-B RGCs. The goals of this application are to understand Tbr2-mediated genetic regulatory network for the formation of ipRGCs and to gain comprehensive understanding of Tbr2+ and Tbr1+ RGCs. Specific Aim 1. a) Test the hypothesis that some Tbr2+ RGCs that have low levels of Opn4 expression can serve as a reservoir for ipRGCs; b) To determine the identity of Spp1+Tbr2+ RGCs and whether Spp1 negatively regulates Opn4 expression. Specific Aim 2. a) Test the hypothesis that Tbr2 may function as a master regulator of ipRGCs by ectopically expressing Tbr2 in Tbr1+ RGCs; b) Test the combinatorial effect of Tbr2 and Isl1 in regulating ipRGCs. Specific Aim 3. Test the hypothesis that Tbr1 marks and regulates unique subsets of RGCs: a) Determine the identity of Tbr1+ RGC types; b) Determine the functions of Tbr1+ RGCs; and c) Investigate the roles of Pou4f1 in regulating Tbr1+ RGCs. The objective of this application-to obtain integrative knowledge regarding RGC subtypes (transcription regulation, morphologies, central projections, and physiological functions)-is important to understand the molecular mechanisms underlying RGC subtype formation, and the structure and function of retinal circuits. It is also in line with the specific research interest described in the recently announced NEI Audacious Goal Initiative (://grants.nih.gov/grants/guide/rfa-files/RFA-EY-14- 001.html).