Project Summary/Abstract The overall goal of this proposal is to elucidate how to regrow and reconnect injured optic nerves and tracts to specific target neurons in the brain. Specifically, this proposal investigates mechanisms that promote the regeneration of connections made by direction selective retinal ganglion cells (DSGC) to their the accessory optic targets in the brainstem (collectively referred to as the “Accessory Optic System,” or “AOS”). The AOS serves a crucial role in vision by generating slip-compensating eye movements whenever the head or the eyes move at slow speeds. In the absence of proper AOS connectivity and function, images appear blurry and perceptual performance is severely degraded. From a practical standpoint, understanding how to regenerate the mammalian AOS, and defining the cellular and molecular underpinnings of that regeneration, represent an ideal model for parsing regeneration of other visual parallel pathways and also mammalian CNS circuits generally. The AOS is comprised of known retinal neurons and circuits, and the central targets and information carried in this pathway are rather well understood. Indeed, significant progress has been made by our and other groups in identifying genetic markers for the DSGCs that drive AOS function and also cellular and molecular pathways that wire them to their targets. Moreover, both of our laboratories have adopted and expanded state-of-the-art approaches to measure AOS function at the whole animal level with quantitative rigor. In parallel to our work, the field of CNS visual system regeneration has reached the crucial milestone of identifying molecular and activity-based manipulations that allow some retinal ganglion cell (RGC) axons to regenerate following axotomy. The next crucial milestone is to figure out how to ensure accurate reconnection of specific RGC types with their correct targets in the brain. Importantly, it remains unclear whether, after damage to the retina or optic nerve, RGCs and/or their targets re-express, or maintain expression of, the receptors or ligands that enabled them to correctly wire up with one another during development. It is also imperative to determine how the specificity of axon-target matching at the level of cell types and targets, impacts circuit function and behavior. Now that the molecular programs for these developmental steps have started to become clear, this essential issue relating to optic nerve regeneration can finally be approached with deep rigor, and we propose here do that in the context of the AOS. The four major aims of this proposal are to: 1) Test the hypothesis that AOS-projecting RGCs are among the cohort of RGC types capable of regenerating in response to increases in mTOR activation and/or RGC firing. 2) Test the hypothesis that damage to AOS-projecting RGCs and their axons triggers robust changes in axon guidance receptors and ligands in the relevant RGCs and targets. 3) . Test the hypothesis that re-introduction...