Project Summary Retinal ganglion cell (RGC) degeneration is the cause of vision loss for millions of individuals worldwide. RGC degeneration follows a sequential paradigm of physiological dysfunction and structural atrophy that is common to neurodegeneration across the central nervous system. This neuronal death program begins with functional deficits associated with a neuron's ability to maintain connections with other neurons via their axons. This first functional deficit defines the most promising window for therapeutic intervention. Due to its association with several steps in this neuronal death program, interleukin-6 (IL-6) has significant interventional potential across neurodegenerative disorders. Unfortunately, ambiguities in reported outcomes and lack of clarity regarding mechanisms of action have impeded development of IL-6 as a therapeutic target. Pro-survival outcomes of IL- 6 signaling in RGCs are most often described for developing RGCs or those with catastrophic injury of their axons. In contrast, detrimental outcomes are often described for disease models where RGC axons remain relatively intact and degeneration is more akin to cell disintegration. Our goal during the proposed award period is to systematically and mechanistically establish the contextual basis for IL-6 outcome ambiguities in RGC neurodegeneration and delineate the IL-6 signaling pathways that mediate them. Our preliminary data indicate that IL-6 directly impacts the development and function of RGC axons. Furthermore, the balance between IL-6 signaling pathways shifts with respect to insult and developmental state. We will test the central hypothesis that the mechanisms and outcomes of IL-6 signaling in RGCs are directly dependent upon the structural and functional state of their axons. In models of RGC development (Aim 1), RGC degeneration with intact axons (Aim 2) and RGC degeneration with structural axon injury (Aim 3), we will: 1) define the functional outcomes of IL-6 signaling in RGCs, 2) Delineate the relative contributions of IL-6 classical and. trans-signaling pathways to those outcomes and 3) Identify downstream targets of signaling. In inducible, conditional IL-6 receptor knockout mice, we will perform pharmacological treatments to induce: native IL-6 signaling, global IL- 6 signaling deficiency, IL-6 trans-signaling alone and IL-6 classical signaling alone. By measuring overall visual function, numerous structural and functional outcomes for RGCs and induction of downstream signaling pathways, we will test the working hypotheses that: 1) IL-6 promotes RGC development and axon formation via classical signaling-mediated induction of cell survival and differentiation pathways, 2) IL-6 contributes to degeneration of mature RGCs and their intact axons via trans-signaling-mediated induction of inflammatory signaling pathways and 3) IL-6 promotes axon repair in mature RGCs with structural axon injury via classical signaling-mediated induction of cell survival...