Axon Guidance Molecules and Optic Nerve Disease

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The chronic and progressive loss of Retinal Ganglion Cells (RGC) and their axons is a hallmark of Glaucoma. Despite its well-appreciated link with elevated intraocular pressure (IOP), the sequence of molecular events that lead to RGC dysfunction and subsequent death is not clearly understood. Although increases in IOP may directly impact RGC cell bodies in the retina, a substantial body of evidence suggests that the site of primary damage may in fact be at the optic nerve head (ONH). It is thought that intrinsic tissue properties in the ONH makes it especially susceptible to mechanical damage, resulting in localized cellular and molecular changes that affect optic nerve axon physiology and survival. The identification of the direct molecular triggers of optic nerve axon dysfunction in glaucoma will significantly enhance our understanding of disease pathogenesis and may also potentially provide new important therapeutic avenues. Axon guidance molecules serve as key developmentalproteins that collectively exert major effects on RGC axons during formation of the optic nerve. Axon guidance molecules can directly activate intracellular signaling pathways in developing RGC axons to trigger cytoskeletal disassembly and the elimination of inappropriate axon branches. Recent work has reported that axon guidance molecules are also present in the adult nervous system particularly in settings of neuronal injury and pathology, where they may have previously unappreciated injurious functions on both neurons and axons. In our own work, we have found that specific guidance molecules reappear after optic nerve trauma and govern the ability of damaged adult RGC axons to regenerate. A possible functional role for axon guidance molecules in causing axon damage in more chronic and progressive forms of optic nerve injury, such as glaucoma, has not been explored. In this application, we propose a set of studies to test the notion that axon guidance molecule expression is up-regulated at the glaucomatous adult ONH region and that these axon guidance molecules are capable of eliciting significant physiological responses in adult RGC axons. These studies form the initial tests of a broader hypothesis that axon guidance molecules at the ONH trigger RGC axon dysfunction and are involved in the development or severity of glaucomatous damage. Our preliminary evidence shows that expression of specific axon guidance molecules are indeed up-regulated at the ONH of DBA/2J glaucomatous mice around the time of onset of axon damage. Furthermore, these molecules are capable of physiologically activating adult RGC axons, consistent with our specific hypothesis that guidance molecules may have a primary role in mediating axon damage in glaucoma.