Glaucoma is a leading cause of blindness that is characterized by the death of retinal ganglion cells (RGCs)— the output neurons of the retina. Ocular hypertension, elevated intraocular pressure (IOP), is an important risk factor for glaucoma. Through unknown mechanisms, chronically elevated IOP leads to RGC axonal injury, ultimately causing RGC death and loss of vision. To date, elevated IOP is the only clinically treatable component of glaucoma, and unfortunately, normalizing IOP does not prevent glaucoma progression or development in many patients. Therefore, identifying the molecular signaling pathways that lead from ocular hypertension to RGC death is critical for understanding the pathobiology of glaucoma. There is growing evidence that the endothelin (EDN) system is an important mediator of glaucomatous neurodegeneration. Components of the EDN system were upregulated in human and animal models of glaucoma. Importantly, EDN ligands (Edn1, Edn2) and receptors (Ednra, Ednrb) were upregulated prior to the onset of glaucomatous neurodegeneration in a mouse model of chronic ocular hypertension (DBA/2J mice). EDN induced RGC death both in vitro and in vivo, and antagonizing EDNRs lessened glaucomatous neurodegeneration in DBA/2J mice. Thus, targeting EDN signaling holds great promise as a neuroprotective treatment for glaucoma. However, it is unknown which retinal and/or optic nerve cell type EDN affects to elicit RGC death. Retinal neurons and macroglia (astrocytes and Müller glia) are the retinal and optic nerve cells known to express EDNRB, and deletion of Ednrb from these cell types did not protect RGCs from intravitreal EDN insult. Therefore, I propose to investigate the role of EDNRA in EDN- induced RGC death. EDNRA is expressed by RGCs and vascular mural cells. The canonical role of EDNRA is to regulate vascular tone, and EDN-activation of EDNRA elicits vasoconstriction. EDN injection caused immediate retinal vasoconstriction and hypoxic RGCs 3hr post-insult. Interestingly, glaucomatous DBA/2J retinas also had hypoxic RGCs, which were not seen in age-matched controls. Therefore, it is possible that EDNRA contributes to glaucoma-relevant EDN-induced RGC death by causing a hypoxic insult to RGCs. To investigate the role of EDNRA in EDN-induced RGC death, I will systematically delete Ednra from glaucoma- relevant cell types, perform intravitreal EDN injections, and assess changes in vasoconstriction, hypoxia, and RGC death. Furthermore, the importance of EDN in ocular hypertension-induced RGC death has not been critically tested in vivo. The Edn2 ligand was robustly upregulated in the optic nerve head and retina (specifically, by RGCs) prior to the onset of glaucomatous neurodegeneration in DBA/2J mice. By deleting Edn from glaucoma-relevant cell types in DBA/2J mice, I will determine the necessity and cellular source of EDN production in glaucomatous neurodegeneration. Identifying the mechanisms controlling EDN-induced RGC death will provide...