Mitochondrial morphology differences and mitophagy deficit in murine glaucomatous optic nerve

PURPOSE. Decreased ATP correlates with intraocular pressure exposure in the optic nerves of mice with glaucoma. To understand what underlies this energy deficit, we examined mitochondria in the myelinated optic nerve axons of the DBA/2J mouse, a model of glaucoma secondary to iris pigment disease, and the DBA/2^wt-gpnmb control strain. METHODS. Mitochondrial length, width, surface area, and health status were measured in 30 electron microscopic fields within the myelinated portion of optic nerves from DBA/2J and DBA/2^wt-gpnmb mice at 3, 6, and 10 months of age. Protein was isolated from optic nerve for analysis of PINK1, Parkin, LC3-I and -II, and lysosome-associated membrane protein 1 (LAMP1) by Western blot. RESULTS. The number of mitochondria in DBA/2J optic nerve was increased, and they had significantly smaller surface area. Mitochondria in DBA/2J were closer to the axolemma, more spatially isolated, and their cristae were more disrupted at every age group as compared to DBA/^2wt-gpnmb. Autophagosomes were significantly increased in DBA/2J optic nerve at all ages. Protein analysis showed higher LC3-II to LC3-I ratio in aged DBA/2J optic nerve than in DBA/^2wt-gpnmb. PINK1 and Parkin levels were not statistically different across age groups. LAMP1 was significantly decreased in the aged DBA/2J optic nerve. CONCLUSIONS. Decreased surface area, combined with reduced oxidative capacity in mitochondria from the aged DBA/2J axon, indicate that mitochondrial pathology may contribute to the energy deficit in glaucomatous optic nerve. Though autophagosomes were increased in DBA/2J optic nerve, the increased mitochondria and decreased LAMP1 suggest deteriorating mitochondria are not being efficiently recycled by mitophagy.