Intracranial Pressure Reduction Is Associated with Mitochondrial OPA1 and Cytochrome c Release in the Retinas of AQP1-Null Mice

Background: Recent studies strongly suggest that low intracranial pressure (ICP) may be involved in the pathogenesis of glaucomatous optic neuropathy. As retinal ganglion cells (RGCs) are highly susceptible to mitochondrial dysfunction, mitochondrial injury may be associated with optic neuropathy related to reduced ICP. In this study, aquaporin-1 (AQP1)-null mice were used to investigate whether reduced ICP is associated with alterations in mitochondrial structure and the release of optic atrophy type 1 (OPA1) and cytochrome c from mitochondria. Methods: Intraocular pressure (IOP) and ICP were measured in AQP1-null mice, and mitochondrial structural changes were examined using transmission electron microscopy (TEM). Total OPA1 and cytochrome c protein levels were evaluated using immunocytochemistry and Western blotting. Cytosolic and mitochondrial fractions were extracted from retinal tissues, and the subcellular distribution of OPA1 and cytochrome c was further analyzed by Western blotting. Bax and Bcl-2 expression levels were also detected. Results: TEM revealed mitochondrial fission, matrix swelling, and abnormal cristae depletion in the retinas of 1-, 3-, and 6-month-old AQP1-null mice. Morphometric quantification further confirmed significantly reduced mitochondrial length across all age groups and increased mitochondrial width at 1 and 6 months in AQP1-null mice compared with wild-type controls. Decreased retinal OPA1 immunoreactivity and protein expression were observed across all age groups of AQP1-null mice compared with age-matched C57BL/6 control mice. Subcellular fractionation showed increased mitochondrial release of OPA1 (at 3 and 6 months) and cytochrome c (at 1, 3, and 6 months) in the retinas of AQP1-null mice. Altered Bax expression was also detected in the retinas of AQP1-null mice with reduced ICP at all examined ages. Conclusions: Mitochondrial ultrastructural abnormalities, including fission and cristae depletion, altered OPA1 distribution, increased mitochondrial release of OPA1 and cytochrome c, and upregulated Bax expression were observed in the retinas of AQP1-null mice with reduced ICP. These concurrent changes indicate a close association between reduced ICP and retinal mitochondrial dysfunction. Maintaining mitochondrial integrity may therefore serve as a potential protective strategy against optic nerve degeneration in patients with chronic low ICP.