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Sahar Bedrood, Fred N. Ross-Cisneros, Kevin R. Tozer, Chun Shi Lin, Mark S. Sharpley, Valerio Carelli, Douglas C. Wallace, Alfredo A. Sadun; A Morphological Study of the Optic Nerve in a Mito-Mouse Model Carrying an ND6 Point Mutation. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5908.
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© ARVO (1962-2015); The Authors (2016-present)
Mitochondrial DNA (mtDNA) mutations can cause optic neuropathies. For example, mutations in the ND6 subunit of complex I may lead to Leber hereditary optic neuropathy (LHON). We generated the first mouse model harboring a mtDNA missense mutation, G13997A, in the ND6 gene inducing the P25L amino acid substitution. Because this mouse model could potentially mimic an optic neuropathy in LHON, we first chose to examine the microscopic features of these optic nerves.
Mouse fibroblasts from a cell line harboring the G13997A ND6 mutation were enucleated and chemically fused with mouse embryonic stem (ES) cells depleted of endogenous mitochondria. After fusion, the clones were screened for both ES nucleus and mutant mtDNA. Positive clones were then injected into pseudo-pregnant C57BL/6J females. Offspring were screened for germline transmission of the ND6 mutation. At 18 months, optic nerves from two wild-type and two mutant homoplasmic ND6 mice were fixed and processed for light (LM) and transmission electron (TEM) microscopic examination. At the LM level, tissue morphology, axon number, and density was evaluated. At the TEM level, nerves were observed for ultrastructural detail of axons and glia, especially for mitochondria. Percentage of normal mitochondria and number per axon was analyzed.
There were no observable nor statistically significant differences between groups at the LM level for morphology, axon number, and axon density. At the ultrastructural level (TEM), we qualitatively observed in the mutants a greater level of abnormal morphological features such as axonal condensation, hypermyelination, apoptotic changes in glia, and abnormal mitochondria (absent matrix, cristae) in axons and glia. The mutant group also had a significantly lower percentage of normal mitochondria per axon as compared to controls (16 ± 0.05 vs 66 ± 0.03) but a greater number of mitochondria per axon as compared to controls (0.39 ± 0.08 vs 0.25 ± 0.03).
Our findings demonstrated pathological features in the mutant mice only at the ultrastuctural level. There was a significant increase in the abnormal morphological features and number of mitochondria per axon in the mutants. Thus, to maintain axonal function and integrity, ND6 mutants may be attempting to compensate with mitochondrial proliferation; yet, most of the mitochondria display abnormal morphology.
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