Leber hereditary optic neuropathy mtDNA mutations disrupt glutamate transport in cybrid cell lines

Leber hereditary optic neuropathy (LHON) is a maternally inherited form of retinal ganglion cell degeneration leading to optic atrophy which is caused by point mutations in the mitochondrial genome (mtDNA). Three pathogenic mutations (positions 11778/ND4, 3460/ND1 and 14484/ND6) account for the majority of LHON cases and they affect genes that encode for different subunits of mitochondrial complex I. Excitotoxic injury to retinal ganglion cells and the optic nerve has been previously hypothesized, especially given the high susceptibility of this neural cell type to glutamate toxicity. Osteosarcoma-derived cytoplasmic hybrids (cybrids) generated from six unrelated LHON patients, two cell lines for each pathogenic mutation, were compared with cybrids obtained from three healthy controls. Molecular and biochemical analyses showed that excitatory amino acid transporter 1 (EAAT1)/GLAST is the most active glutamate transporter in this cellular model. The glutamate uptake maximal velocity was significantly reduced in all LHON cybrids compared with control cybrids. This reduction was correlated in a mutation-specific fashion with the degree of mitochondrial production of reactive oxygen species, which is enhanced in LHON cybrids. Our findings support the hypothesis that the genetically determined mitochondrial dysfunction in LHON patients leads to impaired activity of the EAAT1 glutamate transporter. This observation is particularly relevant since EAAT1 is the major means of glutamate removal in the inner retina and this prevents retinal ganglion cells being damaged as a result of excitotoxicity.