OPA1 mutations associated with dominant optic atrophy impair oxidative phosphorylation and mitochondrial fusion

Abstract

Dominant optic atrophy (DOA) is characterized by retinal ganglion cell degeneration leading to optic neuropathy. A subset of DOA is caused by mutations in the OPA1 gene, encoding for a dynamin-related GTPase required for mitochondrial fusion. The functional consequences of OPA1 mutations in DOA patients are still poorly understood. This study investigated the effect of five different OPA1 pathogenic mutations on the energetic efficiency and mitochondrial network dynamics of skin fibroblasts from patients. Although DOA fibroblasts maintained their ATP levels and grew in galactose medium, i.e. under forced oxidative metabolism, a significant impairment in mitochondrial ATP synthesis driven by complex I substrates was found. Furthermore, balloon-like structures in the mitochondrial reticulum were observed in galactose medium and mitochondrial fusion was completely inhibited in about 50% of DOA fibroblasts, but not in control cells. Respiratory complex assembly and the expression level of complex I subunits were similar in control and DOA fibroblasts. Co-immunoprecipitation experiments revealed that OPA1 directly interacts with subunits of complexes I, II and III, but not IV and with apoptosis inducing factor. The results disclose a novel link between OPA1, apoptosis inducing factor and the respiratory complexes that may shed some light on the pathogenic mechanism of DOA.

Syndromic parkinsonism and dementia associated with OPA1 missense mutations

“…A similar, albeit less severe, pattern was observed for the fibroblasts with the p.A495V mutation (see Fig 6C). These findings resemble those previously reported in fibroblasts carrying different OPA1 mutations predicted to cause haploinsufficiency 31. Interestingly, Western blot analysis revealed a decreased amount of OPA1 protein in fibroblast cell lines from both families, suggesting that these missense mutations were also leading to haploinsufficiency (Fig 7A, B).…”

“…We and others have previously shown this feature in fibroblasts carrying OPA1 mutations leading to haploinsufficiency linked to nonsyndromic DOA 31, 44. Western blot analysis revealed the somewhat surprising observation that, as also reported for DOA‐associated nonsense mutations, both these OPA1 missense mutations led to decreased OPA1 protein amount, suggesting haploinsufficiency.…”

“…After addition of 10μM oligomycin, the chemiluminescence signal was calibrated with an internal ATP standard. Data were normalized for citrate synthase activity 31, 33. The ATP monitoring kit, digitonin, P 1 ,P 5 ‐di(adenosine‐5′) pentaphosphate, oligomycin, rotenone, ADP, and all the substrates of the respiratory complexes were from Sigma‐Aldrich.…”

Stem cell modeling of mitochondrial parkinsonism reveals key functions of OPA1

“…Instead, our data suggest that a late defect in complex I may account for the reduction of electron transport across the respiratory chain as evidenced by the decreased maximal respiration in mutant neurons and the reduction of complex I levels and activity. Functional changes in complex I were previously reported in fibroblasts with OPA1 haploinsufficiency18 or the p.G488R mutation9 and in models of acute OPA1 depletion, where respiratory efficiency was impaired when mitochondria were energized specifically with the complex I substrates, glutamate/malate 17. Therefore, in human iPSC‐derived neurons, OPA1 levels are important for the maintenance of oxidative phosphorylation, at least partly, by regulating the stability of complex I.…”

Mitochondrial Ca2+ uptake correlates with the severity of the symptoms in autosomal dominant optic atrophy

“…Nevertheless, the protein is expressed in all examined human tissues, explaining the accidental development of the so-called 'ADOA +' forms characterized by the association of the blindness with various neuromuscular disorders [17][18][19] or hearing loss [20,21]. The vulnerability of retinal ganglion cells and that of spiral ganglion cells in the inner ear [22] has been attributed to the impairment of ATP production as observed in fibroblasts [23,24] or skeletal muscle [25]. Since retinal ganglion cells are unique among neurons in that the are exposed to direct sunlight, the ensuing oxidative stress may potentiate the consequences of Opa1 dysfunction, leading to apoptosis of these cells [13].…”