Previous studies of LHON fibroblasts have shown significant complex I impairment [3
]. These results attest that, although the clinical expression is mostly limited to retinal ganglion cells, the OXPHOS defect is probably more generalized as has also been shown by Barbiroli et al. [16
] who found defective energy metabolism in the muscle and brain of LHON patients by using 31P-MRS in vivo
. Thus, fibroblasts represent an interesting model to explore the LHON-associated OXPHOS defect and to research molecules able to compensate the defect.
This study demonstrates the impact of idebenone on respiratory chain activity in fibroblasts from nine LHON patients. Indeed, idebenone increased complex I enzymatic activity in these fibroblasts by 42% compared to controls (p
= 0.002). This idebenone effect on complex I enzymatic activity was found to be shared by fibroblasts carrying the same m.11778 G > A mutation in 7 different genetic backgrounds (inter-individual variability); this idebenone effect was also found to be shared by fibroblasts carrying the 3 main LHON mutations (inter-mutation variability). The action of idebenone was not due to an increase either of complex I or of the mitochondrial mass. Indeed, two subunits of complex I, i.e. NDUFA9 and NDUFB8 in western blot, and the citrate synthase activity were not affected by the treatment (Additional file 1
: Figure S1).
Various types of action have been ascribed to idebenone in the literature. Experiments with idebenone have led to some conflicting results. Thus, the oral administration of idebenone for 3 days has been reported to stimulate mitochondrial respiration linked to complexes I and II in the rat brain [11
]. In contrast, idebenone has been described as a weak substrate for complex I activity [17
]. Idebenone has also been shown to bind to a rotenone-insensitive site, i.e. the non-physiological iron-sulphur N2 site, thus leading to higher NADH oxidation but without complete reduction [19
]. However, in these latter observations, idebenone was added in the incubation medium directly only during the experiments. The administration of idebenone over a period of 3 days [11
] may have led to better metabolization of the drug, allowing it to enter cells and activate signalling pathways such as MAP kinase [10
In our study, the polarographic investigation of the effect of idebenone on mitochondrial activity in fibroblasts from six LHON patients led to contradictory results: a 16% increase in complex I-driven respiration in three cases, and a 42% decrease in the other three cases. The increase observed might correspond to the idebenone-induced improvement similar to that reported in intact rat brain mitochondria [11
], whereas the opposite response might be explained by an increase of electron escape du to enhanced idebenone fixation at a non-specific site [19
The variability of the effect of idebenone on complex I activity may be linked to the genetic variability of the LHON patients. However, the number of patients in our study was too small to reveal the possible influence of the haplogroup, the mutation type or the mutation level on the treatment. A larger cohort of LHON patients would be necessary to assess the influence of these factors.
To our knowledge, this is the first demonstration that idebenone directly influences mitochondrial respiratory chain activity in LHON patients. Idebenone increases the enzymatic activity of complex I and acts on the complex I-driven respiration rate, modifying the balance between the electron fluxes provided by complexes I and II. It is tempting to speculate that the variability of the biological response to idebenone in fibroblasts from LHON patients may explain the discordant results observed in some clinical studies [6
]. Our results suggest that only a fraction of LHON patients may benefit from the improved respiration afforded by idebenone. If this finding is substantiated by a more extensive study, a preliminary investigation of the impact of idebenone on fibroblasts from LHON patients may be useful to select potentially "good responders" to the treatment.