Effects of Idebenone on the Mitochondrial Respiration of Neurons, Astrocytes, and Microglia

Abstract

Neuroinflammation and mitochondrial bioenergetic dysfunction are present in most neurodegenerative diseases. Microglia, the resident immune cells of the brain, release cytotoxic factors such as nitric oxide (NO), superoxide, and cytokines following proinflammatory activation. NO mediates mitochondrial respiratory inhibition in microglia and neighboring cells by competing with oxygen at complex IV of the electron transport chain. Additional modes of inhibition by NO include post-translational modifications to upstream complexes I and II. Idebenone is a clinically safe prodrug that, in its reduced form idebenol, can act in place of ubiquinol and donate electrons directly to complex III. This dissertation tested the overarching hypothesis that idebenone can support mitochondrial respiration in cells with sufficient quinone-reduction capacity to convert idebenone to idebenol, despite complex I and II impairment by NO. In astrocytes but not neurons, idebenone and two related quinones could rescue maximal oxygen consumption rate (OCR) when a complex I inhibitor was present. This difference between astrocytes and neurons was due to a disparity in cellular quinone-reduction capacity mediated by the expression of NADPH:quinone oxidoreductase 1 (NQO1). Astrocytes were sensitive to respiratory impairment by an NO donor or co-cultured proinflammatory microglia at a physiologically-relevant oxygen level and idebenone was able to partially reverse this impairment. Interestingly, microglia upregulated their quinone-reduction capacity following proinflammatory stimulation and idebenone was also able to partially reverse respiratory impairment in microglia following activation. Surprisingly, in contrast to astrocytes, NQO1 was not responsible for idebenone reduction in activated microglia. Biochemical isolation of the responsible enzyme identified inducible nitric oxide synthase (iNOS) among the few candidates common to three distinct fractionation approaches. Assays performed with recombinant iNOS revealed a novel idebenone reduction activity with exciting implications for future studies. This dissertation’s findings suggest that insufficient quinone-reduction capacity in diseased target cells may be a mechanistic reason for the failure of idebenone in clinical trials. These results support new strategic approaches for the use of idebenone and similar drugs to overcome mitochondrial bioenergetic dysfunction.