Novel insights into the heterogeneous manifestations of mitochondrial dysfunction

Abstract

Mitochondria are organelles that populate eukaryotic cells. Genetic defects in mitochondrial components are cause of a group of heterogeneous disorders known collectively as mitochondrial diseases. The study of these patients has helped our understanding on how cell respond to mitochondrial dysfunction and contributed greatly to our knowledge about this organelle. Several lines of evidence indicate that different mitochondrial defects activate a similar response at the cell level, however, its consequences are still hard to predict and we don’t know why it manifest in such an heterogenous way. As a result, therapeutic options are scarce for most patients. Defining the molecular basis of the pathogenesis of these disorders is therefore a current challenge for biomedical research. Here, we studied the effects of mitochondrial dysfunction by two complementary approaches. Firstly, we compare the effects on cell growth and respiratory complexes of two known complex I inhibitors, rotenone and diphenyleneiodonium (DPI). We find that, despite both block complex I-dependent respiration and suppressed cell growth in galactose, DPI causes a rapid degradation of complex I and complex Icontaining supercomplexes. We show that DPI reacts irreversibly with cell flavin cofactors FMN and FAD, destabilizing complex I N-module and halting complex I assembly at its very last step. Accordingly, cells grown in riboflavin deficient media or knockout for the mitochondria FAD carrier SLC25A32 show decrease levels of assembled complex I and complex I-dependent respiration. Secondly, we study the phenotypic consequences of alterations in ATP synthase oligomerization in vivo by knocking out two subunits of the FO domain of complex V, Atp5mk and Atp5mj. Atp5mk -/- mice present alteration in fasting glucose levels and decreased exercise capacity. Atp5mj -/- mice develop a progressive neuromyopathy whose manifestation (ataxia, ophthalmopathies and cerebellar atrophy) resemble the clinical syndrome observed in some patient with mutations in MT-ATP6 and MT-ATP8 genes. In Atp5mj -/- mitochondria we observe marked ultrastructure abnormalities and reduced oxygen consumption. Despite the loss of oligomeric complex V, ATPase activity is unaffected, suggesting that complex V has important functions beyond ADP phosphorylation. Moreover, even though Oma1 activity is elevated in Atp5mj -/- mice, deleting Oma1 in Atp5mj -/- mice does not result in relevant phenotypic changes. Collectively, our results reveal a complex interplay between mitochondrial flavin cofactor pools, complex I assembly, ATP synthase oligomerization and mitochondria morphology