Abstract 21073: Succinate Dehydrogenase Assembly Factor 1 Regulates Mitochondrial Reserve Respiratory Capacity

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Under normal conditions, the myocyte runs on a fraction of its mitochondrial bioenergetics’ capacity, where the difference between the maximum respiratory capacity and basal respiratory capacity constitutes the reserve respiratory capacity (RRC). We have shown that the RRC allows the cell to respond to an increase in energy demand by promptly increasing supply, avoiding an “ATP crisis”. Our objective is to understand the source and mechanism of regulation of the RRC, which would allow us to augment it during increases in energy demand. Our data show that activation of AMP-activated kinase (AMPK) by AICAR or adiponectin, increases RRC (1.34-fold to 1.9-fold) in a substrate and Sirt3-dependent fashion, whereas, the level of the RRC positively correlates with cell survival. We have identified complex II (cII, or succinate dehydrogenase complex) as the source of RRC, as its inhibition completely abrogates the reserve with no significant effect on basal oxygen consumption rates (OCR). In corroboration, disassembly of cII via knockdown of succinate dehydrogenase assembly factor 1 (Sdhaf1; mutated in infantile mitochondrial disease), also completely abrogates RRC, as higher doses induce cell death and reactive oxygen species (ROS) production. Conversely, over-expression of Sdhaf1 increases RRC (1.2-1.5-fold) during normoxia, and after non-lethal hypoxia (2 to 6-fold). Moreover, a unique acetylation-resistant Sdhaf1 mutant, has a more robust effect on RRC (4 to10-fold). We were further able to confirm that overexpression of Sdhaf1 or Sirt3 in cardiac myocytes reduced hypoxia-induced disassembly of cII. In summary, our data suggest that the RRC increases the survivability of myocytes after non-lethal periods of hypoxia, and can be manipulated by factors that regulate Sdhaf1 and assembly of cII.

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