| Summary: | Ischaemic heart disease (IHD) is a leading cause of morbidity and mortality worldwide. IHD occurs when blood-flow to the heart is occluded, depriving the tissue of oxygen and glucose. Paradoxically, restoration of blood supply can cause further damage, termed reperfusion injury. The resultant oxidative stress (OS) causes multiple pathologies including profound mitochondrial damage. Mitochondrial function is closely linked to their dynamics; continual fission and fusion facilitate mitochondrial-trafficking and ATP-distribution. Mitochondrial fission is mediated by the dynamin-like GTPase Drp1, which is recruited to mitochondria by its receptor Mff. Over-expression of Drp1 or Mff results in highly fragmented, dysfunctional mitochondria. The regulation of Drp1 by post-translational modifications (PTMs) is well-established, whereas regulation of Mff PTMs is less defined. The aim of this PhD was to determine if, and how, manipulation of Mff PTMs can protect against mitochondrial damage incurred by IHD. I show that OS causes phosphorylation of Mff, which promotes its SUMOylation. Mff SUMOylation subsequently leads to its ubiquitination and degradation. We propose that this is a protective response to limit mitochondrial fragmentation. Non-SUMOylatable Mff mutants have significantly reduced ubiquitination and slower turnover, indicating the activity of a SUMO-targeted ubiquitin ligase (STUbL). I demonstrate that Mff is ubiquitinated by at least three ligases, and test the hypothesis that Parkin, a known ligase of Mff, could be recruited via a SUMO-dependent mechanism. While Parkin interacts non-covalently with SUMO, its recruitment to Mff is SUMO-independent. I then show that Parkin ubiquitinates Mff at its SUMOylatable lysine, whereas Fbxo7, a component of the Skp1-Cul1-Fbx ligase complex, mediates ubiquitination of Mff at a site independent of both SUMO and Parkin. Given its dependence on OS, we propose that specific manipulation of the novel STUbL pathway could be a viable clinical intervention in recovery from IHD. This project therefore provides a solid foundation for further study.
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