Investigating A Mitochondrial Role For Kynurenine 3-Monooxygenase

Kynurenine 3-monooxygenase (KMO) is an enzyme which operates within the kynurenine pathway (KP), catalysing the hydroxylation of kynurenine to produce 3- hydroxykynurenine (3-HK). KMO is a mitochondrial protein, localising at the outermembrane (MOM) due to a C-terminal domain, yet a mitochondrial role for KMO has not been described. However, the KMO encoding gene in Drosophila, cinnabar (cn), was identified in an RNAi screen as a modulator of both mitochondrial morphology and the recruitment of an autosomal recessive juvenile parkinsonism related protein, Parkin, to the MOM. Given the prospective use of KMO inhibition in the treatment of neurodegenerative disorders, of which mitochondrial dysfunction is a well-established aspect of pathology, mitochondrial functions of KMO warrant thorough investigation. This thesis interrogates the mitochondrial role of KMO in Drosophila and mammaliancell models. A range of mitochondrial-related phenotypes were identified in cinnabar deficient Drosophila, including elongated mitochondria and an increase in mitochondrial mass, yet a decrease in capacity of the mitochondrial electron transfer system (ETS) complex I. Supplementation of flies with 3-HK was not sufficient to reverse these phenotypes, indicating a novel role of KMO in mitochondrial quality control, independent from its enzymatic function in the KP. This implicates KMO in two mechanisms which maintain a healthy mitochondrial network - mitochondrial dynamics and mitophagy. The interlay between KMO and components of these pathways was explored by means of genetic epistasis experiments in Drosophila, accompanied by functional assays in Drosophila and human immortalised cells. Deletion of cinnabar in flies lacking Parkin or another juvenile parkinsonism related protein, PTEN-induced kinase 1 (PINK1), caused partial lethality, indicating a functional overlap between these mitophagyrelated proteins and KMO. Furthermore, overexpression of cinnabar or human KMO was sufficient to rescue locomoter defects in PINK1, but not Parkin deficient flies. Parkin interaction and localisation experiments implied a transient interaction between KMO and Parkin at the MOM, however a downstream function of Parkin MOM recruitment, the ubiquitation and degradation of Drosophila Mitofusin (MARF) was unaffected by cinnabar modulation. Overexpression of dynamin related protein 1 (Drp1) reversed locomoter defects in cinnabar deficient flies, indicating that a decrease in mitochondrial fission could be responsible for this phenotype. Furthermore, overexpression of KMO influenced post-translational modification of DRP1. These findings implicate KMO in regulation of mitochondrial dynamics and mitochondrial quality control.