AbstractLeigh syndrome is an untreatable, early-onset mitochondrial disease characterised by psychomotor regression and the presence of symmetrical brain lesions. The mechanisms underlying neuronal dysfunction in Leigh syndrome remain elusive. Mitochondria communicate their functional status to the nucleus through mitochondrial retrograde signalling. The transcriptional responses brought about by these signals can have broad effects on cellular function. In this thesis, I have used Drosophila and mouse models of Leigh syndrome to explore the role of mitochondrial retrograde signalling in neuronal dysfunction and the development of neurological symptoms.
Knockdown of the mitochondrial complex I subunit ND-75 in Drosophila neurons using RNAi recapitulated the severe neurological and lifespan phenotypes characteristic of Leigh syndrome. Use of a weaker ND-75 RNAi produced flies with a milder phenotype, allowing me to use ND-75 RNAis of different strengths to identify mitochondrial retrograde signals that are specific to the development of severe neurological symptoms.
Knockdown of ND-75 in Drosophila neurons using both the stronger and weaker RNAi led to significant decreases in levels of putative mitochondrial retrograde signals ATP and ROS, inferring that these molecules do not drive symptom development in Leigh syndrome. Mitochondria are physically and functionally coupled to the endoplasmic reticulum (ER). Expression of the stronger, but not weaker ND-75 RNAi in Drosophila neurons led to a decrease in the number of ER-mitochondria contacts, and activation of ER unfolded protein response (UPR). Knockdown of the UPR transcription factors ATF4, XBP1 and ATF6 improved symptoms in flies pan-neuronally expressing the strong ND-75 RNAi. UPR activation was not conserved in brains of Ndufs4 knockout mice, however, increased ATF4 expression was observed within the forebrain, cerebellum, and brainstem. Metabolomic analysis of adult fly heads revealed that ATF4 regulates several of the metabolic changes brought about neuronal-specific ND-75 knockdown.
Taken together, my findings reveal that the ER UPR is a detrimental mitochondrial retrograde signalling pathway in neurons of a Drosophila model of Leigh syndrome. The discovery that ATF4 activation is conserved in a mouse model of Leigh syndrome, reveals ATF4 as a potential therapeutic target for mitochondrial disease.
|Date of Award
|1 Jul 2021
|Joseph Bateman (Supervisor) & Christopher Miller (Supervisor)