Dissecting the roles of JNK/AP-1 signalling in the regulation of redox homeostasis within neurons

Ageing is characterised by pathological increases in reactive oxygen species (ROS) termed oxidative stress, a known contributor to the pathology underlying many neurodegenerative diseases. Using Drosophila, we investigated the role of Fos and Jun, transcription factors of the redox-sensitive JNK/AP-1 signalling pathway, in the neuronal response to oxidative stress. Feeding larvae with Diethyl Maleate (DEM) depletes glutathione levels and induces mitochondrial oxidative stress. Synaptic overgrowth is observed following treatment with DEM that was shown to require the permissive action of Fos, Jun and the JNKKK, ASK1. Tandem Affinity Purification (TAP)-tagging and subsequent purification of neuronally expressed Fos and Jun led to the identification of the associated protein Punch. Punch is the Drosophila orthologue of GTP cyclohydrolase 1 (GTPCH1), haploinsufficient mutations of which are found in DOPA-responsive dystonia and Parkinson’s disease patients. Punch is the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin (BH4), an essential cofactor in the synthesis of dopamine and serotonin as well as a potent ROS scavenger. Treatment with DEM appeared to cause dissociation of Punch from AP-1 upon increased oxidative stress. It was found that Punch, Fos and Jun are localised to the motor neuron cell nuclei. Analysis of larval neuromuscular junctions revealed JNK/AP-1-mediated synaptic overgrowth in Punch heterozygote mutants. This is rescued when the oxidative stress or dopamine deficit is relieved. Punch overexpression also rescues synaptic overgrowth associated with elevated JNK/AP-1 signalling in the absence of oxidative stress, suggesting Punch constrains AP-1 action. Punch expression is able to reduce oxidative stress-synaptic overgrowth potentially via synthesis of BH4 and increased ROS scavenging, suggesting a link between standing and transcriptional defences against oxidative stress. Punch may serve as a neuroprotective agent with implications for patients suffering from oxidative stress-induced neurodegenerative diseases.