The role and regulation of class IIa HDACs in neuronal antioxidant responses

Reactive oxygen species (ROS) are highly reactive signalling molecules, produced naturally as by-products of oxidative metabolism, which can lead to a state of oxidative stress when in excess. ROS can be cytotoxic to neurons and high levels are linked to many neurodegenerative diseases. Neurons counter excessive ROS in part through transcriptional upregulation of antioxidant enzymes. Class IIa histone deacetylases (HDACs) are transcriptional co-regulators that can either repress or activate gene transcription. They repress MADS box (MEF2) transcription factor mediated gene expression by promoting chromatin compaction and activate forkhead (FOXO) transcription factors through deacetylation. Class IIa HDACs are regulated by signal-dependent nucleocytoplasmic shuttling whereby increased neuronal firing triggers their nuclear export. Whilst these HDACs are linked to both neuroprotection and neurodegeneration, little is known about their activity and regulation under oxidative stress. This thesis aimed to investigate class IIa HDAC regulation under oxidative stress conditions. Oxidative stress was imposed using either diethyl maleate (DEM) to deplete cellular glutathione levels or paraquat which affects the mitochondrial electron transport chain. Work here shows that both HDAC4 and 5 translocate to the nucleus of rat cortical neurons under DEM-induced oxidative stress, whereas paraquat has different effects on HDAC4/5 localisation. Furthermore, HDAC5 showed a dephosphorylation, essential for its nuclear import. Putative FOXO target genes Tpm2 and Spp1 were induced in neurons during moderate oxidative stress, suggesting that ROS-induced nuclear imports of HDAC4/5 promote deacetylation and activation of FOXOs. Transgenic flies expressing human HDAC5 or a constitutively nuclear HDAC5 mutant, raised on DEM food, displayed strong increases in Drosophila Gadd45 gene mRNA levels, and in silico approaches revealed Gadd45 as a putative FOXO target. Moreover, transgenic flies showed a possible translocation of expressed HDAC5-GFP to the nucleus of Drosophila neuronal cells. Therefore, the proposed mechanism occurring during low oxidative stress may be conserved and exploited therapeutically.