Investigating the role of histone deacetylase HDAC4 in long-term memory formation : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Genetics at Massey University, Manawatu, New Zealand

Abstract

Epigenetic mechanisms are emerging as master regulators of cognitive abilities such as learning and memory. It has been previously shown that the histone deacetylase HDAC4 plays a critical role in memory formation in both mammals and insects although the specific mechanisms through which it acts have not yet been elucidated. HDAC4 undergoes nucleocytoplasmic shuttling and, in neurons, it is largely cytoplasmic implying it may play both nuclear and non-nuclear functions. To identify upstream regulators and downstream targets of HDAC4, a genetic interaction screen was performed in the fruit fly Drosophila melanogaster, a powerful model system to study the genetic mechanisms of neurological disease. Twenty-nine genes were found to interact with HDAC4 suggesting they are part of the same molecular pathway. Functional network analysis revealed that many of the genes could be grouped into three biological categories comprising transcriptional factors, SUMOylation machinery enzymes and cytoskeletal regulators/interactors. Within the latter, Ankyrin2 was selected for further analysis as it is implicated in synaptic stability and in human intellectual disability. In addition HDAC4 harbours a conserved ankyrin binding domain. Immunohistochemical analyses showed widespread distribution of Ankyrin2 throughout the adult brain and coincident distribution with HDAC4 was observed in the axons of the mushroom body, a key structure for memory formation in flies. Both HDAC4 and Ankyrin2 were also found to regulate mushroom body development. RNAi-mediated depletion of Ankyrin2 in the adult brain impaired long-term memory in the courtship suppression assay, a model of associative memory and preliminary evidence of a physical association between HDAC4 and Ankyrin2 was also demonstrated. The genes identified in the screen provide new avenues for investigation of the mechanisms through which HDAC4 regulates memory formation and preliminary analyses suggest that interaction with the cytoskeletal adaptor Ankyrin2 may involve remodelling of the actin/spectrin cytoskeleton, phenomenon that underlies memory related processes like synaptic plasticity and neuronal excitability.