An epigenetic role for Tip60 in synaptic plasticity and neurodegenerative diseases

Jessica B. Sarthi

doi:10.17918/etd-4213

Age-associated cognitive decline and neurodegenerative disorders such as Alzheimer's disease (AD) are associated with misregulation of synaptic plasticity linked genes; however the mechanisms underlying decline of such gene control during aging are unknown. Histone acetylation of chromatin promotes dynamic transcriptional responses in neurons that influence neuroplasticity critical for cognitive ability. Accordingly, aberrant changes to histone acetylation patterns in the aging brain epigenome are linked to memory loss. It is therefore critical to identify and study the histone acetyltransferases (HAT) that create such marks. One such promising candidate is Tip60, a HAT important for various cellular processes and also implicated in AD and shown by our laboratory to be critical in regulating neuronal processes linked to cognition. To explore a direct role for Tip60 in synaptic plasticity, here we explore the consequences of misregulating Tip60 HAT activity in the Drosophila neuromuscular junction (NMJ). The Drosophila NMJ is an extremely well characterized, highly tractable and valuable tool to study synaptic plasticity. In addition, many of the pathways present at the Drosophila NMJ are well conserved and homologous to the mammalian CNS. We show that the HAT dTip60 is concentrated both pre- and post-synaptically within the NMJ. Presynaptic targeted reduction of dTip60 HAT activity significantly increases synaptic bouton number that specifically affects type Is boutons while postsynaptic reduction results in significant loss of these boutons. The excess boutons demonstrate defects in neurotransmission function. Analysis using immunohistochemical staining to the MAP, futsch reveals a significant increase in the rearrangement of microtubule loop architecture that is required for bouton division. Our results are the first to demonstrate a causative role for the HAT dTip60 in the control of synaptic plasticity that is achieved, at least in part, via regulation of the synaptic microtubule cytoskeleton. We also show its post- synaptic role in the muscles and its function in retrograde signaling in addition to anterograde mechanisms. We show that postsynaptic loss of Tip60 HAT activity affects DLG localization, leads to decrease in GluRIIC subunit localization thus suggesting roles in activity dependent mechanisms. We also demonstrate its role in regulating genes involved in activity dependent synaptic plasticity and wingless pathway. Our ChIP-qPCR data suggests regulation of these genes via acetylation of learning and memory marks, H3K9, H4K12 and H4K16. We also report the functional interaction between HAT deficient Tip60 and hAPP at the NMJ, pre- and post-synaptically via the intracellular domain of APP (AICD), the molecule implicated in AD. Presynaptic expression of APP/Tip60 double mutants cause drastic increases in bouton numbers, and decrease in active zone synaptic function marker bruchpilot suggesting defects in neurotransmission. Conversely, postsynaptic expression of the APP/Tip60 double mutants leads to marked decrease in bouton numbers and absence of GluRIIC and decrease in GluRIIB and GluRIIA receptor subunits suggesting defects in signaling mechanisms. These findings have implications for dTip60 HAT dependant epigenetic mechanisms in neurodevelopment and neurodenerative diseases.

An epigenetic role for Tip60 in synaptic plasticity and neurodegenerative diseases

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An epigenetic role for Tip60 in synaptic plasticity and neurodegenerative diseases

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