The Dynamic Neuron: Cellular Mechanisms Maintaining Neural Activity and the Consequences of Phosphatidylinositol 3,5-Bisphosphate Biosynthesis on Synapse Function.

Proper trafficking of neurotransmitter receptors through endosomal compartments is important for the development, maintenance and plasticity of neural circuits in the brain. However, little is known about the signaling cues orchestrating these intracellular events. Phosphoinositide lipids are excellent candidates for these roles because they regulate multiple types of membrane trafficking, but their specific functions are poorly understood. There are seven unique phosphoinositide lipids that are synthesized from the structural lipid, phosphatidylinositol, by lipid kinases and phosphatases. They function in part through the recruitment of complex protein machines to specific membrane subdomains. That multiple neurological disorders are linked to mutations in phosphoinositide lipid-related genes suggests that they may be particularly important in neurons. A major goal of this dissertation was to identify novel roles for phosphoinositide lipids, as well as identify cellular mechanisms involved in maintaining normal neural function. The specific hypothesis tested was that the low abundant phosphoinositide lipid, phosphatidylinositol 3,5-bisphosphate, is important for synapse function. Notably, Vac14, a regulator of phosphatidylinositol 3,5-bisphosphate biosynthesis is enriched at synapses, which raised the possibility that this lipid impacts synaptic function and plasticity. Additionally, this dissertation examined the roles of phosphatidylinositol 3,5-bisphosphate in determining the fate of internalized AMPA-type glutamate receptors following stimulus-induced internalization. Evidence presented shows that phosphatidylinositol 3,5-bisphosphate acts as a negative regulator of synapse strength. These data reveal that phosphatidylinositol 3,5-bisphosphate synthesis is required for maintenance of homeostatic synaptic weakening and suggest that phosphatidylinositol 3,5-bisphosphate impacts synapse function by altering AMPA-type glutamate receptor trafficking. These data identify the activity-dependent synthesis of phosphatidylinositol 3,5-bisphosphate as a novel mechanism for regulating synapse strength and suggest that synaptic dysfunction may contribute to the pathogenesis of neurological diseases arising from loss of dynamic phosphatidylinositol 3,5-bisphosphate regulation. In addition, this dissertation examined the role of intrinsic neuronal excitability in the homeostatic response to AMPA-receptor blockade. The ability to adapt to changes in levels of activity is critical for stability of neural networks. These studies identify that relatively brief blockade of AMPA-type glutamate receptor activity triggers at least three forms of homeostatic compensation: postsynaptic strengthening, increased presynaptic probability of release and increased intrinsic excitability.PHDNeuroscienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108757/1/ajmneuro_1.pd

Shared Visual Attention and Memory Systems in the Drosophila Brain

Background: Selective attention and memory seem to be related in human experience. This appears to be the case as well in simple model organisms such as the fly Drosophila melanogaster. Mutations affecting olfactory and visual memory formation in Drosophila, such as in dunce and rutabaga, also affect short-term visual processes relevant to selective attention. In particular, increased optomotor responsiveness appears to be predictive of visual attention defects in these mutants. Methodology/Principal Findings: To further explore the possible overlap between memory and visual attention systems in the fly brain, we screened a panel of 36 olfactory long term memory (LTM) mutants for visual attention-like defects using an optomotor maze paradigm. Three of these mutants yielded high dunce-like optomotor responsiveness. We characterized these three strains by examining their visual distraction in the maze, their visual learning capabilities, and their brain activity responses to visual novelty. We found that one of these mutants, D0067, was almost completely identical to dunce for all measures, while another, D0264, was more like wild type. Exploiting the fact that the LTM mutants are also Gal4 enhancer traps, we explored the sufficiency for the cells subserved by these elements to rescue dunce attention defects and found overlap at the level of the mushroom bodies. Finally, we demonstrate that control of synaptic function in these Gal4 expressing cells specifically modulates a 20-30 Hz local field potential associated with attention-like effects in the fly brain. Conclusions/Significance: Our study uncovers genetic and neuroanatomical systems in the fly brain affecting both visual attention and odor memory phenotypes. A common component to these systems appears to be the mushroom bodies, brain structures which have been traditionally associated with odor learning but which we propose might be also involved in generating oscillatory brain activity required for attention-like processes in the fly brain

Modulation of synaptic function by VAC14, a protein that regulates the phosphoinositides PI(3,5)P 2 and PI(5)P

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/102191/1/embj2012200.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/102191/2/embj2012200-sup-0001.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/102191/3/embj2012200-reviewer_comments.pd

Parkinson Sac Domain Mutation in Synaptojanin 1 Impairs Clathrin Uncoating at Synapses and Triggers Dystrophic Changes in Dopaminergic Axons

Synaptojanin 1 (SJ1) is a major presynaptic phosphatase that couples synaptic vesicle endocytosis to the dephosphorylation of PI(4,5)P2, a reaction needed for the shedding of endocytic factors from their membranes. While the role of SJ1's 5-phosphatase module in this process is well recognized, the contribution of its Sac phosphatase domain, whose preferred substrate is PI4P, remains unclear. Recently a homozygous mutation in its Sac domain was identified in early-onset parkinsonism patients. We show that mice carrying this mutation developed neurological manifestations similar to those of human patients. Synapses of these mice displayed endocytic defects and a striking accumulation of clathrin-coated intermediates, strongly implicating Sac domain's activity in endocytic protein dynamics. Mutant brains had elevated auxilin (PARK19) and parkin (PARK2) levels. Moreover, dystrophic axonal terminal changes were selectively observed in dopaminergic axons in the dorsal striatum. These results strengthen evidence for a link between synaptic endocytic dysfunction and Parkinson's disease