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

Abstract

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.