Unique patterns of synaptic connectivity between neurons, and the differential strength of those synapses, are fundamental to the information processing capability of the brain. Synaptic strength is determine by the number, composition, and post-translational modifications of post-synaptic AMPA receptors (AMPARs). These features of AMPARs are regulated by a host of second messenger pathways, scaffolding proteins, and trafficking proteins in post-synaptic densities (PSDs). For synapses that display primarily postsynaptic plasticity, the proteins responsible for regulating AMPAR surface expression are thought to be shared across glutamatergic PSDs. Thus, it remains unknown whether fundamental differences in synaptic strength between synapses exist due to unique protein signatures of individual PSDs. Neuron-specific genes (NSG1-3) encode single transmembrane proteins involved in the secretory trafficking of multiple scaffold and signaling proteins, including postsynaptic AMPARs. Studies in cultured cells as well as acute hippocampal slice preparations have established that disrupting NSG1-3 function independently causes severe alterations in basal synaptic activity and plasticity. Interestingly, our published and preliminary evidence show that NSG1 and NSG2 chronically reside within a subset of synapses in excitatory hippocampal neurons. In addition, our data show that knockout (KO) of these proteins differentially affects network function and induces behavioral deficits. This study will be significant because it will identify whether multiple members of the NSG family are restricted to a unique subpopulation of excitatory synapses, and confer unique functional properties via the promotion of AMPAR surface expression. We will use a combination of validated and novel techniques to address these important questions in three specific aims:
Specific Aim 1. Determine whether NSG proteins define unique population(s) of excitatory synapses. We will use in vitro time lapse, and ex vivo imaging to determine whether NSG1 and NSG2 are specifically targeted to a subset of hippocampal synapses or trafficked between them.
Specific Aim 2. Determine whether individual NSG proteins differentially affect synaptic function. Using physiological recordings and glutamate uncaging we will determine whether NSG1/2 are differentially involved in promoting surface AMPAR expression during basal or activity-dependent conditions.
Specific Aim 3 : Determine whether KO of NSG proteins leads to specific, dissociable behavioral deficits. Using established and novel behavioral tests in single and double KO mice we will determine whether NSG1/2 proteins play unique or overlapping roles in shaping motor, affective, and cognitive function in live animals.
While synapses within a single cell have long been known to be independently modified to allow the encoding of new information, the elements that allow these processes are thought to be relatively uniform across synapses. Thus, a fundamental gap exists in our knowledge as to whether unique protein signatures at individual synapses create functional differences in activity. This study seeks to determine whether proteins encoded by members of the neuron-specific gene family confer distinct functional properties to a subset of excitatory synapses, which could alter how neural networks respond to the flow of information during experience-dependent plasticity.
