PROJECT SUMMARY Preclinical and clinical data show elevated glutamatergic transmission and neuronal activity within the hippocampus during the early stages of schizophrenia development, and it is proposed that such elevations drive the progression of the disorder. However, the molecular and cellular mechanisms that give rise to schizophrenia- related elevations in hippocampal glutamatergic transmission are unknown. Recent human genetic data supports a connection between reduced DLG1 gene expression/function and schizophrenia. The DLG1 gene encodes the putative synaptic regulatory protein SAP97. However, a clear synaptic regulatory role for SAP97 has remained elusive. We have now, for the first time, identified a native synapse in the brain where SAP97 plays a critical and direct role in synaptic regulation. The long-term goal of our research is to understand how glutamatergic synapse dysfunction contributes to the development of psychiatric disorders. Our central hypothesis is that disruption of SAP97 function in dentate granule neurons (DG neurons) of the hippocampus produces pathological synaptic strengthening that contributes to schizophrenia etiology. Guided by strong preliminary data, we will pursue this hypothesis in three specific aims. In Aim 1, we will combine electrophysiological methods with brand new imaging tools to characterize pathway-specific roles of SAP97 in regulating glutamatergic neurotransmission and plasticity. In Aim 2, we will use a combination of newly generated antibodies, molecular biological and electrophysiological/imaging approaches to identify the roles of SAP97 subregions in governing SAP97’s subcellular localization and influence over glutamatergic synapse function in DG neurons. In Aim 3, we will deploy cutting-edge in vivo imaging tools in freely behaving animals in order to understand how hippocampal neuron activity is altered in rodents with compromised SAP97 function. The present proposal is innovative because it deploys new and powerful genetic, in vivo imaging, and behavioral analysis tools that will allow a precise characterization of a newly discovered role for SAP97 in regulating the function of native glutamatergic synapses in the hippocampus. The proposal is significant because it stands to identify synaptic pathology in a specific brain region that represents a common component in schizophrenia development. This proposal squarely meets the mission objectives of the NINDS given its focus on understanding glutamatergic synapse regulation and memory formation which are found to be disrupted in complex brain disorders.