PROJECT SUMMARY/ABSTRACT Epilepsy is one of the most common neurological disorders. Approximately one third of patients are drug- resistant, underscoring the need for alternative therapies. One of the biggest challenges in developing disease- modifying therapies is the limited understanding of the underlying mechanisms behind the initiation and propagation of seizures. We propose to develop a novel, translational viral vector that targets dentate gyrus (DG) granule cells, which have been shown to act as a seizure gate. We will make use of a promoter specific to DG granule cells, Prospero-related homeobox 1 (Prox-1), to selectively express a calcium indicator in these cells. We expect that the project will provide an invaluable tool to study behaviors of the seizure gate of DG granule cell. We will first develop the viral vector in the US lab (Aim 1). We will then examine activity of DG granule cells through fiber photometry in freely behaving mice during spontaneously recurring seizures in the intrahippocampal kainic acid (IHKA) model of temporal lobe epilepsy in Japan lab (Aim 2). We believe that this project can help us better understand the mechanisms at play during seizures, as well as improve treatment in patients with drug-resistant epilepsy. In addition to its utility in epilepsy, a Prox1-driven expression vector could have potential applications in Alzheimer’s disease and neuropsychiatric disorders as well as in lymphatic and cancer research. This international collaboration is a unique combination of the expertise of the two groups, namely development of molecular tools and applications of such new technologies in an animal model of epilepsy. This effort will significantly enhance the goals of the parent R21 award.