PROJECT SUMMARY/ABSTRACT The latent period after a severe brain insult such as traumatic brain injury or status epilepticus, and before the onset of spontaneous recurrent seizures, is characterized by changes in adult hippocampal neurogenesis. Profound morphological changes, including hilar ectopic granule cells and abnormal dendritic development (e.g., hilar basal dendrites) is observed, therefore raising the question whether seizure- induced neurogenesis is epileptogenic. Our past work of ablating adult neurogenesis before or after acute seizures has shown a pro- epileptic role of new neurons, however existing ablation strategies in animal models have all suffered from an inability to decipher the mechanisms that promote aberrant adult-born granule cells (abGCs) because the cells are removed from the circuit. In this supplement, we will capitalize on recently published work from our laboratory demonstrating that activity in immature abGCs regulates Ca2+ and gene expression which is necessary and sufficient for the production of aberrant abGCs and disruption of the hippocampal circuitry leading to epilepsy. We propose work to determine the mechanisms that promote aberrant neurogenesis, focusing on the genes and signaling pathways that drive aberrant abGCs as well as identifying the neuronal inputs to the aberrant abGCs. In this Diversity Supplement, we will perform new experiments to expand the scope of Aim 1 of the parent R01 application, which is to determine the function of previously identified hM4Di-identified genes in aberrant neurogenesis and seizure. The proposed project will use human embryonic stem cells (ESCs) and 3D neural organoids to examine the role of TIMP3, a tissue inhibitor of matrix metalloproteinase, by using a small molecule to knockdown its expression. These studies are expected to provide a greater understanding of the mechanisms that promote aberrant neurogenesis, which may offer new strategies to specifically target abnormal new neurons while sparing healthy ones. These studies would be broadly impactful in a variety of neurological disorders including epilepsy.