Project Summary Memory impairment is a defining characteristic of many neuropsychiatric disorders, however, an understanding of the complex neural mechanisms regulating hippocampal memory formation remains elusive. The sex steroid 17β-estradiol (E2) is a powerful modulator of hippocampal plasticity and memory in both males and females, however the neural mechanisms through which this occurs are poorly understood. Therefore, the long-term goal of our research is to pinpoint the neural mechanisms through which E2 regulates hippocampal memory consolidation in males and females. The overall objectives of this application are to determine the mechanisms through which E2 regulates activity of the ubiquitin proteasome system (UPS) and the extent to which proteasomal protein degradation contributes to estrogenic regulation of memory consolidation and hippocampal dendritic spine density in both sexes. Although UPS-mediated protein degradation in the hippocampus is essential for synaptic remodeling and memory consolidation, the role that UPS activity plays in mediating E2’s modulatory effects on memory formation and spine remodeling in either sex remains completely unexplored. Our central hypothesis is that UPS-mediated protein degradation in the DH is triggered by E2 acting at estrogen and glutamate receptors and is a principal mechanism through which E2 promotes memory consolidation and CA1 spine density. This hypothesis is based on previous work suggesting overlapping mechanisms through which E2 and UPS activity contribute to hippocampal plasticity and memory. Our central hypothesis will be evaluated in the following three specific aims: 1) determine the cellular mechanisms through which E2 activates hippocampal UPS activity, 2) identify protein targets of E2-induced UPS activation across subcellular compartments, 3) establish a key role for proteasome activity in E2-induced facilitation of memory consolidation and CA1 spine density. Regionally- and temporally- specific pharmacological manipulations will be used to establish receptor mechanisms underlying E2-UPS interactions in the dorsal hippocampus, as well as the structural and behavioral outcomes of these interactions. Cutting edge ubiquitin-specific mass spectrometry-based proteomics will also be used to identify novel proteins targeted by E2 for degradation. This work is innovative in that it represents a fundamental shift from a conventional focus on protein synthesis as a primary contributor to estrogenic memory modulation to a novel consideration of protein degradation as an equal and opposite counterpart necessary for estrogenic regulation of hippocampal plasticity and memory. This contribution is significant because it will provide essential foundational knowledge about the mechanisms through which E2 regulates memory consolidation in both sexes, which could lead to the development of novel sex-specific treatments to address the memory dysfunction observed in numerous mental disorders.