We are requesting this Administrative Supplement to allow our currently funded R01 project to expand into Alzheimer’s disease (AD) research. The currently funded project was not focused on AD, but recent discoveries in this field have revealed an opportunity that we are uniquely positioned to pursue by leveraging resources generated through the parental project. In the parental R01, we set out to characterize a new molecular mechanism that regulates signal transduction in the context of neurotransmission. More specifically, a protein we classified as a “paradoxical G protein regulator” (PGR) fine-tunes the responses triggered by G protein-coupled receptors (GPCRs), a large group of neurotransmitter receptors. A salient feature of GPCRs is that they are highly “druggable”— e.g., about a third of clinically used drugs target them, including numerous medications for neurological diseases. Recent evidence has now established a connection between this PGR and the progression of Alzheimer’s disease. We posit that further characterization of this connection would pave the way for new therapeutic avenues in AD by identifying new pharmacologically actionable pathways (like those mediated by GPCRs) in the molecular basis of this dementia. This would address a critical unmet medical need because therapeutic options in Alzheimer's disease are very limited. Incidentally, the same PGR we were investigating in our parental project was recently identified in large- scale (n>400 patients) brain proteomic studies as a protein that is downregulated in Alzheimer’s disease, starting from the asymptomatic phase of the disease. Moreover, loss of this PGR in patients was also found to correlate with age-dependent cognitive decline independently of AD status. These observations are exciting because dysregulated neurotransmission underlies cognitive decline in AD, but the specific alterations and mechanisms involved are not well understood. Our goal is to interrogate a causal relationship between loss of PGR and age-dependent cognitive decline, and to map the brain proteomic changes that underlie this relationship. We have gathered preliminary data suggesting that PGR knock-out mice have learning and memory defects when they are one year old (middle age), but not when they are young. Our data also suggest that these defects might be more pronounced in females than in males, an interesting feature that correlates with the higher prevalence of AD in women. Motivated by these compelling preliminary results, we now propose to expand our mouse behavioral studies by evaluating young, middle, and old age mice (4, 12, or 18 months, respectively). The experimental design will be powered to identify differences not only by age but also by sex. In parallel, we will use quantitative proteomics and bioinformatics to assess global proteomic changes in different brain regions across the experimental groups. We anticipate that this project will establish a previously unappreciated causa...