Abstract GPR39 is a G-protein coupled receptor (GPCR) activated by zinc (Zn2+) and implicated in several neurodegenerative symptoms of Alzheimer’s disease (AD), including dyshomeostasis, oxidative stress, microtubule destabilization, and tau phosphorylation. In AD brains, Zn2+ binding to amyloid beta (Aβ) disrupts the zincergic GPR39 signaling cascade, leading to down regulation of critical neuroprotective pathways and poor cognitive function. Targeting GPR39 agonists could inform development of novel drugs to ameliorate AD symptoms, but we have no reliable, noninvasive methods to quantify GPR39 levels. This project will evaluate the first positron emission tomography (PET) radiotracer to accurately measure GPR39 in vivo in rodent and human brain tissues of AD. This PET imaging strategy will significantly advance our understanding of Zn2+ GPR39-mediated dyshomeostasis pathways in AD brains beyond, currently available through terminal ex vivo studies. PET imaging of GPR39 will provide a valuable tool to assess the real-time distribution, target engagement, and dose occupancy of novel GPR39 agonist drugs. Our group has developed [11C]TM-N1324, the first brain-penetrant, PET radiotracer for imaging GPR39 and tested it in rodents. The proposed proof-of-concept R21 project is designed to establish the efficacy of two exciting approaches. First, we will investigate the imaging utility of [11C]TM-N1324 in two rodent models and compare the results in (a) mice that overexpress or do not express GPR39 along age-matched healthy controls (n=8/sex/group); and (b) Aβ-overexpressing APP/PS1 transgenic and age-matched wildtype littermates (n=8/sex/group). We will correlate the PET outcome with associated GPR39 levels via western blot and Aβ levels via immunohistochemical analyses. Second, we will characterize and evaluate binding and pharmacokinetic parameters of [11C]TM-N1324 in the Aβ-overexpressing APP/PS1 transgenic and age- matched wildtype mice and postmortem human AD and age-matched cognitively normal brain tissues (n=8/sex/group) using biodistribution, autoradiography, and metabolite studies. The proposed project will be the first to systematically examine in vivo imaging of GPR39 in rodent models of AD and validate in postmortem human brain AD tissues. Results will inform future imaging studies examining GPR39 modulations in AD pathogenesis and advance the therapeutic potential of GPR39 agonists to ameliorate AD symptoms.