PROJECT SUMMARY/ABSTRACT The efficacy of memory storage is determined by the delicate balance between excitatory and inhibitory synaptic strength and connectivity (E/I balance). The goal of this proposal is to understand how the disruption of this balance in cortical neurons leads to memory loss in mouse models of Alzheimer's disease (AD). Immunohistology of postmortem brains from the AD patients shows that the reduction in excitatory synapse density is the strongest correlate for the severity of memory loss. A reduction in excitatory synapse density would lower neuronal activity. In contrast, brain imaging studies identified neuronal hyperactivity in clinically healthy individuals with a genetic predisposition for AD. Mouse models of AD, with human familial AD- linked mutations in the gene coding for amyloid precursor protein (APP mice), also display reduced excitatory synapses and neuronal hyperactivity. In this proposal, we will experimentally reconcile these contrasting observations and determine the synaptic deficits associated with memory loss in APP mice. Neuronal activity is maintained around a set point within a dynamic range. Any perturbation to this range elicits compensatory synaptic changes to achieve homeostasis. Therefore, we hypothesize that the reduction in excitatory synapse density in APP mice is a homeostatic adaptation to hyperactivity triggered by E/I imbalance. The reduction in excitatory synapses then causes long-term memory loss. We recently developed a novel approach to label and repeatedly image excitatory and inhibitory synaptic proteins in the same cortical neurons in vivo using multicolor two-photon microscopy. This approach has allowed us to simultaneously visualize excitatory and inhibitory synapse dynamics in the mouse brain in vivo with an unprecedented resolution. In addition, we have established a paradigm for assessing accelerated forgetting (normal short-term but an impaired long-term memory) in APP mice. Accelerated forgetting was recently discovered in clinically healthy individuals with APP mutations. Our preliminary studies indicate that the APP mice form a visual recognition memory (VRM) but are unable to stabilize it as long-term memory. Using chronic in vivo synapse imaging and the VRM task in APP mice (J20 and 5X-FAD lines), we will determine 1) whether excitatory synapse loss is a homeostatic adaptation to hyperactivity and whether the initial E/I imbalance is triggered by impairments to excitatory or inhibitory synapses; 2) whether hyperactivity prevents the stabilization of excitatory synapses formed during learning and leads to accelerated forgetting; and 3) the relative contribution of impaired stabilization of new synapses and accelerated destabilization of synaptic proteins in pre-existing synapses in reducing excitatory synapse density. The proposed studies will provide the highest resolution examination of synapses in APP mice in vivo to date and reveal synaptic impairments that precede memory loss. Most importantly, these studies have the potential to identify new targets for the treatment of AD.