Project Summary/Abstract Mutations in the PSEN1 and PSEN2 genes encoding Presenilin-1 (PS1) and Presenilin-2 (PS2) are the most common cause of familial Alzheimer's disease (FAD), highlighting the importance of Presenilin function in disease pathogenesis, but the underlying mechanisms remain unresolved. Aberrant APP processing by γ- secretase and γ-secretase-independent activities of PS1 have been implicated in FAD pathogenesis. Our recent work has shown surprisingly that pathogenic mutations in PS1 can inactivate its function as the catalytic subunit of the γ-secretase complex and produce FAD-related phenotypes through a loss-of-function mechanism. To assess the effects of FAD mutations in vivo, particularly in the brain where the pathogenic process occurs, we generated two independent lines of Psen1 knock-in (KI) mice that precisely reproduce chromosomal PSEN1 mutations identified in FAD patients. Our analysis revealed phenotypes in the resulting homozygous KI mice indistinguishable from those caused by a Psen1 null mutation, accompanied by essentially complete loss of γ-secretase activity in the brain. Heterozygosity for the Psen1 L435F KI mutation produced deficits in hippocampal short- and long-term synaptic plasticity and hippocampal learning and memory reminiscent of those caused by conditional inactivation of Presenilins in the adult brain. Intriguingly, heterozygous KI mice also displayed elevation of the cortical Aβ42/Aβ40 ratio and exacerbation of cortical Aβ deposition on a mutant APP transgenic background. Moreover, the Psen1 L435F KI mutation was unable to support neuronal survival in the aging brain, triggering widespread cerebral cortical neurodegeneration. In this competing renewal application, we propose to investigate the important questions of whether and to what extent these synaptic, behavioral, and neurodegenerative phenotypes caused by the FAD mutation are attributable to aberrant APP processing and impaired γ-secretase activity, or alternatively to APP-independent and/or γ-secretase-independent functions of PS1. We propose to perform multidisciplinary molecular, synaptic, behavioral, and histological analysis using novel mouse models to understand the contributions of APP processing and γ-secretase-independent activity to Presenilin function and FAD-related dysfunction in the adult brain. The results of our studies will have significant impact on understanding of FAD pathogenesis and strategies to devise effective therapies.