PROJECT SUMMARY Late onset Alzheimers disease (LOAD) is a neurodegenerative disease with a multifactorial etiology andintersecting genetic and environmental risks making it a complex systems challenge. Brain functions arehighly energy-dependent with most of which generated by mitochondria via oxidative phosphorylation. Whilethe association between LOAD and an early decline in brain glucose metabolism and changes in mitochondrialfunction is well-established therapeutics that universally enhance brain mitochondrial function have yet resultedin favorable outcomes. As the greatest genetic risk factor for LOAD the e4 variant of APOE (APOE4) was alsofound to affect brain bioenergetics and lipid metabolism via incompletely understood mechanisms. Consideringthe metabolically diverse cellular composition of the brain and APOE as an inter-cellular lipid carrier we proposethat cell type-specific intra-cellular bioenergetic shifts and inter-cellular metabolic uncoupling of fatty acid (FA)metabolism underlie APOE4-driven AD relevant metabolic phenotypes in the brain. Specifically we hypothesizethat APOE4-induced disruption to astrocytic clearance of neuronal FAs and subsequent degradation in astrocyticmitochondria could elicit lipid dysregulation neuronal dysfunction neuroinflammation and cognitive decline. Program of research proposed herein will determine the mechanisms at the cellular level by which APOEpolymorphism alters brain bioenergetics and lipid homeostasis and eventually LOAD risk. To test ourhypotheses we will implement three levels of investigations to understand the complex mechanisms underlyingAPOE regulation of metabolic coupling between neuron and astrocytes. Aim 1 will determine and differentiatethe effect of APOE isoforms on cellular metabolic shift in neurons and astrocytes using single-cell transcriptomicsand in vitro functional assessment. Using a neuron-astrocyte co-culture system Aim 2 is designed to investigatethe impact and mechanism by which different isoforms and origins (neuronal- vs. astrocytic) of APOE affectneuron-astrocyte metabolic coupling focusing on fatty acid metabolism. Aim 3 will test determine howperturbations to neuron-astrocyte metabolic coupling mediate APOE4-induced LOAD at-risk phenotypes duringaging in vivo. Projected outcomes from this research will elucidate how cell types with distinctive bioenergeticphenotypes jointly maintain the brain metabolic homeostasis and how APOE4 increases risk of LOAD bydisrupting the metabolic system of the brain. Translationally this research will shed light on selective cellvulnerability in AD development and has the potential to identify APOE genotype-specific and cell type-specific therapeutic targets to sustain or restore a bioenergetic equilibrium and lipid homeostasis in the brainthat are resilient against synaptic- and cognitive declines in LOAD.