Project Summary Polymorphism in the APOE gene is the leading risk modulator for late onset Alzheimer’s Disease (LOAD). Despite extensive studies, the molecular mechanisms by which Apolipoprotein E (ApoE) influences AD risk remain poorly understood. A growing body of research is pointing towards a significant role for ApoE/heparan sulfate interactions in AD pathogenesis. Heparan sulfate (HS) is ubiquitously present on neuronal and glial surface and facilitates ApoE cell surface binding and cellular uptake. In our recent studies, we have shown that ApoE/HS binding affinity correlates with AD risk and that ApoE recognizes a rare HS modification, 3-O-sulfation. In addition, we demonstrate an increase in the 3-O-sulfated HS in AD brains. In preliminary data, we show that ApoE4 allele further increases the level of 3-O-sulfated HS, leading to differential interaction between ApoE isoforms and HS in the brain. We hypothesize that the differential binding modes between ApoE isoforms and HS contribute to AD risk. This hypothesis will be tested on multiple levels using an interdisciplinary approach in three aims. Aim 1, Delineate the glycan determinants of ApoE-HS interaction in AD. Aim 2, Define isoform specific ApoE-HS interaction at atomic resolution. Aim 3, Study isoform specific ApoE-HS interaction in cellular and animal models. Successful completion of this project will provide novel insights into how ApoE isoforms modulate AD risk through their distinct interactions with HS, and will identify new drug targets for AD.