Characterization of ApoE4 Induced Phospholipid Dysregulation in AD Pathogenesis

PROJECT SUMMARY APOE4 is the strongest genetic risk factor for sporadic AD with Ab-dependent and Ab-independent effects on disease pathogenesis. However, the molecular mechanisms underlying the pathogenic nature of APOE4 are not fully elucidated. In previous funding period, we have demonstrated that APOE4-induced phosphoinositol biphosphate (PIP2) dyshomeostasis through the increased expression of a PIP2-degrading enzyme, synaptojanin 1 (synj1). In parallel, a non-biased multiscale network analysis of human dataset from the AMP-AD consortium identifies synj1 as a key driver in both male and female AD subnetworks. Synj1 reduction has been found to provide several beneficial effects such as rescuing APOE4-associated cognitive impairments and lysosomal defects. Notably, we discovered that APOE4+ microglia have higher synj1 expression at baseline, and that APOE4+ microglia manifests with impaired phagocytic activities and lysosomal defects when compared to APOE3+ microglia which can be rescued by synj1 haploinsufficiency. A pilot study indicates that lowering synj1 expression in E4FAD mice protects against cuprizone-induced chronic neuro-inflammation and cognitive/motor deficits in vivo. We postulate that the APOE-synj1-PIP2 signal pathway plays a functional role in regulating microglial function, and that dysregulated microglial synj1-PIP2 homeostasis induced by APOE4 may lead to dysregulated immune function and excessive synaptic elimination, promoting APOE4-associated neuroinflammation and synaptic dysfunction. Therefore, we will characterize the regulation of microglial function by the APOE-synj1-PIP2 signaling pathway during aging and AD pathogenesis in this renewal application. We propose to 1) investigate microglial synj1 function in modulating APOE4-regulated neuroinflammation in AD in vivo (Aim 1) using cuprizone-induced inflammation in male and female EFAD mouse models (human ApoE4 knock-in at 5xFAD background); 2) to characterize the molecular mechanisms by which the APOE-synj1-PIP2 signaling pathway regulates microglial function (Aim 2) using mouse microglia from EFAD mice (synj1+/+ and synj1+/-), mouse microglia from TREM2-/- or APOE-/- mice, and human iPSC-derived microglia from APOE3+/+ and APOE4+/+ normal and AD subjects; 3) to investigate temporal and spatial relationship between dysregulated microglial function and excessive neurite and synaptic elimination using ex vivo 3-D co-culture of iPSC-derived brain cells; 4) to perform high resolution multiscale network modeling using scRNA-seq dataset from mouse brains and RNA-seq dataset from human iPSC-derived microglia and co-culture to identify microglia-specific molecular signatures driven by APOE-synj1-PIP2 signaling; and 5) to validate identified molecular signature in post-mortem human brain samples (Aim 3). The goals of this application aim to elucidate novel pathways and molecular signatures underlying APOE4-induced microglial dysfunction. These studies will facilitate identification and development of a more personalized targeted therapy to APOE-regulated neuroinflammation in AD.