Understanding how microglial associated Sporadic Alzheimer''''s disease risk genes affect ROS-induced lipopathology and AD phenotypes

Alzheimer’s disease (AD) accounts for 70% of all age-related dementias in humans. While heavily studied for decades, fundamental understanding of the origin of this disease is still lacking. Therapeutic strategies have mainly focused at reducing AD related symptoms in an attempt to halt the devastating neurological deterioration. However, a true curative therapy will remain a distant goal as long as we don’t understand the molecular basis of AD development. Early-onset familial AD accounts for approximately 5% of all AD cases and is related to specific mutations in the amyloid precursor protein (APP) or Presenilin genes (PSEN1, PSEN2). The great majority of AD cases are formed by sporadic or late-onset AD (SAD, LOAD). Advances in meta-analysis of genome wide association studies have led to the identification of around 30 susceptibility loci that increase the risk for development of SAD. Mechanistic explanation for the contribution of these risk polymorphisms to AD development is still largely lacking but might significantly contribute to our understanding of AD. Many of the SAD risk variants are found in genes with roles attributed to cholesterol metabolism. Our lab has recently shown that cholesteryl esters (CE), the storage product of cholesterol, can drive aberrant accumulation of hyperphosphorylated Tau and Aß in neurons. Importantly, lowering CE by pharmacological targeting of the cholesterol synthesis pathway lowers pTau levels and Aß accumulation in human neurons and animal models indicating the important potential for future AD treatment strategies. Yet it is unknown how CE accumulates in AD patients and how different SAD risk variants can contribute to this. Recent studies in drosophila have shown that APOE4, the major risk variant for AD, impairs the transport of excess lipids from neurons to astrocytes and microglia. Neuronal lipogenesis is increased under oxidative stress (as also observed during aging) to scavenge the radioactive oxygen species (ROS). Glial cells play an important role in resolution of these stress-induced neuronal lipids via a neuron-to-glial transport and storage in glia lipid droplets. Impairment of lipid-, and specifically cholesterol transport from neurons to glia (astrocytes and microglia) could drive neuronal accumulation of CE and downstream AD phenotypes. Based on these results, we hypothesize that alterations in lipid shuttling caused by AD risk variants account for aberrant accumulation of lipids, like CE, in the AD brain leading to downstream Aß and Tau pathology. Interestingly, ROS induced lipids are strongly taken up by microglia, indicating that these cells have a major effect in lipid buffering. Microglial function has been extensively linked to AD pathology, yet how these immune cells contribute to AD development is still unknown. In this proposal we want to study how 2 well-documented SAD-risk genes involved in lipid metabolism (TREM2 and APOE), expressed by microglia, affect the ROS-induced lipid shuttle and downstream AD phenotypes. In addition, we want to uncover how lipid metabolism and TREM2 or APOE variants affect the immune response in microglia. To address these issues in a human cell model system, we will generate and characterize hIPSC-derived TREM2R47H microglia and hiPSC-derived microglia with APOE2/2, APOE3/3 or APOE4/4 expression (aim 1). To specifically study lipid shuttle between cell types we will set up a co-culture system of wt (APOE3/3) neurons with microglia harbouring the selected risk variants (aim 2). Finally we will use our system to determine how neuronal-microglial lipid shuttle and the SAD-risk variants affect the microglial immune state. Together we will elucidate how these SAD-risk variants affect neuronal-microglial lipid metabolism and contribute to AD development. This will inform new strategies for the development of AD therapeutics.