Apolipoprotein E (APOE), the most important genetic predisposition factor for Alzheimer’s disease (AD), long has been known for its effect on the formation of amyloid plaques and, more recently, for its importance in glial cell activation. There is evidence for another role of APOE on neuron function independent of amyloid plaques. Since some neurons are more vulnerable than others to neurodegeneration, we ask in this proposal whether APOE also could modulate their vulnerability. Our aim is to both better understand the role of APOE in AD, as well as the vulnerability of specific neurons. The most vulnerable neurons of the brain are the ones from layer II of the entorhinal cortex (ECII), which are crucial for new memory formation. Their early degeneration hinders the ability to form new memories at the onset of the disease. We previously compared the full inventory of all proteins present in these ECII neurons in mice with different alleles of human APOE—the risk allele, the neutral one and the protective one—and found signals of cellular stress in the presence of the risk allele. We also obtained data showing a slightly decreased number of neurons within the entorhinal cortex when mice have the risk allele of APOE. Strikingly, this population corresponds to the absolute most vulnerable area of the entorhinal cortex during the natural course of AD. This is pretty remarkable, since this occurs in the absence of any detectable tau or amyloid pathology, demonstrating that APOE by itself potentiates the vulnerability of these cells. We will test whether the neurons that regulate the activity of ECII neurons are functioning properly. We also will characterize the mechanisms by which APOE mediates pathology in vulnerable neurons by using a novel in vitro model: we will cultivate human ECII neurons from stem cells along with astrocytes that produce different alleles of APOE. We will test whether this model recapitulates the vulnerability observed in vivo. If it does, it will allow us to dissect which cellular processes are responsible for this vulnerability.