Contributions of cell type and exosome signaling to prodromal synaptic and circuit changes in Alzheimer's Disease models

Alzheimer’s Disease (AD) is characterized by loss of synapses, resulting in decline of cognitive function. The pathology of AD is increasingly recognized to involve neuronal interactions with other brain cell types, notably microglia and astrocytes. Extracellular vesicles (EVs) are secreted by all cells in the brain, and carry protein and RNA cargo. EVs have the capacity to signal from donor to recipient cells within brain tissue and modify cell functions, as shown in cancers and multiple neurological diseases. EV propagation of pathologic proteins between cells in the brain is a strong candidate mechanism underlying at least some aspects of AD pathology, suggesting that distinct EV cargos may not only serve as biomarkers for disease but also directly induce vulnerability to or protection from pathologic disease states. Yet, little is known about EV cargo diversity and bioactivity from key brain cell types implicated in AD. Furthermore, the impact of genetic susceptibility loci or other factors such as gender has not been determined with respect to EV content, EV bioactivity or variation in neuronal responses to EVs. To overcome these barriers, we propose to use iPSCs and direct reprogramming to generate purified cultures of human neurons, astrocytes and microglia from iPSCs that vary by their AD-related genetic background or expected susceptibility. Because variation at APOE has a strong effect on both AD risk (APOE4) and protection (APOE2) we will profile EV cargos of EVs from isogenic iPSCs of each APOE genotype using state-of-the-art proteomic and RNA-Seq methods. In parallel, we will address the functional consequences of EV bioactivity on human induced neurons with two sensitive readouts: imaging neuronal connectivity and synaptic dynamics and unbiased transcriptomic profiling. These collaborative studies will establish a novel catalog of cell-type based profiles of EV cargo diversity and signaling bioactivity. Comparing EV contents from pathologic risk and protective variants has the potential to uncover novel mechanisms related to cell-to-cell spread of pathogenic or protective signals, and identify candidate biomarkers to test in clincal samples of human AD patients.