Exosomes:From biogenesis and secretion to the early pathogenesis of Alzheimer's disease

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ABSTRACT Bi-directional communication between astrocytes and neurons regulates synaptic formation, synaptic strength, and participates in the regulation of neural circuitry by coordinating activity among groups of neurons. Astrocyte dysfunction in Alzheimer’s (AD), and other neurodegenerative conditions has been postulated to contribute to perturbations in activity of neural networks involved in memory and executive functions. Although AD associated modifications in the composition and quantity of various cytokine, chemokine and growth factors released from astrocytes have been demonstrated, these observations have thus far been insufficient to explain how astrocyte stress contributes to neuronal dysfunction. Advancements in our understanding of the biology of extracellular vesicles have begun to implicate glial released microvesicles as primary mediators of glia to neuron communication. In preliminary experiments we provide evidence that a variety of stimuli can induce astrocytes to shed microvesicles. The molecular cargo of these astrocyte-shed microvesicles was complex, and contained more than 200 distinct proteins, 100 miRNA, and hundreds of bioreactive lipid species. Moreover, the protein, miRNA and lipid composition of astrocyte exosomes was modified by the stimulus used to induce release and could be further modified by pre-treatment with oligomeric A peptides. These astrocyte- shed exosomes directly interacted with neurons to modify neuronal structure and function. Based on these preliminary findings we reasoned that a scientific focus on any one protein, lipid or miRNA would be unlikely to produce a true representation of the functions regulated by this complex signaling vesicles. Therefore, we used bioinformatic and systems biology approaches to understand how the protein, miRNA and lipid composition of exosomes interacts to regulate neuronal signaling pathways identified by whole genome sequencing of target neurons. In this application we focused our efforts on a small number of the identified pathways. In particular we concentrated on neural pathways associated with synapse formation, spine formation, and neurite outgrowth, as these neuronal structures are damaged in AD. The goals of this application are to understand how endogenous excitatory stimuli and inflammatory stimuli associated with AD modulate the cargo of astrocyte-shed exosomes and how these exosomes regulate/dysregulate the structure and function of target neurons.