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The role of neuronal intracellular traffic in Alzheimer´s disease

Final Report Summary - TRAFFICINAD (The role of neuronal intracellular traffic in Alzheimer´s disease)

Abeta generated by processing of the amyloid precursor protein (APP) by beta- and gamma-secretase is either secreted or retained in endosomes near synapses. APP temporal and subcellular localization is different from its secretases. Therefore intracellular trafficking of APP determines the availability of APP to beta- and gamma-secretase. It remains unknown how the complex geometry, polarity, and aging of neurons can influence APP trafficking.
Understanding how intracellular trafficking contributes to AD and how it changes with aging is necessary to develop new mechanistic insights into neurodegeneration in AD.
Our main question was: does neuronal intracellular trafficking contribute to Abeta accumulation that leads to AD?
By studying the impact of two late-onset risk factors, Bin1 and CD2AP, on the endocytic itinerary of the amyloid precursor protein and of beta-secretase, we found Abeta generation polarized in neurons. Bin1 loss of function increased Abeta generation in axons while CD2AP loss of function increased Abeta in dendrites. Bin1 was polarized to axonal endosomes while CD2AP was polarized to dendritic endosomes. Different mechanisms were utilized by Bin1 and CD2AP to control Abeta generation. Bin1 controlled BACE1 recycling but not APP trafficking, and CD2AP controlled APP sorting for degradation but not BACE1 trafficking. Bin1 was found necessary for the scission of BACE1 tubules off endosomes while CD2AP was necessary for the translocation of APP to the lumen of maturing endosomes. Moreover, we found that Bin1 neuronal isoform could rescue Bin1 loss of function but not ubiquitous Bin1.
Our findings established Bin1 and CD2AP as regulators of intracellular trafficking pathways relevant for the amyloidogenic processing of APP causal to AD.
Our research has highlighted the importance of understanding the cellular mechanisms of neuronal intracellular trafficking to our knowledge of the etiology of late-onset Alzheimer's disease.