UC San Diego

Alzheimer's Disease is a devastating neurodegenerative disorder with no clear etiology. Proteolytic processing of the Amyloid Precursor Protein has been implicated in the disease due to familial mutations that alter that amount or ratios of APP fragments produced. Recent developments have indicated that more specifically, location of APP cleavage products may be important in the disease. Deposition of toxic fragments in synapses from axons or dendrites may contribute to pathology. The intracellular location of processing and secretion of APP fragments in neurons is unclear, and very little information is known about these parameters on endogenous proteins, particularly in human neurons. We have developed a system to differentiate human neurons from human Embryonic Stem cells, which can be cultured in microfluidic culture devices which separate axons from bulk neuronal culture, enabling direct biochemical measurements from axons. Using human neurons or mouse hippocampal neurons cultured in these devices, we have found measurable levels of A[beta], sAPP[alpha] and sAPP[beta] are secreted from axons. Additionally, a majority of the fragments secreted from axons are processed in the soma, and are also dependent on somatic endocytosis for axonal secretion. This implies a ̀transcytotic' pathway for APP fragments to arrive in axons. Full-length APP however, is not dependent on endocytosis for axonal sorting. These data implicate multiple pathways for APP and its fragments delivery into axons, a previously unknown mechanism for regulating APP localization. This pathway may be a potential target for mitigating toxicity in Alzheimer Disease