App interaction with trka is promoted by ngf through pt668 downregulation and is lost in ad

Amyloidogenic processing of Amyloid Precursor Protein (APP) is modulated by binding of the APP interacting proteins, which depends on the specific phosphorylation pattern of the APP C-tail. In particular, the phosphorylation at threonine 668 (APP pT668) favours the APP amyloidogenic processing in vivo, it is neurodegenerative in vitro, and increases in AD dystrophic neurites. Nerve Growth Factor (NGF) is a neuroprotective/antiamyloidogenic factor in AD, previously shown to regulate APP phosphorylation at the tyrosine 682 (Y682) and its binding to the NGF receptor Tyrosine kinase A (TrkA), with effects on cholinergic functions in vivo. However, little is known about the ability of NGF to modulate the pathogenic APP pT668 levels in neurons, and the significance of the APP-TrkA interaction in AD. To address this, we used primary septal neurons in vitro and septo-hippocampal acute brain slices ex vivo, treated or not with rodent NGF (1hr, 100ng/ml); and post-mortem brain tissues from AD and non-demented patients. The study was done by mean of western blotting, immunoprecipitation and immunofluorescence techniques. Here we show that NGF reduces the JNK-mediated phosphorylation of APP at T668 (APP pT668), which in turn inhibits APP interaction with TrkA. NGF reduced BACE binding to APP and increased APP trafficking toward the Golgi and sAPPalpha production. The ShcC neuron specific adaptor protein is also implicated in these events, since ShcC knock-out mice have 1) elevated levels of pAPP T668, 2) minimal APP-TrkA interaction, and 3) increased beta-CTFs levels. Furthermore, elevated levels of pAPP T668 and lack of the APP-TrkA interaction were found in the hippocampus but not in the cerebellum of AD brains. Accumulating evidences prompts the idea that a perturbation in NGF signalling may occur early in AD. In line with this, our data would suggest that early events affecting the NGF/TrkA/shcC signalling system may cause a JNK-driven and pT668-dependent loss of the APP-TrkA interaction, and a consequent shift of the APP processing toward the amyloidogenic pathway in the brain. For the first time, we observed the lack of the APP-TrkA interaction in AD targeted tissues, pinpointing its potential relevance for both early diagnosis of sporadic AD and AD therapy.