Inhibition of neuronal calcium oscillations by cell surface APP phosphorylated on T668

Adenoviral expression of human APP (hAPP), but not of hAPP deleted from its C-terminal intracellular domain, in rat cortical neurons abolishes spontaneous synchronous calcium oscillations. The intracellular domain of APP695 contains several residues that can be phosphorylated. Contrary to non-neuronal cells, a very high phosphorylation of APP on T668 is observed in neurons, which is mediated by JNK, GSK3 and Cdk5 protein kinases. JNK activity, modulated by GSK3, enhances the traffic of phosphorylated APP to nerve terminals, contrary to Cdk5. Here we show that inhibition of GSK3 and JNK restores calcium oscillations in an hAPP expressing neuronal network, whereas inhibition of Cdk5 does not. Expression of mutant hAPPT668A does not inhibit calcium oscillations, and the proportion of hAPPT668A at the plasma membrane is reduced by more than 50%. Altogether, these results indicate that the intracellular domain of APP is needed to inhibit neuronal calcium oscillations because GSK3/JNK phosphorylation of T668 controls APP trafficking at the plasma membrane.