The molecular mechanism of the Tau-Fyn interaction and dendritic targeting in neurons

Abstract

Alzheimer's disease (AD) is the most common form of neurodegenerative disorder that progresses with aging. The AD brain is characterized by a massive loss of neurons and synapses. Two key hallmarks of AD are the aggregation of the amyloid-β (Aβ) peptide as plaques and of the microtubule-associated protein Tau as neurofibrillary tangles. Under physiological conditions, Tau is mainly localized to the axon of neurons, with low levels of expression found in dendrites and spines. Under disease conditions, Tau is hyperphosphorylated and accumulates in the somatodendritic domain of neurons including spines. Recent data suggest that dendritic spine-targeted Tau disrupts synaptic function and causes synaptic loss long before neurofibrillary tangle formation and neuronal loss occurs.Fyn is a non-receptor tyrosine kinase that mainly targets to the postsynaptic membrane in spines. Fyn interacts with and phosphorylates Tau and this interaction has a critical role in mediating Aβ toxicity. Understanding how Tau and Fyn are targeted to the dendritic spine and how their trafficking and interaction is regulated under both physiological and pathological conditions is critical for understanding how AD is initiated in the synapse and for developing novel therapeutic approaches to delay, halt or treat AD.Fyn phosphorylates Tau mainly at Tyr18; however, it is unclear how tyrosine phosphorylation of Tau by Fyn affects serine/ threonine phosphorylation. In the first part of my thesis, I have generated a transgenic animal model that expresses constitutively active Fyn and studied the functional consequence of Fyn activation, especially for Tau. We found that FynCA animals showed premature lethality and hyperactive phenotypes reflecting the synaptic over-excitation by Fyn as expected. A biochemical analysis of isolated synaptosomes revealed increased levels of the NMDA receptor subunit NR2b phosphorylated at residue Y1472, and of Tau phosphorylated at the 12E8 phosphoepitope S262/S356. Besides, Tau phosphorylation at the AT8 epitope S202/T205 was strongly increased in FynCA mice as revealed by both immunohistochemistry and Western blot analysis. Our results indicate that an increased tyrosine kinase activity of Fyn has a role in serine/threonine-directed phosphorylation of Tau, which may contribute to the tauopathy in AD. 3 Under physiological conditions, low levels of Tau are targeted to dendritic spines in a process that is regulated by synaptic activity and Aβ levels. It has also been reported that hyperphosphorylated Tau mislocalizes into the spine, although it is not clear what kind of phosphorylation is required. In order to understand how Tau enters the spines and the role of Fyn in dendritic spine targeting of Tau, we performed an extensive mutagenesis study by overexpressing different mutant forms of Tau in cultured wildtype or FynKO neurons.Our results revealed that the spine localization of Tau is facilitated by deletion of the microtubule-binding repeat domain. Distinct pseudophosphorylation at AD related sites AT180(...