Premature lethality, hyperactivity, and aberrant phosphorylation in transgenic mice expressing a constitutively active form of Fyn

Di Xia,JÃ¼rgen GÃ¶tz

doi:10.3389/fnmol.2014.00040

Di Xia, JÃ¼rgen GÃ¶tz

Open Access

https://doi.org/10.3389/fnmol.2014.00040

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Abstract

The kinase Fyn, the microtubule-associated protein tau and the peptide amyloid-β (Aβ) constitute a toxic triad in Alzheimer's disease (AD). Tau's subcellular localization is mainly regulated by phosphorylation whereas Fyn's localization is dictated by palmitoylation targeting it to the plasma membrane in a reversible manner. We have previously shown that tau is required for Fyn to be targeted to the dendritic spine. We had also shown that a truncated form of tau (Δtau) that accumulates in the cell soma is capable of trapping Fyn and preventing it from entering the spine. Here we determined that palmitoylation is required for Fyn's membrane and spine localization. We further evaluated the functional consequences of neuronal over-expression of the constitutively active Y531F mutant form of Fyn (FynCA) in transgenic mice. We found that the FynCA transgenic mice displayed a reduced weight, a massively reduced lifespan and a high level of hyperactivity. The lifespan of the FynCA mice was only slightly extended by crossing them with Δtau transgenic mice, possibly reflecting differences in expression patterns of the transgenes and high levels of transgenic FynCA compared to endogenous Fyn. Analysis of synaptosomes revealed that FynCA accumulated at high levels in the spine, resulting in increased levels of the NMDA receptor subunit NR2b phosphorylated at residue Y1472. Tau was strongly phosphorylated at the AT8 epitope S202/T205 as shown by Western blot and immunohistochemistry indicating that an increased tyrosine kinase activity of Fyn has down-stream consequences for serine/threonine-directed phosphorylation.

Highlights

In Alzheimer’s disease (AD), serine/threonine-directed phosphorylation has attracted significantly more attention than tyrosinedirected phosphorylation (Götz et al, 2013)
One of the reasons is that the microtubule-associated protein tau, a protein implicated in AD, contains 80 serines and threonines, many of which have been shown to be phosphorylated under disease conditions, compared to only 5 tyrosine residues (Chen et al, 2004)
It has been shown that Aβ-induced dendritic spine loss requires both PrPc and Fyn, and that in mice lacking PrPc, human familial AD transgene-induced convulsive seizures do not occur (Um et al, 2012)

Summary

Introduction

In Alzheimer’s disease (AD), serine/threonine-directed phosphorylation has attracted significantly more attention than tyrosinedirected phosphorylation (Götz et al, 2013). One of the reasons is that the microtubule-associated protein tau, a protein implicated in AD, contains 80 serines and threonines, many of which have been shown to be phosphorylated under disease conditions, compared to only 5 tyrosine residues (Chen et al, 2004). When tau transgenic mice as a model of AD are analyzed this routinely includes the assessment of the phosphorylation status of relevant serine and threonine residues, but rarely that of tyrosines. To establish the sequence of tau-related cytoskeletal changes in AD, the serine/threoninedirected antibody AT8 and not a tyrosine-directed antibody has been used (Braak et al, 1994). It is increasingly appreciated that tyrosine phosphorylation has an important role in AD (Boehm, 2013). Of tau’s five tyrosine residues, Y18 is interesting because it is phosphorylated by the tyrosine-directed kinase Fyn, a member of the Src family of non-receptor tyrosine kinases (http://cnr.iop.kcl.ac.uk/hangerlab/tautable) (Figure 1A)

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