Improved long-term potentiation and memory in young tau-P301L transgenic mice before onset of hyperphosphorylation and tauopathy

Abstract

The microtubule binding protein tau is implicated in neurodegenerative tauopathies, including frontotemporal dementia (FTD) with Parkinsonism caused by diverse mutations in the tau gene. Hyperphosphorylation of tau is considered crucial in the age-related formation of neurofibrillary tangles (NFTs) correlating well with neurotoxicity and cognitive defects. Transgenic mice expressing FTD mutant tau-P301L recapitulate the human pathology with progressive neuronal impairment and accumulation of NFT. Here, we studied tau-P301L mice for parameters of learning and memory at a young age, before hyperphosphorylation and tauopathy were apparent. Unexpectedly, in young tau-P301L mice, increased long-term potentiation in the dentate gyrus was observed in parallel with improved cognitive performance in object recognition tests. Neither tau phosphorylation, neurogenesis, nor other morphological parameters that were analyzed could account for these cognitive changes. The data demonstrate that learning and memory processes in the hippocampus of young tau-P301L mice are not impaired and actually improved in the absence of marked phosphorylation of human tau. We conclude that protein tau plays an important beneficial role in normal neuronal processes of hippocampal memory, and conversely, that not tau mutations per se, but the ensuing hyperphosphorylation must be critical for cognitive decline in tauopathies.

Figure 1.

Immunohistochemistry for human tau and phosphorylated tau in the hippocampus of young nontransgenic and of tau-4R and tau-P301L transgenic mice. Human protein tau was detected with mAb HT7 in the hippocampal formation of tau-P301L and tau-4R transgenic mice (8 weeks of age) but not in nontransgenic mice (FVB/N). IHC with the phosphorylation-specific antibodies AD2, AT8, and AT180 demonstrated less phosphorylation in tau-P301L mice. Except for IHC with HT7, all sections were counterstained with hematoxylin (original magnification, 5×).

Figure 2.

Western blotting for human tau and phosphorylated tau in the hippocampus of young nontransgenic and tau-4R and tau-P301L transgenic mice. Protein extracts from the hippocampus of nontransgenic mice (FVB) and tau-P301L and tau-4R transgenic mice (n = 3, each) were analyzed by Western blotting with mAb Tau5 to detect total tau (i.e., mouse tau and transgenic human tau). Western blotting with phosphorylation-dependent antibodies AT8, AT180, and AD2 demonstrated that transgenic tau-P301L was less phosphorylated than wild-type mouse tau and transgenic tau-4R. Only tau-4R isoforms were present as demonstrated with antibody 2R (bottom panel) directed against the second microtubule binding domain in protein tau-4R (Takuma et al., 2003). The different tau-4R isoforms are indicated. Protein loaded was threefold higher for the nontransgenic than for the transgenic mice to compensate for the overexpression relative to endogenous mouse tau for different levels of the epitopes and for differences in titer and avidity of the respective antibodies. Samples loaded for tau-P301L and tau-4R transgenic mice were equivalent to 2.8 μg of protein for mAbs Tau-5 and AD2, 9 μg for mAb AT180, 18 μg for mAb AT8, and 5.6 μg for 2R antibody.

Figure 3.

Long-term potentiation in CA1 and DG of young mice at 9 weeks of age. A, LTP is similar in CA1 of nontransgenic and tau-P301L transgenic mice. B, LTP in the DG is significantly increased in tau-P301L transgenic mice compared with nontransgenic mice (WT) over the whole 60 min period (*p = 0.031) as well as over the last 5 min (right panel; *p = 0.036). Although potentiation was low in the WT, it was still significantly increased between 55 and 60 min compared with the pretetanus situation (p = 0.027). The level of 100% was defined as the average of the slope of 20 fEPSP recordings before the induction of LTP by theta burst stimulation (arrow). Error bars represent SEM.

Figure 4.

Novel object recognition in young mice at 9 weeks of age. Young age-matched nontransgenic and tau-P301L transgenic mice were assessed in the novel object recognition task with 1 h and 3.5 h delay intervals after acquisition. Both genotypes showed a similar preference for the novel object at 1 h, whereas at 3.5 h, the preference of the tau-P301L transgenic mice for the novel object remained high, as opposed to the decline in nontransgenic (WT) mice (*p = 0.02) This demonstration of improved memory was also observed in younger tau-P301L mice (at 5 weeks of age) (for details, see Results, Behavioral testing). Error bars represent SEM.

Figure 5.

Volume of the hippocampal formation. Total volume of three hippocampal subareas per hemisphere was not different in tau-P301L transgenic mice relative to age-matched nontransgenic mice (WT). Cell density in the DG granular layer was similar in age-matched tau-P301L and nontransgenic mice (WT) (see Results for details and discussion of other parameters). Error bars represent SEM.

Figure 6.

Dendritic morphology in young tau-P301L transgenic mice. Examples of Golgi–Cox impregnated cells in DG and CA1 are shown. Quantitative analysis of Golgi–Cox impregnated cells in CA1 revealed no major dendritic changes in different cellular parameters in the tau-P301L transgenic mice relative to age-matched nontransgenic mice (WT) (see Results for details). Error bars represent SEM.

Figure 7.

Newborn cells and survival in young tau-P301L transgenic mice. Different markers were examined to define putative changes in cell genesis and/or turnover. A, D, Immunohistochemistry for doublecortin marks young neurons. B, E, BrdU was analyzed 4 weeks after injection as a measure of cell age (see Results for details). C, Immunohistochemistry for the Ki-67 antigen as a marker of proliferating cells. A–C show an overview of the dentate gyrus, and representative individual cells or groups of cells are illustrated in D and E. Sections were counterstained with hematoxylin.