Effects of altered tau expression on dentate granule cell excitability in mice

Abstract

Tauopathies, including Alzheimer's disease, are characterized by progressive accumulation of hyperphosphorylated and pathologic tau protein in association with onset of cognitive and behavioral impairment. Tau pathology is also associated with increased susceptibility to seizures and epilepsy, with tau^-/- mice showing seizure resistance in some epilepsy models. To better understand how tau pathology is related to neuronal excitability, we performed whole-cell patch-clamp electrophysiology in dentate gyrus granule cells of tau^-/- and human-tau expressing, htau mice. The htau mouse is unique from other transgenic tau models in that the endogenous murine tau gene has been and replaced with readily phosphorylated human tau. We assessed several measures of neuronal excitability, including evoked action potential frequency and excitatory synaptic responses in dentate granule cells from tau^-/-, htau, and non-transgenic control mice at 1.5, 4, and 9 months of age. Compared to age matched controls, dentate granule cells from both tau^-/- and htau mice had a lower peak frequency of evoked action potentials and greater paired pulse facilitation, suggesting reduced neuronal excitability. Our results suggest that neuronal excitability is more strongly influenced by the absence of functional tau than by the presence of pathologic tau. These results also suggest that tau's effect on neuronal excitability is more complex than previously understood.

Keywords: Dentate granule cell; EPSC; Hippocampus; Microtubule associated protein tau; Tau(−/−); Tauopathy; htau.

Figure 1.. Membrane voltage response to injected current in DGCs from tau^−/− and htau mice compared to non-transgenic control mice.

(A) Sample recordings showing voltage response from −100, 0, and 200pA current steps in DGCs from 1.5 month old tau^−/−, htau, and control mice. Inset: representative Western blot showing absence of tau protein in tau^−/− and presence of human tau protein in htau mice. (B) Comparison of maximum evoked action potential frequency (250pA current injection). At 1.5 months, DGCs from control mice fire more actions potentials than those from tau^−/− and htau mice. In tau^−/− mice, DGCs from 9 month old mice fire more action potentials than at 1.5 or 4 months. In htau mice, DGCs from 9 month old mice fire more action potentials than at 1.5 months. In control mice, DGCs from 4 month old mice fire more action potentials than at 1.5 months.*: Different from control at same age (p<0.05) †: Different from 1.5 month of same genotype (p<0.05) ǂ: Different from 4 month of same genotype (p<0.05). Error bars indicate SEM. Statistical comparisons were made between all groups by two-way ANOVA with age and genotype as factors (Tukey post hoc).

Figure 2.. Average spontaneous EPSC frequency in tau^−/−, htau, and non-transgenic control mice.

No differences sEPSC frequency were found between DGCs from tau^−/− or htau mice and control mice at any age. sEPSC frequency did not change with age in DGCs from tau^−/−, htau, or control mice (p>0.05). A Shapiro-Wilk test found the data were not normally distributed because they were were positive and right-skewed, so raw data were log transformed prior to further analysis. Statistical comparisons were made between all groups by two-way ANOVA with age and genotype as factors. (C) Representative traces of sEPSCs in DGCs from tau^−/−, htau, or control mice at 1.5, 4, and 9 months. Error bars indicate SEM.

Figure 3.. Paired pulse ratio in tau^−/− mice compared to non-transgenic control mice.

(A) The paired pulse ratio is higher in DGCs from 1.5 month old tau^−/− mice compared to 1.5 month old control mice. *: p<0.05 (B, C) No differences in in paired pulse ratio were found between DGCs from tau^−/− and control mice at 4 (B) or 9 (C) months old. (D) The paired pulse ratio is lower in DGCs from 9 month old tau^−/− mice compared to 1.5 month old tau^−/− mice. The paired pulse ratio did not change with age in DGCs from control mice. Statistical comparisons were made between all groups by two-way ANOVA with age and genotype as factors. *: different from 1.5 month (p<0.05) Error bars indicate SEM.

Figure 4.. Paired pulse ratio in htau mice compared to non-transgenic control mice.

(A) The paired pulse ratio is higher in DGCs from 1.5 month old htau mice compared to 1.5 month old control mice. *: p<0.05 (B, C) No differences in in paired pulse ratio were found between DGCs from htau and control mice at 4 (B) or 9 (C) months old. (D) The paired pulse ratio in DGCs from htau mice compared to 1.5 month old htau mice. The paired pulse ratio did not change with age in DGCs from control mice. Statistical comparisons were made between all groups by two-way ANOVA with age and genotype as factors. *: different from 1.5 month (p<0.05) Error bars indicate SEM.