Genetic ablation of tau mitigates cognitive impairment induced by type 1 diabetes

Abstract

Patients affected by diabetes show an increased risk of developing Alzheimer disease (AD). Similarly, patients with AD show impaired insulin function and glucose metabolism. However, the underlying molecular mechanisms connecting these two disorders are still not well understood. Herein, we investigated the microtubule-associated protein tau as a new link between AD and diabetes. To determine whether diabetes causes cognitive decline by a tau-dependent mechanism, we treated non-transgenic (Ntg) and tau-knockout mice with streptozotocin, causing type 1 diabetes-like disease (T1D). Interestingly, although induction of T1D in Ntg mice led to cellular and behavioral deficits, it did not do so in tau-knockout mice. Thus, data suggest that tau is a fundamental mediator of the induction of cognitive impairments in T1D. Tau dysregulation, which causes a reduction in synaptic protein levels, may be responsible for the cognitive decline observed in Ntg streptozotocin-treated mice. Concomitantly, we demonstrate the novel finding that depletion of endogenous tau mitigates behavioral impairment and synaptic deficits induced in T1D-like mice. Overall, our data reveal that tau is a key molecular factor responsible for the induction of cognitive deficits observed in T1D and represents a potential therapeutic target for diabetes and patients with AD.

Figure 1

Streptozotocin treatment induces hippocampal cognitive impairment in Ntg mice through a tau-dependent mechanism. Mice were trained on the spatial reference version of the MWM (n = 10 to 12 per group) at 4 months of age. Acquisition curves (A) are shown for the 5 days of training on the MWM. *P < 0.05. Mixed analysis of variance: trials [F(4,148) = 27.75, P < 0.0001], treatment [F(3,37) = 8.84, P < 0.001], and interaction [F(12,148) = 2.05, P < 0.05]. B and C: Time spent in the platform quadrant (B) and number of crosses (C) of NTg, NTg-STZ, tauKO, and tauKO-STZ groups. Time spent in the target zone for NTg-STZ was 58.48% ± 5.75%. Two-way analysis of variance: genotype [F(1,31) = 3.20], treatment [F(1,31) = 3.67, P = 0.006], and interaction [F(1,31) = 5.95, P < 0.05]. Number of platform crossings for NTg-STZ was 52.89% ± 5.15%. Two-way analysis of variance: genotype [F(1,28) = 6.50, P < 0.05], treatment [F(1,28) = 1.90], and interaction [F(1,28) = 3.53, P = 0.05]. Pairwise comparisons: ^∗∗P < 0.01 (B and C). Speed swim (D) and traveled distance (E). The values represent the means ± SEM.

Figure 2

Streptozotocin treatment impairs memory-related intracellular signaling and levels of synaptic-related proteins. A: Immunoblot analyses of PSD-95, synaptophysin, p-CREB, and CREB of protein extracts from whole-brain homogenates of Ntg, Ntg-STZ, tauKO, and tauKO-STZ at 5 months of age are shown in alternating lanes. B: Quantification normalized to GAPDH and expressed as percentage of control. Pairwise comparisons: ^∗P < 0.05, ^∗∗P < 0.001 for PSD-95 (29.3% ± 7.4%), genotype [F(1,11) = 1.85], treatment [F(1,11) = 6.53, P < 0.05], interaction [F(1,11) = 5.53, P < 0.05]; synaptophysin (26.4% ± 8.9%), genotype [F(1,11) = 0.51], treatment [F(1,11) = 1.98], interaction [F(1,11) = 7.21, P < 0.05]; and phosphorylated CREB expression (24.9% ± 3.5%), genotype [F(1,20) = 3.61], treatment [F(1,20) = 11.62, P < 0.01], interaction [F(1,20) = 8.09, P < 0.05]. The values represent the means ± SEM. MW, molecular weight.

Figure 3

Streptozotocin treatment leads to tau hyperphosphorylation in Ntg mice. A: Immunoblot analyses of phospho-tau epitopes, including pSer199/202 tau (AT8), pSer212/Thr214 (AT100), pThr231 (AT180), pThr181 (AT270), and pSer396/404 (PHF-1) of protein extracts from whole-brain homogenates of Ntg and Ntg-STZ mice at 5 months of age, are shown in alternating lanes. B: Quantification normalized to GAPDH and expressed as percentage of control. A significant increase was observed in p-tau epitopes at pSer199/202 [47.02% ± 11.97%, ^∗P < 0.05, unpaired t-test (17) = 2.53] and Thr231 [53.37% ± 11.24%, ^∗∗P < 0.01, unpaired t-test (20) = 3.03] in Ntg-STZ compared with Ntg-vehicle mice. Furthermore, increases of tau phosphorylation at residues Ser212/Thr214, Thr181, and Ser396/404 were found in Ntg-STZ compared with Ntg-vehicle mice, although they were not statistically significant. The values represent the means ± SEM. MW, molecular weight.

Figure 4

Streptozotocin treatment alters the IR/PI3K/AKT pathway in Ntg and tauKO mice. A: Immunoblot analyses of pIR, IR, pPI3k(p85), PI3k, pAKT(Ser473), AKT, GSK3β (Ser9), and GSK3β of protein extracts from whole-brain homogenates of Ntg, Ntg-STZ, tauKO, and tauKO-STZ mice at 5 months of age are shown on alternating lanes. B: Quantification normalized to GAPDH and expressed as percentage of control. Pairwise comparisons: ^∗P < 0.05, ^∗∗P < 0.01 for phosphorylated IR (Ntg-STZ, 31.8% ± 5.1%; tauKO-STZ, 20.2% ± 7.2%; two-way analysis of variance: genotype [F(1,10) = 2.23], treatment [F(1,10) = 20.03, P < 0.01], interaction [F(1,10) = 0.43]) and phosphorylated PI3K (Ntg-STZ, 14.3% ± 8.0%; tauKO-STZ, 18.9% ± 3.0%). Two-way analysis of variance: genotype [F(1,11) = 0.11], treatment [F(1,11) = 9.93, P < 0.01], interaction [F(1,10) = 0.45]. Phosphorylated AKT at residue Ser473 (Ntg-STZ, 26.3% ± 6.0%; tauKO-STZ, 13.5% ± 4.5%; two-way analysis of variance: genotype [F(1,10) = 2.78], treatment [F(1,10) = 17.12, P < 0.01], interaction [F(1,10) = 0.42]) and GSK3β-Ser9 (NTg-STZ, 30.4% ± 6.0%; tauKO-STZ, 24.7% ± 5.0%; two-way analysis of variance: genotype [F(1,12) = 0.53], treatment [F(1,12) = 27.83, P < 0.001], interaction [F(1,12) = 0.10]). The values represent the means ± SEM. MW, molecular weight.

Figure 5

Streptozotocin treatment does not alter Cdk5, ERK, and p38-MAPK kinases. A: Immunoblot analyses of cdk5, p25/p35, ERK 1/2, phosphorylated ERK 1/2, p38-MAPK, and phosphorylated p38-MAPK of protein extracts from whole-brain homogenates of Ntg, Ntg-STZ, tauKO, and tauKO-STZ mice at 5 months of age are shown in alternating lanes. B: Quantification normalized to GAPDH and expressed as percentage of control. The values represent the means ± SEM. MW, molecular weight.