Effect size of memory deficits in mice with adult-onset P301L tau expression

Abstract

Transgenic mice expressing mutations in tau have yielded essential discoveries for Alzheimer's disease. One of the most commonly used tau mouse models is the tet-off Tg(tauP301L)4510 model that expresses P301L human tau driven by the calcium-calmodulin kinase IIα (CaMKIIα) promoter system. Tau expression in this model is regulatable, allowing for suppression of mutant tau expression until adulthood and prevention of possible developmental alterations resulting from P301L tau expression during development. Here, we compared the effect and sample sizes needed for three learning and memory tasks in mice with adult-onset P301L tau expression. Our findings indicate that the Incremental Repeated Acquisition (IRA) and trace fear conditioning tasks, neither of which have previously been published with these mice, were highly sensitive to P301L tau expression, whereas the Morris water maze, the most commonly used task with this model, was the least sensitive. Memory deficits were observed at a time when tau pathology was subtle and prior to readily detectable neuronal loss. Thus, we provide essential information (effect and sample sizes needed) for establishing experimental designs at a time point when memory deficits are likely to go undetected if inadequate sample sizes are used. Our work also suggests the tet-off Tg4510 model provides a way to avoid mutant tau expression during the perinatal and early postnatal stages, thereby preventing possible developmental alterations unrelated to Alzheimer's disease.

Keywords: Alzheimer's disease; Fear conditioning; Incremental repeated acquisition; Memory; Tau; Tg4510.

Figure 1

Regulation of Tau Expression. (A) Both TauWT and TauP301L responder lines encode for human four repeat tau lacking the amino terminal sequences (4R0N). The activator line uses a forebrain specific promoter (CaMKIIα) to restrict tau expression to the forebrain. (B) The addition of doxycycline (DOX 40 ppm) to the drinking water prevents the transcription of tau by preferentially binding to the tetracycline transactivator (tTA) protein. In the absence of DOX, transcription occurs. (C) In the absence of DOX, TauP301L mice exhibit strong tau expression in the hippocampus at 3.5 months of age when examined using an antibody that recognizes both mouse and human tau (Tau-5), or just human tau (T-14). In the presence of DOX, tau expression is suppressed in tauP301L mice at 3.5 months of age. Equal loading was observed across the lanes (α-tubulin). (D) Whole and hemi-forebrain weights were similar for controls and tauP301L mice on or off DOX at 3.5 months of age.

Figure 2

TauP301L mice exhibit spatial reference memory deficits after tau expression (6M) but not before tau expression (2.5M). At 2.5M, prior to expression of tau, there were no differences between controls or tauP301L mice in (A) pathlength during hidden platform training or (B) platform crossing index (PIC) during probe trials. In contrast, at 6M, after 3 months of tau expression, tauP301L mice exhibited (C) longer pathlengths during hidden platform training and (D) spatial reference memory deficits, as assessed by PCI. Months, M; Platform crossing index, PIC. *p<.05; **p<.01; ***p<.001.

Figure 3

TauP301L mice exhibit deficits during acquisition of the performance chain. TauP301L mice made fewer total responses (A) and earned fewer reinforcers (B) across all sessions. In contrast, chain length (C), accuracy (D), and PQ (E) were similar during initial sessions, but diverged as training progressed.

Figure 4

Responding and reinforcers earned in the IRA. Responding did not differ between tauP301L and controls for performance (A) or learning (B) sessions. Though reinforcers earned were similar in the performance sessions (C), tauP301L mice earned significantly fewer reinforcers during learning sessions (D).

Figure 5

TauP301L mice are impaired in the IRA task. Maximum chain length reached (A,B), accuracy (C,D), and PQ (E,F) were lower in tauP301L mice compared to controls in both performance (left) and learning (right) sessions.

Figure 6

TauP301L mice were impaired in trace and contextual fear conditioning. Baseline, tone CS, and trace CS freezing was similar between controls and tauP301L mice during training with a 10-s (A) and 30-s (D) trace interval. Baseline freezing and retention of the tone CS memory were similar in controls and tauP301L mice at the 10-s (B) and 30-s (E) trace intervals. Though trace fear retention was similar between controls and tauP301L mice in the 10-s trace group (B), tauP301L mice exhibited a significant decrease in mean percentage of freezing during trace fear retention testing after training with the 30-s trace (E). TauP301L mice exhibited a significant decrease in mean percentage of freezing during contextual fear retention for both the 10-s (C) and 30-s (F) traces. Baseline, BL; conditioned stimulus, CS. *p < 0.05.

Figure 7

Western blot analysis of total CREB, pCREB, and the ratio of pCREB to CREB in the hippocampus and prefrontal cortex (PFC) of tauP301L mice compared to controls. Total CREB expression did not differ between controls and tauP301L mice in the hippocampus (A) or PFC (B), but pCREB and pCREB/CREB was decreased in the hippocampus (A) and PFC (B) of tauP301L mice. *p<.05; ***p<.001.

Figure 8

Representative images of tau pathology in tauP301L mice. At 6.5 months of age, after 3 months of tau expression, IHC studies revealed abnormal tau conformation and phosphorylation when using the early pathological markers of AD, CP-13 and MC-1 (A) and the pretangle marker AT-8, but not PG-5 (B). Conformational changes determined using: MC-1, amino acids 7 to 9 and amino acids 326 to 300, and phosphorylation changes detected using: CP-13, pSer²⁰²; AT-8, pSer²⁰²/pThr²⁰⁵; PG-5, pSer⁴⁰⁹. No positive labeling was observed after parallel processing of control tissue (n = 4). Staining was consistent across all tauP301L mice (n = 4). Boxes indicate area shown at higher magnification. Original magnifications: x5 for hippocampus (Hippo.) and frontal cortex (F.Ctx.) and x40 for CA1 or F.Ctx.

Figure 9

No readily detectable neuron loss was observed in tauP301L mice when compared using hematoxylin and eosin (H&E) or a neuron-specific antibody (NeuN). Boxes indicate area shown at higher magnification. Representative images shown and were consistent across all tauP301L mice (n = 4). Original magnifications: x5 for hippocampus (Hippo.) and frontal cortex (F.Ctx.) and x40 for CA1.