Introduction
Alzheimer
’s disease (AD) is the most common cause of dementia in the elderly, which affects over 50 million people worldwide [
1,
2]. Intraneuronal formation of neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau protein is a pathological hallmark of AD [
3,
4] and is well recognized to correlate with cognitive deficits of patients [
4,
5]. Therefore, increasing attention has been paid on the mechanisms underlying how tau pathology contributes to AD, as well as the tau-targeted drug discovery [
6‐
9]. However, studies of tau pathology and drug development are largely limited by the lack of readily available mouse models that mimic the AD-specific tau pathology.
Several tau transgenic (Tg) mouse models have been used for decades in the research of tau pathology [
10], including: (1) lines overexpressing wild-type (WT) or mutant human tau (hTau), such as ALZ17 (WT) [
11] and PS19 (P301S) [
12]; (2) Tg mice with expression of full-length WT or mutant hTau in the absence of endogenous murine tau, such as the hTau line (JAX005491) [
13‐
15]; and (3) mice with expression of WT or mutant full-length hTau under the induction of tetracycline, such as rTg4510 (CaMK2a:P301L) [
16] and rT2 (Col1a1:P301L) [
17]. However, these Tg lines show limitations in imitating the tau pathology in AD [
18]. First, most non-inducible hTau-Tg lines do not exhibit overt phosphorylated tau accumulation in the brain until 6–9 months of age [
11,
19], so researchers have to spend much time and resources to keep mice before experiments. Second, the commonly studied tau mutations in tau lines such as P301L and P301S are only found in frontotemporal dementia but not in AD patients [
20,
21]. In addition, some other mutated tau lines like rTg4510 (CaMK2a:P301L) show serious motor impairments, which may disturb the evaluation of cognitive functions in behavioral tests [
17,
22‐
24].
To generate an easy-to-use animal model with accurate simulation of the actual AD tau pathology, we generated a novel hTau368-Tg mouse line, in which the AD-like truncated hTau N1–368 (hTau368) was expressed under the neuron-specific enolase 2 (Eno2) promoter with induction of tetracycline (tet-on). The neurotoxic tau fragment N1–368 is cleaved by asparagine endopeptidase (AEP), and it accumulates and mediates NFT formation during aging and AD [
25]. Recently, we have used hTau368 mice as a model to test the effectiveness of a peptide drug designed to specifically facilitate tau dephosphorylation [
26]. For better use of this model, we here report more detailed characterization of AD-like pathologies in this mouse line.
Materials and methods
Animals
All mice were housed in groups of three to four per cage, under a 12 h light/dark cycle at 23–25 °C. Food and water were available ad libitum. Doxycycline hyclate (Beyotime, ST0398) was dissolved in drinking water (2 mg/l) and administered ad libitum. Equal numbers of male and female mice were randomly assigned to vehicle (Veh) or dox-treatment (Dox) groups. Mice were sacrificed at age of 7 months unless otherwise specified. All experiments and data analyses were conducted by experimenters blind to the groupings. All animal experiments were conducted in accordance with relevant ethical regulations for animal testing and research, and were approved by institutional guidelines and the Animal Care and Use Committee of Tongji Medical College, Huazhong University of Science and Technology.
Tg hTau368 generation
The hTau368 mice were generated jointly by our laboratory and Nanjing Biomedical Research Institute of Nanjing University. Specifically, an Eno2-h
MAPT vector, Insulator-pTRE3G-Kozak-ATG-hMapt N1-368-CDS-TGA-polyA-bGH-polyA-TAA-rtTA3G-Kozak-ATG-Rat Eno2 promoter-Insulator, was constructed by linking reactive cloning. pTRE is a tetracycline-induced promoter. h
MAPT is the targeted gene searched through
https://www.ncbi.nlm.nih.gov/gene query (Gene ID: 4137).
MAPT contains 12 transcripts, of which the 2N4R transcript NM_001123066.4 and the sequences encoding tau N1-368 were selected. PolyA was used as the termination signal. rtTA3G was the tetracycline-regulated transcription activator. Rat Eno2 promoter (Gene ID: 24,334, transcript xM_0062373304) was used to control the expression of rtTA3G.
The Eno2-h
MAPT vector was confirmed by DNA sequencing and microinjected into the nucleus of fertilized eggs of C57BL/6 J mice. Surviving fertilized eggs were implanted into the uterus of pseudo-pregnant C57BL/6 J mice. Founder mice in the F0 offspring were identified by PCR at 1 month old. Whole-genome sequencing was performed to find the insertion site of the targeted gene (Additional file
1: Fig. S1). Identification of homozygous and hemizygous hTau368 was conducted by PCR using primers shown in Additional file
1: Table S1. F1 hTau368 mice were used for expand reproduction and F2–4 offspring with transgene were used for experiments in this study. C57BL/6 J mice were used as wild-type controls.
Antibodies
Antibodies used in the present study are summarized in Additional file
1: Table S2. The polyclonal rabbit anti-TauN368 antibody gifted by Professor Keqiang Ye was developed in his laboratory and had been reported previously [
25]. The new monoclonal mouse anti-tauN368 antibody was jointly developed by AtaGenix (Wuhan, China). In brief, a keyhole limpet hemocyanin (KLH)-conjugated tau peptide Cry-
358DNITHVPGGGN
368 was synthesized and used as an antigen to immunize C57BL/6 J mice for 4 times within two months. Antisera were pooled and the immunoreactivities to tauN368 and full-length hTau were tested by ELISA. Two mice with well serum immunization validated by Western blotting were selected, whose spleens were harvested for cell fusion. Positive hybridoma cells were selected by hypoxanthine-aminopterin-thymidin (HAT) medium, and the positive hybridoma cells were subcloned by limited dilution method to obtain monoclonal cell lines. During each subcloning, indirect ELISA screening was performed for 2–3 rounds to obtain positive monoclonal cell lines and then the antibody subtypes of the constructed cell lines were identified. Balb/c mice were injected with selected cell line, and ascitic fluid was purified for TauN368 antibody (titer > 1:64,000). All TauN368 Western blotting results were obtained using the newly generated antibody, while all TauN368 immunostaining results were obtained using the polyclonal rabbit anti-TauN368 antibody.
Western blotting
Mouse brains were removed, and the cortex and hippocampus were dissected on ice, respectively. Samples were homogenized with RIPA lysis buffer containing 50 mM Tris (pH 7.4), 150 mM NaCl, 1% Triton X-100, 1% sodium deoxycholate, and 0.1% SDS (#P0013B, Beyotime, Shanghai, China) mixed with a cocktail of protease and phosphatase inhibitors (Thermo Scientific, Waltham, MA), at a ratio of 10 μl/mg tissue. Then the tissue homogenates were centrifuged for 20 min at 12,000 ×
g, resulting in RIPA-soluble and RIPA-insoluble parts. Protein concentration in the RIPA-soluble lysate was quantitated using BCA protein assay kit (Thermo Fisher), and equal amounts of proteins were loaded on SDS–PAGE gels. The RIPA-insoluble pellets were further dissolved in 90–120 μl of loading buffer containing 200 mM Tris–HCl pH 6.8, 8% SDS and 40% glycerol. Proteins were separated by SDS-PAGE (10%), transferred onto nitrocellulose membranes (Merck Millipore, Darmstadt, Germany) and then blocked with 5% bovine serum albumin (BSA). Membranes were incubated with primary and secondary antibodies (Additional file
1: Table S2), in sequence. Blots were visualized by an enhanced chemiluminescence substrate system (Santa Cruz Biotech, Santa Cruz, CA), imaged by an Odyssey Imaging System (LI-COR Biosciences, Lincoln, NE), and quantified by the ImageJ software. β-Actin was used as a loading control.
Immunostaining and quantification
Mice were anesthetized with 2% isoflurane (RWD Life Science, Shenzhen, China) one day after completion of the final behavioral trial, and perfused via the ventriculus sinister with 0.9% NaCl for 5 min followed by PBS containing 4% paraformaldehyde for 5 min. Brains were cryoprotected sequentially in 25% and 30% sucrose, for 2 days in total, and then cut into 30-μm sections using a cryostat microtome (CM1900, Leica, Wetzlar, Germany). For immunohistochemistry, free floating sections were immersed in 3% H2O2 in anhydrous methanol for 30 min, and non-specific sites were blocked with BSA for 30 min at room temperature. Brain slices were then incubated overnight at 4 °C with primary antibodies. Immunoreactions were developed using a DAB-staining kit (ZSGB-BIO, Beijing, China). Images were taken by an automatic slice scanning system (SV120, Olympus, Tokyo, Japan) at 20 × magnification and analyzed with ImageJ software. Areas of various regions of the brain were measured.
For immunofluorescence, sections were thoroughly washed with PBST (PBS containing 0.1% Triton X-100) and incubated overnight with primary antibodies under 4 °C. After that, the sections underwent PBST washes for 15 min, followed by 1-h incubation with the secondary antibody at 37 °C, and finally counterstained with DAPI. Images were taken by an automatic slice scanning system (SV120, Olympus) and a two-photon laser-scanning confocal microscope (LSM 800, Zeiss, Oberkochen, Germany) at 20 × magnification, and analyzed with the ImageJ software. The areas of hTau368 staining in various regions, the mean intensity of NeuN staining in the CA1 pyramidal layer, as well as the numbers of cells positive for MAP2, GFAP, Iba1 or DCX staining were measured or counted in a single 20 × magnification view and averaged per 0.1 mm2. One section from each mouse and a total of four to six mice per group were analyzed.
Gallyas silver staining
Free-floating brain sections were washed with Tris-buffered saline (TBS) for 3 × 5 min, and then placed in 5% periodic acid followed by alkaline silver iodide solution and developer solution (#G1052, Servicebio, Wuhan, China). After washing with acetic acid and water, they were placed in 0.1% gold chloride, followed by sodium thiosulphate solution. The sections were placed in absolute ethanol for 3 × 5 min and in xylene for 2 × 5 min to become transparent. Images were taken by an automatic slice scanning system (SV120, Olympus, Tokyo, Japan) and analyzed at 100 × magnification.
Thioflavin S staining
Free-floating brain sections were washed with TBS for 3 × 5 min, and then incubated with 0.3% Thioflavin S (#1326-12-1, Sigma, Darmstadt, Germany) dissolved in 50% ethanol. The sections were then decolorized in 50% ethanol for 3 × 5 min, washed in TBS, and subsequently co-stained with DAPI for 10 min.
Sholl analysis
Fluorescent images of GFAP and Iba1 immunostaining were analyzed as reported previously [
27]. Processes were traced by the plugin Sholl Analysis in ImageJ. The intersections of processes were counted within concentric circles at 10-μm intervals from the center of the soma. Ending radius and the sum of intersections were used to indicate the complexity of processes.
Electron microscopy
Synaptic density and neuronal axon width were determined by electron microscopy as described previously [
28]. After deep anesthesia, mice were perfused transcardially with 2% glutaraldehyde and 3% paraformaldehyde in PBS. Hippocampal slices were post-fixed in cold 1% OsO
4 for 1 h. Samples were prepared and examined using standard procedures. Ultrathin sections (90 nm) were stained with uranylacetate and lead acetate and viewed at 200 kV in a Tecnai electron microscope. The slices were from three mice in each group. Synaptic contacts were identified by the presence of synaptic vesicles and post-synaptic densities with high electron density.
Transcriptomic analysis
Hippocampal tissues were isolated on ice. The detailed procedures of total RNA extraction, mRNA library construction and quality assurance has been reported previously [
29]. The differentially expressed genes (DEG) between vehicle-treated (
n = 3) and Dox-treated hTau368 mice (
n = 3) (FDR-adjusted
P < 0.05, fold change > 1.5) were analyzed by DESeq2 (v1.4.5) [
30]. Kyoto encyclopedia of genes and genomes (KEGG) enrichment and the relationship network of KEGG pathways of DEGs were analyzed by Dr. Tom software (BGI, Shenzhen, China). Gene network analysis was performed using the STRING database (
https://string-db.org/).
Novel location recognition test
Mice were handled and habituated before tests. On day 1, each mouse was placed in the center of a plastic box in which two identical objects (A and B) were located in two corners. The mouse was allowed 5 min to explore freely. After 24 h, the mouse was placed back to the box with object A in the same corner while object B placed in a new location. Again, each mouse was allowed 5 min to explore. The time the mouse spent exploring objects A and B was recorded as TA and TB, respectively. Exploration was identified by a video tracking system (Anymaze Technology SA, Stoelting Co., IL) once the mouse head stayed close (< 3 cm) to either object. Mice with TA or TB less than 2 s were excluded from analysis. The discrimination index was calculated as (TB−TA)/(TB + TA). A higher discrimination index indicates better spatial memory.
Morris water maze test
Mice were kept in the test room for 24 h before tests. In the learning phase, mice were trained in the maze to find the hidden platform. The learning phase consisted of 3 trials per day with an interval of 30 min, from 14:00 to 17:00, for 5 consecutive days. In each trial, a mouse was placed in one of the three quadrants without the platform, facing the wall of the pool. If the mouse found the hidden platform within 60 s, another 15 s was left for learning consolidation. If the mouse did not find the platform within 60 s, it was guided to the platform and allowed to stay on the platform for 15 s. The time to find the platform during the 5-day training was recorded as the escape latency. On day 6, a testing trial was performed. The hidden platform was removed and each mouse was placed in the quadrant opposite to the target quadrant. The time/distance and trajectory of each mouse traveling in the pool were recorded and analyzed by a video tracking system (Chengdu Taimeng Software Co., Ltd, China). Mice disabled in eyes or limbs were excluded from analysis.
Open field test
Mice were handled for 1 day, and placed in the test room the day before the behavioral test to get acclimated to the environment. The open field apparatus was a 60 × 60 × 50 cm3 white plastic box, with the floor divided virtually into 16 equal squares in the monitoring system, including a central field (the central 4 square regions) and 12 periphery fields. Each mouse was allowed to explore freely in the box for 5 min. The time and distance each mouse travelled in different zones were recorded and analyzed by the ANY-maze video tracking system (Stoelting Co., WoodDale, IL).
Elevated-plus maze test
The elevated-plus maze consisted of two enclosed arms (65 × 5 × 20 cm3) and two open arms (65 × 5 cm2). The apparatus was elevated to 50 cm above the floor. Each mouse was placed in the center of the maze, facing the open arm opposite to the experimenter, and allowed to explore freely for 5 min. The time and place mice traveled in the maze were recorded and analyzed using a video tracking system (Chengdu Taimeng Software Co., Ltd, China).
Statistical analyses
All data were analyzed and plotted using GraphPad Prism 8 (La Jolla, CA). Comparisons between two groups were made by two-tailed unpaired Student’s t-tests, and comparisons between multiple groups were conducted with one-way, two-way or repeated measures ANOVA followed by post-hoc tests for multiple comparisons. P < 0.05 was considered statistically significant. All experiments and analysis were performed in a blind manner. All values are shown as mean ± SEM or min-median-max unless otherwise specified.
Discussion
In the present study, we generated a hTau368 transgenic mouse line with inducible and efficient expression of truncated hTau368, which naturally exists in the brain and increases during aging and AD [
25,
29,
49]. Dox induced hTau368 expression most predominantly in the hippocampus, the best-recognized area responsible for the spatial learning and memory loss in the early stage of AD, together with AD-like gliosis, neurodegeneration and cognitive deficits. In addition to the hippocampus, minor expression of hTau368 was also detected in other brain regions, including piriform, entorhinal cortex, cortex, cerebellum, brainstem and spinal cord. Coincidentally, prominent tau pathology in the brains of AD patients has been also observed in the entorhinal cortex-hippocampus, from which tau pathology gradually propagates to the limbic system and finally to the whole brain [
6,
50]. Therefore, the established hTau368 mouse model in the present study well mimicked the initiation and progression of tau pathology in AD.
To precisely control the starting age and the duration of hTau expression to meet different experimental requirements, we designed the tet-on system to control the neuron-specific Eno2 promoter-driven expression of hTau368. Taking advantage of this system, here we found that dox treatment at the age of 8 weeks, consecutively for 2 months, was sufficient to induce overt tau hyperphosphorylation and accumulation in the hippocampus. On the one hand, starting dox administration in adulthood avoids the potential risks of artificial perturbation of embryonic and infantile development by hTau expression. On the other hand, dox treatment at relatively young ages of adulthood is time- and resource-saving in establishing overt tau pathology, which may otherwise occur at 6–9 months of age in most traditional tau-related mouse models [
13,
14,
19]. The highly efficient expression of hTau in the hTau368 mice under dox induction may be due to strong power of TRE in driving gene expression [
31].
It should be noted that we used the tet-on system to express the truncated hTau N1-368 in hTau368 mice, instead of intact full-length of WT or mutant hTau as seen in other models. This model has the following advantages. First, the tau N1-368 fragment, generated from AEP cleavage, accumulates in neurons during aging and AD, and shows stronger toxicity to neurons than full-length hTau and other hTau fragments [
25,
29]. The truncated tau could interact with full-length tau to gain neurotoxicity [
51,
52]. We also previously found that expressing hTau368 in normal C57 mice caused more significant hippocampal neurodegeneration and spatial cognitive impairments than expressing WT full-length hTau [
29]. Second, Unlike other types of tauopathy, such as frontotemporal dementia with parkinsonism-17 (FTDP-17) which bears R5L, P301L and R406W mutations in tau [
20,
53], to date, no mutations in the tau gene have been identified in AD. Serious motor deficits and reduction of spinal cord motor neurons, associated with neurogenic muscle atrophy and peripheral neuropathy, have been reported in mice with tau mutations at 4–6 months of age, while these symptoms are not observed in patients at early stages of AD [
22‐
24] nor in our hTau368 mouse model. Third, by using the dox-on and off system in this mouse line, precise control of the abnormal tau expression was achieved. Therefore, this mouse model could be used for in-depth exploration of the molecular mechanisms underlying tau accumulation and its neural toxicity, and for tauopathy-related drug development.
However, it should be noted that when measuring tau levels in this hTau368 line, the increases of total and phosphorylated tau appeared to be most prominently at the molecular weight of 55 kD, while previous studies had recognized tau fragments cleaved at N368 at about 15–55 kD, with the most dominant change found at 37 kD [
25,
26]. This discrepancy might be partly due to the different antibodies used. In fact, to our knowledge, the accurate molecular weight of tau cleaved at N368 is quite variable and difficult to define. Tau proteins have at least 6 isoforms plus multiple posttranslational modifications, therefore they cover a wide range of apparent molecular mass. In particular, the molecular weight of tau1-368 might vary a lot when AEP or other enzymes cleave 0N4R, 1N4R and 2N4R tau at the same N368 site. Therefore, it is hard to precisely discriminate which blot indicates the tau1-368 fragment simply by Western blotting. Therefore, it would be more objective in future studies to measure all tau bands on the Western blotting membrane, including the 37 and 55 kD blots, to evaluate the tau level when using the hTau368 mouse model.
Several studies have identified “tau filament cores” for its aggregation or toxicity. For instance, a previous study demonstrated that the tau 306–379 region, which contains 10 more amino acids than hTau368, is required for PHF/SF formation in AD [
54]. However, it remains unknown whether the additional tau
369KKIETHKLTF
379 residues are necessary for the formation of tau filament core. Other studies have revealed a pivotal role of
306VQIVYK
311, which is included in the tau 1–368 fragment, in the aggregation of tau [
55], and peptides designed against this
306VQIVYK
311 fragment are capable of inhibiting tau fibril formation [
56,
57]. In postmortem brains of AD patients, tau fragments ending at the N368 site are present in NFTs [
58]. These data together strongly support the feasibility of tau 1–368 fragment in mimicking the AD-like tau model.
Unexpectedly, we found here that the tau-associated pathologies and cognitive impairment induced by 2-month dox treatment in hTau368 mice gradually relieved following dox withdrawn for 3 months. This is consistent with a previous report that in a TAU62 transgenic line with dox-dependent 3R tau 151–421 expression, the nerve cell dysfunction and severe paralysis generally recovered when the expression of tau was halted [
51]. In another line rTg4510 which uses a tet-off system to control the expression of mutant P301L hTau, tau-associated pathologies like NFTs, neuronal loss, forebrain atrophy and memory impairments were also ameliorated following the cessation of hTau expression [
16]. The mechanism for this phenomenon may involve activation of a compensatory system to remove pathological tau, such as activation of the proteasome or lysosomal proteolysis system, and modulation of tau-associated protein kinases and phosphatases which regulate tau phosphorylation and thus indirectly regulate tau degradation. Simultaneously, synaptic remodeling and loss of hippocampal neurons ceased when hTau had been eliminated, which confirmed the toxic role of hTau368. It remains to be determined whether the gradual and autonomic alleviation of tau pathology will still happen when dox-on duration persists for a much longer time, like 6–9 months; the underlying mechanisms warrant further investigations to understand the etiology and unravel the drug targets of AD from the perspective of tauopathy. Additionally, we only presented the results of dox-induction in young animals. As AD is an ageing-associated disorder, the dox-induced tau pathology in older animals needs to be studied. We anticipate that the time to the appearance of similar pathologies and behavioral deficits caused by dox induction should be shorter in older mice.
Moreover, in the brains of the elderly and AD patients, there should be more tau x–368 fragments, which are all cleaved at N368 but at different sites from the C-terminal on the other end. It remains to be defined the content of each fragment in human AD brains, especially the longest 1–368 fragment. We designed the tau368 mice here to express hTau 1–368 since it showed relatively higher cytotoxicity than many other tau fragments [
25], while it is also possible for the hTau1-368 fragment to be further cleaved by murine AEP or other endogenous enzymes to generate smaller-size and toxic tau species, like what happens in human AD brains. In addition to the hTau368 mice, other transgenic mouse lines expressing truncated hTau, such as tau159–391 which is also found in AD patients [
59], might also be good tools to study tau-associated pathologies in AD.
In previously reported amyloid models, such as PDAPP [
60], APPswe/PSEN1dE9 (JAX034832) and 5 × FAD (JAX034848), the age-dependent increase of Aβ pathology is significant while tau pathologies are minor or appear at a relatively old age [
36,
61]. As for tau models such as Tau3R0N (hMAPT3R0N, JAX003741), P301L-Tau0N4R (rTg4510, JAX015815 and 016198) and P301S-Tau1N4R (PS19, JAX008169), no significant amyloid deposition is reported [
10,
36,
37]. In the present study, we observed that hTau368 mice shared several phenotypes with the amyloid-driven lines, including the increased gliosis, synaptic degeneration and cognitive deficits, but without significant amyloid deposition. These data support that tau pathology could occur independently or downstream of amyloid deposition [
62]. Nevertheless, future studies should detect whether induction of hTau368 expression in old animals or its expression for longer times could induce amyloid pathology.