Escitalopram Alleviates Alzheimer’s Disease-Type Tau Pathologies in the Aged P301L Tau Transgenic Mice

Abstract

Background:

The inhibition of tau hyperphosphorylation is one of the most promising therapeutic targets for the development of Alzheimer’s disease (AD) modifying drugs. Escitalopram, a kind of selective serotonin reuptake inhibitor antidepressant, has been previously reported to ameliorate tau hyperphosphorylation in vitro.

Objective:

In this study, we determined whether escitalopram alleviates tau pathologies in the aged P301L mouse.

Methods:

Mice were intraperitoneal injected with either escitalopram or saline for 4 weeks, and a battery of behavioral tests were conducted before tissue collection and biochemical analyses of brain tissue with western blot and immunohistochemistry.

Results:

Wild-type (Wt) mice statistically outperformed the aged pR5 mice in the Morris water maze, while escitalopram treatment did not significantly rescue learning and memory deficits of aged pR5 mice. Tau phosphorylation at different phosphorylation sites were enhanced in the hippocampus of aged pR5 mice, while escitalopram treatment significantly decreased tau phosphorylation. The levels of phosphorylated GSK-3β and phosphorylated Akt were significantly decreased in the hippocampus of aged pR5 mice, while escitalopram administration markedly increased the expression level. The aged pR5 mice showed significant decreases in PSD95 and PSD93, while the administration of escitalopram significantly increased PSD95 and PSD93 to levels comparable with the Wt mice.

Conclusion:

The protective effects of escitalopram exposure during advanced AD are mainly associated with significant decrease in tau hyperphosphorylation, increased numbers of neurons, and increased synaptic protein levels, which may via activation of the Akt/GSK-3β signaling pathway.

Keywords

Alzheimer’s disease selective serotonin reuptake inhibitor synapse tau hyperphosphorylation transgenic mice

INTRODUCTION

Alzheimer’s disease (AD) is the most common progressive neurodegenerative disorder in the elderly, affecting millions of people worldwide [1]. With the development of disease, patients with AD show deterioration of cognitive functions, especially progressive loss of memory [2]. Currently available drugs can only ameliorate symptoms of AD but are unable to reverse or even slow down the disease process [3]. Extracellular senile plaques formed by deposits of amyloid-β (Aβ) and intracellular neurofibrillary tangles formed by the accumulation of abnormally phosphorylated intracellular tau filaments are two core neuropathological features of AD [4]. Aβ and tau oligomers have been shown to disrupt cellular cytoskeleton and synaptic integrity as well as long term potentiation and, most importantly, synaptic spines and synaptic communication that leads to progressive cognitive decline [5]. However, targeting Aβ was shown to be ineffective in early intervention studies in patients with mild cognitive impairment and prodromal AD [6]. The role of tau is more agreed upon compared to the controversy around the consequential role of Aβ in the disease process. In fact, tau pathology follows the Braak and Braak staging of AD and correlates well with the timeline of neurodegeneration and dementia progression [7]. Neurofibrillary tangles positively correlated with the severity of clinical dementia in AD, implicating hyperphosphorylation as a potent inducer of tau pathology [8]. Tau-targeted immunotherapy is the new direction after the failure of Aβ-targeted immunotherapy and the outcomes with tau immunotherapy appear effective and promising [9]. Therefore, the inhibition of tau hyperphosphorylation is one of the most promising therapeutic targets for the development of AD modifying drugs.

As a selective serotonin reuptake inhibitor (SSRI), the antidepressant escitalopram has been used to treat major depressive disorder [10]. A large body of research indicates that SSRI antidepressants have a variety of neuroprotective effects [11]. Preclinical studies have also demonstrated a favorable cognitive-improving effect of SSRIs [12, 13]. SSRIs are also reported to increase neurotrophic factors including brain-derived neurotrophic factor (BDNF), promote neurogenesis in the hippocampus, and reduce levels of toxic Aβ [14 –16]. Due to the broad neuroprotective effects of SSRI, accumulating studies have attempted to use SSRI to treat neurodegenerative disorders such as AD. It has been shown that SSRI treatment can ameliorate cognitive deficits in patients with mild cognitive impairment or in AD patients with depression complications [17 –19]. However, whether SSRIs ameliorates AD pathological processes, especially tau pathology, is not investigated yet.

Interestingly, our previous study showed that escitalopram, one of the SSRIs, attenuated forskolin-induced tau hyperphosphorylation in human embryonic kidney cells that stably express human longest tau isoform tau441 (HEK293/tau441 cells) and Aβ-induced tau hyperphosphorylation in primary neurons [20, 21]. In the current study, we employed P301L tau transgenic pR5 model of tauopathy to investigate the effects of escitalopram on tau pathologies and cognitive impairments. We found that in aged pR5 mice, escitalopram attenuates tau hyperphosphorylation, enhances synaptic plasticity, and increases Akt/GSK-3β activity. Hence, our findings implicate that escitalopram might be a promising drug for treating tau-related diseases including AD.

MATERIALS AND METHODS

Animals

PR5 mice, which overexpress the longest human tau isoform together with the P301L mutation on the C57BL/6 background, were gifts from Dr. Yan-Jiang Wang (Department of Neurology, Daping Hospital, Third Military Medical University, Chongqing, China). Male non-transgenic wild-type (Wt) control mice (a hybrid of 129/Sv and C57BL/6 mice) and pR5 mice were housed in clean polypropylene cages (4– 5 mice per cage) and maintained in a light cycle-controlled (12 h light/12 h dark cycle) environment with free food and water. Animal experiments were performed according to the ‘Policies on the Use of Animals and Humans in Neuroscience Research’ revised and approved by the Society for Neuroscience in 1995, and all animal studies were approved by the Academic Review Board of Medical College of Southeast University.

Experimental procedures

The 10-month-old pR5 mice were treated by escitalopram with a dosage of 10 mg/kg body weight, which was dissolved in distilled water at a concentration of 1 mg/ml. The control animals were administered with the same volume (1 ml/100 g) of the saline. The drug or saline was used to treat the animals for 4 weeks by intraperitoneal injection.

Behavioral tests

Morris water maze

Briefly, Morris water maze (MWM) tests were conducted in a circular pool (122 cm in diameter and 58 cm high), which was virtually divided into four quadrants and filled with ∼23– 25°C opaque water of 22 cm in depth. An escape platform was placed in a constant position 1.2 cm below the water surface in the third quadrant. An opaque curtain surrounding the pool eliminated external environment cues, and some visual cues such as a triangle, circle, or other shapes of various colors, were fixed on the inner wall of the pool. The animals underwent four training trials (one trial per quadrant) per day in 30 min intervals for 5 consecutive days. In each training trial, mice were faced towards the pool wall and placed into the water at a semi-random start location. If the mice found the platform within 60 s, or if searching time was longer more than 60 s, the training trial was automatically terminated. The animals which could not find the platform within 60 s were manually guided to the platform and stayed there for 30 s. The platform was removed, and mice were tested to record the latency to find the platform in the target zone, as well as the number of times crossing the platform during test trials. The swimming traces and times of every mouse were recorded using a computerized tracking system.

Sucrose preference test

All animals were first acclimatized over 48 h to drink a 1% sucrose solution. During the test, rats were given a 24 h exposure to two bottles, one containing 1% (wt/vol) sucrose and the other tap water. The positions of the two bottles (right/left) were interchanged and were randomly varied from trial to trial. Sucrose preference was calculated according to the following ratio: sucrose preference=sucrose intake (g)/[sucrose intake (g)+water intake (g)] × 100%.

Open field test

The open field test was conducted in a 50 × 50 × 30 cm black Plexiglas box with a black floor. The times of standing on the hind legs (rearing) of rats and total distance traveled were set up with a digital video camera in a 5 min session. The apparatus was cleaned with a detergent and dried after occupancy by each mice. The observer that was blinded to experimental groups manually assessed rearing behavior using a computerized system provided in the software.

HE staining

After air-drying for 1 h, paraformaldehyde-fixed frozen sections were placed onto the slides and immersed into HE dye liquor for 5 min. The sections were then decolored in 95% alcohol twice, followed by transparency in xylene twice. The sections were sealed, dried in a fume hood, and imaged using a light microscope. Every 10th section through the entire region of the hippocampus was used for neuronal area analysis. Six sections corresponding to the brain region of interest (Bregma 1.5 mm to 2.8 mm) were analyzed per mouse. The analysis of neuronal area was measured using the mean optical density and performed by two independent investigators (blinded to mouse identity) by using the color threshold function of ImageJ 1.49 m software (National Institutes of Health, Bethesda, MD, USA).

Western blotting

Mice were deeply anaesthetized and decapitated. Then, the hippocampus was rapidly removed from the brain and homogenized at 4°C with 50 mM Tris-HCl (pH 7.4), 150 mM NaCl, 10 mM NaF, 1 mM Na3VO4, 5 mM EDTA, 2 mM benzamidine, and 1 mM phenylmethylsulfonyl fluoride. The extract was mixed with sample buffer (3 : 1 vol/vol; 200 mM Tris-HCl [pH 7.6], 8% sodium dodecyl sulfate, 40% glycerol, and 40 mM dithiothreitol) boiled for 10 min, and the total protein concentration was measured with the Pierce BCA Protein Assay Kit (ThermoFisher Scientific). Equal amounts of protein were isolated on 10% SDS polyacrylamide gel (SDS-PAGE) and transferred to polyvinylidene difluoride (PVDF) membranes (Immobilon Transfer Membrane, Millipore, USA). The membranes were blocked with 5% nonfat milk at room temperature and incubated with the following primary antibodies at 4°C overnight. The primary antibodies were diluted as follows: synaptophysin (1 : 1000; Abcam), PSD93 (1 : 2000; Abcam), PSD95 (1 : 1000; Abcam), pT231 (1 : 1000; Abcam), pS396 (1 : 1000; Abcam), Tau-1 (1 : 1000; Millipore), Tau-5 (1 : 1000; Abcam), GSK3β (1 : 1000; Cell Signaling), pGSK3β (ser9) (1 : 1000; Cell Signaling), Akt (1 : 1000; Cell Signaling), pAkt (ser473) (1 : 1000; Cell Signaling), or GAPDH (1 : 4000; Sigma). The blots were developed with horseradish peroxidase– conjugated secondary antibodies (1 : 5000; Pierce), visualized by an enhanced chemiluminescent substrate kit, and exposed to CL-XPosure film. The immunoreactivity of the protein bands was quantitatively analyzed by a LAS400 mini system (GE Healthcare, USA). The levels of GAPDH protein were expressed as the relative level of the mean optical density against the control.

Immunohistochemistry

Mice were deeply anaesthetized and transcardially perfused with 300 ml normal saline, followed by 400 ml ice-cold phosphate buffer containing 4% paraformaldehyde. The brain was removed and post-fixed at 4°C in 4% paraformaldehyde for 24 h. Coronal serial sections were cut at 25 mm thickness with a vibratome (VT1000S, Leica, Nussloch, Germany) through the extent of the hippocampus and collected in 0.1 M phosphate buffer (PB, pH 7.5) containing 0.02% NaN3 and stored at 4°C. Each section anatomically matched to the hippocampus was used for all staining procedures and for cell counting.

After washing in phosphate-buffered saline (PBS, pH 7.5), the sections were permeabilized with PBS containing 3% H₂O₂ and 0.5% Triton X-100, and blocked with 5% BSA for 1 h. Subsequently, the sections were incubated with primary antibodies at 4°C overnight, followed by incubation with secondary and tertiary antibodies for 1 h each at room temperature. The sections were dyed for 10 min in the dark using diaminobenzidine (DAB), and mounted on the slide, followed by drying in a fume hood. After dehydration with successive alcohol washes, the sections were immersed in xylene to produce transparency, and the slide was sealed. The sections were imaged using a light microscope.

Fig.1

The effects of escitalopram treatment on learning and spatial memory abilities detected by Morris water maze.A represents the latency to find the platform during training. B and C represent the distance percent in target quadrant and the number of crossing platforms in different groups respectively. D represents the average speed of different groups. All data were expressed as mean±SEM (n = 12 mice each group in MWM test). WT + Saline: the wild-type mice administered with the saline vehicle; WT + Escit: the wild-type mice administered with escitalopram; Tg + saline: pR5 AD mice administered with the saline vehicle; Tg + Escit: pR5 AD mice administered with escitalopram. *p < 0.05.

Statistical analysis

Data were presented as mean±standard error of the mean (SEM). One-way analysis of variance (ANOVA) followed by Tukey post hoc test were used to compare the differences between means in more than two groups by GraphPad Prism 6.01. A probability value of p < 0.05 was considered to be statistically significant.

RESULTS

Effects of escitalopram on behavioral changes in aged pR5 mice

To study whether escitalopram rescues cognitive defects in the P301L tau transgenic pR5 model of tauopathy, MWM test was carried out. During the training period, compared with the Wt mice, the pR5 mice exhibited significantly prolonged times in finding the hidden platform on days 5 of the training (Fig. 1A). In addition, the percentage of the time spent in the target quadrant and the number of target crossings of the pR5 mice were reduced compared with those of the Wt mice (Fig. 1B,C). However, treatment of escitalopram had no significant effect on restoring the latency to hidden platform on days 5 (Fig. 1A), distance percent in target quadrant, and numbers of crossing platform to control level (Fig. 1B,C). There was no difference in swimming speed among control and experimental groups (Fig. 1D), indicating that did not affect the motor activity. Thus, our data suggest that escitalopram treatment did not significantly rescued learning and memory deficits of aged pR5 mice.

Fig.2

The effects of escitalopram on depression and anxiety-related behaviors.A) The sucrose preference test was used to assess the reward sensitivity behavior. B– D) The open field test was used to test the anxiety/exploration-like levels of the mice. The results are expressed as the mean±SEM (n = 12), WT + Saline: the wild-type mice administered with the saline vehicle; WT + Escit: the wild-type mice administered with escitalopram; Tg + saline: pR5 AD mice administered with the saline vehicle; Tg + Escit: pR5 AD mice administered with escitalopram.

Escitalopram is an SSRI that is primarily used as an antidepressive or anti-anxiety therapy in the clinic; thus, we examined the depression- and anxiety-like behaviors of aged pR5 mice. The sucrose consumption test was used to assess anhedonia, which is considered as a core symptom of human depression. No significant difference in the intake of the 1% sucrose solution was observed between the four groups (Fig. 2A). Next, we investigated anxiety-like behavior using the open field test. No significant difference in the total distance travelled, the distance percentage and the time percentage travelled in the central area within 5 min between the four groups (Fig. 2B– D). Our data indicated that there is no depression-like and anxiety-like behavior in aged pR5 mice.

Fig.3

The effects of escitalopram treatment on tau proteins in mice hippocampus. Western blotting and quantitative analysis of tau phosphorylation at pS396, pT231, Tau-1 epitopes, and total tau measured in mice hippocampus. The phosphorylation level of tau was normalized to GAPDH. The results are expressed as the mean±SEM (n = 3); WT + saline: the wild-type mice administered with the saline vehicle; WT + Escit: the wild-type mice administered with escitalopram; Tg + Saline: pR5 AD mice administered with the saline vehicle; Tg + Escit: pR5 AD mice administered with escitalopram. *p < 0.05; **p < 0.01.

Escitalopram decreased tau hyperphosphorylation in aged pR5 mice

To further investigate whether escitalopram treatment decreases tau phosphorylation in aged pR5 mice, we firstly carried out western blot to detect the level and phosphorylation of tau in the hippocampus. The expression levels of pS396, pT231, and tau-5 were enhanced, and that of Tau-1 was decreased in the hippocampus of pR5 mice compared with the levels in the Wt mice (Fig. 3). Escitalopram treatment significantly decreased levels of pS396 and pT231, and increased the levels of Tau-1 in the hippocampus of pR5 mice (Fig. 3). Immunohistochemistry showed positive immunostaining of pS396 in hippocampal DG, CA1, and CA3 regions of pR5 mice treated with the vehicle (Fig. 4), while escitalopram treatment significantly ameliorated the immunostaining of pS396 in the hippocampal DG, CA1, and CA3 regions of pR5 mice (Fig. 4), which is consistent with western blot results. These data strongly indicate that escitalopram treatment decreased tau hyperphosphorylation in aged pR5 mice.

Fig.4

The effects of escitalopram treatment on tau phosphorylation in mice hippocampus detected by immunohistochemistry staining with pS396 antibody.A– D) Representative images of pS396 immunostaining in the hippocampal DG region of mice. Scale bars = 100μm. E– H) Representative images of pS396 immunostaining in the hippocampal CA1 region of mice. Scale bars = 100μm. I– L) Representative images of pS396 immunostaining in the hippocampal CA3 region of mice. Scale bars = 100μm. WT + saline: the wild-type mice administered with the saline vehicle; WT + Escit: the wild-type mice administered with escitalopram; Tg + Saline: pR5 AD mice administered with the saline vehicle; Tg + Escit: pR5 AD mice administered with escitalopram.

Fig.5

The effects of escitalopram treatment on Akt/GSK3β pathway in mice hippocampus. Western blotting and quantitative analysis for the levels of ser-9-phosphorylated GSK-3β, the Ser-473-phosphorylated Akt, total GSK-3β, and total Akt measured in mice hippocampus. GAPDH was used as a loading control in each sample. The results are expressed as the mean±SEM (n = 3); WT + saline: the wild-type mice administered with the saline vehicle; WT + Escit: the wild-type mice administered with escitalopram; Tg + Saline: pR5 AD mice administered with the saline vehicle; Tg + Escit: pR5 AD mice administered with escitalopram. *p < 0.05; **p < 0.01.

Escitalopram upregulated the Akt/GSK-3β pathway in aged pR5 mice

GSK-3β is a key kinase that regulates tau phosphorylation. Akt is the upstream regulator of GSK-3β that is responsible for its phosphorylation in mammalian cells. We measured the levels of GSK-3β, phosphorylated GSK-3β and Akt, phosphorylated Akt in the hippocampus of pR5 mice and Wt mice by western blotting. We found that, compared with Wt mice, the levels of phosphorylated GSK-3β and phosphorylated Akt were significantly decreased in the hippocampus of aged pR5 mice (Fig. 5), while escitalopram administration markedly increased the expression of phosphorylated GSK-3β and phosphorylated Akt in the hippocampus (Fig. 5). No changes in the total level of GSK-3β and Akt were detected between the different groups (Fig. 5). These data indicate that upregulation of the Akt/GSK-β pathway may underlie the escitalopram-decreased tau pathology in hippocampus.

Escitalopram recovered a panel of synaptic proteins in aged pR5 mice

To further investigate whether tau overexpression and escitalopram treatment affect the synapses, we carried out western blot to examine synapse-associated proteins (synaptophysin, PSD93, and PSD95) in the hippocampus. The results revealed significant decreases in PSD95 and PSD93 in the pR5 mice (Fig. 6). The administration of escitalopram significantly increased PSD95 and PSD93 to levels comparable with the control (Fig. 6).

Fig.6

The effects of escitalopram treatment on synapse-associated proteins in mice hippocampus.Western blotting and quantitative analysis for the levels of PSD93, PSD 95, and synaptophysin measured in mice hippocampus. GAPDH was used as a loading control in each sample. The results are expressed as the mean±SEM (n = 3); WT + saline: the wild-type mice administered with the saline vehicle; WT + Escit: the wild-type mice administered with escitalopram; Tg + Saline: pR5 AD mice administered with the saline vehicle; Tg + Escit: pR5 AD mice administered with escitalopram. *p < 0.05; **p < 0.01.

We also detected the total cell numbers by HE staining. The results showed that the cell numbers in hippocampus of aged pR5 mice was significantly reduced compared with Wt mice (Fig. 7), and treatment of escitalopram increased the cell numbers in hippocampus of pR5 mice (Fig. 7). Thus, our data strongly support that escitalopram treatment had a potential pharmacological interference in restoring the synaptic plasticity.

Fig.7

The effects of escitalopram treatment on cell numbers in mice hippocampus detected by HE staining. A– D) Representative images of HE staining in the hippocampus of mice. Scale bars = 100μm. E– H) Representative images of HE staining in the hippocampal DG region of mice. Scale bars = 100μm. I– L) Representative images of HE staining in the hippocampal CA1 region of mice. Scale bars = 100μm. M-N) Quantification of the cellular area in hippocampal DG and CA1. Data were expressed as mean±SEM (n = 3 mice per group). WT + saline: the wild-type mice administered with the saline vehicle; WT + Escit: the wild-type mice administered with escitalopram; Tg + Saline: pR5 AD mice administered with the saline vehicle; Tg + Escit: pR5 AD mice administered with escitalopram. *p < 0.05; **p < 0.01.

DISCUSSION

Currently, treatment for AD focuses on the cholinergic hypothesis and provides limited symptomatic effects. Research currently focuses on other factors that are thought to contribute to AD development such as tau proteins and Aβ deposits. As a series of new drugs for AD failed in clinical trials, it is necessary to choose drugs with both an established safety profile and a mechanism-based rationale for future clinical trials.

The neurotransmitter serotonin has attracted increasing interest due to its involvement in cognition, memory and learning [22], posing the question whether SSRIs, which are approved by the Food and Drug Administration, represent a rational and novel strategy for AD. In the present study, we took such a screen approach and investigated the potential therapeutic effect of escitalopram, a kind of SSRIs that is currently used for the treatment of depression. We found that escitalopram is a potent drug that can rescue the phenotypes of several major AD hallmarks in an AD mouse model. Injection of escitalopram after the onset of tau deposition in these mice reduced the pathology of phosphorylated tau, suppressed neuronal apoptosis, preserved synaptic proteins, though not rescued the cognitive impairment of pR5 tau mice.

PR5 mice, which serve as a model of the neurofibrillary tangle pathology of AD and show hippocampus-dependent behavioral impairments related to AD, have been widely used as AD models. At the age of 3 months, pR5 mice already display high levels of hyperphosphorylated human tau. Neurofibrillary tangles first appear at 6 months in the amygdala, followed by the CA1 region of the hippocampus. At around 9 months of age, pR5 mice present with obvious cognitive impairment [23 –25]. In the current study, we used the 10-month-old pR5 mice with remarkable pathological and behavioral impairments to study whether escitalopram could improve the cognitive abilities and tau pathologies of the mice. The selected dose and treatment protocol used in the present study are referenced by previous studies [16 , 27]. The therapeutic dose of escitalopram for humans is approximately 0.2– 1.0 mg/kg/day, and rodents metabolize escitalopram about ten times faster than humans [28]. Therefore, the currently used dose in mice is equivalent to 20 mg/day in humans, which is a common dose of escitalopram prescribed to treat depression [29]. Besides, escitalopram is one of the fast-acting antidepressant and it has been reported to exert antidepressant actions within 2 weeks in mice [30]. So, here we treated pR5 mice with escitalopram for consecutive 4 weeks to investigate the effects of escitalopram on cognition and tau phosphorylation.

Interestingly, we found that escitalopram alleviates tau pathologies but not cognitive deficits in the 10-month-old pR5 mice treated with escitalopram for consecutive 4 weeks. The reason may be that the time of our intervention is too late or the course of treatment is not enough. AD is a neurodegenerative disease and the pathophysiological process is thought to begin decades before the diagnosis. It is widely accepted that synaptic damage and tau hyperphosphorylation are early events in disease process. Previous reports showed that fluoxetine administration during adolescence attenuates cognitive and synaptic deficits in adult 3×TgAD mice [31]. In the phase III trials of the first monoclonal antibodies bapinezumab, cerebrospinal fluid tau protein was reduced but no significant clinical benefit was seen in the cognitive or functional endpoints assessed [32], which is similar with our results. The clinical failure of bapinezumab could derive from recruiting the wrong population (patients with advanced AD). It posits that some of the patients that are being selected are too advanced in terms of pathology within the disease process, and that therapy may be too late for these patients. A previous study reported that fluoxetine administration to APP/PS1 mice beginning at 2 months and continuing to 9 months significantly suppressed the production of soluble Aβ and improved behavioral performance [33]. In the same APP/PS1 mice, fluoxetine administration beginning at 2 months and continuing for 4 months lowered Aβ products and improved behavioral performance [34]. So, our data suggested that early intervention may be critical for good prognosis, and that cognitive improvement may require longer treatment than recovering tau-related pathology markers. Reinforce the timing and course of intervention will be a critical consideration in our future study.

There is a high prevalence rate (30– 50%) of AD and depression comorbidity. Depression can be a risk factor for the development of AD or it can be developed secondary to the neurodegenerative process [35]. Recent studies have found that SSRIs reduce the risk of AD in aged depressed individuals and have a positive role in hindering the progression. In preclinical studies, a favorable cognitive-improving effect of SSRIs has been proved. SSRIs are also reported to increase neurotropic factors including BDNF, promote neurogenesis in the hippocampus, and reduce levels of toxic Aβ [11]. Accumulation of hyperphosphorylated tau forming neurofibrillary tangles is positively correlated with memory loss in AD patients; therefore, reducing tau phosphorylation may prevent AD progression. Our previous study has also revealed that escitalopram ameliorated forskolin-induced tau hyperphosphorylation in HEK293/tau441 cells [20] and Aβ_1–42-induced tau hyperphosphorylation in primary hippocampal neurons [21]. In the present study, we also found tau hyperphosphorylation at multiple AD-associated sites in aged pR5 mice, while escitalopram supplementation remarkably attenuated tau hyperphosphorylation, which further demonstrated that SSRIs could lessen tau pathology.

The mechanism by which SSRIs inhibit tau hyperphosphorylation is unclear. Akt/GSK-3β is the most implicated signaling pathway in regulating tau phosphorylation [36]. It was demonstrated that stimulation of GSK-3β both in vitro and in vivo induces tau hyperphosphorylation with impairments of the cognitive functions, whereas inhibition of GSK-3β improves tau pathologies and memory deficit [37]. The activation effects of escitalopram, paroxetine, sertraline, and fluoxetine on Akt have been previously reported in hippocampal neuron cultures, neural stem cells, and rat brain [38, 39]; the inhibition effects of fluoxetine on GSK-3β was also reported in mice brain and cultured neural precursor cells [40, 41]. Our present study showed that Akt was activated and GSK-3β was inhibited following escitalopram administration, in agreement with previous results and suggesting that the effect of escitalopram on tau hyperphosphorylation is related to the activation of Akt/GSK-3β signaling pathway. In the past, targeting Aβ pathology has been the major avenue pursued in developing treatments for AD. Several studies have reported that the antidepressant SSRIs suppressed the production of Aβ in AD mice. Fluoxetine administration decreased Aβ levels in the 3×TgAD mice and the underlying molecular mechanisms involving activation of the CREB/BDNF signaling pathway [31]. Another study showed that citalopram administration blocked the amyloid plaque growth in APP/PS1 mice and serotonin-mediated activation of extracellular regulated kinase (ERK) was necessary for this regulation [27]. Here we found that the effect of escitalopram on tau hyperphosphorylation is related to the activation of Akt/GSK-3β signaling pathway. The mechanisms by which SSRIs mediate Aβ pathology and tau pathology are likely to be different. We thus speculate that SSRI could target multiple pathways of the AD pathogenesis and SSRI holds a promise as a therapeutic agent for AD. However, future experiments are still necessary to further define the entire intracellular signaling pathway responsible for the reduced tau hyperphosphorylation following escitalopram treatment.

Synapses are the functional units of neuronal communication, and synaptic dysfunction is directly linked to cognitive disturbances [42]. In AD, synaptic failure strongly correlates with cognitive decline [43]. AD brains show a marked reduction in synaptic density and a loss of dendritic spines in the cortex and the hippocampus [44]. Synaptic impairment is likely to be the major contributor to memory loss in AD [45, 46]. In our research, we assessed synapse-associated protein expression after escitalopram treatment. We found that escitalopram is a strong neuroprotective agent that protects neurons from apoptosis and downregulation of synaptic proteins in pR5 mice. Our previous study has reported that escitalopram significantly enhanced dendritic outgrowth and increased dendritic spines in hippocampal neuron cultures exposed to Aβ_1–42 [21]. Thus, it appears that the SSRIs target multiple key pathways in the pathogenesis of AD, providing greater support to its potential in AD treatment.

In conclusion, we demonstrated that the protective effects of escitalopram exposure during advanced AD are mainly associated with significant decrease in tau hyperphosphorylation, increased numbers of neurons, and increased synaptic protein levels, which may via activation of the Akt/GSK- 3β signaling pathway. Finding effective drugs for the prevention and treatment of AD remains a holy grail for doctors and scientists. Comparing with traditional de novo drug discovery, drug repurposing may speed the process by which insights and inventions are honed, eliminating dead-end approaches and saving time, effort, and cost. Because escitalopram is currently used for depression and proven safe and well tolerated in humans, the promising data from our current study warrant the further clinical trials to test its efficacy in the treatment or prevention of AD.

Footnotes

ACKNOWLEDGMENTS

This work was supported by grants from the National Natural Science Foundation of China (81801075 Yan-juan Wang), Natural Science Foundation of Jiangsu Province (BK20180379 Yan-juan Wang) and Foundation of Jiangsu Commission of Health (Z2018023).

Authors’ disclosures available online ().

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