Thiamme2-G,a Novel O-GlcNAcase Inhibitor,Reduces Tau Hyperphosphorylation and Rescues Cognitive Impairment in Mice

Abstract

Background:

Abnormal hyperphosphorylation of microtubule-associated protein tau plays a pivotal role in Alzheimer’s disease (AD). We previously found that O-GlcNAcylation inversely correlates to hyperphosphorylation of tau in AD brain, and downregulation of brain O-GlcNAcylation promotes tau hyperphosphorylation and AD-like neurodegeneration in mice.

Objective:

Herein we investigated the effect of increasing O-GlcNAcylation by using intermittent dosing with low doses of a potent novel O-GlcNAcase (OGA) inhibitor on AD-like brain changes and cognitive function in a mouse model of sporadic AD (sAD) induced by intracerebroventricular (ICV) injection of streptozotocin (STZ).

Methods:

STZ was injected into the lateral ventricle of C57BL/6J mice. From the second day, Thiamme2-G (TM2G) or saline, as a vehicle control, was orally administered to the ICV-STZ mice three times per week for five weeks. A separate group of ICV-saline mice treated with saline was used as a baseline control. Behavioral tests, including open field and novel object recognition, were conducted three weeks after the first dose of the TM2G or saline. Protein O-GlcNAcylation, tau hyperphosphorylation, synaptic proteins, and neuroinflammation in the mouse brain were assessed by western blotting.

Results:

ICV-STZ caused decreased protein O-GlcNAcylation. Enhancement of O-GlcNAcylation to moderate levels by using low-dose OGA inhibitor in ICV-STZ mice prevented STZ-induced body weight loss, rescued cognitive impairments, and restored AD-like pathologies, including hyperphosphorylation of tau and abnormalities in synaptic proteins and neuroinflammation.

Conclusion:

These findings suggest that moderately increasing protein O-GlcNAcylation by using low doses of OGA inhibitor may be a suitable therapeutic strategy for sAD.

Keywords

Alzheimer’s disease O-GlcNAcylation OGA OGA inhibitor tau hyperphosphorylation Thiamme2-G

INTRODUCTION

Intracellular neurofibrillary tangles (NFTs) composed of abnormally hyperphosphorylated tau protein are a histopathological hallmark of Alzheimer’s disease (AD) and related neurodegenerative diseases known as tauopathies, which include corticobasal degeneration, frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17), chronic traumatic encephalopathy, progressive supranuclear palsy, Pick disease, and parkinsonism dementia complex of Guam [1, 2]. Abnormally hyperphosphorylated tau promotes its assembly into paired helical filaments/NFTs [3, 4], and sequestration of normal tau into tangles of filaments and disassembly microtubules [5]. Importantly, the density of NFTs strongly correlates with cognitive impairment in patients with AD [6, 7]. Mutations of tau protein, as seen in FTDP-17, make tau more abnormally hyperphosphorylated than the wild-type tau protein [8] and drive neurodegeneration in human and animal models of tauopathies [9]. Thus, inhibition of abnormal hyperphosphorylation of tau offers a promising therapeutic target for AD and related tauopathies. Previous studies from our lab and others suggest that decreases in O-GlcNAcylation of tau protein contribute to tau hyperphosphorylation in AD [10 –14].

O-GlcNAcylation is a dynamic posttranslational modification of serine/threonine residues of various nuclear and cytosolic proteins [15, 16]. The enzyme O-GlcNAc transferase (OGT) transfers GlcNAc from the sugar nucleotide UDP-GlcNAc to the hydroxyl group of serine/threonine residues of proteins [15, 17]. The enzyme OGA hydrolyzes O-GlcNAc from modified proteins, liberating the hydroxyl group of the protein and GlcNAc [15, 18]. O-GlcNAcylation is involved in diverse cellular processes such as cell cycle, nutrient sensing, stress response, proteasomal regulation, and the response to insulin [19]. Given that phosphorylation also occurs on serine and threonine residues of proteins, O-GlcNAcylation and phosphorylation can cross-talk to regulate protein functions [15, 19]. We and others discovered previously that O-GlcNAcylation regulates tau hyperphosphorylation [11, 20]. Significant reduction of O-GlcNAcylation of global brain proteins and of tau is also seen in AD [11 , 21]. The decrease in O-GlcNAcylation negatively correlates to hyperphosphorylation of tau in AD [12]. Importantly, tau is thought to be extensively modified by O-GlcNAc [11, 22]. Decreased brain glucose metabolism induced by fasting leads to decreased O-GlcNAcylation and increased hyperphosphorylation of tau at several AD-associated sites in rodents [23 –25]. Several independent studies reported that dramatic and sustained elevation of protein O-GlcNAcylation via daily administration of high doses of OGA inhibitors alleviates tau pathology, prevents neuronal loss, and improves cognitive function in transgenic mouse models overexpressing human mutant tau [26 –31]. Little is known, however, about the potential for increased O-GlcNAc offering protection in sporadic models of AD. In addition, the effects of increasing O-GlcNAc to moderate levels, as opposed to the high-level increases seen in previous studies, not been well explored.

Periphery application of streptozotocin (STZ), a glucosamine-nitrosourea compound derived from soil bacteria and originally developed as an anticancer agent, has been widely used experimentally for generating rodent models of diabetes mellitus because of its ability to kill pancreatic β cells and to induce insulin resistance [32]. In addition, intracerebroventricular (ICV) injection of STZ in rodents can cause AD-like phenotypes, such as memory impairment [33, 34], hypometabolism of glucose, oxidative stress and mitochondrial abnormalities [33, 35], cholinergic deficiency [36], astrogliosis and neuronal degeneration [37], interruption of brain insulin signaling and tau hyperphosphorylation [34, 38], and increase in Aβ level [34]. Thus, the ICV-STZ mouse model has been used as a model of sporadic AD (sAD) [39].

Inefficient insulin signaling activity and glucose transporters can both lead to decreased intracellular glucose metabolism that, in turn, can lead to deficient intracellular UDP-GlcNAc and thus a decrease in protein O-GlcNAcylation [40]. Glucose is the primary source of the energy required for the brain. In mammalian brain, glucose transporter 1 (GLUT1) and 3 (GLUT3) are the predominant glucose transporters responsible for glucose transport from blood to the neurons [41]. Decrease of GLUT1 and GLUT3 was found in AD brain [42 –44]. Importantly, we found that this decrease associated with decreased O-GlcNAcylation and abnormal hyperphosphorylation of tau in AD brain [45]. Furthermore, we found that insulin signaling is impaired and the GLUT1 and GLUT3 levels are also decreased in the brains of ICV-STZ–injected rats [38]. These alterations were speculated to lead to down-regulation of protein O-GlcNAcylation in the ICV-STZ rat model of sAD.

Supporting this view, we and others found that ICV-STZ can cause downregulation of protein O-GlcNAcylation in animal models [38, 46]. ICV-STZ–induced dysregulation of insulin signaling and the decreased levels of GLUT1 and GLUT3 in treated brains can lead to decreased intracellular glucose metabolism that, in turn, can lead to deficient intracellular UDP-GlcNAc and thus a decrease in protein O-GlcNAcylation [38, 40]. Downregulation of OGT levels in ICV-STZ rodents can directly lead to the reduction of protein O-GlcNAc modification [38, 46]. Given that dysregulation of O-GlcNAc homeostasis has been implicated in multiple human diseases including AD, enhancement of O-GlcNAc levels may present a promising therapeutic strategy for treatment of sAD. Here, we treated the sAD ICV-STZ mouse model with a new potent OGA inhibitor, Thiamme2-G (TM2G; compound 15b) [47], three times per week for 5 weeks starting 24 h after STZ injection, to restore O-GlcNAc levels, and investigated whether elevation of protein O-GlcNAcylation can protect the mice from STZ-induced abnormalities. We found that increasing protein O-GlcNAcylation can rescue STZ-induced cognitive decline, decrease abnormal hyperphosphorylation of tau, and alleviate STZ-induced synaptic deficit. These results indicate that increasing protein O-GlcNAcylation might serve as a broadly useful therapeutic strategy for treatment of AD and other tauopathies.

MATERIALS AND METHODS

Antibodies and reagents

The primary antibodies used in this study are listed in Table 1. Chemicals and other reagents were purchased from Sigma-Aldrich (St. Louis, MO, USA) unless otherwise noted.

Table 1

Primary antibodies used in this study

Antibody	Type	Specificity	Phosphorylation sites	Source / Reference
CTD110.6	Mono-	O-GlcNAc		Covance
R134d	Poly-	Tau		50, 67
pThr181	Poly-	P-tau	Thr181	Invitrogen
pSer199	Poly-	P-tau	Ser199	Invitrogen
AT8	Mono-	P-tau	Ser202/Thr205	Thermo Fisher Scientific
pThr205	Poly-	P-tau	Thr205	Invitrogen
pThr212	Poly-	P-tau	Thr212	Invitrogen
pThr217	Poly-	P-tau	Thr217	Invitrogen
12E8	Mono-	P-tau	Ser262/356	Dr. D. Schenk [68]
PHF-1	Mono-	P-tau	Ser396/404	Dr. P. Davies [69]
Synapsin-1	Poly-	Synapsin-1		Santa Cruz Biotechnology
Synaptophysin	Mono-	Synaptophysin		Millipore
PSD95	Mono-	PSD95		Cell Signaling Technology
GFAP	Mono-	GFAP		Millipore
Iba 1	Poly-	Iba 1		Abcam
GAPDH	Poly-	GAPDH		Sigma-Aldrich

Mice

C57BL/6 mice were initially obtained from the Jackson Laboratory (New Harbor, ME, USA). The mice were bred in our air-conditioned animal facility and housed with a 12/12 h light/dark cycle and with ad libitum access to food and water. Animal studies were approved by the Animal Care and Use Committee (IACUC) of the New York State Institute for Basic Research in Developmental Disabilities (Staten Island, NY, USA) and were in compliance with the US PHS NIH guidelines.

Ten- to 11-month-old C57BL/6 mice were first grouped according to their body weight and age, and the mice from the same litter were evenly assigned to different groups. Then the grouped mice were randomized into (A) ICV-saline treated with saline (saline/saline), (B) ICV-STZ treated with saline (STZ/saline), and (C) ICV-STZ treated with TM2G (STZ/TM2G) (Fig. 1).

Fig. 1

Study design. Male 10- to 11-month-old C57BL/6J mice were intracerebroventricularly (ICV) injected with 3μl of saline or 3μl of streptozotocin (STZ) (3 mg/kg) in saline on day 0. A) ICV-saline mice (saline/saline) and B) ICV-STZ mice (STZ/saline) were orally administered saline, and C) ICV-STZ mice (STZ/TM2G) were orally administered Thiamme2-G (TM2G, 2 mg/kg) three times per week from day 1. Behavioral tests were conducted from days 21 to 35. Animals administered the last dose of TM2G or saline on day 35 were euthanized on day 37. A) Saline/Saline, n = 14; B) STZ/Saline, n = 15; and C) STZ /TM2G, n = 15.

ICV injection of STZ

Mice were anesthetized by intraperitoneal injection of ketamine (100 mg/kg)/xylazine (10 mg/kg). STZ (Sigma-Aldrich Co.) dissolved in saline was injected into the left ventricle of the brain at a dose of 3.0 mg/kg. A separate group of mice, as an injection control, was injected with the same volume (3μl) of saline. The stereotaxic coordinates for the ICV injection were 0.9 mm posterior, 1.8 mm lateral, and 3.8 mm ventral from the bregma.

Treatment with TM2G

Twenty-four hours after ICV injection, the ICV-STZ mice were orally administered by gavage with TM2G (2 mg/kg) or with saline as a vehicle control, three times per week for five weeks. The ICV-saline mice were administered saline (Fig. 1).

Behavioral tests

Open field and one-trial novel object recognition tests were carried out from day 21 to day 35 (Fig. 1).

Open field test

The open field test was used to assess locomotion, exploration, and anxiety in rodents [48, 49]. The animals were transferred to the test room for habituation 1 h before starting the test. Each mouse was placed in the open field arena (made of opaque white plastic material, 50 cm×50 cm×40 cm) and allowed to explore the arena for 15 min. The distances travelled (meters) in the open field arena and central area (10 cm×10 cm), as well as entries and times spent in the central area, were automatically recorded by a video tracking system (ANY-maze version 4.5 software, Stoelting Co., Wood Dale, IL, USA).

One-trial novel object recognition task

One-trial novel object recognition test was performed as described previously [50]. The testing consisted of a habituation phase, a sample phase, and a test phase. Following initial exposure, four additional 10 min daily habituation sessions were performed for mice to become familiar with the apparatus (50 cm×50 cm×40 cm) and the surrounding environment. On the fifth day, each mouse was first submitted to the sample phase in which two identical objects were placed in a symmetric position from the center of the arena. The mouse was allowed to freely explore the objects for 5 min. After a 20 min delay during which the mouse was returned to its home cage, the mouse was reintroduced in the arena to perform the test phase. The mouse was then exposed to two objects for another 5 min: a familiar object (previously presented during the sample phase) and a novel object, which was of similar size to the familiar one but had a different color and shape. Data were collected by using a video tracking system (ANY-maze version 4.5 software). Object discrimination index was calculated as follows: [(time spent exploring the novel object)/(time spent exploring both familiar and novel objects)×100%] during the test phase.

Western blot analysis

Mice were sacrificed by cervical dislocation two days after completion of the behavioral tests. The mouse brains were immediately dissected, and the brain tissue was homogenized in pre-chilled buffer containing protease and phosphatase inhibitors: 50 mM Tris-HCl (pH 7.4), 150 mM sodium chloride, 100 mM sodium fluoride, 1 mM sodium ortho-vanadate, 1 mM ethylene glycol-bis(β-aminoethyl ether)-N,N,N^′,N^′-tetraacetic acid (EGTA), 0.5 mM 4-(2-aminoethyl) benzenesulfonyl fluoride hydro-chloride (AEBSF), 10μg/ml aprotinin, 10μg/ml leupeptin, and 10μg/ml pepstatin. The homogenates were boiled in 2×Laemmli sample buffer for 10 min. The protein concentrations of the samples were measured by Pierce^TM 660 nm protein assay (Thermo Scientific, Rockford, IL, USA). The samples were resolved by 10%or 14%SDS-PAGE and electrotransferred onto Immobilon-P membranes (EMD Millipore, Billerica, MA, USA). The blots were then probed with primary antibodies (Table 1) and developed with the corresponding HRP-conjugated secondary antibody and enhanced chemiluminescence kit (Thermo Scientific). Densitometric quantification of protein bands in Western blots was analyzed by using Multi Gauge version 3.0 software (FUJIFILM North America, Valhalla, NY, USA).

Statistical analysis

Data were analyzed by using Prism version 5.0 software (GraphPad Software Inc., La Jolla, CA, USA) and one-way or two-way analysis of variance (ANOVA) (as appropriate) followed by a Bonferroni post hoc test. Further intergroup comparisons were also performed by using t tests. All data are presented as mean±SEM. p < 0.05 was considered statistically significant.

RESULTS

TM2G is an orally bioavailable OGA inhibitor and exerts its effect in the brain

TM2G is a new OGA inhibitor (K_i = 2.4 nM for human OGA) [47]. Firstly, we evaluated whether TM2G can cross the blood-brain barrier and act in the brain via peripheral administration. We orally administered a single dose of TM2G at 0.2, 2.0, 10, and 20 mg/kg, respectively, to 12-month-old C57BL/6J mice and assessed the global protein O-GlcNA-cylation level in the brain at different time points (6–96 h post-administration) with CTD110.6 antibody by Western blots. We found that TM2G at 0.2mg/kg had no effect on the brain protein O-GlcNA-cylation level. Treatment of mice with TM2G with higher doses resulted in dose- and time-dependent increases in brain protein O-GlcNAcylation, which peaked approximately 24 h post-administration (Supplementary Figure 1). The brain protein O-GlcNAcylation level was increased by ∼1-fold with 2 mg/kg of TM2G and by ∼5-fold with 10 mg/kg of TM2G at 24 h post-administration, and the levels decreased thereafter, as determined by measurements made 48 h after administration. Nevertheless, the increases in brain protein O-GlcNAcylation levels lasted for at least 96 h with high-dose TM2G treatment (∼1.5-fold with 10 or 20 mg/kg). No apparent adverse effects were observed in the mice during these dosing studies. These data demonstrated that TM2G is a potent orally bioavailable OGA inhibitor that promotes an increase in protein O-GlcNAcylation level in the brain in a dose- and time-dependent manner. Based on these studies, we chose to orally administer TM2G at a conservatively low dose (2.0 mg/kg) three times per week in the present study as this level of dosing leads to modest increases in O-GlcNAc levels.

Treatment with TM2G reduces STZ-induced body weight loss

We monitored body weight over the course of the whole study. We found that the saline/saline mice did not show any significant change in body weight during the study (Fig. 2A), which eliminated significant effect of the ICV injection itself on the mice. However, the STZ/saline mice showed time-dependent body weight loss (Fig. 2A). Interestingly, treatment with TM2G partially prevented STZ-induced body weight loss (Fig. 2A). Biological analysis demonstrated that ICV-STZ led to decreases in global protein O-GlcNAcylation in the brain, and TM2G treatment fully restored protein O-GlcNAcylation level in ICV-STZ mice, and protein O-GlcNAcylation levels were even higher in inhibitor treatment mice than in saline/saline-treated mice (Fig. 2B). These results indicate that ICV-STZ not only decreases protein O-GlcNAcylation but also leads to body weight loss. Interestingly, TM2G can restore the STZ-induced decreases in protein O-GlcNAcylation and prevent the STZ-induced body weight loss.

Fig. 2

Treatment with TM2G prevents STZ-induced body weight loss. A) Percent of body weight loss. Saline/saline, n = 14; STZ/Saline, n = 15; STZ/TM2G, n = 15. Two-way ANOVA, ^*p < 0.05, ^***p < 0.001. B) Protein O-GlcNAcylation level tested by CTD110.6 antibody. C) Densitometric quantification of blots in B. Saline/Saline, n = 8; STZ/Saline, n = 8; STZ/TM2G, n = 10. Unpaired t-test, ^*p < 0.05, ^***p < 0.001.

Treatment with TM2G may alleviate ICV-STZ–induced anxiety in mice

Our recent studies demonstrated that ICV-STZ increased the anxiety and enhanced the locomotive activity of 6-month-old wild-type mice and exacerbated anxiety in 3xTg-AD mice as assessed by the open field test [39, 49]. Given that anxiety itself affects cognitive function, we first evaluated the anxiety of the mice by using the open field test to investigate whether enhancement of protein O-GlcNAcylation could alleviate ICV-STZ-induced anxiety. We found that the STZ/saline mice traveled longer distances in the open field apparatus than the saline/saline mice (Fig. 3A), which suggest that ICV-STZ enhanced the locomotive activity of the mice. The STZ/saline mice entered the central area less often (Fig. 3B) and spent less time in the central area (Fig. 3C) compared to the saline/saline mice. To avoid the possible confounding effects of the difference in locomotive activity on anxiety, we analyzed the ratio of distance travelled in the central area to total distance travelled in the open field apparatus. As shown in Fig. 3D, the ratio of the STZ/saline mice is smaller than that of the saline/saline mice. These results are consistent with previous studies [39] and demonstrate that ICV-STZ increases locomotive activity and induces anxiety in C57BL/6J mice. Although the STZ/TM2G mice did not show any difference in total travelled distance (Fig. 3A) compared with the STZ/saline mice, the STZ/TM2G mice exhibited a tendency to increase in the numbers of entries into the central area, more time spent in the central area, and enhancement of the ratio of distance travelled in the central area (Fig. 3B-D). These results suggest that enhancement of protein O-GlcNAcylation may protect these animals from ICV-STZ–induced anxiety.

Fig. 3

Treatment with TM2G alleviates anxiety induced by STZ in the open field test. A) Total distance travelled in open field. B) Number of the central area entries. C) Time in the central area. D) Percent of distance travelled in central area. Saline/Saline, n = 14; STZ/Saline, n = 15; STZ/TM2G, n = 15. Unpaired t-test, ^*p < 0.05, ^**p < 0.01.

Treatment with TM2G rescues ICV-STZ–induced short-term memory impairment

A one-trial novel object recognition test was used to evaluate the cognitive function of the different groups of mice. During the sample phase, all the mice spent similar times with two identical objects, which indicates that the mice did not show any location and/or object bias (Fig. 4A, B). During the test phase, the saline/saline mice spent more time with the novel object; however, the STZ/saline mice spent a similar amount of time with the novel object and the familiar object (Fig. 4C, D). These results suggest that ICV-STZ impaired short-term memory. Importantly, the STZ/TM2G mice also spent more time with the novel object than the familiar object and showed a significant increase in the discrimination index as compared with the STZ/saline mice (Fig. 4C, D). These results indicate that enhancement of protein O-GlcNAcylation can rescue ICV-STZ–induced short-term memory impairment.

Fig. 4

Treatment with TM2G rescues ICV-STZ–induced short-term memory impairment in a one-trial novel object recognition test. A) The time spent exploring two identical objects, object 1 and object 2, during the sample phase. B) The percentage of time spent exploring two identical objects during the sample phase. C) The time spent exploring a novel object and a familiar object during the test phase. D) Discrimination index (time spent exploring novel object/time spent exploring novel and familiar objects)×100%in the test phase. Paired t-test was used for panels A and C, and unpaired t test for panels B and D. Saline/Saline, n = 14; STZ/Saline, n = 15; STZ/TM2G, N = 15. ^**p < 0.01, ^***p < 0.01.

Treatment with TM2G decreases ICV-STZ–induced site-specific hyperphosphorylation of tau

We previously found that downregulation of O-GlcNAcylation in the brain contributes to site-specific hyperphosphorylation of tau [11, 12]. We assessed the level of phosphorylated tau by Western blotting among three groups of mice. Treatment with STZ upregulated the level of tau hyperphosphorylation at Thr181, Ser199, Ser202/Thr205 (AT8), Ser214, and Thr217 sites, downregulated tau hyperphosphorylation level at Thr212, Ser262/356 (12E8), and Ser 396/404 (PHF1) sites, and had no effect at the Ser422 site (Fig. 5A, B). Given that tau hyperphosphorylation is crucial to the pathogenesis and the cognitive impairment seen in AD and related tauopathies [51], STZ-induced hyperphosphorylation of tau at multiple sites may contribute to the cognitive impairment in mice.

Fig. 5

Treatment with TM2G decreases ICV-STZ–induced tau hyperphosphorylation. A) Western blots developed with R134d against total tau, and several phosphorylation-dependent and site-specific tau antibodies. B) Levels of phosphorylated tau normalized with total tau. The data are presented as mean±SEM. Saline/saline, n = 8; STZ/Saline, n = 8; STZ/TM2G, n = 10. Unpaired t test, ^*p < 0.05, ^***p < 0.01, ^***p < 0.001.

Importantly, treatment with TM2G decreased tau hyperphosphorylation at most of the sites studied, including the STZ-induced hyperphosphorylation sites, Thr181, Ser199, Thr205, Ser214, and Thr217 sites (Fig. 5A, B). These results indicate that enhancement of protein O-GlcNAcylation can prevent STZ-induced, site-specific tau hyperphosphory-lation, which may contribute to its beneficial effect on cognitive improvement in STZ/TM2G mice.

Treatment with TM2G increases levels of synaptic proteins

We analyzed levels of presynaptic proteins sy-napsin 1 and synaptophysin, and of postsynaptic protein PSD95 by western blotting. Compared to the saline/saline mice, the STZ/saline mice exhibited lower levels of synapsin 1 and synaptophysin, and similar levels of PSD95 (Fig. 6A, B). These results indicate that ICV-STZ decreases presynaptic proteins, and this effect may underlie the cognitive deficits. Elevation of O-GlcNAcylation did not affect the level of synaptophysin and increased the levels of PSD95, but although a trend was observed, they had no significant effect on synapsin 1 in the STZ/TM2G mice. These results suggest that amelioration of STZ-induced synaptic deficit may also contribute to the beneficial effect of TM2G on behavioral performance in the STZ/TM2G mice.

Fig. 6

Treatment with TM2G alleviates ICV-STZ–induced synaptic deficit. A) Western blots of cerebral cortex developed with antibodies indicated at the left side of the blots. B) Densitometric quantification of the blots after normalization. Saline/saline, n = 8; STZ/Saline, n = 8; STZ/TM2G, n = 10. Unpaired t-test, ^*p < 0.05.

The effect of enhancement of protein O-GlcNAcylation on neuroinflammation

We previously demonstrated neuroinflammation in the brain, especially in the hippocampus, of the ICV-STZ mouse model of sAD [39]. To assess whether enhancement of protein O-GlcNAcylation can reduce ICV-STZ–induced neuroinflammation, we analyzed the levels of Iba 1, a microglia marker, and GFAP, an astrocyte marker, by using Western blotting. We found increased Iba 1 levels in the ICV-STZ mouse brains (Fig. 7A-C), which is consistent with our previous findings [39]. Surprisingly, a decreased GFAP level was found in the cerebral cortex of the ICV-STZ mice (Fig. 7A-C). Importantly, treatment of the ICV-STZ mice with TM2G was found to restore levels of GFAP (Fig. 7A, B). These results indicate that enhancement of protein O-GlcNAcylation can repair the STZ-induced disruption of astrocytes.

Fig. 7

The effect of enhancement of protein O-GlcNAcylation on STZ-induced neuroinflammation. A) Western blots developed with Iba1 and GFAP in the cerebral cortex. B and C) Densitometric quantification of blots in A after normalization with GAPDH is shown. Saline/saline, n = 8; STZ/Saline, n = 8; STZ/TM2G, n = 10. D) Western blots of mouse brain hippocampal tissue developed with GFAP. E) Densitometric quantification of blots in D after normalization with GAPDH is shown. Saline/saline, n = 8; STZ/Saline, n = 8; STZ/TM2G, n = 9 (The last mouse excluded from quantification). Unpaired t test, ^*p < 0.05, ^***p < 0.001.

Because we previously found marked increases in the GFAP levels in the hippocampus of ICV-STZ mice [39], which is inconsistent with the decreased GFAP levels in the ICV-STZ mouse cerebral cortex found in the present study, we determined the GFAP levels in hippocampal tissue and found that they were indeed increased in ICV-STZ mice (Fig. 7D, E). TM2G treatment of the ICV-STZ mice did not have a significant effect on the GFAP level in the hippocampus.

DISCUSSION

We previously demonstrated a reciprocal relationship between O-GlcNAcylation and hyperphosphorylation of tau protein in the brain [11, 12]. However, whether maintaining brain protein O-GlcNAcylation is a broadly applicable therapeutic approach for inhibiting tau hyperphosphorylation and neurodegeneration in sAD remains elusive. Here, we found that ICV-STZ induced decreases of protein O-GlcNAcylation, body weight loss, site-specific hyperphosphorylation of tau, synaptic deficit, neuroinflammation, and cognitive impairment. Importantly, enhancement of protein O-GlcNAcylation via moderate levels of inhibition of OGA was found to protect the mice from STZ-induced abnormalities in body weight loss, tau hyperphosphorylation, and especially cognitive impairment.

Recent studies suggest that elevation of protein O-GlcNAcylation via inhibition of OGA can alleviate tau pathology in different transgenic mouse models overexpressing human mutant tau [26 –30]. However, these studies did not compare protein O-GlcNAcylation levels between transgenic mice and age-matched wild-type mice. It is quite likely that overexpressing human mutant tau, not dysregulation of protein O-GlcNAcylation, leads to the tau pathology in these transgenic animals. Especially, mutant tau is known to be more easily hyperphosphorylated than wild-type tau protein [8]. If protein O-GlcNAcylation is not disrupted in these transgenic animals, it is expected that chronic elevation of protein O-GlcNAcylation will not show any effect on tau hyperphosphorylation, especially in these human mutant tau transgenic mice that exhibit aggressive progression of tau pathology. This finding can, at least partially, explain why only acute increase of protein O-GlcNAcylation can reduce tau hyperphosphorylation [26, 28], and chronic elevation of protein O-GlcNAcylation only reduces tau pathology, but does not affect the level of tau hyperphosphorylation [27, 29]. The phosphorylation of tau is regulated by kinases and phosphatases. GSK-3β is the main kinase that regulates tau phosphorylation [52]. We have shown previously that activation of GSK-3β attributes to tau phosphorylation in ICV-STZ rats [38]. However, elevation of protein O-GlcNAcylation in the STZ/TM2G mice did not affect GSK-3β activity under our conditions, as assessed by the phosphorylation level of GSK-3β at Ser 9 sites (data not shown). Additionally, we did not observe any change of protein phosphatase 2A (PP2A) protein level in brain among three groups of mice (data not shown). These results imply that decreases in tau phosphorylation in the STZ/TM2G mice may not via the effect of O-GlcNAcylation on tau kinase and phosphatase. In the present study, the STZ/saline mice exhibited both decreases in brain protein O-GlcNAcylation and increased tau hyperphosphorylation at several sites implicated in tau pathology. Importantly, our results also showed that chronic elevation of protein O-GlcNAcylation decreased STZ-induced hyperphosphorylation of tau, which suggests the in vivo cause-effect relationship between O-GlcNAcylation and tau phosphorylation. Although the exact underlying mechanisms by which upregulation of protein O-GlcNAcylation decreases tau hyperphosphorylation remains elusive, O-GlcNAcylation has been proposed to modulate hyperphosphorylation either directly, by modifying phosphorylation sites, or indirectly, through glycosylation of residues proximal to these sites [10, 19]. It was reported that long-term inhibition of OGA strongly increases tau O-GlcNAcylation at Ser400 residue and can transiently reduce tau hyperphosphorylation at several sites [26].

We found that STZ/saline mice exhibited time-dependent body weight loss, which is consistent with a recent finding that ICV-STZ caused body weight loss at 1 and 4 weeks after ICV-STZ injection [46]. At present, the underlying mechanism(s) driving this effect are unknown. Given that elevation of protein O-GlcNAcylation could partially prevent STZ-induced body weight loss, STZ-induced decreases of protein O-GlcNAcylation might be involved in this observed body weight loss. Consistent with this finding, treatment with another potent OGA inhibitor, Thiamet-G (500 mg/kg/day) for 36 weeks, also prevented body weight loss in both JNPL3 mice [28] and Tau.P301L mice [27]. Additionally, it is known that STZ can enter beta cells via glucose transporter 2 and kill beta cells in the pancreas [53]. Taking into consideration that glucose transporter 2 is also present in hypothalamic neurons and serves as a glucose sensor in regulation of food intake [54], STZ might enter these neurons and disrupt the feeding behavior of these mice. Unfortunately, we did not monitor the food intake in the present study. Therefore, it is possible that ICV-STZ might cause neuronal loss within the hypothalamus, with the result being reduced food intake and consequent body weight loss. Neuronal loss was observed previously in rats after ICV-injection of STZ [55, 56].

ICV-STZ can cause neuroinflammation [37, 49], which may contribute to the STZ-induced cognitive impairments. We also found that ICV-STZ increased the expression of Iba 1 in the brain. Previous studies reported that ICV-STZ increased the GFAP protein level in the corpus callosum of rats [37] and in the hippocampus of 3xTg-AD mice [49]. We also found that STZ increased the level of GFAP in the hippocampus. Unexpectedly, STZ dramatically decreased the level of GFAP within the cerebral cortex. The exact reason for this apparent discrepancy is, at present, unknown. Importantly, elevation of protein O-GlcNAcylation restored the GFAP protein levels within the cerebral cortex (Fig. 7), although it did not affect the STZ-induced increase of GFAP in hippocampus.

Synaptic loss and dysfunction are an early event and play a pivotal role in the cognitive impairment in AD and many other diseases associated with dementia [57 –59]. Here, we found that ICV-STZ decreased the levels of presynaptic proteins synaptophysin and synapsin 1, which are consistent with previous reports that ICV-STZ reduced the expression of synaptophysin [60 –62]. We did not observe an alteration of postsynaptic protein PSD95 in the STZ/saline mice compared with the Saline/saline mice. These results indicate that ICV-STZ may mainly cause a major presynaptic disruption which might attribute to the cognitive deficits of the ICV-STZ mice. Elevation of O-GlcNAcylation increased PSD95 level and synapsin 1 level, although the latter did not reach a statistical significance. These data imply that postsynaptic protein PSD95 might play major beneficial effects in the cognitive improvement of the STZ/TM2G mice.

We and others reported that ICV-STZ can cause anxiety and cognitive impairment in rodents [35 , 63–66]. Our results showed that ICV-STZ caused anxiety and cognitive deficits, as evaluated by open field test and novel objected recognition test, respectively. Importantly, chronic elevation of protein O-GlcNAcylation not only restored the cognitive function but also showed a tendency to reduce STZ-induced anxiety. Enhancement of protein O-GlcNAcylation restored STZ-induced abnormalities in protein O-GlcNAc modification. Rescue of tau hyperphosphorylation, synaptic deficit, and neuroinflammation could together contribute to the cognitive improvement observed in STZ/TM2G mice.

Finally, we also assessed spatial learning and memory by using the Morris water maze test. Unfortunately, all the mice enjoyed swimming and did not show significant learning during four consecutive days of the acquisition phase. We thus were unable to proceed further with the water maze test. The exact cause of the unexpected outcome of the Morris water maze test remains unknown. One possibility is the hot and humid weather of the testing days, which may have led the mice to enjoy swimming in the relatively cool test pool that has 20±1°C water during the 90 s training.

In summary, we found that restoration of brain protein O-GlcNAcylation in mice whose O-GlcNAc levels were reduced by ICV-STZ treatment through oral administration of modest levels of an OGA inhibitor can prevent body weight loss, tau hyperphosphorylation, and astrocyte disruption as well as rescue cognitive impairments. These results obtained using a mouse model of sAD provided evidence that maintenance of protein O-GlcNAc homeostasis could be a potentially effective therapeutic strategy for AD.

Footnotes

ACKNOWLEDGMENTS

This work was supported in part by the New York State Office for People With Developmental Disabilities, an Alzheimer’s Association grant (2015-NIRG-339945), and grants from Brain Canada—Multi-Investigator Research Initiative and a CIHR Operating grant (MOP-123341).

Authors’ disclosures available online ().

The supplementary material is available in the electronic version of this article: .

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