Obesity-induced diabetes is associated with chronic inflammation and is considered a risk factor for neurodegeneration. We tested the hypothesis that an AMP-activated protein kinase activator, resveratrol (RES), which is known to exert potent anti-inflammatory effects, would attenuate peripheral and central inflammation and improve memory deficit in mice fed a high-fat diet (HFD). C57BL/6J mice were fed an HFD or an HFD supplemented with RES for 20 weeks. Metabolic parameters in serum were evaluated, and Western blot analysis and immunohistochemistry in peripheral organs and brain were completed. We used the Morris water maze test to study the role of RES on memory function in HFD-treated mice. RES treatment reduced hepatic steatosis, macrophage infiltration, and insulin resistance in HFD-fed mice. In the hippocampus of HFD-fed mice, the protein levels of tumor necrosis factor-α and Iba-1 expression were reduced by RES treatment. Choline acetyltransferase was increased, and the phosphorylation of tau was decreased in the hippocampus of HFD-fed mice upon RES treatment. In particular, we found that RES significantly improved memory deficit in HFD-fed mice. These findings indicate that RES reverses obesity-related peripheral and central inflammation and metabolic derangements and improves memory deficit in HFD-fed diabetic mice.
Obesity is a major risk factor for the development of insulin resistance, type 2 diabetes, stroke (1), and Alzheimer disease (AD) (2). It is well established that obesity is characterized by abnormal adipokine production and activation of some proinflammatory signaling pathways (3,4). Chronic inflammation associated with obesity is also characterized by macrophage infiltration into adipose tissues (5). Insulin resistance in peripheral tissues could influence central insulin resistance with reduced brain insulin levels (6). Therefore, peripheral insulin resistance could affect cognition (7). A number of epidemiological studies provide direct evidence to strengthen the link between type 2 diabetes and AD (8). The progressive worsening of insulin resistance with AD is correlated with increased oxidative stress, lipid peroxidation manifested by 4-hydroxynonenal (4-HNE), and inflammation (9). A recent study demonstrates that resveratrol (RES), a polyphenolic compound enriched in grapes and red wine, prevents memory deficits and the increase in acetylcholinesterase activity in streptozotocin (STZ)-induced diabetic rats (10).
RES has attracted wide attention because of its antioxidant and anti-inflammatory effects (11). Recent evidence shows that treatment with RES ameliorates elevated levels of tumor necrosis factor (TNF)-α, interleukin (IL)-6, and cyclooxygenase-2 in experimental diabetic neuropathy (12). AICAR, an activator of AMP-activated protein kinase (AMPK), has been reported to show anti-inflammatory and immunomodulatory effects in experimental autoimmune encephalomyelitis or lipopolysaccharide (LPS)-stimulated rats (13,14). RES has also been demonstrated to promote degradation of amyloid-β (A-β) peptide in AD (15).
RES increases AMPK activity and improves insulin sensitivity (16). AMPK is an energy sensor that regulates energy homeostasis and metabolic stress (17). By affecting glucose and lipid metabolism, energy balance is closely related to obesity and type 2 diabetes. The activation of AMPK is responsible for metabolic changes via phosphorylation of downstream substrates, such as acetyl CoA carboxylase (ACC) and glycogen synthase kinase (GSK)-3β, which are directly related to glycogen synthesis and fatty acid oxidation, respectively (18,19). In particular, data from AMPK-deficiency models suggest that AMPK activity might influence the pathophysiology and therapy of diabetes (17). Interactions between insulin and the AMPK signaling pathway were reported in liver, skeletal muscle, and adipocytes (19). In the central nervous system (CNS), it has been demonstrated that the ability of leptin to modulate both tau-phosphorylation and A-β production is mediated through AMPK (20).
Although many studies show neuroprotective effects of RES in STZ-administered rodents, the effects of RES on the hippocampus in obesity-induced diabetes are not fully known. Therefore, the purpose of the current study was to determine whether chronic, dietary RES administration not only enhances insulin resistance and peripheral inflammation but also prevents neuroinflammation and memory deficits in mice fed a high-fat diet (HFD). Our findings suggest that obesity- or diabetes-related systemic inflammation, neuroinflammation, and memory deficits in type 2 diabetic mice can be prevented or delayed by RES supplementation.
RESEARCH DESIGN AND METHODS
Animals and obesity model.
Male C67BL/6J mice (3 weeks old) were purchased from KOATECH (Pyeongtaek, South Korea) and maintained in the animal facility at Gyeongsang National University. The experiments were performed in accordance with the National Institutes of Health Guidelines on the Use of Laboratory Animals. The university animal care committee for animal research of Gyeongsang National University approved the study protocol (GLA-101104-M0108). Mice were individually housed with an alternating 12-h light/dark cycle. Mice, starting at age 4 weeks, were randomly divided into four groups (n = 30 per group). Mice were fed for 20 weeks with either HFD (60%) or low-fat diet (LFD, 10%) chow (Research Diets, Inc., New Brunswick, NJ). Mice were dosed with 200 mg/kg of trans-RES (ChromaDex, Inc., Irvine, CA) daily in either LFD or HFD chow. Mice were weighed four times weekly and just prior to killing at age 24 weeks.
Glucose tolerance test.
Mice were fasted overnight (16 h) before a glucose tolerance test (GTT). d-glucose (2 g/kg; Sigma-Aldrich, St. Louis, MO) was injected intraperitoneally, and blood samples were taken before and 30, 60, 90, and 120 min after the injection of glucose. Blood glucose was measured using an Accu-Chek glucometer (Roche Diagnostics GmbH, Mannheim, Germany).
Insulin tolerance test.
The insulin tolerance test (ITT) was performed on mice at approximately 2:00 p.m. Mice were injected with insulin (0.75 units/kg; Humulin-R; Eli Lilly and Company, Indianapolis, IN) in 0.1 mL 0.9% normal saline. A drop of blood was taken from the cut tail vein before and 15, 30, 45, and 60 min after the injection of insulin for the determination of blood glucose with a glucometer (Accu-Chek).
Enzyme-linked immunosorbent assay.
Serum adiponectin, leptin, insulin, and TNF-α concentrations (n = 5–6 per group) were measured using adiponectin, leptin, and insulin mouse ELISA kits (Alpco Diagnostics, Salem, NH) and a mouse TNF-α ELISA kit (BD, Franklin Lakes, NJ) according to the manufacturers’ protocols.
Tissue collection and sample preparations.
For tissue analysis, mice (n = 7 per group) were anesthetized with zoletil (5 mg/kg, Virbac Laboratories, Carros, France) and then perfused transcardially with heparinized saline followed by 4% paraformaldehyde in 0.1 mol/L PBS. Six hours after postfixation in the same fixative, the brains were sequentially immersed in 0.1 mol/L PBS containing 15% sucrose and then in PBS containing 30% sucrose at 4°C until they sank. The brains were cut into 40 μm–thick coronal sections. Other tissues, including liver, epididymal fat pads, and pancreas, were processed for paraffin embedding and sectioned (5 μm).
Oil Red O staining.
Oil Red O staining is commonly used to identify lipid deposits. To determine hepatic lipid accumulation, frozen sections (5 μm) of liver were stained with 0.5% Oil Red O (Sigma-Aldrich) for 10 min, washed, and counterstained with Mayer’s hematoxylin (Sigma-Aldrich) for 45 s. The sections were visualized under a BX51 light microscopy (Olympus, Tokyo, Japan), and digital images were captured and documented.
Rapid Golgi staining.
For rapid Golgi staining, mice (n = 3 per group) were intraperitoneally anesthetized with 5 mg/kg zoletil (Virbac Laboratories) and brains were removed from the skull as quickly as possible and rinsed briefly in distilled water. Golgi staining was performed with an FD rapid GolgiStain kit (PK 401; FD NeuroTechnologies, Inc., Ellicott City, MD) according to the manufacturer’s protocol. Coronal sections (150 μm) of Golgi-stained brains were cut with a slicetome (Meiwa Shoji Co., Osaka, Japan). Slices were mounted on microscope slides (Fisherbrand; Fisher Scientific, Waltham, MA). The sections were visualized under a BX51 light microscopy (Olympus).
Deparaffinized sections from liver, pancreas, and epididymal fat pads or frozen sections from brains were placed in a solution of 0.3% H2O2 for 10 min. After washing, sections were treated with diluted blocking serum for 20 min. Slides were incubated overnight at 4°C in a humidified chamber with anti–mouse-4-HNE (1:100; Abcam, Cambridge, MA), anti–rabbit-F4/80 (1:100; Santa Cruz Biotechnology, Santa Cruz, CA), anti–rabbit-CD68 (1:100; Santa Cruz Biotechnology), and anti–guinea pig-insulin (1:100; Abcam) diluted in blocking serum. After washing three times with 0.1 mol/L PBS, sections were incubated for 1 h at room temperature with a secondary biotinylated antibody (1:200). After washing, sections were incubated in avidin-biotin-peroxidase complex solution (Vector Laboratories, Burlingame, CA). Sections were developed with 0.05% diaminobenzidine (Sigma-Aldrich) containing 0.05% H2O2 and were dehydrated through graded alcohols, cleared in xylene, and coverslipped with Permount (Sigma-Aldrich). Sections were visualized under a BX51 light microscopy (Olympus). Photomicrographs were assessed by densitometry using the analySIS FIVE program (Olympus Soft Imaging Solutions, Münster, Germany). For immunostaining for Iba-1 (1:200; Wako Pure Chemical Industries, Osaka, Japan) and choline acetyltransferase (ChAT; 1:100; Millipore, Billerica, MA), free-floating sections were labeled with Iba-1 antibody overnight at 4°C. Sections were incubated with Alexa Fluor 594–conjugated donkey anti-rabbit antibody (1:1,000; Invitrogen, Carlsbad, CA). Fluorescence was visualized under a confocal microscope (FV-1000; Olympus).
Western blot analysis.
For protein extraction, frozen hippocampi were homogenized in a lysis buffer. The following antibodies were used: TNF-α (Santa Cruz Biotechnology); Iba-1 (Wako Pure Chemical Industries); p–insulin receptor (IR) and IR (both from Millipore); p-AMPK, AMPK, p-ACC, ACC, p–GSK-3β, and GSK-3β (all from Cell Signaling Technology, Danvers, MA); and p-tau, tau, ChAT, and 4-HNE (all from Santa Cruz Biotechnology). The membranes were probed with each antibody or α-tubulin (Sigma-Aldrich) and visualized using an enhanced chemiluminescence substrate (Pierce, Rockford, IL). The Multi-Gauge version 3.0 image analysis program (Fujifilm, Tokyo, Japan) was used to measure band densitometry.
Morris water maze test.
Morris water maze testing was performed as previously described (21), with the following modifications. In brief, mice (n = 10 per group) were first trained to find a randomly positioned visible platform in a 100 cm–diameter swimming pool, maintained at 25.0 ± 1°C. All mice were subjected to four trials per day for 4 consecutive days. The escape latency and swimming distance to find the platform was recorded by a video-tracking program (Noldus EthoVision XT7; Noldus Information Technology, Wageningen, the Netherlands). On the day of testing, the platform was removed and time spent in the target quadrant, where the platform had been located during training, was analyzed. The starting position was changed with each trial.
Differences between mice fed an LFD, HFD, HFD plus RES, or LFD plus RES were determined by two-way ANOVA, followed by Bonferroni post hoc analysis. Values are expressed as the mean ± SEM. P < 0.05 was considered statistically significant.
RES does not reduce the body weight of HFD-fed mice.
Mice were fed an HFD for 20 weeks. After 2 weeks, the body weight gradually increased in HFD-fed mice compared with LFD-fed mice (Fig. 1A). However, RES did not cause any reduction in body weight in HFD-fed mice. Next, whole brain weights were measured at the time of killing. Although brain weights were unchanged (data not shown), the ratio of brain to body weight was significantly decreased in HFD-fed mice compared with LFD-fed mice (Fig. 1B). However, the ratio was not changed by RES treatment.
RES improves insulin sensitivity in HFD-fed mice.
To determine the effects of RES on serum adipokines in the LFD- and HFD-fed mice, we measured the concentration of adiponectin and leptin using enzyme-linked immunosorbent assay (ELISA) (Fig. 2A and B). HFD-induced hypoadiponectinemia and hyperleptinemia were significantly reversed by RES treatment. Next, to explore the role of RES in obesity-induced insulin resistance, GTTs and ITTs were performed (Fig. 2C and D). HFD-fed mice were significantly more glucose tolerant after a 20-week HFD challenge (Fig. 2C); however, administration of RES reduced glucose tolerance 30 min after intraperitoneal injection of glucose in HFD-fed mice. Consistent with the effects of RES on GTTs, the reduction in glucose levels during ITTs was greater in mice fed an HFD plus RES (Fig. 2D). Hyperinsulinemia in HFD-fed mice was significantly reduced by RES treatment (Fig. 2E). Finally, to examine islet morphology and production of insulin, immunostaining for insulin was performed on pancreas sections from each group of mice (Fig. 2F). Although there was no difference in the diameter of pancreatic islets between the groups, the relative densities (10.48 ± 1.52) of immunostained insulin-positive cells in the pancreatic islets of HFD-fed mice were higher (1.0 ± 0.34) than that of LFD-fed mice (P < 0.001). However, the relative density (0.99 ± 0.20) in HFD-fed mice was significantly reduced by RES treatment (P < 0.001).
RES reduces hepatic steatosis, lipid peroxidation, and macrophage infiltration in HFD-fed mice.
Hepatic steatosis as fatty liver disease is characterized by the accumulation of fats, oxidative stress–induced lipid peroxidation, and macrophage infiltration (22). To examine the effects of RES on hepatic steatosis, we performed hematoxylin and eosin and Oil Red O staining (Fig. 3A). Histologic examination showed that hepatocytes of HFD-fed mice were distended by large cytoplasmic lipid droplets. This change in cellular morphology was prevented by RES treatment (Fig. 3A). Next, we confirmed the effect of RES on lipid peroxidation in the liver of HFD-fed mice using immunohistochemistry for 4-HNE (Fig. 3B). There were significant increases in immune densities in HFD-fed mice compared with LFD-fed mice, which were reduced by RES treatment (Fig. 3C). Finally, we found that immunostaining for F4/80, a macrophage marker, in HFD-fed mice was also significantly reduced by RES treatment (Fig. 3D and E).
RES reduces serum TNF-α and macrophage infiltration of adipose tissue in HFD-fed mice.
We measured serum concentration of TNF-α levels using ELISA (Fig. 4A). The circulation levels of TNF-α were significantly increased in HFD-fed mice compared with LFD-fed mice (P = 0.046). However, this HFD-induced increase in TNF-α was attenuated by RES treatment (P = 0.023). To investigate the effect of RES on macrophage accumulation in HFD-fed mouse epididymal fat pads, we performed immunohistochemistry with anti-CD68 antibody (Fig. 4B). Figure 4B shows that many CD68-expressing cells were deposited in the adipose tissue of HFD-fed mice. In accordance with the role of RES on serum TNF-α, RES reduced immune density of CD68 in epididymal fat pads of HFD-fed mice (Fig. 4C).
RES attenuates neuroinflammation and oxidative stress in the hippocampus of HFD-fed mice.
In addition to peripheral tissues, obesity is associated with an increased risk for dementia and neuroinflammation (23). Among brain regions, the hippocampus is particularly vulnerable in AD (24). Western blot analysis showed that HFD-induced TNF-α and Iba-1 expression was significantly decreased by RES treatment (Fig. 5A and B). Immunohistochemical analysis showed activated microglia in the hippocampus of HFD-fed mice (Fig. 5C). However, there were many ramified, resting microglia in the hippocampus of mice fed an LFD or HFD plus RES. We also evaluated the effect of RES on lipid peroxidation in the hippocampus of HFD-fed mice (Fig. 5D). Western blot analysis revealed that HFD resulted in increased hippocampal 4-HNE expression compared with LFD-fed mice. However, RES significantly attenuated this HFD-induced increased expression of 4-HNE. It is interesting that there was also a significant decrease in 4-HNE expression in mice fed an LFD plus RES.
RES improves insulin resistance and energy metabolism in the hippocampus of HFD-fed mice.
Hyperphosphorylation of tau-protein is potentiated by a disturbance of glucose and energy metabolism (25). In Western blot analysis with anti–p-IR and total IR antibodies, the expression levels of p-IR were significantly decreased in HFD-fed mice compared with LFD-fed mice, whereas the ratio of p-IR to IR protein expression was increased by RES treatment (Fig. 6A). In accordance with serum adiponectin levels, the decreased hippocampal expression of adiponectin in the HFD-fed mice was increased by RES (Fig. 6B). Next, we evaluated the effects of RES on the adiponectin-mediated AMPK signaling pathway in HFD-fed mice (Fig. 6C and D). Western blot analysis showed that levels of hippocampal p-AMPK and p-ACC were significantly decreased in the HFD-fed mice compared with LFD-fed mice. However, RES increased the phosphorylation of p-AMPK and p-ACC in HFD-fed mice. Finally, to test whether RES affects AMPK-mediated phosphorylation of GSK-3β and tau in the hippocampus of HFD-fed mice, we performed Western blot analysis (Fig. 6E and F). Mice fed an HFD plus RES showed neither an HFD-induced decrease in the ratio of p–GSK-3β to GSK-3β nor an increase in the ratio of p-tau to tau in the hippocampus.
RES inhibits neurodegeneration in the hippocampus of HFD-fed mice.
Golgi staining has been commonly used for neuronal morphometry in neurodegenerative disorders (26). In serial coronal sections of the hippocampus (Fig. 7A), we found that neurons from HFD-fed mice showed less dendritic branching and decreased branch length and complexity of neuronal dendritic trees compared with mice fed an HFD plus RES (Fig. 7B).
RES enhances cognitive function in HFD-fed mice.
We evaluated the effect of RES on hippocampal ChAT expression in HFD-fed mice using Western blot analysis and immunohistochemistry (Fig. 8A and B). Western blot analysis showed that hippocampal ChAT expression in HFD-fed mice was decreased compared with LFD-fed mice, and RES attenuated this HFD-induced effect (Fig. 8A). In immunohistochemical analysis, ChAT-positive cells were distributed in the neurites of neurons in the CA1 region in the hippocampus of mice fed an LFD or HFD plus RES (Fig. 8B). However, ChAT-immunoreactive positive neurons and neurites had comparatively the least immunoreactivity in the HFD-treated mice. Finally, to assess whether RES affects memory deficits in HFD-induced obesity, we performed the Morris water maze test after 20 weeks of HFD administration (Fig. 8C–F). HFD-fed mice exhibited increased escape latencies and swimming distance (Fig. 8C and D) during training trials and reduced time spent in target quadrant during Morris water maze testing (Fig. 8E), whereas RES treatment significantly decreased escape latency and swimming distance and increased time spent in the target quadrant. As shown in Fig. 8F, examination of swim paths revealed that HFD-fed mice with RES showed remarkable improvements in spatial bias compared with HFD-fed mice.
Previous studies indicate close associations between diabetes-induced chronic inflammation and AD (27). Aged rodents show exaggerated neuroinflammation and memory deficits in response to peripheral inflammation (28). RES has recently received considerable attention because of its antioxidant, anti-inflammatory, and neuroprotective effects, including cognitive deficits associated with type 1 diabetes (10,29). Although some studies have investigated the neuroprotective effects of RES in type 1 diabetes, the actions of RES on the neuronal AMPK pathway and memory in HFD-induced type 2 diabetic mice have not been reported in the literature. In the current study, we found that RES improves not only peripheral inflammation and insulin sensitivity but also neuroinflammation and memory deficit in HFD-fed mice by activation of the AMPK signaling pathway.
RES is known to reduce fat accumulation and improve glucose tolerance and insulin sensitivity in mice with HFD-induced obesity (30). RES reduces body weight through increased mitochondrial biogenesis and physical endurance (31). Um et al. (32) recently demonstrated that RES (400 mg/kg) increases insulin sensitivity and reduces fat mass in 40% HFD-fed mice. In the current study, although RES (200 mg/kg) did improve insulin sensitivity and metabolic parameters in 60% HFD-fed mice, body weight was not reduced by supplementation with RES for 20 weeks. The reasons for this discrepancy are not clear but may be due to differences in the percentage of fat in the respective diets, dose of RES, and duration of diabetes.
There is evidence that a state of chronic, low-grade inflammation is a key link between obesity and the associated metabolic syndrome (33). In general, HFD is a nutritional condition that accounts for the largest incidence of metabolic syndrome in the world (34). Obesity leads to the infiltration of fats into multiple organs, including the epididymis, heart, liver, and pancreas. In particular, macrophages, which infiltrate the adipose tissue of obese rodents and humans, are a major source for proinflammatory cytokines, such as TNF-α and IL-6 (35). Obesity is associated with macrophage accumulation in adipose tissue and increased serum TNF-α, and we found that chronic HFD feeding induced higher circulating levels of TNF-α and macrophage accumulation in epididymal fat pads and liver (Figs. 3 and 4). In addition to these peripheral organs, although there were no significant differences in the levels of IL-6, IL-1β, or TNF-α mRNA in temporal tissues from LFD- and HFD-fed mice (36), we found that HFD feeding increased hippocampal TNF-α expression and activated microglia compared with LFD-fed mice (Fig. 5). In accordance with our result, another study demonstrates that proinflammatory cytokines, such as TNF-α and IL-1β, were released in the hypothalamus and activated apoptotic signaling in the hypothalamus of rodents on an HFD (37). In AD, an abundance of reactive astrocytes and activated microglial cells are found in the proximity of neuritic plaques (38). However, we found no significant difference in glial fibrillary acidic protein expression between LFD- and HFD-fed mice using Western blot and immunohistochemistry (data not shown). The reasons for this finding are not clear but may be due to the dominant response of microglia cells against chronic fat diet–induced neuroinflammation.
Recent findings show a neuroprotective effect whereby RES reduces free radical formation in LPS-activated microglia of rats (39). Furthermore, RES has been shown to inhibit the production of nitric oxide and TNF-α by LPS-activated microglia (40). In addition, increased 4-HNE (a marker of lipid peroxidation) expression was found in the hippocampus after 20 weeks of HFD. Our results are in agreement with those finding that 4-HNE expression was increased in the cerebellum of HFD-fed rats (36). Oxidative stress in the brain coincided with hepatic steatosis, increased circulating levels of TNF-α, and increased hepatic 4-HNE in HFD-fed mice. Thus, our results indicate that HFD-induced peripheral inflammation and oxidative stress may be closely associated with neuroinflammation and lipid peroxidation during chronic diabetes, whereas dietary supplementation with RES can minimize these deleterious effects.
In general, insulin is synthesized by β-cells of the pancreatic islets of Langerhans and acts by reducing blood glucose levels in the periphery. In the CNS, insulin can be partially formed in the hippocampus, cortex, and olfactory bulb (41). Insulin in the CNS exerts its pleiotropic effects through binding to IRs and triggers the IR to undergo phosphorylation (42). There is evidence that insulin is capable of modulating of AMPK. As suggested by Tezapsidis et al. (43), we confirmed that activation of AMPK by IR leads to the phosphorylation of GSK-3β, which deactivates it and decreases the phosphorylation of tau (Fig. 6G).
Here, AMPK signaling–mediated systemic and brain metabolic dysfunction are associated with neurodegenerative diseases, such as AD. In our study, decreased adiponectin-mediated AMPK/ACC signaling in the hippocampus of HFD-fed mice was consistent with the reduction of renal adiponectin–mediated AMPK/ACC in STZ-induced diabetes (44). In agreement with results in peripheral organs, phosphorylation of AMPK was decreased in the hippocampus of rats fed the HFD (40%) compared with rats fed a regular diet (45). Hyperinsulinemia leads to a decrease in adiponectin concentrations in HFD-fed rats (46). Our current finding shows that decreased adiponectin expression in the hippocampus of HFD-fed mice is closely correlated with circulating levels of adiponectin. These data show that systemic and brain adiponectin levels may play an important role in the regulation of the AMPK/ACC signaling pathway. In addition, reduced levels of AMPK in our results may be a product of the high caloric content of the HFD that can lead to oxidative stress. However, our results showed that RES supplementation restored levels of AMPK and 4-HNE in the hippocampus. Adiponectin exhibits potent anti-inflammatory responses in vascular tissue in addition to its insulin-sensitizing effects in tissues involved in glucose and lipid metabolism (47). In endothelial cells, adiponectin suppresses the adverse effects of inflammatory cytokines and reduces the oxidative stress induced by oxidized LDL (48). Our results showed that hypoadiponectinemia and neuroinflammation are reversed by RES administration and improvement of diabetes. Thus, these findings suggest that inactivation of adiponectin-mediated AMPK signaling in the hippocampus of HFD-fed mice may contribute to brain energy dysregulation and neurodegeneration. Finally, this inactivation of AMPK may lead to activation of GSK-3β and increase phosphorylation of tau (Fig. 6G).
Impaired brain insulin signaling, which is causally linked to neurodegeneration, is helpful to understand the possible mechanisms of diabetes-induced AD. Using a mouse model of HFD-induced obesity, Moroz et al. (36) demonstrated that chronic HFD feeding of C57BL/6 mice induces mild neuropathological lesions but significant impairments in IR binding in the temporal lobe region. In accordance with mild neurodegeneration, although the phosphorylation of tau-protein was increased by HFD, we found no change in hippocampal A-β levels between LFD- and HFD-treated mice using ELISA, including Fluoro-Jade staining (data not shown). These data indicate that chronic HFD feeding for 20 weeks does not induce A-β–mediated neurodegeneration. On the other hand, the reduction of ChAT is correlated with pathological changes, whereas the increased activity of insulin improves the expression of ChAT (49). Recent findings show that insulin signaling may play an important role in the activities of cholinergic neurons through insulin-related proteins colocalized with ChAT (50). In the current study, we confirmed that RES inhibits downregulation of HFD-induced ChAT expression and Golgi staining in the hippocampus. These results are consistent with the findings from the Morris water maze. Thus, our findings support the hypothesis that the impairment of insulin and adiponectin-mediated AMPK signaling in cholinergic neurons results in dysregulation of energy metabolism and impairs repair and cell survival.
In conclusion, our findings suggest that chronic dietary supplementation with RES may play an important role in reversing the deleterious actions of obesity on inflammation and memory in type 2 diabetic mice. Since obesity-induced insulin resistance also has been implicated in the progression of neurodegenerative diseases, natural compounds such as RES may have beneficial effects in retarding the progression of diabetes-induced neurodegenerative diseases.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (No. 2011-0006196).
No potential conflicts of interest relevant to this article were reported.
B.T.J. and G.S.R. researched data, contributed to discussion, and wrote, reviewed, and edited the manuscript. E.A.J., H.J.S., and Y.L. researched data. D.H.L., H.J.K., S.S.K., G.J.C., and W.S.C. contributed to discussion and reviewed and edited the manuscript. G.S.R. is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
The authors thank Jeong Bin Kim for technical support.