Changes in Synaptic Plasticity and Glutamate Receptors in Type 2 Diabetic KK-Ay Mice

Targeting microbiota-immune-synaptic plasticity to explore the effect of tea polyphenols on improving memory in the aged type 2 diabetic rat model

OBJECTIVES

The study aimed to explore whether TP could improve memory in the aged type 2 diabetic rat model by regulating microbiota-immune-synaptic plasticity axis.

METHODS

The experiment was divided into two parts. Firstly, to investigate the effects of TP on the physiopathology of the aged T2DM model rats, rats were randomly divided into the Normal control group, the aged group, the Aged T2DM model group, the TP 75, 150, 300 mg/kg groups, the 150 mg/kg Piracetam group and the 3 mg/kg Rosiglitazone group. Then, to further verify whether TP improved memory in aged T2DM rat model by regulating intestinal flora, the fecal microbiota transplantation (FMT) from the rats in the 300 mg/kg TP group into the rats in the aged T2DM model group was carried out. Effects on gut microbiota, colonic integrity (epithelial tight junction proteins), and endotoxemia (serum LPS) were examined, along with synaptic structure, synaptic plasticity-related structural proteins and inflammation signaling of the hippocampus in our study.

RESULTS

Our results demonstrated that TP alleviated memory impairments in the aged T2DM rat model. The specific outcomes were as follows: TP 300 mg/kg corrected the gut dysbacteriosis, alleviated intestinal permeability reduction and peripheral/central inflammation, inhibited the TLR4/NF-κB signaling pathway. Meanwhile, TP improved the synaptic plasticity in the hippocampus of the aged T2DM model rats, whose expressions of SYN, PSD 95, NMDAR1 and GluR1 in hippocampus were significantly up-regulated. Surprisingly, rats of the FMT group displayed the same changes.

DISCUSSION

TP improves the memory in aged T2DM rat model. The mechanism may be related to the alteration of gut flora, which can inhibit hippocampal TLR4/NF-κB signaling to attenuate neuroinflammation, then improve synaptic plasticity. The study proposes that TP interventions aimed at manipulating the gut microbiota may hold great potential as an effective approach for preventing and treating this disease.

Alterations of the glutamatergic system in diabetes mellitus

Diabetes mellitus (DM) is a chronic disease characterized by elevated blood glucose levels caused by a lack of insulin production (type 1 diabetes) or insulin resistance (type 2 diabetes). It is well known that DM is associated with cognitive deficits and metabolic and neurophysiological changes in the brain. Glutamate is the main excitatory neurotransmitter in the central nervous system that plays a key role in synaptic plasticity, learning, and memory processes. An increasing number of studies have suggested that abnormal activity of the glutamatergic system is implicated in the pathophysiology of DM. Dysfunction of glutamatergic neurotransmission in the central nervous system can provide an important neurobiological substrate for many disorders. Magnetic resonance spectroscopy (MRS) is a non-invasive technique that allows a better understanding of the central nervous system factors by measuring in vivo the concentrations of brain metabolites within the area of interest. Here, we briefly review the MRS studies that have examined glutamate levels in the brain of patients with DM. The present article also summarizes the available data on abnormalities in glutamatergic neurotransmission observed in different animal models of DM. In addition, the role of gut microbiota in the development of glutamatergic alterations in DM is addressed. We speculate that therapeutic strategies targeting the glutamatergic system may be beneficial in the treatment of central nervous system-related changes in diabetic patients.

The regulatory role of Gnao1 protein in diabetic encephalopathy in KK-Ay mice and streptozotocin-induced diabetic rats

AIMS

To investigate the regulation and functional role of Gnao1 in the brain of diabetic encephalopathy (DE) in various animal models.

RESULTS

Data from the biochemical and behavioral studies showed that DE models were successful induced in streptozotocin treatment animals and KK-Ay mice. Gnao1 was down regulated in the brain tissues of these two diabetes animal models with significant cognition deficiency. It suggested that the changes in DE are also related to dementia such as Alzheimer's disease (AD). Our study also showed that the expression of adrenergic α2 receptor (Adr-α2R), the upstream protein of Gnao1, was decreased in DE animal models. Furthermore, many downstream proteins of Gnao1 also altered, among which cAMP and PKA proteins were increased, CREB and BDNF proteins were decreased both in animal models and in the cell levels. In addition, Gnao1 silencing leads to the increase of reactive oxygen species (ROS) and the decreased proliferation in cultured primary astrocytes, which means that the deficiency of Gnao1 might not be benefit for DE.

CONCLUSION

Our findings demonstrated the importance of Gnao1 in DE and suggested Gnao1 as a novel marker and a promising therapeutic target for DE and dementia in animal models.

The attenuation effect of potassium 2-(1-hydroxypentyl)-benzoate in a mouse model of diabetes-associated cognitive decline: The protein expression in the brain

Aims

dl‐PHPB (potassium 2‐(1‐hydroxypentyl)‐benzoate) has been shown to have neuroprotective effects against acute cerebral ischemia, vascular dementia, and Alzheimer's disease. The aim of this study was to investigate the effects of dl‐PHPB on memory deficits and preliminarily explore the underlying molecular mechanism.

Methods

Blood glucose and behavioral performance were evaluated in the KK‐A^y diabetic mouse model before and after dl‐PHPB administration. Two‐dimensional difference gel electrophoresis (2D‐DIGE)‐based proteomics was used to identify differentially expressed proteins in brain tissue. Western blotting was used to study the molecular mechanism of the related signaling pathways.

Results

Three‐month‐old KK‐A^y mice were given 150 mg/kg dl‐PHPB by oral gavage for 2 months, which produced no effect on the level of serum glucose. In the Morris water maze test, KK‐A^y mice treated with dl‐PHPB showed significant improvements in spatial learning and memory deficits compared with vehicle‐treated KK‐A^y mice. Additionally, we performed 2D‐DIGE to compare brain proteomes of 5‐month KK‐A^y mice treated with and without dl‐PHPB. We found 14 altered proteins in the cortex and 11 in the hippocampus; two of the 25 altered proteins and another four proteins that were identified in a previous study on KK‐A^y mice were then validated by western blot to further confirm whether dl‐PHPB can reverse the expression levels of these proteins. The phosphoinositide 3‐kinase/protein kinase B/glycogen synthase kinase‐3β (PI3K/Akt/GSK‐3β) signaling pathway was also changed in KK‐A^y mice and dl‐PHPB treatment could reverse it.

Conclusions

These results indicate that dl‐PHPB may play a potential role in diabetes‐associated cognitive impairment through PI3K/Akt/GSK‐3β signaling pathway and the differentially expressed proteins may become putative therapeutic targets.

The Role of Glucagon-Like Peptide-1 Receptor Agonists (GLP-1 RA) in Diabetes-Related Neurodegenerative Diseases

Recent clinical guidelines have emphasized the importance of screening for cognitive impairment in older adults with diabetes, however, there is still a lack of understanding about the drug therapy. Glucagon-like peptide 1 receptor agonists (GLP-1 RAs) are widely used in the treatment of type 2 diabetes and potential applications may include the treatment of obesity as well as the adjunctive treatment of type 1 diabetes mellitus in combination with insulin. Growing evidence suggests that GLP-1 RA has the potential to treat neurodegenerative diseases, particularly in diabetes-related Alzheimer’s disease (AD) and Parkinson’s disease (PD). Here, we review the molecular mechanisms of the neuroprotective effects of GLP-1 RA in diabetes-related degenerative diseases, including AD and PD, and their potential effects.

Animal Models of Metabolic Disorders in the Study of Neurodegenerative Diseases: An Overview

The incidence of metabolic disorders, as well as of neurodegenerative diseases—mainly the sporadic forms of Alzheimer’s and Parkinson’s disease—are increasing worldwide. Notably, obesity, diabetes, and hypercholesterolemia have been indicated as early risk factors for sporadic forms of Alzheimer’s and Parkinson’s disease. These conditions share a range of molecular and cellular features, including protein aggregation, oxidative stress, neuroinflammation, and blood-brain barrier dysfunction, all of which contribute to neuronal death and cognitive impairment. Rodent models of obesity, diabetes, and hypercholesterolemia exhibit all the hallmarks of these degenerative diseases, and represent an interesting approach to the study of the phenotypic features and pathogenic mechanisms of neurodegenerative disorders. We review the main pathological aspects of Alzheimer’s and Parkinson’s disease as summarized in rodent models of obesity, diabetes, and hypercholesterolemia.

From Obesity to Hippocampal Neurodegeneration: Pathogenesis and Non-Pharmacological Interventions

High-caloric diet and physical inactivity predispose individuals to obesity and diabetes, which are risk factors of hippocampal neurodegeneration and cognitive deficits. Along with the adipose-hippocampus crosstalk, chronically inflamed adipose tissue secretes inflammatory cytokine could trigger neuroinflammatory responses in the hippocampus, and in turn, impairs hippocampal neuroplasticity under obese and diabetic conditions. Hence, caloric restriction and physical exercise are critical non-pharmacological interventions to halt the pathogenesis from obesity to hippocampal neurodegeneration. In response to physical exercise, peripheral organs, including the adipose tissue, skeletal muscles, and liver, can secret numerous exerkines, which bring beneficial effects to metabolic and brain health. In this review, we summarized how chronic inflammation in adipose tissue could trigger neuroinflammation and hippocampal impairment, which potentially contribute to cognitive deficits in obese and diabetic conditions. We also discussed the potential mechanisms underlying the neurotrophic and neuroprotective effects of caloric restriction and physical exercise by counteracting neuroinflammation, plasticity deficits, and cognitive impairments. This review provides timely insights into how chronic metabolic disorders, like obesity, could impair brain health and cognitive functions in later life.

Studies of pathology and pharmacology of diabetic encephalopathy with KK-Ay mouse model

AIMS

Pathogenesis of diabetic encephalopathy (DE) is not completely understood until now. The purposes of this study were to illustrate the changes in morphology, function, and important transporters in neurons and glia during DE, as well as to reveal the potential therapeutic effects of medicines and the diet control on DE.

METHODS

Spontaneous obese KK-Ay mice were used to investigate diabetes-induced cognitive disorder, the morphology, function, and protein expression changes in impact animal and the cell level studies. The new drug candidate PHPB, donepezil, and low-fat food were used to observe the therapeutic effects.

RESULTS

KK-Ay mice at 5 months of age showed typical characteristics of type 2 diabetes mellitus (T2DM） and appeared significant cognitive deficits. Morphological study showed microtubule-associated protein 2 (MAP2) expression was increased in hippocampal neurons and glial fibrillary acidic protein (GFAP) expression decreased in astrocytes. Meanwhile, the vesicular glutamate transporter 1 (vGLUT1) expression was increased and glucose transporter 1 (GLUT1) decreased, and the expression of brain-derived neurotrophic factor (BDNF) and glial cell-derived neurotrophic factor (GDNF) was also reduced in KK-Ay mice. Microglia were activated, and IL-1β and TNF-α were increased obviously in the brains of the KK-Ay mice. Most of the above changes in the KK-Ay mice at 5 months of age could be relieved by diet intervention (DR) or by treatment of donepezil or new drug candidate PHPB.

CONCLUSION

KK-Ay mouse is a useful animal model for studying DE. The alterations of morphology, structure, and function of astrocyte and microglia in KK-Ay mice might be rescued by DR and by treatment of medicine. The proteins we reported in this study could be used as biomarkers and the potential drug targets for DE study and treatment.

HMGB1/TLR4 promotes apoptosis and reduces autophagy of hippocampal neurons in diabetes combined with OSA

AIMS

Obstructive sleep apnea (OSA) combined with type 2 diabetes (T2DM) may lead to cognitive dysfunction. We previously reported that cognitive impairment is exacerbated in KKAy mice exposed to intermittent hypoxia (IH), during which the DNA binding protein HMGB1 mediates hippocampal neuronal apoptosis by maintaining microglia-associated neuroinflammation, but the underlying mechanism remains largely unknown.

MATERIALS AND METHODS

We performed immunofluorescence, Western blotting, and immunohistochemistry experiments in mouse hippocampal tissues and HT22 cells. KKAy type 2 diabetes model mice and normal C57BL/6J mice were exposed to IH or intermittent normoxia. HT22 cells were cultured in high glucose medium and exposed to IH or intermittent normoxia. We transfected HMGB1 siRNA into HT22 cells and then treated them with high glucose combined with intermittent hypoxia.

KEY FINDINGS

In conclusion, IH aggravated apoptosis and autophagy defects in T2DM mice, and increased the protein expression of HMGB1 and TLR4. This was also confirmed in HG + IH-treated hippocampal HT22 cells. HMGB1 siRNA can significantly reduce the protein expression of HMGB1 and TLR4, reverse neuronal apoptosis and enhance autophagy.

SIGNIFICANCE

We believe that HMGB1 is a key factor in the regulation of hippocampal neuronal apoptosis and autophagy defects in T2DM combined with OSA. Targeting HMGB1/TLR4 signaling as a novel approach may delay or prevent the increased apoptosis and decreased autophagy induced by T2DM combined with OSA, and may ultimately improve cognitive dysfunction.

Calcium Dobesilate Restores Autophagy by Inhibiting the VEGF/PI3K/AKT/mTOR Signaling Pathway

Objective: Calcium dobesilate (CaD), an effective drug for the treatment of diabetic microvascular complications, especially diabetic retinopathy, is widely used in the clinic. Interestingly, several studies have indicated that CaD is therapeutic for diabetic kidney disease (DKD). Recently, evidence has indicated that altered vascular endothelial growth factor (VEGF) expression and decreased autophagy are the main pathological mechanisms of proteinuria. Thus, this study was conducted to explore the effect of CaD on restoring autophagy in DKD and the possible signaling pathway between VEGF and autophagy. Methods: Obese mice with spontaneous diabetes (KK-Ay) and high-fat diet- and streptozotocin-induced diabetic mice (HFD/STZ) were used in this study. Biochemical staining, western blotting, and immunohistochemistry were conducted to determine the angioprotective effect of CaD and the underlying mechanism between autophagy and VEGF/VEGFR. Results: Our results showed that CaD was capable of reducing albuminuria and restoring renal histological changes in KK-Ay and HFD/STZ-induced diabetic mice. CaD restored autophagy by decreasing the protein expression of LC3 II, Atg5, and beclin 1 and increasing the expression of P62. Moreover, CaD reduced the activation of the autophagy-related PI3K/AKT/mTOR pathway possibly via decreasing VEGF and downregulating VEGF receptor 2. Conclusion: Overall, CaD, as a novel potential therapeutic drug for DKD, plays a key role in protecting renal function and restoring autophagy by blocking VEGF/VEGFR2 and inhibiting the PI3K/AKT/mTOR signaling pathway.

Strain and sex-based glucocentric & behavioral differences between KK/HlJ and C57BL/6J mice

INTRODUCTION

Small-animal models are the most widely used preclinical model for studying the etiology, pathology and treatment of diabetes, prediabetes and diabetic comorbidities. Diabetic patients are burdened with higher rates of depression, anxiety and cognitive decline due to inadequate control of blood glucose levels, vascular damage and aberrant CNS insulin signaling. The C57BL/6J model is amongst the most widely used mouse model due to its susceptibility to diet-induced obesity (DIO). This strain has also been well-characterized in behavioral research studies. However the C57BL/6J model has a number of limitations: [1] overt fasting hyperglycemia can only be induced by dietary manipulation and/or chemical ablation of the pancreatic beta cells. [2] There is heterogeneity in the obesogenic response to hypercaloric feeding as well as sex-dependent differences, with males being more responsive. The KK inbred strain has been used to study aspects of the metabolic syndrome and prediabetes due to inherent glucose intolerance, hyperinsulinemia and insulin resistance. However KK/HlJ mice are less well-characterized and there have been fewer behavioral studies reported. The aim of this study was to examine differences in male and female glucocentric parameters between KK/HlJ and C57BL/6J mice, and to compare their performance in a variety of standard behavioral tests relating to general, anxiogenic and cognitive paradigms.

METHODS

Strain differences in male and female KK/HlJ and C57BL/6J mouse adiposity, glucose and insulin parameters were studied together with group differences in standard Open Field, Object Recognition, Elevated Plus Maze, Light-Dark Transition, Porsolt test, Marble Burying, Social Recognition and Morris Water Maze tests. Correlations between behavioral variables were analyzed.

RESULTS AND CONCLUSION

In addition to being uniformly larger, hyperinsulinemic and more insulin intolerant than C57BL/6J mice, we observed marked strain and sex-differences in KK/HlJ behavior. KK/HlJ mice exhibited less locomotor and vertical exploratory behavior in comparison to C57BL/6J, whereas object exploration and novel object discrimination were superior in KK/HlJ mice. Female KK/HlJ mice were faster swimmers, whereas the males exhibited greater spatial cognition and place-learning during the MWM test.

Hydrogen sulfide inhibits Ca2+-induced mitochondrial permeability transition pore opening in type-1 diabetes

Hydrogen sulfide (H2S) attenuates N-methyl-d-aspartate receptor-R1 (NMDA-R1) and mitigates diabetic renal damage; however, the molecular mechanism is not well known. Whereas NMDA-R1 facilitates Ca2+ permeability, H2S is known to inhibit L-type Ca2+ channel. High Ca2+ activates cyclophilin D (CypD), a gatekeeper protein of mitochondrial permeability transition pore (MPTP), thus facilitating molecular exchange between matrix and cytoplasm causing oxidative outburst and cell death. We tested the hypothesis of whether NMDA-R1 mediates Ca2+ influx causing CypD activation and MPTP opening leading to oxidative stress and renal injury in diabetes. We also tested whether H2S treatment blocks Ca2+ channel and thus inhibits CypD and MPTP opening to prevent renal damage. C57BL/6J and Akita (C57BL/6J-Ins2Akita) mice were treated without or with H2S donor GYY4137 (0.25 mg·kg-1·day-1 ip) for 8 wk. In vitro studies were performed using mouse glomerular endothelial cells. Results indicated that low levels of H2S and increased expression of NMDA-R1 in diabetes induced Ca2+ permeability, which was ameliorated by H2S treatment. We observed cytosolic Ca2+ influx in hyperglycemic (HG) condition along with mitochondrial-CypD activation, increased MPTP opening, and oxidative outburst, which were mitigated with H2S treatment. Renal injury biomarker KIM-1 was upregulated in HG conditions and normalized following H2S treatment. Inhibition of NMDA-R1 by pharmacological blocker MK-801 revealed similar results. We conclude that NMDA-R1-mediated Ca2+ influx in diabetes induces MPTP opening via CypD activation leading to increased oxidative stress and renal injury, and H2S protects diabetic kidney from injury by blocking mitochondrial Ca2+ permeability through NMDA-R1 pathway.

DNA binding protein HMGB1 secreted by activated microglia promotes the apoptosis of hippocampal neurons in diabetes complicated with OSA

Type 2 diabetes mellitus (T2DM) complicated with obstructive sleep apnea (OSA) may cause neuronal apoptosis and cognitive deficits, but the underlying mechanisms remain unclear. We aimed to determine the relationship between the activation of microglia and the apoptosis of hippocampal neurons, specifically in terms of high mobility group box-1 (HMGB1), after high glucose (HG) and intermittent hypoxia (IH) exposure. Diabetic KK-Ay mice and non-diabetic C57BL/6J mice (C57 mice) underwent IH or normoxia (control) exposure for 4 weeks. Cognitive function, microglial activation and hippocampal neuronal apoptosis were assessed after IH or normoxia exposure. Compared with C57 control mice, KK-Ay control mice exhibited increased cognitive dysfunction, microglial activation and hippocampal neuronal apoptosis. There were no differences between untreated KK-Ay control mice and C57 mice that had been exposed to IH. The abovementioned responses were aggravated in IH-exposed KK-Ay mice compared with control KK-Ay mice. In vitro, a cellular co-culture experiment showed that HG combined with IH could activate BV2 microglia, leading to the release of neuroinflammatory factors (ROS, TNF-α, IL-1β) and mediating the apoptosis of HT22 cells via the PI3K/Akt/GSK-3β signaling pathway. Meanwhile, HMGB1 was actively secreted into the extracellular environment from activated BV2 microglia. As a proinflammatory factor, it was able to sustain microglial activation by directly acting on those cells. The activation promoted positive feedback and aggravated neuronal damage further. In a cellular monoculture or co-culture system, HMGB1 siRNA was able to alleviate the activation of BV2 cells and the apoptosis of HT22 cells induced by HG combined with IH. Our object is to show that inhibition of HMGB1 may break the vicious cycle to prevent or treat neuroinflammation and hippocampal neuronal apoptosis caused by T2DM complicated with OSA.

Hyperglycemia induced the Alzheimer's proteins and promoted loss of synaptic proteins in advanced-age female Goto-Kakizaki (GK) rats

Although both type 2 diabetes mellitus (T2DM) and aging are related with Alzheimer's disease (AD), the effects of aging on the Alzheimer's proteins and the synaptic markers in T2DM have not been investigated. This study, we hypothesized that T2DM rats with advanced-age, aggravates the reduction of synaptic proteins and an increase in the Alzheimer's protein markers. Goto-Kakizaki rats (GK) were used as a T2DM group and wild-type rats (WT) were used as a control group. Rats in each group were categorized by age into young-adult (7 months) and advanced-age rats (12.5 months). Blood was collected in all rats to determine plasma glucose and insulin levels. The brains were used for determining the level of Alzheimer's and synaptic proteins. Our data demonstrated that GK rats had a decreased body weight and increased blood glucose levels, compared to their age-matched WT. p-Tau was increased in both advanced-age WT and GK, compared to their young-adult rats. Moreover, amyloid-beta (Aβ) level was higher in advanced-age GK than their age-matched WT. The synaptic proteins were decreased in advanced-age GK, compared to young-adult GK rats. However, no difference in the level of Alzheimer's proteins and synaptic proteins in the brains of young-adult GK compared to age-matched WT was found. Our data suggested that aging contributes to the pathogenesis of AD and the reduction of synaptic proteins to greater extent in a diabetic than in a healthy condition.