Chronic hyperglycemia induces tau hyperphosphorylation by downregulating OGT-involved O-GlcNAcylation in vivo and in vitro

Irreversible atrophy in memory brain regions over 7 years is predicted by glycemic control in type 2 diabetes without mild cognitive impairment

Memory-related impairments in type 2 diabetes may be mediated by insulin resistance and hyperglycemia. Previous cross-sectional studies have controversially suggested a relationship between metabolic control and a decrease in hippocampal volumes, but only longitudinal studies can test this hypothesis directly. We performed a longitudinal morphometric study to provide a direct test of a possible role of higher levels of glycated hemoglobin with long term brain structural integrity in key regions of the memory system - hippocampus, parahippocampal gyrus and fusiform gyrus. Grey matter volume was measured at two different times - baseline and after ~7 years. We found an association between higher initial levels of HbA1C and grey matter volume loss in all three core memory regions, even in the absence of mild cognitive impairment. Importantly, these neural effects persisted in spite of the fact that patients had significantly improved their glycemic control. This suggests that early high levels of HbA1c might be irreversibly associated with subsequent long-term atrophy in the medial temporal cortex and that early intensive management is critical.

The comorbidity and co-medication profile of patients with progressive supranuclear palsy

BACKGROUND

Progressive supranuclear palsy (PSP) is usually diagnosed in elderly. Currently, little is known about comorbidities and the co-medication in these patients.

OBJECTIVES

To explore the pattern of comorbidities and co-medication in PSP patients according to the known different phenotypes and in comparison with patients without neurodegenerative disease.

METHODS

Cross-sectional data of PSP and patients without neurodegenerative diseases (non-ND) were collected from three German multicenter observational studies (DescribePSP, ProPSP and DANCER). The prevalence of comorbidities according to WHO ICD-10 classification and the prevalence of drugs administered according to WHO ATC system were analyzed. Potential drug-drug interactions were evaluated using AiDKlinik®.

RESULTS

In total, 335 PSP and 275 non-ND patients were included in this analysis. The prevalence of diseases of the circulatory and the nervous system was higher in PSP at first level of ICD-10. Dorsopathies, diabetes mellitus, other nutritional deficiencies and polyneuropathies were more frequent in PSP at second level of ICD-10. In particular, the summed prevalence of cardiovascular and cerebrovascular diseases was higher in PSP patients. More drugs were administered in the PSP group leading to a greater percentage of patients with polypharmacy. Accordingly, the prevalence of potential drug-drug interactions was higher in PSP patients, especially severe and moderate interactions.

CONCLUSIONS

PSP patients possess a characteristic profile of comorbidities, particularly diabetes and cardiovascular diseases. The eminent burden of comorbidities and resulting polypharmacy should be carefully considered when treating PSP patients.

O-GlcNAcylation and Its Roles in Neurodegenerative Diseases

 As a non-classical post-translational modification, O-linked β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) is widely found in human organ systems, particularly in our brains, and is indispensable for healthy cell biology. With the increasing age of the global population, the incidence of neurodegenerative diseases is increasing, too. The common characteristic of these disorders is the aggregation of abnormal proteins in the brain. Current research has found that O-GlcNAcylation dysregulation is involved in misfolding or aggregation of these abnormal proteins to mediate disease progression, but the specific mechanism has not been defined. This paper reviews recent studies on O-GlcNAcylation's roles in several neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, Machado-Joseph's disease, and giant axonal neuropathy, and shows that O-GlcNAcylation, as glucose metabolism sensor, mediating synaptic function, participating in oxidative stress response and signaling pathway conduction, directly or indirectly regulates characteristic pathological protein toxicity and affects disease progression. The existing results suggest that targeting O-GlcNAcylation will provide new ideas for clinical diagnosis, prevention, and treatment of neurodegenerative diseases.

Low glucose induced Alzheimer's disease-like biochemical changes in human induced pluripotent stem cell-derived neurons is due to dysregulated O-GlcNAcylation

INTRODUCTION

Sporadic Alzheimer's disease (sAD) is the leading type of dementia. Brain glucose hypometabolism, along with decreased O-GlcNAcylation levels, occurs before the onset of symptoms and correlates with pathogenesis. Heretofore, the mechanisms involved and the roles of O-GlcNAcylation in sAD pathology largely remain unknown due to a lack of human models of sAD.

METHODS

Human cortical neurons were generated from pluripotent stem cells (PSCs) and treated with glucose reduction media.

RESULTS

We found a narrow window of glucose concentration that induces sAD-like phenotypes in PSC-derived neurons. With our model, we reveal that dysregulated O-GlcNAc, in part through mitochondrial dysfunction, causes the onset of sAD-like changes. We demonstrate the therapeutic potential of inhibiting O-GlcNAcase in alleviating AD-like biochemical changes.

DISCUSSION

Our results suggest that dysregulated O-GlcNAc might be a direct molecular link between hypometabolism and sAD-like alternations. Moreover, this model can be exploited to explore molecular processes and for drug development.

HIGHLIGHTS

Lowering glucose to a critical level causes AD-like changes in cortical neurons. Defective neuronal structure and function were also recapitulated in current model. Dysregulated O-GlcNAcylation links impaired glucose metabolism to AD-like changes. Mitochondrial abnormalities correlate with O-GlcNAcylation and precede AD-like phenotype. Our model provides a platform to study sAD as a metabolic disease in human neurons.

DPP-4 Inhibitors Suppress Tau Phosphorylation and Promote Neuron Autophagy through the AMPK/mTOR Pathway to Ameliorate Cognitive Dysfunction in Diabetic Mellitus

Dipeptidyl peptidase-4 (DPP-4) inhibitors have been considered as incretin-based agents that signal through GLP-1R. Our high-throughput RNA sequencing (RNA-seq) and bioinformatics methods indicated that GLP-1R, downregulated in diabetes mellitus (DM), was a potential target of DPP-4 inhibitors, which was further confirmed in DM rats. Thus, this study illuminated the alleviatory mechanism of DPP-4 on cognitive dysfunction in diabetes mellitus (DM), which may be associated with GLP-1R signaling. DM rats were administered with DPP-4 inhibitors, Chloroquine (an autophagy inhibitor), Exendin 9-39 (a GLP-1R antagonist), or Compound C (a specific inhibitor of AMPK). An in vitro model of DM was induced in rat hippocampal neuronal cell line H19-7 by exposure to high glucose (HG) and high fat (HF), followed by treatment with the above inhibitors and antagonists. It was found that cognitive dysfunction was promoted, and LC3 expression was lowered in DM rats by an autophagy inhibitor. The DPP-4 inhibitors decreased cognitive dysfunction, repressed Tau phosphorylation, and enhanced GLP-1R protein level, LC3 expression, and AMPK and mTOR phosphorylation in DM rats, while GLP-1R antagonist, an autophagy inhibitor, or AMPK inhibitor counteracted these effects. Such effects were also observed in HG/HF-induced neurons. In conclusion, our data elucidated the alleviatory mechanism of DPP-4 inhibitors in the cognitive dysfunction of DM rats via the AMPK/mTOR pathway.

Cognitive dysfunction and increased phosphorylated tau are associated with reduced O-GlcNAc signaling in an aging mouse model of metabolic syndrome

Metabolic syndrome (MetS), characterized by hyperglycemia, obesity, and hyperlipidemia, can increase the risk of developing late-onset dementia. Recent studies in patients and mouse models suggest a putative link between hyperphosphorylated tau, a component of Alzheimer's disease-related dementia (ADRD) pathology, and cerebral glucose hypometabolism. Impaired glucose metabolism reduces glucose flux through the hexosamine metabolic pathway triggering attenuated O-linked N-acetylglucosamine (O-GlcNAc) protein modification. The goal of the current study was to investigate the link between cognitive function, tau pathology, and O-GlcNAc signaling in an aging mouse model of MetS, agouti KKAy+/- . Male and female C57BL/6, non-agouti KKAy-/- , and agouti KKAy+/- mice were aged 12-18 months on standard chow diet. Body weight, blood glucose, total cholesterol, and triglyceride were measured to confirm the MetS phenotype. Cognition, sensorimotor function, and emotional reactivity were assessed for each genotype followed by plasma and brain tissue collection for biochemical and molecular analyses. Body weight, blood glucose, total cholesterol, and triglyceride levels were significantly elevated in agouti KKAy+/- mice versus C57BL/6 controls and non-agouti KKAy-/- . Behaviorally, agouti KKAy+/- revealed impairments in sensorimotor and cognitive function versus age-matched C57BL/6 and non-agouti KKAy-/- mice. Immunoblotting demonstrated increased phosphorylated tau accompanied with reduced O-GlcNAc protein expression in hippocampal-associated dorsal midbrain of female agouti KKAy+/- versus C57BL/6 control mice. Together, these data demonstrate that impaired cognitive function and AD-related pathology are associated with reduced O-GlcNAc signaling in aging MetS KKAy+/- mice. Overall, our study suggests that interaction of tau pathology with O-GlcNAc signaling may contribute to MetS-induced cognitive dysfunction in aging.

Serum Uric Acid Levels Are Related to Diabetic Peripheral Neuropathy, Especially for Motor Conduction Velocity of Tibial Nerve in Type 2 Diabetes Mellitus Patients

Background

Oxidative stress is one of the most critical factors that contribute to the pathogenesis of neuronal damage, including diabetic peripheral neuropathy (DPN). Uric acid is a kind of natural antioxidant that plays a major role in the antioxidant capacity against oxidative stress. Here, we aim to determine the role of serum uric acid (SUA) in the DPN of patients with type 2 diabetes mellitus (T2DM). Patients and Methods. 106 patients with T2DM were recruited and divided into the DPN group and the control group. Clinical parameters, especially for motor nerve fiber conduction velocity and sensory nerve fiber conduction velocity, were collected. Differences between T2DM patients with and without DPN were compared. Correlation and regression analyses were performed to explore the association between SUA and DPN.

Results

Compare with 57 patients with DPN, 49 patients without DPN showed lower HbA1c and elevated SUA levels. Additionally, SUA levels are negatively associated with the motor conduction velocity of tibial nerve with or without adjusting for HbA1c. Besides, it is suggested that decreased SUA levels may influence the motor conduction speed of the tibial nerve by multiple linear regression analysis. Moreover, we demonstrated that decreased SUA level is a risk factor for DPN in patients with T2DM by binary logistic regression analysis.

Conclusion

Lower SUA is a risk factor for DPN in patients with T2DM. Additionally, decreased SUA may influence the damage of peripheral neuropathy, especially for motor conduction velocity of the tibial nerve.

Ultrasensitive assay of O-GlcNAc transferase using capillary electrophoresis-laser induced fluorescence

O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) is directly associated with the level of O-GlcNAc glycosylation of biomolecules and various diseases, and it is expected to be a promising potential new therapeutic target. Here, we develop a robust and sensitive method for OGT assay based on capillary electrophoresis-laser induced fluorescence (CE-LIF) method. AF-488-modified peptide containing serine active group is designed as substrate for OGT-catalyzed reaction, and nonradioactive UDP-GlcNAc is employed as sugar donor to perform O-GlcNAc glycosylation modification. The enzyme activity of OGT is measured by quantitative determination of glycosylated peptide produced by the reaction. Large volume sample stacking technique for sample injection and a unique fluorescence collection system for LIF detection are adopted to greatly enhance the detection sensitivity, thus a low limit of detection down to 0.23 pM for OGT detection is achieved. The method is successfully applied to detect OGT activity in clinical blood samples with satisfactory accuracy. Our study provides a simple, accurate, and sensitive method with great potential application in clinical diagnosis of O-GlcNAc-related diseases.

Targeting Alzheimer's Disease: The Critical Crosstalk between the Liver and Brain

Alzheimer's disease (AD), an age-related neurodegenerative disorder, is currently incurable. Imbalanced amyloid-beta (Aβ) generation and clearance are thought to play a pivotal role in the pathogenesis of AD. Historically, strategies targeting Aβ clearance have typically focused on central clearance, but with limited clinical success. Recently, the contribution of peripheral systems, particularly the liver, to Aβ clearance has sparked an increased interest. In addition, AD presents pathological features similar to those of metabolic syndrome, and the critical involvement of brain energy metabolic disturbances in this disease has been recognized. More importantly, the liver may be a key regulator in these abnormalities, far beyond our past understanding. Here, we review recent animal and clinical findings indicating that liver dysfunction represents an early event in AD pathophysiology. We further propose that compromised peripheral Aβ clearance by the liver and aberrant hepatic physiological processes may contribute to AD neurodegeneration. The role of a hepatic synthesis product, fibroblast growth factor 21 (FGF21), in the management of AD is also discussed. A deeper understanding of the communication between the liver and brain may lead to new opportunities for the early diagnosis and treatment of AD.

Expert Consensus on Cognitive Dysfunction in Diabetes

The incidence of diabetes is gradually increasing in China, and diabetes and associated complications, such as cognitive dysfunction have gained much attention in recent time. However, the concepts, clinical treatment, and prevention of cognitive dysfunction in patients with diabetes remain unclear. The Chinese Society of Endocrinology investigated the current national and overseas situation of cognitive dysfunction associated with diabetes. Based on research both in China and other countries worldwide, the Expert Consensus on Cognitive Dysfunction in Diabetes was established to guide physicians in the comprehensive standardized management of cognitive dysfunction in diabetes and to improve clinical outcomes in Chinese patients. This consensus presents an overview, definition and classification, epidemiology and pathogenesis, risk factors, screening, diagnosis, differential diagnosis, treatment, and prevention of cognitive dysfunction in patients with diabetes.

O-GlcNAcylation increases PYGL activity by promoting phosphorylation

O-GlcNAcylation is a post-translational modification that links metabolism with signal transduction. High O-GlcNAcylation appears to be the general characteristic of cancer cells. It promotes the invasion, metastasis, proliferation and survival of tumor cells, and alters many metabolic pathways. Glycogen metabolism increases in a wide variety of tumors, suggesting that it is an important aspect of cancer pathophysiology. Herein we focused on the O-GlcNAcylation of liver glycogen phosphorylase (PYGL), an important catabolism enzyme in the glycogen metabolism pathway. PYGL expressed in both HEK 293 T and HCT116 were modified by O-GlcNAc. And both PYGL O-GlcNAcylation and phosphorylation of Ser15 (pSer15) were decreased under glucose and insulin, while increased under glucagon and Na2S2O4 (hypoxia) conditions. Then, we identified the major O-GlcNAcylation site to be Ser430, and demonstrated that pSer15 and Ser430 O-GlcNAcylation were mutually reinforced. Lastly, we found that Ser430 O-GlcNAcylation was fundamental for PYGL activity. Thus, O-GlcNAcylation of PYGL positively regulated pSer15 and therefore its enzymatic activity. Our results provided another molecular insight into the intricate post-translational regulation network of PYGL.

Glucagon-like peptide-1 attenuated carboxymethyl lysine induced neuronal apoptosis via peroxisome proliferation activated receptor-γ

Backgrounds and aims: The role of peroxisome proliferator activated receptor-γ (PPAR-γ) in neuronal apoptosis remains unclear. We aim to investigate the role of PPAR-γ in glucagon-like peptide-1 (GLP-1) alleviated neuronal apoptosis induced by carboxymethyl-lysine (CML).

Materials and Methods: In vitro, PC12 cells were treated by CML/GLP-1. Moreover. the function of PPAR-γ was blocked by GW9662. In vivo, streptozotocin (STZ) was used to induce diabetic rats with neuronal apoptosis. The cognitive function of rats was observed by Morris water maze. Apoptosis was detected by TUNEL assay. Bcl2, Bax, PPAR-γ and receptor of GLP-1 (GLP-1R) were measured by western blotting or immunofluorescence.

Results: In vitro experiment, CML triggered apoptosis, down-regulated GLP-1R and PPAR-γ. Moreover, GLP-1 not only alleviated the apoptosis, but also increased levels of PPAR-γ. GW9662 abolished the neuroprotective effect of GLP-1 on PC12 cells from apoptosis. Furthermore, GLP-1R promoter sequences were detected in the PPAR-γ antibody pulled mixture. GPL-1 levels decreased, while CML levels increased in diabetic rats, compared with control rats. Additionally, we observed elevated bax, decreased bcl2, GLP-1R and PPAR-γ in diabetic rats.

Conclusions: GLP-1 could attenuate neuronal apoptosis induced by CML. Additionally, PPAR-γ involves in this process.

MALAT1 Regulated mTOR-Mediated Tau Hyperphosphorylation by Acting as a ceRNA of miR144 in Hippocampus Cells Exposed to High Glucose

Aim

High glucose (HG)-induced activation of mTOR promotes tau phosphorylation and leads to diabetes-associated dementia. This study aimed to explore the role of metastasis associated in lung adenocarcinoma transcript 1 (MALAT1) in HG-induced neuronal cell injury.

Methods

Hippocampus cells were isolated from C57BL/6J mice. After 6 days of culture, the cells were incubated with 5.5 mM glucose in normal medium or 75 mM glucose for 4 days. Cells were transfected with miR-144 mimic, miR-144 inhibitor, siRNA for MALAT1 or corresponding controls. Gene expression was detected by PCR and Western blot analysis.

Results

HG increased the levels of MALAT1 and p-tau in hippocampal cells. Knockdown of MALAT1 partially reversed the effects of HG on mTOR activity and p-tau protein levels. MALAT1 functioned as competing endogenous RNA (ceRNA) for miR-144, and pre-treatment with MALAT1 siRNA decreased mTOR activity and p-tau protein level in HG-treated hippocampal cells, which was significantly attenuated by miR-144 mimics. Moreover, miR-144 negatively regulated the expression of mTOR and knockdown of MALAT1 suppressed mTOR, while overexpression of mTOR abrogated protective effects of MALAT1 knockdown in HG-treated hippocampal cells.

Conclusion

MALAT1 knockdown prevented HG-induced mTOR activation and inhibited tau phosphorylation. MALAT1 may be a therapy target for diabetes associated dementia.

High-Fat Diet Leads to Reduced Protein O-GlcNAcylation and Mitochondrial Defects Promoting the Development of Alzheimer's Disease Signatures

The disturbance of protein O-GlcNAcylation is emerging as a possible link between altered brain metabolism and the progression of neurodegeneration. As observed in brains with Alzheimer's disease (AD), flaws of the cerebral glucose uptake translate into reduced protein O-GlcNAcylation, which promote the formation of pathological hallmarks. A high-fat diet (HFD) is known to foster metabolic dysregulation and insulin resistance in the brain and such effects have been associated with the reduction of cognitive performances. Remarkably, a significant role in HFD-related cognitive decline might be played by aberrant protein O-GlcNAcylation by triggering the development of AD signature and mitochondrial impairment. Our data support the impairment of total protein O-GlcNAcylation profile both in the brain of mice subjected to a 6-week high-fat-diet (HFD) and in our in vitro transposition on SH-SY5Y cells. The reduction of protein O-GlcNAcylation was associated with the development of insulin resistance, induced by overfeeding (i.e., defective insulin signaling and reduced mitochondrial activity), which promoted the dysregulation of the hexosamine biosynthetic pathway (HBP) flux, through the AMPK-driven reduction of GFAT1 activation. Further, we observed that a HFD induced the selective impairment of O-GlcNAcylated-tau and of O-GlcNAcylated-Complex I subunit NDUFB8, thus resulting in tau toxicity and reduced respiratory chain functionality respectively, highlighting the involvement of this posttranslational modification in the neurodegenerative process.

Implications of Glycosylation in Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting millions of people worldwide, and no cure is currently available. The major pathological hallmarks of AD are considered to be amyloid beta plaques and neurofibrillary tangles, generated by respectively APP processing and Tau phosphorylation. Recent evidence imply that glycosylation of these proteins, and a number of other AD-related molecules is altered in AD, suggesting a potential implication of this process in disease pathology. In this review we summarize the understanding of glycans in AD pathogenesis, and discuss how glycobiology can contribute to early diagnosis and treatment of AD, serving as potential biomarkers and therapeutic targets. Furthermore, we look into the potential link between the emerging topic neuroinflammation and glycosylation, combining two interesting, and until recent years, understudied topics in the scope of AD. Lastly, we discuss how new model platforms such as induced pluripotent stem cells can be exploited and contribute to a better understanding of a rather unexplored area in AD.

Liraglutide improved the cognitive function of diabetic mice via the receptor of advanced glycation end products down-regulation

Background and aims Advanced glycation end products (AGEs) and receptor of advanced glycation end products (RAGE), are associated with cognition decline. We aim to investigate the effect of liraglutide on cognitive function in diabetic mice.

Results Diabetic mice showed decreased cognitive function. Moreover, lower glucagon like peptide-1 (GLP-1) levels in plasma were detected in db/db mice. Additionally, up-regulated RAGE and down-regulated glucagon like peptide-1 (GLP-1R) levels were observed in db/db mice. However, decreased GLP-1R and increased RAGE were reversed by liraglutide. We also found decreased cellular activity in cells with AGEs. Moreover, AGEs up-regulated RAGE in PC12 and HT22 cells. However, liraglutide improved the cell activity damaged by AGEs. Although we did not discover the direct-interaction between RAGE and GLP-1R, elevated RAGE levels induced by AGEs were restored by liraglutide.

Conclusion We demonstrated that the cognitive function of diabetic mice was improved by liraglutide via the down-regulation of RAGE.

Methods db/db mice and db/m mice were used in this study. Liraglutide was used to remedy diabetic mice. Neurons and RAGE in hippocampus were shown by immunofluorescence. And then, PC12 cells or HT22 cells with AGEs were treated with liraglutide. GLP-1R and RAGE were measured by western blotting.