Insulin resistance: an emerging link in Alzheimer's disease

Deep Sea Water Alleviates Tau Phosphorylation and Cognitive Impairment via PI3K/Akt/GSK-3β Pathway

Deep sea water (DSW), as a noticeable natural resource, has been demonstrated to contain high levels of beneficial minerals and exert marked anti-diabetes effects. Epidemiological studies show that type 2 diabetes mellitus (T2DM) is closely related to high danger of Alzheimer's disease (AD); moreover, Akt/GSK-3β signaling is the main underlying pathway that connects these two diseases. Besides, it has been demonstrated that minerals in DSW, such as Mg, Se, and Zn, could effectively treat cognitive deficits associated with AD. Herein, we first observed the protection of DSW against cognitive dysfunction in T2DM rats, then furtherly explored the neuroprotective mechanism in SH-SY5Y cell model. In T2DM rats, DSW obviously elevated the concentrations of elements Mg, V, Cr, Zn, and Se in brain and improved learning and memory dysfunction in behavior assays, including Morris water maze (MWM) and new object recognition (NOR). Western blot (WB) results demonstrated that DSW could stimulate PI3K/Akt/GSK-3β signaling, arrest Tau hyperphosphorylation at serine (Ser) 396 and threonine (Thr)231, which was confirmed by immunohistochemistry (IHC). In order to further confirm the mechanism, we employed wortmannin to inhibit PI3K in SH-SY5Y cells; results showed that pretreatment with wortmannin almost abolished DSW-induced decreases in phosphorylated Tau. Taken together, these data elucidated that DSW could improve Tau hyperphosphorylation and cognitive impairment, which were closely related with the stimulation of Akt/GSK-3β signaling, and the neuroprotective effects of DSW should be contributed to the synergistic effects of major and trace elements in it, such as Mg, V, Cr, Zn, and Se. These experimental evidence indicated that DSW may be explored as natural neuroprotective food for the prevention and treatment of AD.

Hypothesis: JNK signaling is a therapeutic target of neurodegenerative diseases

The exact signaling leading to neurological dysfunction in neurodegenerative diseases is currently unknown. We hypothesize that the c-Jun N-terminal kinase (JNK) signaling pathway is a potential therapeutic target for neurodegenerative diseases. This postulate rests on extensive data from cell and animal experimental studies, demonstrating that JNK signaling plays a crucial role in the pathogenesis of neurodegenerative diseases. The sustained activation of JNK leads to synaptic dysfunction and even neuronal apoptosis, ultimately resulting in memory deficits and neurodegeneration. JNK phosphorylates the amyloid precursor protein and tau, ultimately resulting in the formation of extraneuronal senile plaques and intraneuronal neurofibrillary tangles. Our hypothesis could be validated by investigating the cerebral cortex of elderly chimpanzees injected with phosphorylated JNK or transgenic pig and chimpanzee models established using gene editing technology including CRISPR. This hypothesis provides clues for further understanding the molecular mechanisms of neurodegenerative diseases and the development of potential target therapeutic drugs.

Increased Brain Iron Detection by Voxel-Based Quantitative Susceptibility Mapping in Type 2 Diabetes Mellitus Patients With an Executive Function Decline

Purpose

Brain iron accumulation has been suggested as a pathomechanism in patients with type 2 diabetes mellitus (T2DM) with cognitive impairment. This research aims to examine the total-brain pattern of iron accumulation in relation to executive function decline in patients with T2DM by voxel-based quantitative susceptibility mapping (QSM) analysis.

Materials and Methods

A total of 32 patients with T2DM and 34 age- and sex-matched healthy controls (HCs) were enrolled in this study. All participants underwent brain magnetic resonance examination, and 48 individuals underwent cognitive function assessments. Imaging data were collected with three-dimensional fast low-angle shot sequences to achieve magnitude as well as phase images. Using voxel-based QSM analysis, we compared the voxel-wise susceptibility values of the whole brain among groups and explored whether the susceptibility values had correlations with cognitive data.

Results

Among the 66 participants, cognitive function was estimated in 23 patients with T2DM (11 males and 12 females; average age, 64.65 ± 8.44 years) and 25 HCs (13 males and 12 females; average age, 61.20 ± 7.62 years). T2DM patients exhibited significantly (t = 4.288, P < 0.001) lower Montreal Cognitive Assessment (MoCA) scores [T2DM, 27 (27, 28); HCs, 29 (28, 29); normal standard ≥ 26)] and higher Trail-making Test (TMT)-A/TMT-B scores [71 (51, 100)/185 (149, 260)] than HCs [53 (36.5, 63.5)/150 (103, 172.5)] (Z = 2.612, P = 0.009; Z = 2.797, P = 0.005). Subjects with T2DM showed significantly higher susceptibility values than HCs in the caudate/putamen/pallidum, frontal inferior triangular gyrus, and precentral gyrus on the right hemisphere. In contrast (HC > T2DM), no region showed a significant difference in susceptibility values between the groups. The correlation analysis between susceptibility values and cognitive function scores was tested by voxel-based susceptibility value with sex and age as covariates. After multiple comparison correction, in T2DM patients, the left thalamus showed a significant relationship with TMT-A (R ² = 0.53, P = 0.001). The right thalamus and left thalamus showed a significant relationship with TMT-B (R ² = 0.35, P = 0.019; and R ² = 0.38, P = 0.017, respectively). In HCs, the cluster of right precentral/middle frontal gyrus/inferior frontal gyrus/inferior triangular gyrus showed a significant relationship with TMT-B (R ² = 0.59, P = 0.010). No relationship was found between the susceptibility values with MoCA in the brain region in both two groups.

Conclusion

Patients with T2DM presented declined cognitive assessments and elevated iron deposition in the striatum and frontal lobe, suggesting that executive function decline in T2DM might be associated with the cerebral iron burden and that changes in susceptibility values may represent a latent quantitative imaging marker for early assessment of cognitive decline in patients with T2DM.

Increased Brain Iron Deposition in the Putamen in Patients with Type 2 Diabetes Mellitus Detected by Quantitative Susceptibility Mapping

BACKGROUND

The underlying brain structural changes in type 2 diabetes mellitus (T2DM) patients have attracted increasing attention. The insulin-resistant state causes iron overload in neurons and leads to lesions in the central nervous system. Quantitative susceptibility mapping (QSM) can provide a noninvasive quantitative analysis of brain iron deposition. We aimed to compare the difference of brain iron deposition in the gray matter nucleus between T2DM patients and healthy elderly individuals using QSM.

METHODS

Thirty-two T2DM patients and thirty-two age- and gender-matched healthy controls (HCs) were enrolled in this research. Twenty-three patients and twenty-six HCs underwent cognitive assessments. Brain QSM maps were computed from multiecho GRE data using morphology-enabled dipole inversion with automatic uniform cerebrospinal fluid zero reference algorithm (MEDI+0). ITK-SNAP was used to measure the susceptibility values reflecting the content of iron in the regions of interest (ROIs).

RESULTS

The study included thirty-two T2DM patients (20 males and 12 females; mean age of 61.09 ± 9.99 years) and 32 HCs (14 males and 18 females; mean age of 59.09 ± 9.77 years). These participants had no significant difference in age or gender (P > 0.05). Twenty-three patients with T2DM (11 males and 12 females; mean age, 64.65 ± 8.44 years) and twenty-six HCs (14 males and 12 females; mean age, 62.30 ± 6.13 years) received an assessment of cognitive function. T2DM patients exhibited an obviously (t = 3.237, P = 0.003) lower Montreal Cognitive Assessment (MoCA) score (26.78 ± 2.35; HCs, 28.42 ± 0.64; normal standard ≥26) and a higher Stroop color-word test (SCWT)-C score [87(65,110); HC, 63(60,76.75), Z = -2.232, P = 0.003] than HCs. The mean susceptibility values in the putamen appeared obviously higher in T2DM patients than in HCs (t = -3.994, P < 0.001). The susceptibility values and cognitive assessment scores showed no obvious association (P > 0.05). However, an obvious correlation was observed between the changes in the susceptibility values in the putamen and the thalamus/dentate nucleus (r = 0.404, P < 0.001; r = 0.423, P < 0.001).

CONCLUSION

T2DM patients showed increased susceptibility values in the putamen and had declines in executive functions, but the linear association between them was not statistically significant. Changes in susceptibility values in the putamen indicated increased iron deposition and might be used as a quantitative imaging marker of central nervous system injury in T2DM patients. QSM might be able to help probe micro neuronal damage in gray matter and provide information on diabetic encephalopathy.

Non-alcoholic fatty liver disease, liver fibrosis score and cognitive function in middle-aged adults: The Framingham Study

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is common and has been recently related to brain health. We aimed to assess the relationships of NAFLD and its severity, using the NAFLD fibrosis score (NFS), with cognitive performance.

METHODS

Framingham study Offspring and 3rd generation participants were included if they attended exams 9 (2002-2008) and 2 (2008-2011), respectively, were free of dementia and stroke, and did not have excessive alcohol intake. Between 2008 and 2011, participants underwent Multi-detector computed tomography scans of the abdomen to determine NAFLD diagnosis and the NFS was used to categorize the severity of fibrosis. Cross-sectional relationships of NAFLD and the NFS with cognitive testing of memory, abstract reasoning, visual perception, attention and executive function were assessed, while adjusting for multiple cardiometabolic variables including visceral adipose tissue, diabetes and insulin resistance.

RESULTS

Of the 1287 participants (mean age = 61±12 years, 48% men), 378 (29%) had NAFLD. The presence of NAFLD was not associated with cognitive function. However, among those with NAFLD (mean age = 61±12 years; 58% men), high compared to low risk of advanced fibrosis was associated with poorer performance on similarities (β = -2.22 ± 0.83; P = 0.009) and trail-making B minus A (β = -0.11 ± 0.05; P = 0.028), independently of potential confounders.

CONCLUSIONS

Participants with high risk of advanced fibrosis may have poorer cognitive function compared to those with low risk, particularly in executive function and abstract reasoning. Future findings are necessary to evaluate the value of the NFS as a biomarker that predicts cognitive impairment and dementia and to explore the role of hepatic fibrosis in brain health.

Iron Deposition Leads to Hyperphosphorylation of Tau and Disruption of Insulin Signaling

Iron deposition in the brain is an early issue in Alzheimer's disease (AD). However, the pathogenesis of iron-induced pathological changes in AD remains elusive. Insulin resistance in brains is an essential feature of AD. Previous studies determined that insulin resistance is involved in the development of pathologies in AD. Tau pathology is one of most important hallmarks in AD and is associated with the impairment of cognition and clinical grades of the disease. In the present study, we observed that ferrous (Fe²⁺) chloride led to aberrant phosphorylation of tau, and decreased tyrosine phosphorylation levels of insulin receptor β (IRβ), insulin signal substrate 1 (IRS-1) and phosphoinositide 3-kinase p85α (PI3K p85α), in primary cultured neurons. In the in vivo studies using mice with supplemented dietary iron, learning and memory was impaired. As well, hyperphosphorylation of tau and disrupted insulin signaling in the brain was induced in iron-overloaded mice. Furthermore, in our in vitro work we identified the activation of insulin signaling following exogenous supplementation of insulin. This was further attenuated by iron-induced hyperphosphorylation of tau in primary neurons. Together, these data suggest that dysfunctional insulin signaling participates in iron-induced abnormal phosphorylation of tau in AD. Our study highlights the promising role of insulin signaling in pathological lesions induced by iron overloading.

Metabolism-Centric Overview of the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is a degenerative brain disease and the most common cause of dementia. AD is characterized by the extracellular amyloid beta (Aβ) plaques and intraneuronal deposits of neurofibrillary tangles (NFTs). Recently, as aging has become a familiar phenomenon around the world, patients with AD are increasing in number. Thus, many researchers are working toward finding effective therapeutics for AD focused on Aβ hypothesis, although there has been no success yet. In this review paper, we suggest that AD is a metabolic disease and that we should focus on metabolites that are affected by metabolic alterations to find effective therapeutics for AD. Aging is associated with not only AD but also obesity and type 2 diabetes (T2DM). AD, obesity, and T2DM share demographic profiles, risk factors, and clinical and biochemical features in common. Considering AD as a kind of metabolic disease, we suggest insulin, adiponectin, and antioxidants as mechanistic links among these diseases and targets for AD therapeutics. Patients with AD show reduced insulin signal transductions in the brain, and intranasal injection of insulin has been found to have an effect on AD treatment. In addition, adiponectin is decreased in the patients with obesity and T2DM. This reduction induces metabolic dysfunction both in the body and the brain, leading to AD pathogenesis. Oxidative stress is known to be induced by Aβ and NFTs, and we suggest that oxidative stress caused by metabolic alterations in the body induce brain metabolic alterations, resulting in AD.

Docosahexaenoic Acid and Cognition throughout the Lifespan

Docosahexaenoic acid (DHA) is the predominant omega-3 (n-3) polyunsaturated fatty acid (PUFA) found in the brain and can affect neurological function by modulating signal transduction pathways, neurotransmission, neurogenesis, myelination, membrane receptor function, synaptic plasticity, neuroinflammation, membrane integrity and membrane organization. DHA is rapidly accumulated in the brain during gestation and early infancy, and the availability of DHA via transfer from maternal stores impacts the degree of DHA incorporation into neural tissues. The consumption of DHA leads to many positive physiological and behavioral effects, including those on cognition. Advanced cognitive function is uniquely human, and the optimal development and aging of cognitive abilities has profound impacts on quality of life, productivity, and advancement of society in general. However, the modern diet typically lacks appreciable amounts of DHA. Therefore, in modern populations, maintaining optimal levels of DHA in the brain throughout the lifespan likely requires obtaining preformed DHA via dietary or supplemental sources. In this review, we examine the role of DHA in optimal cognition during development, adulthood, and aging with a focus on human evidence and putative mechanisms of action.

Docosahexaenoic Acid and Cognition throughout the Lifespan

Docosahexaenoic acid (DHA) is the predominant omega-3 (n-3) polyunsaturated fatty acid (PUFA) found in the brain and can affect neurological function by modulating signal transduction pathways, neurotransmission, neurogenesis, myelination, membrane receptor function, synaptic plasticity, neuroinflammation, membrane integrity and membrane organization. DHA is rapidly accumulated in the brain during gestation and early infancy, and the availability of DHA via transfer from maternal stores impacts the degree of DHA incorporation into neural tissues. The consumption of DHA leads to many positive physiological and behavioral effects, including those on cognition. Advanced cognitive function is uniquely human, and the optimal development and aging of cognitive abilities has profound impacts on quality of life, productivity, and advancement of society in general. However, the modern diet typically lacks appreciable amounts of DHA. Therefore, in modern populations, maintaining optimal levels of DHA in the brain throughout the lifespan likely requires obtaining preformed DHA via dietary or supplemental sources. In this review, we examine the role of DHA in optimal cognition during development, adulthood, and aging with a focus on human evidence and putative mechanisms of action.

Geniposide Attenuates the Phosphorylation of Tau Protein in Cellular and Insulin-deficient APP/PS1 Transgenic Mouse Model of Alzheimer's Disease

Our previous studies have shown that geniposide plays an essential role in glucose-stimulated insulin secretion from pancreatic β cells and also regulates the metabolism of Aβ and its deposition in neurons. In this study, we reported that insulin deficiency induced significant increase of tau phosphorylation. Administration of geniposide for 4 weeks significantly decreased the phosphorylated level of tau and the acceleration of GSK-3β phosphorylation in the brain of APP/PS1 transgenic mice induced by insulin deficiency. We also observed that geniposide decreased the phosphorylation of tau protein directly and increased the phosphorylation of Akt in primary cultured cortical neurons. Furthermore, geniposide enhanced the role of insulin on the phosphorylation of Akt, GSK-3β, and tau in primary cultured cortical neurons. And these effects of geniposide in cortical neurons could be prevented by preincubation with LY294002, an inhibitor of PI3K. Taken together, our findings provide a mechanistic and perhaps a foundational link between diabetes and Alzheimer's disease and are consistent with the notion that geniposide might play an essential role on the phosphorylation of tau protein via enhancing insulin signaling and may convey a therapeutic benefit in Alzheimer's disease.

Role of insulin resistance in Alzheimer's disease

A critical role of insulin resistance (IR) in Alzheimer's disease (AD) includes beta-amyloid (Aβ) production and accumulation, the formation of neurofibrillary tangles (NFTs), failure of synaptic transmission and neuronal degeneration. Aβ is sequentially cleavaged from APP by two proteolytic enzymes: β-secretase and γ-secretase. IR could regulate Aβ production via enhancing β- and γ-secretase activity. Meanwhile, IR induces oxidative stress and inflammation in the brain which contributes to Aβ and tau pathology. Aβ accumulation can enhance IR through Aβ-mediated inflammation and oxidative stress. IR is a possible linking between amyloid plaques and NFTs pathology via oxidative stress and neuroinflammation. Additionally, IR could disrupt acetylcholine activity, and accelerate axon degeneration and failures in axonal transport, and lead to cognitive impairment in AD. Preclinical and clinical studies have supported that insulin could be useful in the treatment of AD. Thus, an effective measure to inhibit IR may be a novel drug target in AD.

The role of nutrient-based epigenetic changes in buffering against stress, aging, and Alzheimer's disease

Converging evidence identifies stress-related disorders as putative risk factors for Alzheimer Disease (AD). This article reviews evidence on the complex interplay of stress, aging, and genes-epigenetics interactions. The recent classification of AD into preclinical, mild cognitive impairment, and AD offers a window for intervention to prevent, delay, or modify the course of AD. Evidence in support of the cognitive effects of epigenetics-diet, and nutraceuticals is reviewed. A proactive epigenetics diet and nutraceuticals program holds promise as potential buffer against the negative impact of aging and stress responses on cognition, and can optimize vascular, metabolic, and brain health in the community.

Is Alzheimer's disease related to metabolic syndrome? A Wnt signaling conundrum

Alzheimer's disease (AD) is the most common cause of dementia, affecting more than 36 million people worldwide. AD is characterized by a progressive loss of cognitive functions. For years, it has been thought that age is the main risk factor for AD. Recent studies suggest that life style factors, including nutritional behaviors, play a critical role in the onset of dementia. Evidence about the relationship between nutritional behavior and AD includes the role of conditions such as obesity, hypertension, dyslipidemia and elevated glucose levels. The coexistence of some of these cardio-metabolic risk factors is generally known as metabolic syndrome (MS). Some clinical studies support the role of MS in the onset of AD. However, the cross-talk between the molecular signaling implicated in these disorders is unknown. In the present review, we focus on the molecular correlates that support the relationship between MS and the onset of AD. We also discuss relevant issues such as the role of leptin, insulin and renin-angiotensin signaling in the brain and the possible role of Wnt signaling in both MS and AD. We discuss the evidence supporting the use of ob/ob mice, high-fructose diets, aortic coarctation-induced hypertension and Octodon degus, which spontaneously develops β-amyloid deposits and metabolic derangements, as suitable animal models to address the relationships between MS and AD. Finally, we examine emergent data supporting the role of Wnt signaling in the modulation of AD and MS, implicating this pathway as a therapeutic target in both conditions.

The nature, significance, and glucagon-like peptide-1 analog treatment of brain insulin resistance in Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disease leading over the course of decades to the most common form of dementia. Many of its pathologic features and cognitive deficits may be due in part to brain insulin resistance recently demonstrated in the insulin receptor→insulin receptor substrate-1 (IRS-1) signaling pathway. The proximal cause of such resistance in AD dementia and amnestic mild cognitive impairment (aMCI) appears to be serine inhibition of IRS-1, a phenomenon likely due to microglial release of inflammatory cytokines triggered by oligomeric Aβ. Studies on animal models of AD and on human brain tissue from MCI cases at high risk of AD dementia have shown that brain insulin resistance and many other pathologic features and symptoms of AD may be greatly reduced or even reversed by treatment with FDA-approved glucagon-like peptide-1 (GLP-1) analogs such as liraglutide (Victoza). These findings call attention to the need for further basic, translational, and clinical studies on GLP-1 analogs as promising AD therapeutics.