Physiological and pathological changes in glucose regulate brain Akt and glycogen synthase kinase-3

Sex-dimorphic effects of glucose transporter-2 gene knockdown on hypothalamic primary astrocyte phosphoinositide-3-kinase (PI3K)/protein kinase B (PKB/Akt)/mammalian target of rapamycin (mTOR) cascade protein expression and phosphorylation

Glucose transporter-2 (GLUT2), a unique high capacity/low affinity, highly efficient membrane transporter and sensor, regulates hypothalamic astrocyte glucose phosphorylation and glycogen metabolism. The phosphoinositide-3-kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR) signaling pathway participates in glucose homeostasis, but its sensitivity to glucose-sensory cues is unknown. Current research used a hypothalamic astrocyte primary culture model to investigate whether glucoprivation causes PI3K/Akt/mTOR pathway activation in one or both sexes by GLUT2-dependent mechanisms. Glucoprivation did not alter astrocyte PI3K levels, yet up-regulated both phosphorylated derivatives in female and down-regulated male p60 phosphoprotein expression. GLUT2 siRNA pretreatment diminished glucoprivic patterns of PI3K and phospho-PI3K expression in each sex. Astrocyte Akt and phospho-Akt/Thr308 proteins exhibited divergent, sex-contingent responses to GLUT2 gene knockdown or glucoprivation. GLUT2 siRNA pretreatment exacerbated glucoprivic-associated Akt diminution in the female, and either amplified (male) or reversed (female) glucoprivic regulation of phospho-Akt/Thr308 expression. GLUT2 gene silencing down- (male) or up-(female) regulated mTOR protein, and phospho-mTOR protein in male. Male astrocyte mTOR and phospho-mTOR profile were refractory to glucoprivation, but glucose-deprived females showed GLUT2-independent mTOR inhibition and GLUT2-dependent phospho-mTOR up-augmentation. Results identify a larger number of glucoprivic-sensitive PI3K/Akt/mTOR pathway proteins in female versus male astrocytes, and document divergent responses of common glucose-sensitive targets. GLUT2 stimulates phosphoPI3K protein expression in each sex, but imposes differential control of PI3K, Akt, phospho-Akt/Thr308, mTOR, and phospho-mTOR profiles in male versus female. Data implicate GLUT2 as a driver of distinctive pathway protein responses to glucoprivation in female, but not male.

Distinct Hippocampal Expression Profiles of lncRNAs in Obese Type 2 Diabetes Mice Exhibiting Cognitive Impairment

Cognitive dysfunction has been accepted as a possible complication of type 2 diabetes (T2D), but few studies revealed the potential roles of Long non‑coding RNAs (lncRNAs) in cognitive dysfunction in T2D. The current research aims to demonstrate the specific expression patterns of lncRNA-mRNA in the hippocampi of T2D db/db mice exhibiting cognitive impairment. In this study, the results from behavioral tests showed that T2D db/db mice displayed short-term and spatial working memory deficits compared to db/m mice. Furthermore, western blot analysis demonstrated that compared with db/m mice, p-GSK3β (ser9) protein levels were markedly elevated in T2D db/db mice (P < 0.01). In addition, though not statistically significant, the ratio of p-Tau (Ser396) to Tau 46, α-Synuclein expression, and p-GSK3α (ser21) expression were also relatively higher in T2D db/db mice than in db/m mice. The microarray profiling revealed that 75 lncRNAs and 26 mRNAs were dysregulated in T2D db/db mice (> 2.0 fold change, P < 0.05). GO analysis demonstrated that the differentially expressed mRNAs participated in immune response, extracellular membrane-bounded organelle, and extracellular region. KEGG analysis revealed that the differentially expressed mRNAs were mainly involved in one carbon pool by folate, glyoxylate and dicarboxylate metabolism, autophagy, glycine, serine and threonine metabolism, and B cell receptor signaling pathway. A lncRNA‑mRNA coexpression network containing 71 lncRNAs and 26 mRNAs was built to investigate the interaction between lncRNA and mRNA. Collectively, these results revealed the differential hippocampal expression profiles of lncRNAs in T2D mice with cognitive dysfunction, and the findings from this study provide new clues for exploring the potential roles of lncRNAs in the pathogenesis of cognitive dysfunction in T2D.

Exploring the translational impact of type 1 diabetes on cerebral neurovascular function through ECoG-LSCI

Type 1 diabetes mellitus (T1DM) can result in complications such as retinopathy, nephropathy, and peripheral neuropathy, which can lead to brain dysfunction. In this study, we investigated the effects of T1DM on cerebral neurovascular function in mice. Streptozotocin (STZ) is known to induce T1DM in animals; thus, we used an STZ-induced diabetes model to evaluate the effects of hyperglycemia on brain morphology and neurovascular tissue. Neurovascular coupling is the connection between neuronal activity and cerebral blood flow that maintains brain function. The ECoG-LSCI technique combines electrocorticography (ECoG) and laser speckle contrast imaging (LSCI) to detect cortical spreading depression (CSD) as a marker of neurovascular coupling and measure corresponding neurovascular function. Our results suggested that in the STZ group, hyperglycemia affected excitatory neurotransmission and metabolism, leading to reductions in intercellular signaling, somatosensory evoked potential (SSEP) amplitudes, and CSD transmission rates. Western blot data further revealed that brain-derived neurotrophic factor (BDNF) and neuronal nuclear antigen levels were reduced in the STZ group. Abnormalities in glucose metabolism in the brain and increased phosphorylation of AKT and GSK3 are hypothesized to be responsible for these decreases. Overall, this study highlights the importance of glucose metabolism in normal brain physiology and demonstrates that hyperglycemia disrupts neurovascular coupling and affects cerebral neurovascular function and that the degree of CSD is positively correlated with the extent of brain tissue damage. Further research is essential to gain a complete understanding of the related mechanisms and the implications of these findings.

State of the Science on Brain Insulin Resistance and Cognitive Decline Due to Alzheimer's Disease

Type 2 diabetes mellitus (T2DM) is common and increasing in prevalence worldwide, with devastating public health consequences. While peripheral insulin resistance is a key feature of most forms of T2DM and has been investigated for over a century, research on brain insulin resistance (BIR) has more recently been developed, including in the context of T2DM and non-diabetes states. Recent data support the presence of BIR in the aging brain, even in non-diabetes states, and found that BIR may be a feature in Alzheimer's disease (AD) and contributes to cognitive impairment. Further, therapies used to treat T2DM are now being investigated in the context of AD treatment and prevention, including insulin. In this review, we offer a definition of BIR, and present evidence for BIR in AD; we discuss the expression, function, and activation of the insulin receptor (INSR) in the brain; how BIR could develop; tools to study BIR; how BIR correlates with current AD hallmarks; and regional/cellular involvement of BIR. We close with a discussion on resilience to both BIR and AD, how current tools can be improved to better understand BIR, and future avenues for research. Overall, this review and position paper highlights BIR as a plausible therapeutic target for the prevention of cognitive decline and dementia due to AD.

Temporal and region-specific tau hyperphosphorylation in the medulla and forebrain coincides with development of functional changes in male obese Zucker rats

Metabolic syndrome (MetS) is associated with development of tauopathies that contribute to cognitive decline. Without functional leptin receptors, male obese Zucker rats (OZRs) develop MetS, and they have increased phosphorylated tau (ptau) with impaired cognitive function. In addition to regulating energy balance, leptin enhances activation of the hippocampus, which is essential for spatial learning and memory. Whether spatial learning and memory are always impaired in OZRs or develop with MetS is unknown. We hypothesized that male OZRs develop MetS traits that promote regional increases in ptau and functional deficits associated with those brain regions. In the medulla and cortex, tau-pSer199,202 and tau-pSer396 were comparable in juvenile (7-8 wk old) lean Zucker rats (LZRs) and OZRs but increased in 18- to 19-wk-old OZRs. Elevated tau-pSer396 was concentrated in the dorsal vagal complex of the medulla, and by this age OZRs had hypertension with increased arterial pressure variability. In the hippocampus, tau-pSer199,202 and tau-pSer396 were still comparable in 18- to 19-wk-old OZRs and LZRs but elevated in 28- to 29-wk-old OZRs, with emergence of deficits in Morris water maze performance. Comparable escape latencies observed during acquisition in 18- to 19-wk-old OZRs and LZRs were increased in 28- to 29-wk-old OZRs, with greater use of nonspatial search strategies. Increased ptau developed with changes in the insulin/phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway in the hippocampus and cortex but not medulla, suggesting different underlying mechanisms. These data demonstrate that leptin is not required for spatial learning and memory in male OZRs. Furthermore, early development of MetS-associated autonomic dysfunction by the medulla may be predictive of later hippocampal dysfunction and cognitive impairment.NEW & NOTEWORTHY Male obese Zucker rats (OZRs) lack functional leptin receptors and develop metabolic syndrome (MetS). At 16-19 wk, OZRs are insulin resistant, with increased ptau in dorsal medulla and impaired autonomic regulation of AP. At 28-29 wk OZRs develop increased ptau in hippocampus with deficits in spatial learning and memory. Juvenile OZRs lack elevated ptau and these deficits, demonstrating that leptin is not essential for normal function. Elevated ptau and deficits emerge before the onset of diabetes in insulin-resistant OZRs.

Novel insights into D-Pinitol based therapies: a link between tau hyperphosphorylation and insulin resistance

Alzheimer's disease is a neurodegenerative disorder characterized by the amyloid accumulation in the brains of patients with Alzheimer's disease. The pathogenesis of Alzheimer's disease is mainly mediated by the phosphorylation and aggregation of tau protein. Among the multiple causes of tau hyperphosphorylation, brain insulin resistance has generated much attention, and inositols as insulin sensitizers, are currently considered candidates for drug development. The present narrative review revises the interactions between these three elements: Alzheimer's disease-tau-inositols, which can eventually identify targets for new disease modifiers capable of bringing hope to the millions of people affected by this devastating disease.

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.

New Insights into the Regulation of mTOR Signaling via Ca2+-Binding Proteins

Environmental factors are important regulators of cell growth and proliferation. Mechanistic target of rapamycin (mTOR) is a central kinase that maintains cellular homeostasis in response to a variety of extracellular and intracellular inputs. Dysregulation of mTOR signaling is associated with many diseases, including diabetes and cancer. Calcium ion (Ca²⁺) is important as a second messenger in various biological processes, and its intracellular concentration is tightly regulated. Although the involvement of Ca²⁺ mobilization in mTOR signaling has been reported, the detailed molecular mechanisms by which mTOR signaling is regulated are not fully understood. The link between Ca²⁺ homeostasis and mTOR activation in pathological hypertrophy has heightened the importance in understanding Ca²⁺-regulated mTOR signaling as a key mechanism of mTOR regulation. In this review, we introduce recent findings on the molecular mechanisms of regulation of mTOR signaling by Ca²⁺-binding proteins, particularly calmodulin (CaM).

Insulin Deficiency Increases Sirt2 Level in Streptozotocin-Treated Alzheimer's Disease-Like Mouse Model: Increased Sirt2 Induces Tau Phosphorylation Through ERK Activation

Accumulating evidence suggests that insulin deficiency is a risk factor for Alzheimer's disease (AD); however, the underlying molecular mechanisms are not completely understood. Here, we investigated the effects of insulin deficiency on AD-like pathologies using an insulin-deficient amyloid-β (Aβ) precursor protein (APP) transgenic mouse model (Tg2576 mice). Female Tg2576 mice were injected intraperitoneally with streptozotocin (STZ) to induce insulin deficiency, and their body weights, serum glucose levels, and serum insulin levels were evaluated. STZ-treated mice showed exacerbated Aβ accumulation, tau hyperphosphorylation, glial activation, neuroinflammation, and increased Sirt2 protein levels in the brain, as determined by two-dimensional gel electrophoresis (2-DE) coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) and Western blotting. Furthermore, our in vitro experiments revealed that insulin depletion or interleukin-6 treatment increased Sirt2 protein levels in both Neuro2a and Neuro2a-P301L cells. The overexpression of Sirt2 in these cells induced tau hyperphosphorylation through extracellular signal-regulated kinase (ERK) activation. Conversely, Sirt2 knockdown reversed tau hyperphosphorylation in these cells. We showed for the first time that Sirt2 is upregulated in the brains of STZ-treated Tg2576 mice and is involved in tau phosphorylation through ERK activation. Our findings suggest that Sirt2 is a promising therapeutic target for the treatment of AD.

Effects of hypercaloric diet-induced hyperinsulinemia and hyperlipidemia on the ovarian follicular development in mice

Long-term hypercaloric diets may adversely affect the development of ovarian follicles. We investigated the effects of high sugar (HS), high fat low sugar (HFLS), and high fat normal sugar (HFNS) diets on the ovarian follicle development in mice fed with these diets as compared to those fed with normal diet (control) for 180 days. Body weight, gonadal fat, glucose, lipid, insulin, estrous cycle, sex hormones and ovarian tissues were examined, and metabolism-related protein expression in the ovaries was evaluated by immunoblotting. The mice fed with hypercaloric diets showed hyperinsulinemia and hyperlipidemia, and exhibited heavier body and gonadal fat weights, longer estrous cycles, and fewer preantral and antral follicles than mice fed with normal diet. The sex hormone levels in the blood were similar to those in controls, except for significantly elevated estradiol levels in the HS diet group. The AMPKα phosphorylation was reduced, while AKT phosphorylation and caspase-3 levels were increased in the ovarian tissues of mice in all three hypercaloric diet groups than those in control. Taken together, the results suggest hyperinsulinemia and hyperlipidemia as possible mechanisms that impair the development of ovarian follicles in response to long-term exposure to unhealthy hypercaloric diets.

Insulin blood-brain barrier transport and interactions are greater following exercise in mice

Exercise has multiple beneficial effects including improving peripheral insulin sensitivity, improving central function such as memory, and restoring a dysregulated blood-brain barrier (BBB). Central nervous system (CNS) insulin resistance is a common feature of cognitive impairment, including Alzheimer's disease. Delivery of insulin to the brain can improve memory. Endogenous insulin must cross the BBB to directly act within the CNS and this transport system can be affected by various physiological states and serum factors. Therefore, the current study sought to investigate whether exercise could enhance insulin BBB transport as a mechanism for the underlying benefits of exercise on cognition. We investigated radioactive insulin BBB pharmacokinetics following an acute bout of exercise in young, male and female CD-1 mice. In addition, we investigated changes in serum levels of substrates that are known to affect insulin BBB transport. Finally, we measured the basal level of a downstream protein involved in insulin receptor signaling in various brain regions as well as muscle. We found insulin BBB transport in males was greater following exercise, and in males and females to both enhance the level of insulin vascular binding and alter CNS insulin receptor signaling, independent of changes in serum factors known to alter insulin BBB transport.NEW & NOTEWORTHY Central nervous system (CNS) insulin and exercise are beneficial for cognition. CNS insulin resistance is present in Alzheimer's disease. CNS insulin levels are regulated by transport across the blood-brain barrier (BBB). We show that exercise can enhance insulin BBB transport and binding of insulin to the brain's vasculature in mice. There were no changes in serum factors known to alter insulin BBB pharmacokinetics. We conclude exercise could impact cognition through regulation of insulin BBB transport.

Wheel running leads to sex-specific effects on Western diet-associated glucose homeostasis and brain insulin signaling without altering food-related impulsive choice

OBJECTIVES

There is a clear association between obesity and impulsivity. While exercise can suppress weight gain and decrease impulsive choice (IC), the relationship between impulsivity, the consumption of palatable, energy dense diets, and exercise is unclear. We examined IC before and after Western diet (WD) exposure in rats of both sexes and whether exercise would rescue any diet-mediated increases in IC. Our hypotheses were twofold: first, increased impulsivity would be associated with higher WD preference in a positive feedback loop and second, increased WD consumption would impair both peripheral and central insulin signaling, both of which exercise would attenuate.

METHODS

Following baseline assessment of IC through a delay discounting task, rats were divided into naïve, sedentary (Sed), or wheel running (WR) groups for a 5-week WR and two-diet choice period after which rats underwent an oral glucose (OGTT) and insulin tolerance test (ITT) in addition to a re-test of IC. Insulin induced Akt-GSK3β signaling in the brain was examined using western blot.

RESULTS

All Sed rats preferred the WD diet, and all WR rats initially avoided the WD but subsequently reversed their avoidance to preference with females reversing earlier than males. Exercise suppressed weight gain and adiposity to a greater extent in males than females. Only WR males showed improved glucose clearance during OGTT, but both male and female WR rats had a faster recovery of hypoglycemia during ITT. Furthermore, WR rescued WD-induced deficits in hypothalamic Akt-GSK3β signaling in males but not females. In the prefrontal cortex, however, WD and WR both reduced Akt-GSK3β signaling in males but not females. There were no sex differences in IC at baseline, and all rats made more impulsive choices during the re-test independent of diet, sex, or exercise.

DISCUSSION

The results suggest that while exercise may have a greater efficacy at attenuating diet-mediated metabolic dysregulation in males, it has some beneficial effects for females and highlights the need to develop sex-specific interventions for restoring energy balance.

Hyperinsulinemia or Insulin Resistance: What Impacts the Progression of Alzheimer's Disease?

Type 2 diabetes mellitus (T2D), which is often accompanied by hyperinsulinemia and insulin resistance, is associated with an increased risk for developing mild cognitive impairment and Alzheimer's disease (AD); however, the underlying mechanisms for this association are still unclear. Recent findings have shown that hyperinsulinemia and insulin resistance can coexist or be independent events. This makes it imperative to determine the contribution of these individual conditions in impacting AD. This literature review highlights the recent developments of hyperinsulinemia and insulin resistance involvement in the progression and pathogenesis of AD.

Electroacupuncture Protects Cognition by Regulating Tau Phosphorylation and Glucose Metabolism via the AKT/GSK3β Signaling Pathway in Alzheimer's Disease Model Mice

Background

Alzheimer’s disease (AD) is mainly manifested as a continuous and progressive decline in cognitive ability. Neurofibrillary tangles (NFTs) are pathological hallmarks of AD and due to accumulated phosphorylated Tau. Glycogen synthase kinase-3β (GSK3β), as a major Tau kinase and a downstream target of the serine protein kinase B (AKT) signaling pathway, can regulate Tau phosphorylation in AD. Importantly, the AKT/GSK3β signaling pathway is involved in glucose metabolism, and abnormal glucose metabolism is found in the AD brain. Numerous studies have shown that electroacupuncture (EA), which is thought to be a potential complementary therapeutic approach for AD, can protect cognitive ability to a certain extent.

Objective

The purpose of this experiment was to investigate whether the protective and beneficial mechanism of EA on cognition was mediated by the AKT/GSK3β signaling pathway, thereby improving glucose metabolism and Tau phosphorylation in the brain.

Methods

EA was applied to the Baihui (GV20) and Yintang (GV29) acupoints of 6-month-old amyloid precursor protein (APP)/presenilin-1 (PS1) mice for 20 min, and then quickly prick Shuigou (GV26) acupoint. The intervention was performed once every other day for 28 days. The Morris water maze (MWM) test was performed on C57BL/6N (Non-Tg) mice, APP/PS1 (Tg) mice and EA-treated Tg (Tg + EA) mice to evaluate the effect of EA therapy on cognitive function. ¹⁸F-FDG positron emission tomography (PET), immunohistochemistry, and western blotting (WB) were used to investigate the possible mechanism underlying the effect of EA on AD.

Results

EA treatment significantly improved the cognition of APP/PS1 mice and the glucose uptake rate in the hippocampus. Furthermore, EA inhibited the phosphorylation of Tau (Ser199 and Ser202) proteins by inducing AKT (Ser473) and GSK3β (Ser9) phosphorylation.

Conclusion

These results demonstrate that EA intervention protects cognition by enhancing glucose metabolism and inhibiting abnormal phosphorylation of Tau protein in the AD model mice, and the AKT/GSK3β pathway might play an irreplaceable role in the regulation process.

Insulin Resistance at the Crossroad of Alzheimer Disease Pathology: A Review

Insulin plays a major neuroprotective and trophic function for cerebral cell population, thus countering apoptosis, beta-amyloid toxicity, and oxidative stress; favoring neuronal survival; and enhancing memory and learning processes. Insulin resistance and impaired cerebral glucose metabolism are invariantly reported in Alzheimer's disease (AD) and other neurodegenerative processes. AD is a fatal neurodegenerative disorder in which progressive glucose hypometabolism parallels to cognitive impairment. Although AD may appear and progress in virtue of multifactorial nosogenic ingredients, multiple interperpetuative and interconnected vicious circles appear to drive disease pathophysiology. The disease is primarily a metabolic/energetic disorder in which amyloid accumulation may appear as a by-product of more proximal events, especially in the late-onset form. As a bridge between AD and type 2 diabetes, activation of c-Jun N-terminal kinase (JNK) pathway with the ensued serine phosphorylation of the insulin response substrate (IRS)-1/2 may be at the crossroads of insulin resistance and its subsequent dysmetabolic consequences. Central insulin axis bankruptcy translates in neuronal vulnerability and demise. As a link in the chain of pathogenic vicious circles, mitochondrial dysfunction, oxidative stress, and peripheral/central immune-inflammation are increasingly advocated as major pathology drivers. Pharmacological interventions addressed to preserve insulin axis physiology, mitochondrial biogenesis-integral functionality, and mitophagy of diseased organelles may attenuate the adjacent spillover of free radicals that further perpetuate mitochondrial damages and catalyze inflammation. Central and/or peripheral inflammation may account for a local flood of proinflammatory cytokines that along with astrogliosis amplify insulin resistance, mitochondrial dysfunction, and oxidative stress. All these elements are endogenous stressor, pro-senescent factors that contribute to JNK activation. Taken together, these evidences incite to identify novel multi-mechanistic approaches to succeed in ameliorating this pandemic affliction.

Common Pathological Mechanisms and Risk Factors for Alzheimer's Disease and Type-2 Diabetes: Focus on Inflammation

BACKGROUND

Diabetes is considered as a risk factor for Alzheimer's Disease, but it is yet unclear whether this pathological link is reciprocal. Although Alzheimer's disease and diabetes appear as entirely different pathological entities affecting the Central Nervous System and a peripheral organ (pancreas), respectively, they share a common pathological core. Recent evidence suggests that in the pancreas in the case of diabetes, as in the brain for Alzheimer's Disease, the initial pathological event may be the accumulation of toxic proteins yielding amyloidosis. Moreover, in both pathologies, amyloidosis is likely responsible for local inflammation, which acts as a driving force for cell death and tissue degeneration. These pathological events are all inter-connected and establish a vicious cycle resulting in the progressive character of both pathologies.

OBJECTIVE

To address the literature supporting the hypothesis of a common pathological core for both diseases.

DISCUSSION

We will focus on the analogies and differences between the disease-related inflammatory changes in a peripheral organ, such as the pancreas, versus those observed in the brain. Recent evidence suggesting an impact of peripheral inflammation on neuroinflammation in Alzheimer's disease will be presented.

CONCLUSION

We propose that it is now necessary to consider whether neuroinflammation in Alzheimer's disease affects inflammation in the pancreas related to diabetes.

Short-term fasting differentially regulates PI3K/AkT/mTOR and ERK signalling in the rat hypothalamus

It is known that insulin secreted by pancreatic β-cells enters the brain by crossing the blood-brain barrier. However, it was demonstrated that insulin expression occurs in various brain regions as well. Albeit the list of insulin actions in the brain is long and it includes control of energy homeostasis, neuronal survival, maintenance of synaptic plasticity and cognition, not much is known about the adaptive significance of insulin synthesis in brain. We previously reported that short-term fasting promotes insulin expression and subsequent activation of insulin receptor in the rat periventricular nucleus. In order to uncover a physiological importance of the fasting-induced insulin expression in hypothalamus, we analyzed the effect of short-term food deprivation on the expression of several participants of PI3K/AKT/mTOR and Ras/MAPK signaling pathways that are typically activated by this hormone. We found that the hypothalamic content of total and activated IRS1, IRS2, PI3K, and mTOR remained unchanged, but phosphorylated AKT1/2/3 was decreased. The levels of activated ERK1/2 were increased after six-hour fasting. Moreover, activated ERK1/2 was co-expressed with activated insulin receptor in the nucleus arcuatus. Our previously published and current findings suggest that the ERK activation in hypothalamus was at least partially initiated by the centrally produced insulin.

Role of Nampt-Sirt6 Axis in Renal Proximal Tubules in Extracellular Matrix Deposition in Diabetic Nephropathy

Nicotinamide adenine dinucleotide (NAD⁺) metabolism plays a critical role in kidneys. We previously reported that decreased secretion of a NAD⁺ precursor, nicotinamide mononucleotide (NMN), from proximal tubules (PTs) can trigger diabetic albuminuria. In the present study, we investigated the role of NMN-producing enzyme nicotinamide phosphoribosyltransferase (Nampt) in diabetic nephropathy. The expression of Nampt in PTs was downregulated in streptozotocin (STZ)-treated diabetic mice when they exhibited albuminuria. This albuminuria was ameliorated in PT-specific Nampt-overexpressing transgenic (TG) mice. PT-specific Nampt-conditional knockout (Nampt CKO) mice exhibited TBM thickening and collagen deposition, which were associated with the upregulation of the profibrogenic gene TIMP-1. Nampt CKO mice also exhibited the downregulation of sirtuins, particularly in Sirt6. PT-specific Sirt6-knockout mice exhibited enhanced fibrotic phenotype resembling that of Nampt CKO mice with increased Timp1 expression. In conclusion, the Nampt-Sirt6 axis in PTs serves as a key player in fibrogenic extracellular matrix remodeling in diabetic nephropathy.

Effect of Peripheral Insulin Administration on Phosphorylation of Tau in the Brain

BACKGROUND

Abnormally hyperphosphorylated tau is the major protein of neurofibrillary tangles in Alzheimer's disease. Insulin activates PI3K-AKT signaling and regulates tau phosphorylation. Impaired brain insulin signaling is involved in Alzheimer's disease pathogenesis. However, the effect of peripheral insulin on tau phosphorylation is controversial.

OBJECTIVE

In the present study, we determined the effect of peripheral insulin administration on tau phosphorylation in brain.

METHODS

We intraperitoneally injected a super physiological dose of insulin to mice and analyzed PI3K-AKT signaling and tau phosphorylation in brains by western blots.

RESULTS

We found that peripherally administered insulin activated the PI3K-AKT signaling pathway immediately in the liver, but not in the brain. Tau phosphorylation in the mouse brain was found to be first decreased (15 min) and then increased (30 min and 60 min) after peripheral insulin administration and these changes correlated inversely with body temperature and the level of brain protein O-GlcNAcylation. Maintaining body temperature of mice post peripheral insulin administration prevented the insulin/hypoglycemia-induced tau hyperphosphorylation after peripheral insulin administration.

CONCLUSION

These findings suggest that peripheral insulin can induce tau hyperphosphorylation through both hypothermia and downregulation of brain protein O-GlcNAcylation during hypoglycemia.

Rapeseed oil fortified with micronutrients improves cognitive alterations associated with metabolic syndrome

Metabolic syndrome represents a major risk factor for severe comorbidities such as cardiovascular diseases or diabetes. It is also associated with an increased prevalence of emotional and cognitive alterations that in turn aggravate the disease and related outcomes. Identifying therapeutic strategies able to improve those alterations is therefore a major socioeconomical and public health challenge. We previously reported that both hippocampal inflammatory processes and neuronal plasticity contribute to the development of emotional and cognitive alterations in db/db mice, an experimental model of metabolic syndrome that displays most of the classical features of the syndrome. In that context, nutritional interventions with known impact on those neurobiological processes appear as a promising alternative to limit the development of neurobiological comorbidities of metabolic syndrome. We therefore tested here whether n-3 polyunsaturated fatty acids (n-3 PUFAs) associated with a cocktail of antioxidants can protect against the development of behavioral alterations that accompany the metabolic syndrome. Thus, this study aimed: 1) to evaluate if a diet supplemented with the plant-derived n-3 PUFA α-linolenic acid (ALA) and antioxidants (provided by n-3 PUFAs-rich rapeseed oil fortified with a mix of naturally constituting antioxidant micronutrients, including coenzyme Q10, tocopherol, and the phenolic compound canolol) improved behavioral alterations in db/db mice, and 2) to decipher the biological mechanisms underlying this behavioral effect. Although the supplemented diet did not improve anxiety-like behavior and inflammatory abnormalities, it reversed hippocampus-dependent spatial memory deficits displayed by db/db mice in a water maze task. It concomitantly changed subunit composition of glutamatergic AMPA and NMDA receptors in the hippocampus that has been shown to modulate synaptic function related to spatial memory. These data suggest that changes in local neuronal plasticity may underlie cognitive improvements in db/db mice fed the supplemented diet. The current findings might therefore provide valuable data for introducing new nutritional strategies for the treatment of behavioral complications associated with MetS.

GluT4: A central player in hippocampal memory and brain insulin resistance

Insulin is now well-established as playing multiple roles within the brain, and specifically as regulating hippocampal cognitive processes and metabolism. Impairments to insulin signaling, such as those seen in type 2 diabetes and Alzheimer's disease, are associated with brain hypometabolism and cognitive impairment, but the mechanisms of insulin's central effects are not determined. Several lines of research converge to suggest that the insulin-responsive glucose transporter GluT4 plays a central role in hippocampal memory processes, and that reduced activation of this transporter may underpin the cognitive impairments seen as a consequence of insulin resistance.

Diet control to achieve euglycaemia induces tau hyperphosphorylation via AMPK activation in the hippocampus of diabetic rats

Alzheimer's disease (AD) is a chronic neurodegenerative disease, and typical pathologic findings include abnormally hyperphosphorylated tau aggregation and neurofibrillary tangles.

Urtica Dioica and Lamium Album Decrease Glycogen Synthase Kinase-3 beta and Increase K-Ras in Diabetic Rats

Objectives

The aim of the present work is evaluating the special effects of Urtica Dioica and Lamium Album on the serum level of K-Ras and GSK-3 beta in diabetic rats.

Methods

In the present experimental study, 32 male Wistar rats randomly divided into 4 groups (Group I: normal control rats; receiving daily PBS, Group 2: diabetic control rats; receiving single dose of streptozotocin (60 mg/kg) and daily PBS, Group 3: Diabetic rats treated with 100 mg/kg of hydroalcoholic extract of the U. dioica, Group 4: Diabetic rats treated with 100 mg/kg of hydroalcoholic extract of L. Album. Diabetes-induced by an intraperitoneal injection of streptozotocin (60 mg/ kg). On the 14 th day of treatment, the weight, fasting blood sugar (FBS) and on 28 th day blood glucose, K-Ras and GSK3 beta was measured.

Results

In diabetic group blood GSK-3 beta increase in comparison to control group (P < 0.05), also blood K-Ras decrease in the diabetic group (P < 0.05). Both extracts reduced GSK-3 beta level, however, this reduction was only statistically significant by U. dioica (P < 0.05). Compared to diabetic group, blood K-Ras level increased by both extract (P < 0.05). Also diabetes induction increase blood glucose levels and both extracts decrease its level significantly (P < 0.05). there is no significant differences among both extract effects on blood glucose, and K-Ras.

Conclusion

For the first time shown that both extracts by regulating GSK-3 beta and K-Ras improve blood glucose level. More studies are needed to determine all the effects of these herbs.

A Possible Neurodegeneration Mechanism Triggered by Diabetes

Several conditions result in neurodegeneration; among which diabetes mellitus (DM) is of crucial importance. Tau (τ) malfunction is a major pathological process participating in neurodegeneration. Despite extensive considerations, the actual causative link between DM and τ abnormalities remains uncertain thus far. Phosphorylated (p)-τ at Thr-Pro motifs can exist in the two distinct cis and trans conformations. cis is neurotoxic, and is accumulated upon various stress conditions, such as nutrition depletion. We assume that pathogenic cis p-τ is the central mediator of neurodegeneration in DM, and propose why different brain areas give various responses to stress conditions. We herein juxtapose recent approaches in diabetic neurodegeneration and propose a therapeutic target to stop neuronal loss during DM.

Antipsychotics differentially regulate insulin, energy sensing, and inflammation pathways in hypothalamic rat neurons

INTRODUCTION

Second generation antipsychotic (AP)s remain the gold-standard treatment for schizophrenia and are widely used on- and off-label for other psychiatric illnesses. However, these agents cause serious metabolic side-effects. The hypothalamus is the primary brain region responsible for whole body energy regulation, and disruptions in energy sensing (e.g. insulin signaling) and inflammation in this brain region have been implicated in the development of insulin resistance and obesity. To elucidate mechanisms by which APs may be causing metabolic dysregulation, we explored whether these agents can directly impact energy sensing and inflammation in hypothalamic neurons.

METHODS

The rat hypothalamic neuronal cell line, rHypoE-19, was treated with olanzapine (0.25-100 uM), clozapine (2.5-100 uM) or aripiprazole (5-20 uM). Western blots measured the energy sensing protein AMPK, components of the insulin signaling pathway (AKT, GSK3β), and components of the MAPK pathway (ERK1/2, JNK, p38). Quantitative real-time PCR was performed to determine changes in the mRNA expression of interleukin (IL)-6, IL-10 and brain derived neurotrophic factor (BDNF).

RESULTS

Olanzapine (100 uM) and clozapine (100, 20 uM) significantly increased pERK1/2 and pJNK protein expression, while aripiprazole (20 uM) only increased pJNK. Clozapine (100 uM) and aripiprazole (5 and 20 uM) significantly increased AMPK phosphorylation (an orexigenic energy sensor), and inhibited insulin-induced phosphorylation of AKT. Olanzapine (100 uM) treatment caused a significant increase in IL-6 while aripiprazole (20 uM) significantly decreased IL-10. Olanzapine (100 uM) and aripiprazole (20 uM) increased BDNF expression.

CONCLUSIONS

We demonstrate that antipsychotics can directly regulate insulin, energy sensing, and inflammatory pathways in hypothalamic neurons. Increased MAPK activation by all antipsychotics, alongside olanzapine-associated increases in IL-6, and aripiprazole-associated decreases in IL-10, suggests induction of pro-inflammatory pathways. Clozapine and aripiprazole inhibition of insulin-stimulated pAKT and increases in AMPK phosphorylation (an orexigenic energy sensor) suggests impaired insulin action and energy sensing. Conversely, olanzapine and aripiprazole increased BDNF, which would be expected to be metabolically beneficial. Overall, our findings suggest differential effects of antipsychotics on hypothalamic neuroinflammation and energy sensing.

Brain insulin action: Implications for the treatment of schizophrenia

Insulin action in the central nervous system is a major regulator of energy balance and cognitive processes. The development of central insulin resistance is associated with alterations in dopaminergic reward systems and homeostatic signals affecting food intake, glucose metabolism, body weight and cognitive performance. Emerging evidence has highlighted a role for antipsychotics (APs) to modulate central insulin-mediated pathways. Although APs remain the cornerstone treatment for schizophrenia they are associated with severe metabolic complications and fail to address premorbid cognitive deficits, which characterize the disorder of schizophrenia. In this review, we first explore how the hypothesized association between schizophrenia and CNS insulin dysregulation aligns with the use of APs. We then investigate the proposed relationship between CNS insulin action and AP-mediated effects on metabolic homeostasis, and different domains of psychopathology, including cognition. We briefly discuss a potential role of CNS insulin signaling to explain the hypothesized, but somewhat controversial association between therapeutic efficacy and metabolic side effects of APs. Finally, we propose how this knowledge might inform novel treatment strategies to target difficult to treat domains of schizophrenia. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

Structure and Regulation of Glycogen Synthase in the Brain

Brain glycogen synthesis is a regulated, multi-step process that begins with glucose transport across the blood brain barrier and culminates with the actions of glycogen synthase and the glycogen branching enzyme to elongate glucose chains and introduce branch points in a growing glycogen molecule. This review focuses on the synthesis of glycogen in the brain, with an emphasis on glycogen synthase, but draws on salient studies in mammalian muscle and liver as well as baker's yeast, with the goal of providing a more comprehensive view of glycogen synthesis and highlighting potential areas for further study in the brain. In addition, deficiencies in the glycogen biosynthetic enzymes which lead to glycogen storage diseases in humans are discussed, highlighting effects on the brain and discussing findings in genetically modified animal models that recapitulate these diseases. Finally, implications of glycogen synthesis in neurodegenerative and other diseases that impact the brain are presented.

Vanadium pentoxide increased PTEN and decreased SHP1 expression in NK-92MI cells, affecting PI3K-AKT-mTOR and Ras-MAPK pathways

Vanadium is an air pollutant that imparts immunosuppressive effects on NK cell immune responses, in part, by dysregulating interleukin (IL)-2/IL-2R-mediated JAK signaling pathways and inducing apoptosis. The aim of the present study was to evaluate effects of vanadium pentoxide (V₂O₅) on other IL-2 receptor-mediated signaling pathways, i.e. PI3K-AKT-mTOR and Ras-MAPK. Here, IL-2-independent NK-92MI cells were exposed to different V₂O₅ doses for 24 h periods. Expression of PI3K, Akt, mTOR, ERK1/2, MEK1, PTEN, SHP1, BAD and phosphorylated forms, as well as caspases-3, -8, -9, BAX and BAK in/on the cells were then determined by flow cytometry. The results show that V₂O₅ was cytotoxic to NK cells in a dose-related manner. Exposure increased BAD and pBAD expression and decreased that of BAK and BAX, but cell death was not related to caspase activation. At 400 µM V₂O₅, expression of PI3K-p85 regulatory subunit increased 20% and pPI3K 50%, while that of the non-pPI3K 110α catalytic subunit decreased by 20%. At 200 μM, V₂O₅ showed significant decrease in non-pAkt expression (p < 0.05); the decrease in pAkt expression was significant at 100 μM. Non-pmTOR expression displayed a significant downward trend beginning at 100 μM. Expressions of pMEK-1/2 and pERK-1/2 increased substantially at 200 μM V₂O₅. No differences were found with non-phosphorylated ERK-1/2. PTEN expression increased significantly at 100 μM V₂O₅ exposure whereas pPTEN decreased by 18% at 25 μM V₂O₅ concentrations, but remained unchanged thereafter. Lastly, V₂O₅ at all doses decreased SHP1 expression and increased expression of its phosphorylated form. These results indicated a toxic effect of V₂O₅ on NK cells that was due in part to dysregulation of signaling pathways mediated by IL-2 via increased PTEN and decreased SHP1 expression. These results can help to explain some of the known deleterious effects of this particular form of vanadium on innate immune responses_.

Alzheimer's Disease and Type 2 Diabetes: A Critical Assessment of the Shared Pathological Traits

Alzheimer's disease (AD) and Type 2 Diabetes Mellitus (T2DM) are two of the most prevalent diseases in the elderly population worldwide. A growing body of epidemiological studies suggest that people with T2DM are at a higher risk of developing AD. Likewise, AD brains are less capable of glucose uptake from the surroundings resembling a condition of brain insulin resistance. Pathologically AD is characterized by extracellular plaques of Aβ and intracellular neurofibrillary tangles of hyperphosphorylated tau. T2DM, on the other hand is a metabolic disorder characterized by hyperglycemia and insulin resistance. In this review we have discussed how Insulin resistance in T2DM directly exacerbates Aβ and tau pathologies and elucidated the pathophysiological traits of synaptic dysfunction, inflammation, and autophagic impairments that are common to both diseases and indirectly impact Aβ and tau functions in the neurons. Elucidation of the underlying pathways that connect these two diseases will be immensely valuable for designing novel drug targets for Alzheimer's disease.

Glucagon like peptide-1 (GLP-1) likes Alzheimer's disease

Glucagon-like peptide-1 (GLP-1) is a 30 amino acid long peptide hormone derived from the proglucagon gene and secreted in the distal small intestine when food enters the duodenum. GLP-1 is also produced in the central nervous system (CNS), predominantly in the brainstem, and subsequently transported to a large number of regions in the CNS. Neuronal cells in nucleus tractus solitarius (NTS) can synthesize GLP-1 and extends to hypothalamus, some thalamic and cortical areas. A G protein coupled receptor (GPCR) provides the majority of GLP-1 actions. GLP-1 receptor activation triggers some in vivo signaling pathways. GLP-1 receptor agonists (GLP-1 RA) are used in the treatment diabetes and obesity. GLP-1 stimulates insulin secretion, inhibits glucagon secretion, decreases food intake, reduces appetite, delays gastric emptying, provides weight reduction, and protects β cells from apoptosis. Alzheimer's disease (AD) is the most prevalent form of dementia. It is characterized by cognitive insufficiencies and behavioral changes that impact memory and learning abilities, daily functioning and quality of life. Hyperinsulinemia and insulin resistance, which are known as pathophysiological features of the T2DM, have also been demonstrated to have significant impact on cognitive impairment. It is thought that GLP-1 affects neurological and cognitive functions, as well as its regulatory effect on glucose metabolism. The pathophysiological relationship between GLP-1 and AD is discussed in this review.

The biphasic effect of extracellular glucose concentration on carbachol-induced fluid secretion from mouse submandibular glands

Cholinergic agonists evoke elevations of the cytoplasmic free-calcium concentration ([Ca²⁺ ]_i ) to stimulate fluid secretion in salivary glands. Salivary flow rates are significantly reduced in diabetic patients. However, it remains elusive how salivary secretion is impaired in diabetes. Here, we used an ex vivo submandibular gland perfusion technique to characterize the dependency of salivary flow rates on extracellular glucose concentration and activities of glucose transporters expressed in the glands. The cholinergic agonist carbachol (CCh) induced sustained fluid secretion, the rates of which were modulated by the extracellular glucose concentration in a biphasic manner. Both lowering the extracellular glucose concentration to less than 2.5 mM and elevating it to higher than 5 mM resulted in decreased CCh-induced fluid secretion. The CCh-induced salivary flow was suppressed by phlorizin, an inhibitor of the sodium-glucose cotransporter 1 (SGLT1) located basolaterally in submandibular acinar cells, which is altered at the protein expression level in diabetic animal models. Our data suggest that SGLT1-mediated glucose uptake in acinar cells is required to maintain the fluid secretion by sustaining Cl^- secretion in real-time. High extracellular glucose levels may suppress the CCh-induced secretion of salivary fluid by altering the activities of ion channels and transporters downstream of [Ca²⁺ ]_i signals.

Secreted amyloid precursor protein alpha activates neuronal insulin receptors and prevents diabetes-induced encephalopathy

Secreted amyloid precursor protein alpha (sAPPα) is a potent neurotrophin in the CNS but a dedicated receptor has not been found. However, protein interactions involving amyloid beta (Aβ), a peptide cleaved from the same parent peptide as sAPPα, indicate that insulin receptors (IRs) could be a target of amyloid peptides. In this study, in vitro analysis of cortical neuronal cultures revealed that exogenous sAPPα increased IR phosphorylation in the absence of insulin. Furthermore, in an APP overexpressing mouse model, sAPPα bound IRs in the cortex with significantly greater binding in hypoinsulinemic animals. To further examine the effects of sAPPα on the diabetic brain, we next rendered sAPPα overexpressing mice insulin depleted and found that sAPPα blocked aberrant tau phosphorylation (T231) in cortical tissue after 16 weeks diabetes. sAPPα overexpression also prevented hyperphosphorylation of AKT/GSK3 and activation of the unfolded protein response (UPR). In total, these data show sAPPα binds and activates neuronal IRs and that sAPPα has a protective effect on diabetic brain tissue.

Novel Roles for the Insulin-Regulated Glucose Transporter-4 in Hippocampally Dependent Memory

The insulin-regulated glucose transporter-4 (GluT4) is critical for insulin- and contractile-mediated glucose uptake in skeletal muscle. GluT4 is also expressed in some hippocampal neurons, but its functional role in the brain is unclear. Several established molecular modulators of memory processing regulate hippocampal GluT4 trafficking and hippocampal memory formation is limited by both glucose metabolism and insulin signaling. Therefore, we hypothesized that hippocampal GluT4 might be involved in memory processes. Here, we show that, in male rats, hippocampal GluT4 translocates to the plasma membrane after memory training and that acute, selective intrahippocampal inhibition of GluT4-mediated glucose transport impaired memory acquisition, but not memory retrieval. Other studies have shown that prolonged systemic GluT4 blockade causes insulin resistance. Unexpectedly, we found that prolonged hippocampal blockade of glucose transport through GluT4-upregulated markers of hippocampal insulin signaling prevented task-associated depletion of hippocampal glucose and enhanced both working and short-term memory while also impairing long-term memory. These effects were accompanied by increased expression of hippocampal AMPA GluR1 subunits and the neuronal GluT3, but decreased expression of hippocampal brain-derived neurotrophic factor, consistent with impaired ability to form long-term memories. Our findings are the first to show the cognitive impact of brain GluT4 modulation. They identify GluT4 as a key regulator of hippocampal memory processing and also suggest differential regulation of GluT4 in the hippocampus from that in peripheral tissues.

SIGNIFICANCE STATEMENT

The role of insulin-regulated glucose transporter-4 (GluT4) in the brain is unclear. In the current study, we demonstrate that GluT4 is a critical component of hippocampal memory processes. Memory training increased hippocampal GluT4 translocation and memory acquisition was impaired by GluT4 blockade. Unexpectedly, whereas long-term inhibition of GluT4 impaired long-term memory, short-term memory was enhanced. These data further our understanding of the molecular mechanisms of memory and have particular significance for type 2 diabetes (in which GluT4 activity in the periphery is impaired) and Alzheimer's disease (which is linked to impaired brain insulin signaling and for which type 2 diabetes is a key risk factor). Both diseases cause marked impairment of hippocampal memory linked to hippocampal hypometabolism, suggesting the possibility that brain GluT4 dysregulation may be one cause of cognitive impairment in these disease states.

In vivo regulation of glycogen synthase kinase 3β activity in neurons and brains

Glycogen synthase kinase 3β (GSK3β) is a multifunctional protein kinase involved in many cellular activities including development, differentiation and diseases. GSK3β is thought to be constitutively activated by autophosphorylation at Tyr216 and inactivated by phosphorylation at Ser9. The GSK3β activity has previously been evaluated by inhibitory Ser9 phosphorylation, but it does not necessarily indicate the kinase activity itself. Here, we applied the Phos-tag SDS-PAGE technique to the analysis of GSK3β phosphoisotypes in cells and brains. There were three phosphoisotypes of GSK3β; double phosphorylation at Ser9 and Tyr216, single phosphorylation at Tyr216 and the nonphosphorylated isotype. Active GSK3β with phosphorylation at Tyr216 represented half or more of the total GSK3β in cultured cells. Although levels of phospho-Ser9 were increased by insulin treatment, Ser9 phosphorylation occurred only in a minor fraction of GSK3β. In mouse brains, GSK3β was principally in the active form with little Ser9 phosphorylation, and the phosphoisotypes of GSK3β changed depending on the regions of the brain, age, sex and disease conditions. These results indicate that the Phos-tag SDS-PAGE method provides a simple and appropriate measurement of active GSK3β in vivo, and the activity is regulated by the mechanism other than phosphorylation on Ser9.

The Effects of Peripheral and Central High Insulin on Brain Insulin Signaling and Amyloid-β in Young and Old APP/PS1 Mice

Hyperinsulinemia is a risk factor for late-onset Alzheimer's disease (AD). In vitro experiments describe potential connections between insulin, insulin signaling, and amyloid-β (Aβ), but in vivo experiments are needed to validate these relationships under physiological conditions. First, we performed hyperinsulinemic-euglycemic clamps with concurrent hippocampal microdialysis in young, awake, behaving APP_swe/PS1_dE9 transgenic mice. Both a postprandial and supraphysiological insulin clamp significantly increased interstitial fluid (ISF) and plasma Aβ compared with controls. We could detect no increase in brain, ISF, or CSF insulin or brain insulin signaling in response to peripheral hyperinsulinemia, despite detecting increased signaling in the muscle. Next, we delivered insulin directly into the hippocampus of young APP/PS1 mice via reverse microdialysis. Brain tissue insulin and insulin signaling was dose-dependently increased, but ISF Aβ was unchanged by central insulin administration. Finally, to determine whether peripheral and central high insulin has differential effects in the presence of significant amyloid pathology, we repeated these experiments in older APP/PS1 mice with significant amyloid plaque burden. Postprandial insulin clamps increased ISF and plasma Aβ, whereas direct delivery of insulin to the hippocampus significantly increased tissue insulin and insulin signaling, with no effect on Aβ in old mice. These results suggest that the brain is still responsive to insulin in the presence of amyloid pathology but increased insulin signaling does not acutely modulate Aβ in vivo before or after the onset of amyloid pathology. Peripheral hyperinsulinemia modestly increases ISF and plasma Aβ in young and old mice, independent of neuronal insulin signaling.

SIGNIFICANCE STATEMENT

The transportation of insulin from blood to brain is a saturable process relevant to understanding the link between hyperinsulinemia and AD. In vitro experiments have found direct connections between high insulin and extracellular Aβ, but these mechanisms presume that peripheral high insulin elevates brain insulin significantly. We found that physiological hyperinsulinemia in awake, behaving mice does not increase CNS insulin to an appreciable level yet modestly increases extracellular Aβ. We also found that the brain of aged APP/PS1 mice was not insulin resistant, contrary to the current state of the literature. These results further elucidate the relationship between insulin, the brain, and AD and its conflicting roles as both a risk factor and potential treatment.

Agmatine ameliorates type 2 diabetes induced-Alzheimer's disease-like alterations in high-fat diet-fed mice via reactivation of blunted insulin signalling

The risk of Alzheimer's disease (AD) is higher in patients with type 2 diabetes mellitus (T2DM). Previous studies in high-fat diet-induced AD animal models have shown that brain insulin resistance in these animals leads to the accumulation of amyloid beta (Aβ) and the reduction in GSK-3β phosphorylation, which promotes tau phosphorylation to cause AD. No therapeutic treatments that target AD in T2DM patients have yet been discovered. Agmatine, a primary amine derived from l-arginine, has exhibited anti-diabetic effects in diabetic animals. The aim of this study was to investigate the ability of agmatine to treat AD induced by brain insulin resistance. ICR mice were fed a 60% high-fat diet for 12 weeks and received one injection of streptozotocin (100 mg/kg/ip) 4 weeks into the diet. After the 12-week diet, the mice were treated with agmatine (100 mg/kg/ip) for 2 weeks. Behaviour tests were conducted prior to sacrifice. Brain expression levels of the insulin signal molecules p-IRS-1, p-Akt, and p-GSK-3β and the accumulation of Aβ and p-tau were evaluated. Agmatine administration rescued the reduction in insulin signalling, which in turn reduced the accumulation of Aβ and p-tau in the brain. Furthermore, agmatine treatment also reduced cognitive decline. Agmatine attenuated the occurrence of AD in T2DM mice via the activation of the blunted insulin signal.

Antidiabetic Drugs in Alzheimer's Disease: Mechanisms of Action and Future Perspectives

Diabetes mellitus (DM) and Alzheimer's disease (AD) are two highly prevalent conditions in the elderly population and major public health burden. In the past decades, a pathophysiological link between DM and AD has emerged and central nervous system insulin resistance might play a significant role as a common mechanism; however, other factors such as inflammation and oxidative stress seem to contribute to the shared pathophysiological link. Both preclinical and clinical studies have evaluated the possible neuroprotective mechanisms of different classes of antidiabetic medications in AD, with some promising results. Here, we review the evidence on the mechanisms of action of antidiabetic drugs and their potential use in AD.

Estrogen is neuroprotective against hypoglycemic injury in murine N38 hypothalamic cells

Estrogen (E2) has been demonstrated to possess protective effects from hypoglycemic toxicity, particularly in the pancreas. In the central nervous system, several brain regions, such as the hypothalamus, are highly vulnerable to hypoglycemic injuries that may lead to seizures, coma, and mortality. The present study performed a novel in vitro assay of hypoglycemic injury to hypothalamic cells, and is the first study, to the best of our knowledge, to demonstrate that E2 protects hypothalamic cells from hypoglycemic toxicity. The toxic effects of hypoglycemia on hypothalamic cells in vitro was determined by performing cell counts, together with MTT and lactate dehydrogenase assays, using the N38 murine hypothalamic cell line. Following 24 and 48 h in hypoglycemic conditions, a 60 and 75% reduction in cell number and mitochondrial function was observed, which reached 80 and ~100% by 72 and 96 h, respectively. E2 treatment prevented the hypoglycemia‑induced loss in cell number and mitochondrial toxicity at 24 and 48 h. However at 72 and 96 h of hypoglycemic conditions, the neuroprotective effects of E2 on cell number or mitochondrial function was not significant or not present at all. In order to determine whether E2 exerted its effects through the AKT signaling pathway, the expression of proline‑rich AKT substrate of 40 kDa (PRAS40) was analyzed. No alterations in PRAS40 expression were observed when N38 cells were exposed to hypoglycemic shock. From the biochemical and molecular data obtained, the authors speculated that E2 exhibits neuroprotective effects against hypoglycemic shock in hypothalamic cells, which dissipates with time. Despite demonstrating no significant effect on total AKT/PRS40 activity, it is possible that E2 may mediate these neuroprotective effects by upregulating the phosphorylated‑AKT/pPRAS40 signaling pathway. The present study presented, to the best of our knowledge, the first in vitro model for hypoglycemic toxicity to hypothalamic cells, and provided evidence to suggest that E2 may protect hypothalamic cells from the damaging effects of hypoglycemia.

Middle-Aged Diabetic Females and Males Present Distinct Susceptibility to Alzheimer Disease-like Pathology

Type 2 diabetes (T2D) is a highly concerning public health problem of the twenty-first century. Currently, it is estimated that T2D affects 422 million people worldwide with a rapidly increasing prevalence. During the past two decades, T2D has been widely shown to have a major impact in the brain. This, together with the cognitive decline and increased risk for dementia upon T2D, may arise from the complex interaction between normal brain aging and central insulin signaling dysfunction. Among the several features shared between T2D and some neurodegenerative disorders (e.g., Alzheimer disease (AD)), the impairment of insulin signaling may be a key link. However, these may also involve changes in sex hormones' function and metabolism, ultimately contributing to the different susceptibilities between females and males to some pathologies. For example, female sex has been pointed as a risk factor for AD, particularly after menopause. However, less is known on the underlying molecular mechanisms or even if these changes start during middle-age (perimenopause). From the above, we hypothesized that sex differentially affects hormone-mediated intracellular signaling pathways in T2D brain, ultimately modulating the risk for neurodegenerative conditions. We aimed to evaluate sex-associated alterations in estrogen/insulin-like growth factor-1 (IGF-1)/insulin-related signaling, oxidative stress markers, and AD-like hallmarks in middle-aged control and T2D rat brain cortices. We used brain cortices homogenates obtained from middle-aged (8-month-old) control Wistar and non-obese, spontaneously T2D Goto-Kakizaki (GK) male and female rats. Peripheral characterization of the animal models was done by standard biochemical analyses of blood, plasma, or serum. Steroid sex hormones, oxidative stress markers, and AD-like hallmarks were given by specific ELISA kits and colorimetric techniques, whereas the levels of intracellular signaling proteins were determined by Western blotting. Albeit the high levels of plasma estradiol and progesterone observed in middle-aged control females suggested that they were still under their reproductive phase, some gonadal dysfunction might be already occurring in T2D ones, hence, anticipating their menopause. Moreover, the higher blood and lower brain cholesterol levels in female rats suggested that its dysfunctional uptake into the brain cortex may also hamper peripheral estrogen uptake and/or its local brain steroidogenic metabolism. Despite the massive drop in IGF-1 levels in females' brains, particularly upon T2D, they might have developed some compensatory mechanisms towards the maintenance of estrogen, IGF-1, and insulin receptors function and of the subsequent Akt- and ERK1/2-mediated signaling. These may ultimately delay the deleterious AD-like brain changes (including oxidative damage to lipids and DNA, amyloidogenic processing of amyloid precursor protein and increased tau protein phosphorylation) associated with T2D and/or age (reproductive senescence) in female rats. By demonstrating that differential sex steroid hormone profiles/action may play a pivotal role in brain over T2D progression, the present study reinforces the need to establish sex-specific preventive and/or therapeutic approaches and an appropriate time window for the efficient treatment against T2D and AD.

Early detection of hepatocellular carcinoma co-occurring with hepatitis C virus infection: A mathematical model

AIM: To develop a mathematical model for the early detection of hepatocellular carcinoma (HCC) with a panel of serum proteins in combination with α-fetoprotein (AFP).

METHODS: Serum levels of interleukin (IL)-8, soluble intercellular adhesion molecule-1 (sICAM-1), soluble tumor necrosis factor receptor II (sTNF-RII), proteasome, and β-catenin were measured in 479 subjects categorized into four groups: (1) HCC concurrent with hepatitis C virus (HCV) infection (n = 192); (2) HCV related liver cirrhosis (LC) (n = 96); (3) Chronic hepatitis C (CHC) (n = 96); and (4) Healthy controls (n = 95). The R package and different modules for binary and multi-class classifiers based on generalized linear models were used to model the data. Predictive power was used to evaluate the performance of the model. Receiver operating characteristic curve analysis over pairs of groups was used to identify the best cutoffs differentiating the different groups.

RESULTS: We revealed mathematical models, based on a binary classifier, made up of a unique panel of serum proteins that improved the individual performance of AFP in discriminating HCC patients from patients with chronic liver disease either with or without cirrhosis. We discriminated the HCC group from the cirrhotic liver group using a mathematical model (-11.3 + 7.38 × Prot + 0.00108 × sICAM + 0.2574 ×β-catenin + 0.01597 × AFP) with a cutoff of 0.6552, which achieved 98.8% specificity and 89.1% sensitivity. For the discrimination of the HCC group from the CHC group, we used a mathematical model [-10.40 + 1.416 × proteasome + 0.002024 × IL + 0.004096 × sICAM-1 + (4.251 × 10^-4) × sTNF + 0.02567 ×β-catenin + 0.02442 × AFP] with a cutoff 0.744 and achieved 96.8% specificity and 89.7% sensitivity. Additionally, we derived an algorithm, based on a binary classifier, for resolving the multi-class classification problem by using three successive mathematical model predictions of liver disease status.

CONCLUSION: Our proposed mathematical model may be a useful method for the early detection of different statuses of liver disease co-occurring with HCV infection.

Sugar Consumption Produces Effects Similar to Early Life Stress Exposure on Hippocampal Markers of Neurogenesis and Stress Response

Adverse early life experience is a known risk factor for psychiatric disorders. It is also known that stress influences food preference. We were interested in exploring whether the choice of diet following early life stress exerts long-lasting molecular changes in the brain, particularly the hippocampus, a region critically involved in stress regulation and behavioral outcomes. Here, we examined the impact of early life stress induced by limited nesting material (LN) and chronic sucrose availability post-weaning on an array of hippocampal genes related to plasticity, neurogenesis, stress and inflammatory responses and mitochondrial biogenesis. To examine mechanisms underlying the impact of LN and sugar intake on hippocampal gene expression, we investigated the role of DNA methylation. As females are more likely to experience adverse life events, we studied female Sprague-Dawley rats. After mating LN was imposed from days 2 to 9 postpartum. From 3 to 15 weeks of age, female Control and LN siblings had unlimited to access to either chow and water, or chow, water and 25% sucrose solution. LN markedly reduced glucocorticoid receptor (GR) and neurogenic differentiation 1 (Neurod1) mRNA, markers involved in stress and hippocampal plasticity respectively, by more than 40%, with a similar effect of sugar intake in control rats. However, no further impact was observed in LN rats consuming sugar. Hippocampal Akt3 mRNA expression was similarly affected by LN and sucrose consumption. Interestingly, DNA methylation across 4 CpG sites of the GR and Neurod1 promoters was similar in LN and control rats. In summary, early life stress and post-weaning sugar intake produced long-term effects on hippocampal GR and Neurod1 expression. Moreover we found no evidence of altered promoter DNA methylation. We demonstrate for the first time that chronic sucrose consumption alone produces similar detrimental effects on the expression of hippocampal genes as LN exposure.

Reduced cortical BACE1 content with one bout of exercise is accompanied by declines in AMPK, Akt, and MAPK signaling in obese, glucose-intolerant mice

Obesity and type 2 diabetes are significant risk factors in the development of neurodegenerative diseases, such as Alzheimer's disease. A variety of cellular mechanisms, such as altered Akt and AMPK and increased inflammatory signaling, contribute to neurodegeneration. Exercise training can improve markers of neurodegeneration, but the underlying mechanisms remain unknown. The purpose of this study was to determine the effects of a single bout of exercise on markers of neurodegeneration and inflammation in brains from mice fed a high-fat diet. Male C57BL/6 mice were fed a low (LFD; 10% kcal from lard)- or a high-fat diet (HFD; 60% kcal from lard) for 7 wk. HFD mice underwent an acute bout of exercise (treadmill running: 15 m/min, 5% incline, 120 min) followed by a recovery period of 2 h. The HFD increased body mass and glucose intolerance (both P < 0.05). This was accompanied by an approximately twofold increase in the phosphorylation of Akt, ERK, and GSK in the cortex (P < 0.05). Following exercise, there was a decrease in beta-site amyloid precursor protein cleaving enzyme 1 (BACE1; P < 0.05) and activity (P < 0.001). This was accompanied by a reduction in AMPK phosphorylation, indicative of a decline in cellular stress (P < 0.05). Akt and ERK phosphorylation were decreased following exercise in HFD mice to a level similar to that of the LFD mice (P < 0.05). This study demonstrates that a single bout of exercise can reduce BACE1 content and activity independent of changes in adiposity. This effect is associated with reductions in Akt, ERK, and AMPK signaling in the cortex.

Early intervention with glucagon-like peptide 1 analog liraglutide prevents tau hyperphosphorylation in diabetic db/db mice

Increasing evidence has shown that type 2 diabetes (T2D) is a risk factor for Alzheimer's disease. Neurofibrillary tangles, which consist of hyperphosphorylated tau and misfolded microtubules, is one of the neuropathological hallmarks of Alzheimer's disease. Db/db mice, a rodent model of T2D, also exhibited age-dependent tau hyperphosphorylation. Glucagon-like peptide-1 (GLP-1) mimetics, a type of drug used in T2D, has been found to have neuroprotective effects. The aim of this study was to explore the potential effects of liraglutide (a GLP-1 analog), or insulin, on tau phosphorylation in T2D animals. Male db/db mice (3-3.5 weeks) were daily injected subcutaneously with liraglutide (n = 27), insulin (n = 27), or saline (n = 26), and five to seven mice were killed every 2 weeks for analysis of plasma and cerebrospinal (CSF) insulin levels by ELISA, and protein levels in the hippocampal formation by western blot. We found that db/db mice treated with saline exhibited an age-dependent decrease in CSF insulin and an increase in hippocampal tau phosphorylation. Liraglutide injection reversed the CSF insulin to ~1 mIU/L by the end of 8 weeks treatment, and prevented the hyperphosphorylation of tau protein in the hippocampal formation. By contrast, insulin injection had no effects on CSF insulin or phosphorylation of tau protein. In summary, this study indicates that early GLP-1 analog intervention prevented the age-dependent tau hyperphosphorylation in T2D mice brain, probably by facilitating sequential activation in an insulin signaling pathway reflected in increased basal activation of Akt and basal suppression of glycogen synthase kinase-3 beta.

Biliverdin reductase: a target for cancer therapy?

Biliverdin reductase (BVR) is a multifunctional protein that is the primary source of the potent antioxidant, bilirubin. BVR regulates activities/functions in the insulin/IGF-1/IRK/PI3K/MAPK pathways. Activation of certain kinases in these pathways is/are hallmark(s) of cancerous cells. The protein is a scaffold/bridge and intracellular transporter of kinases that regulate growth and proliferation of cells, including PKCs, ERK and Akt, and their targets including NF-κB, Elk1, HO-1, and iNOS. The scaffold and transport functions enable activated BVR to relocate from the cytosol to the nucleus or to the plasma membrane, depending on the activating stimulus. This enables the reductase to function in diverse signaling pathways. And, its expression at the transcript and protein levels are increased in human tumors and the infiltrating T-cells, monocytes and circulating lymphocytes, as well as the circulating and infiltrating macrophages. These functions suggest that the cytoprotective role of BVR may be permissive for cancer/tumor growth. In this review, we summarize the recent developments that define the pro-growth activities of BVR, particularly with respect to its input into the MAPK signaling pathway and present evidence that BVR-based peptides inhibit activation of protein kinases, including MEK, PKCδ, and ERK as well as downstream targets including Elk1 and iNOS, and thus offers a credible novel approach to reduce cancer cell proliferation.

Contribution of the hypothalamus and gut to weight gain susceptibility and resistance in mice

Obesity susceptibility in humans and in rodent strains varies in response to the consumption of high-energy density (HED) diets. However, the exact mechanism(s) involved in this susceptibility remain(s) unresolved. The aim of the present study was to gain greater insight into this susceptibility by using C57BL/6J (B6) mice that were separated into obesity-prone (diet-induced obese (DIO)) and obesity-resistant (diet-induced resistant (DR)) groups following an HED diet for 6 weeks. Physiological, biochemical and gene expression assessments of energy balance were performed in the DIO and DR mice on an HED diet and chow-fed mice. The increased weight gain of the DIO mice as compared to the DR mice was associated with increased energy intake and higher plasma leptin and adiponectin levels but not with reduced physical activity or resting energy expenditure. Hypothalamic Pomc gene expression was elevated, but there were no changes in Npy or Agrp expression. Adipose tissue leptin and adiponectin gene expression were significantly reduced in the DIO group as compared to the DR group. Interestingly, ileum expression of G protein-coupled receptor (Gpr) 40 (Gpr40) was significantly increased, whereas Gpr120, Gpr119, Gpr41, and glucagon-like peptide 1 (Glp1) were reduced. Contrastingly, the lower weight gain of the DR group was associated with elevated adipose tissue leptin and adiponectin gene expression, but there were no differences in plasma hormone or hypothalamic gene expression levels as compared to chow-fed mice. Therefore, the present data demonstrate that susceptibility and resistance to diet-induced weight gain in B6 mice appears to be predominantly driven by peripheral rather than hypothalamic modifications, and changes in gut-specific receptors are a potentially important contributor to this variation.