Insulin: an emerging treatment for Alzheimer's disease dementia?

Impact of chronic hyperglycemia and high-fat diet on Alzheimer's disease-related pathology in CX3CR1 knockout mice

Diabetes mellitus (DM), obesity, and metabolic syndrome are related disorders with wide-ranging and devastating effects that are comorbid with many other diseases. Clinical and epidemiological studies have found that type II diabetes mellitus (T2DM), including chronic hyperglycemia and hyperinsulinemia, significantly increases the risk of Alzheimer's disease (AD) and other forms of dementia in the elderly. Insulin has slightly different functions in the peripheral body than in the central nervous system and the dysregulation of these functions may contribute to the onset and progression of late-life neurodegenerative disease. To investigate cognitive function and AD-related disease pathology, we utilized two different models of key features of diabetes, one model characterized by hyperglycemia resulting from a diabetogenic compound that selectively targets insulin-producing pancreatic β-cells, and the other model based on diet-induced obesity. Additionally, these diabetic models were combined with fractalkine receptor knockout mice (CX3CR1-/-), a genetic mouse model of inflammation, to explore the additive effects of multiple AD risk factors. The CX3CR1 receptor has been implicated in modulating neuroinflammation associated with AD, and its dysregulation can exacerbate metabolic disturbances and neurodegenerative markers. We found that diabetic-status, regardless of whether it was drug- or diet-induced, resulted in profound impairments in learning and memory and AD-related alterations within the hippocampus.

Skeletal muscle proteome differs between young APOE3 and APOE4 targeted replacement mice in a sex-dependent manner

Introduction

Apolipoprotein E4 (APOE4) is the strongest genetic risk factor for Alzheimer's disease (AD), yet it's unclear how this allele mediates risk. APOE4 carriers experience reduced mobility and faster decline in muscle strength, suggesting skeletal muscle involvement. Mitochondria are critical for muscle function and although we have reported defects in muscle mitochondrial respiration during early cognitive decline, APOE4-mediated effects on muscle mitochondria are unknown.

Methods

Here, we sought to determine the impact of APOE4 on skeletal muscle bioenergetics using young, male and female APOE3 (control) and APOE4 targeted replacement mice (n = 8 per genotype/sex combination). We examined the proteome, mitochondrial respiration, fiber size, and fiber-type distribution in skeletal muscle.

Results

We found that APOE4 alters mitochondrial pathway expression in young mouse muscle in a sex-dependent manner without affecting respiration and fiber size or composition relative to APOE3. In both sexes, the expression of mitochondrial pathways involved in electron transport, ATP synthesis, and heat production by uncoupling proteins and mitochondrial dysfunction significantly differed between APOE4 and APOE3 muscle. For pathways with predicted direction of activation, electron transport and oxidative phosphorylation were upregulated while mitochondrial dysfunction and sirtuin signaling were downregulated in female APOE4 vs. APOE3 muscle. In males, sulfur amino acid metabolism was upregulated in APOE4 vs. APOE3 muscle.

Discussion

This work highlights early involvement of skeletal muscle in a mouse model of APOE4-linked AD, which may contribute to AD pathogenesis or serve as a biomarker for brain health.

Amino Acid Compound 2 (AAC2) Treatment Counteracts Insulin-Induced Synaptic Gene Expression and Seizure-Related Mortality in a Mouse Model of Alzheimer's Disease

Diabetes is a major risk factor for Alzheimer's disease (AD). Amino acid compound 2 (AAC2) improves glycemic and cognitive functions in diabetic mouse models through mechanisms distinct from insulin. Our goal was to compare the effects of AAC2, insulin, and their nanofiber-forming combination on early asymptomatic AD pathogenesis in APP/PS1 mice. Insulin, but not AAC2 or the combination treatment (administered intraperitoneally every 48 h for 120 days), increased seizure-related mortality, altered the brain fat-to-lean mass ratio, and improved specific cognitive functions in APP/PS1 mice. NanoString and pathway analysis of cerebral gene expression revealed dysregulated synaptic mechanisms, with upregulation of Bdnf and downregulation of Slc1a6 in insulin-treated mice, correlating with insulin-induced seizures. In contrast, AAC2 promoted the expression of Syn2 and Syp synaptic genes, preserved brain composition, and improved survival. The combination of AAC2 and insulin counteracted free insulin's effects. None of the treatments influenced canonical amyloidogenic pathways. This study highlights AAC2's potential in regulating synaptic gene expression in AD and insulin-induced contexts related to seizure activity.

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.

Effect and Mechanism of Rapamycin on Cognitive Deficits in Animal Models of Alzheimer's Disease: A Systematic Review and Meta-analysis of Preclinical Studies

Background

Alzheimer's disease (AD), the most common form of dementia, remains long-term and challenging to diagnose. Furthermore, there is currently no medication to completely cure AD patients. Rapamycin has been clinically demonstrated to postpone the aging process in mice and improve learning and memory abilities in animal models of AD. Therefore, rapamycin has the potential to be significant in the discovery and development of drugs for AD patients.

Objective

The main objective of this systematic review and meta-analysis was to investigate the effects and mechanisms of rapamycin on animal models of AD by examining behavioral indicators and pathological features.

Methods

Six databases were searched and 4,277 articles were retrieved. In conclusion, 13 studies were included according to predefined criteria. Three authors independently judged the selected literature and methodological quality. Use of subgroup analyses to explore potential mechanistic effects of rapamycin interventions: animal models of AD, specific types of transgenic animal models, dosage, and periodicity of administration.

Results

The results of Morris Water Maze (MWM) behavioral test showed that escape latency was shortened by 15.60 seconds with rapamycin therapy, indicating that learning ability was enhanced in AD mice; and the number of traversed platforms was increased by 1.53 times, indicating that the improved memory ability significantly corrected the memory deficits.

CONCLUSIONS

Rapamycin therapy reduced age-related plaque deposition by decreasing AβPP production and down-regulating β-secretase and γ-secretase activities, furthermore increased amyloid-β clearance by promoting autophagy, as well as reduced tau hyperphosphorylation by up-regulating insulin-degrading enzyme levels.

Current Perspectives: Obesity and Neurodegeneration - Links and Risks

Obesity is increasing in prevalence across all age groups. Long-term obesity can lead to the development of metabolic and cardiovascular diseases through its effects on adipose, skeletal muscle, and liver tissue. Pathological mechanisms associated with obesity include immune response and inflammation as well as oxidative stress and consequent endothelial and mitochondrial dysfunction. Recent evidence links obesity to diminished brain health and neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Both AD and PD are associated with insulin resistance, an underlying syndrome of obesity. Despite these links, causative mechanism(s) resulting in neurodegenerative disease remain unclear. This review discusses relationships between obesity, AD, and PD, including clinical and preclinical findings. The review then briefly explores nonpharmacological directions for intervention.

A multi-layered network model identifies Akt1 as a common modulator of neurodegeneration

The accumulation of misfolded and aggregated proteins is a hallmark of neurodegenerative proteinopathies. Although multiple genetic loci have been associated with specific neurodegenerative diseases (NDs), molecular mechanisms that may have a broader relevance for most or all proteinopathies remain poorly resolved. In this study, we developed a multi-layered network expansion (MLnet) model to predict protein modifiers that are common to a group of diseases and, therefore, may have broader pathophysiological relevance for that group. When applied to the four NDs Alzheimer's disease (AD), Huntington's disease, and spinocerebellar ataxia types 1 and 3, we predicted multiple members of the insulin pathway, including PDK1, Akt1, InR, and sgg (GSK-3β), as common modifiers. We validated these modifiers with the help of four Drosophila ND models. Further evaluation of Akt1 in human cell-based ND models revealed that activation of Akt1 signaling by the small molecule SC79 increased cell viability in all models. Moreover, treatment of AD model mice with SC79 enhanced their long-term memory and ameliorated dysregulated anxiety levels, which are commonly affected in AD patients. These findings validate MLnet as a valuable tool to uncover molecular pathways and proteins involved in the pathophysiology of entire disease groups and identify potential therapeutic targets that have relevance across disease boundaries. MLnet can be used for any group of diseases and is available as a web tool at http://ssbio.cau.ac.kr/software/mlnet.

Hexosamine biosynthetic pathway and O-GlcNAc cycling of glucose metabolism in brain function and disease

Impaired brain glucose metabolism is considered a hallmark of brain dysfunction and neurodegeneration. Disruption of the hexosamine biosynthetic pathway (HBP) and subsequent O-linked N-acetylglucosamine (O-GlcNAc) cycling has been identified as an emerging link between altered glucose metabolism and defects in the brain. Myriads of cytosolic and nuclear proteins in the nervous system are modified at serine or threonine residues with a single N-acetylglucosamine (O-GlcNAc) molecule by O-GlcNAc transferase (OGT), which can be removed by β-N-acetylglucosaminidase (O-GlcNAcase, OGA). Homeostatic regulation of O-GlcNAc cycling is important for the maintenance of normal brain activity. Although significant evidence linking dysregulated HBP metabolism and aberrant O-GlcNAc cycling to induction or progression of neuronal diseases has been obtained, the issue of whether altered O-GlcNAcylation is causal in brain pathogenesis remains uncertain. Elucidation of the specific functions and regulatory mechanisms of individual O-GlcNAcylated neuronal proteins in both normal and diseased states may facilitate the identification of novel therapeutic targets for various neuronal disorders. The information presented in this review highlights the importance of HBP/O-GlcNAcylation in the neuronal system and summarizes the roles and potential mechanisms of O-GlcNAcylated neuronal proteins in maintaining normal brain function and initiation and progression of neurological diseases.

Crosstalk between Alzheimer's disease and diabetes: a focus on anti-diabetic drugs

Alzheimer's disease (AD) and Type 2 diabetes mellitus (T2DM) are two of the most common age-related diseases. There is accumulating evidence of an overlap in the pathophysiological mechanisms of these two diseases. Studies have demonstrated insulin pathway alternation may interact with amyloid-β protein deposition and tau protein phosphorylation, two essential factors in AD. So attention to the use of anti-diabetic drugs in AD treatment has increased in recent years. In vitro, in vivo, and clinical studies have evaluated possible neuroprotective effects of anti-diabetic different medicines in AD, with some promising results. Here we review the evidence on the therapeutic potential of insulin, metformin, Glucagon-like peptide-1 receptor agonist (GLP1R), thiazolidinediones (TZDs), Dipeptidyl Peptidase IV (DPP IV) Inhibitors, Sulfonylureas, Sodium-glucose Cotransporter-2 (SGLT2) Inhibitors, Alpha-glucosidase inhibitors, and Amylin analog against AD. Given that many questions remain unanswered, further studies are required to confirm the positive effects of anti-diabetic drugs in AD treatment. So to date, no particular anti-diabetic drugs can be recommended to treat AD.

Dementia in Diabetes: The Role of Hypoglycemia

Hypoglycemia, a common consequence of diabetes treatment, is associated with severe morbidity and mortality and has become a major barrier to intensifying antidiabetic therapy. Severe hypoglycemia, defined as abnormally low blood glucose requiring the assistance of another person, is associated with seizures and comas, but even mild hypoglycemia can cause troubling symptoms such as anxiety, palpitations, and confusion. Dementia generally refers to the loss of memory, language, problem-solving, and other cognitive functions, which can interfere with daily life, and there is growing evidence that diabetes is associated with an increased risk of both vascular and non-vascular dementia. Neuroglycopenia resulting from a hypoglycemic episode in diabetic patients can lead to the degeneration of brain cells, with a resultant cognitive decline, leading to dementia. In light of new evidence, a deeper understating of the relationship between hypoglycemia and dementia can help to inform and guide preventative strategies. In this review, we discuss the epidemiology of dementia among patients with diabetes, and the emerging mechanisms thought to underlie the association between hypoglycemia and dementia. Furthermore, we discuss the risks of various pharmacological therapies, emerging therapies to combat hypoglycemia-induced dementia, as well as risk minimization strategies.

Selective breeding for physical inactivity produces cognitive deficits via altered hippocampal mitochondrial and synaptic function

Physical inactivity is the 4th leading cause of death globally and has been shown to significantly increase the risk for developing Alzheimer's Disease (AD). Recent work has demonstrated that exercise prior to breeding produces heritable benefits to the brains of offspring, suggesting that the physical activity status of previous generations could play an important role in one's brain health and their subsequent risk for neurodegenerative diseases. Thus, our study aimed to test the hypothesis that selective breeding for physical inactivity, or for high physical activity, preference produces heritable deficits and enhancements to brain health, respectively. To evaluate this hypothesis, male and female sedentary Low Voluntary Runners (LVR), wild type (WT), and High Voluntary Runner (HVR) rats underwent cognitive behavioral testing, analysis of hippocampal neurogenesis and mitochondrial respiration, and molecular analysis of the dentate gyrus. These analyses revealed that selecting for physical inactivity preference has produced major detriments to cognition, brain mitochondrial respiration, and neurogenesis in female LVR while female HVR display enhancements in brain glucose metabolism and hippocampal size. On the contrary, male LVR and HVR showed very few differences in these parameters relative to WT. Overall, we provide evidence that selective breeding for physical inactivity has a heritable and detrimental effect on brain health and that the female brain appears to be more susceptible to these effects. This emphasizes the importance of remaining physically active as chronic intergenerational physical inactivity likely increases susceptibility to neurodegenerative diseases for both the inactive individual and their offspring.

Alterations in Peripheral Metabolites as Key Actors in Alzheimer's Disease

Growing evidence supports that Alzheimer's disease (AD) could be regarded as a metabolic disease, accompanying central and peripheral metabolic disturbance. Nowadays, exploring novel and potentially alternative hallmarks for AD is needed. Peripheral metabolites based on blood and gut may provide new biochemical insights about disease mechanisms. These metabolites can influence brain energy homeostasis, maintain gut mucosal integrity, and regulate the host immune system, which may further play a key role in modulating the cognitive function and behavior of AD. Recently, metabolomics has been used to identify key AD-related metabolic changes and define metabolic changes during AD disease trajectory. This review aims to summarize the key blood- and microbial-derived metabolites that are altered in AD and identify the potential metabolic biomarkers of AD, which will provide future targets for precision therapeutic modulation.

Changes in aggression and locomotor behaviors in response to zinc is accompanied by brain cell heterogeneity and metabolic and circadian dysregulation of the brain-liver axis

Zinc is an essential nutrient for life, but over-accumulation can result in toxicity. Anthropogenic activities can increase zinc concentrations in aquatic environments (e.g., to ∼0.46-1.00 mg/L), which are above the safe level of 0.1 mg/L. We investigated the behavior and physiology of zebrafish (Danio rerio) in response to environment-related exposure to zinc chloride at 0.0 (Ctrl), 1.0 (ZnCl₂-low) and 1.5 (ZnCl₂-high) mg/L for 6 weeks (the zinc conversion ratio of zinc chloride is ∼0.48 and the nominal (measured) values were: Ctrl, 0 (∼0.01); ZnCl₂-low, 0.48 (∼0.51); ZnCl₂-high, 0.72 (∼0.69) mg/L). Low-zinc exposure resulted in significantly increased locomotion and fast moving behaviors, while high-zinc exposure resulted in significantly increased aggression and freezing frequency. Single cell RNA-seq of neurons, astrocytes, and oligodendrocytes of the brain revealed expression of genes related to ion transport, neuron generation, and immunomodulation that were heterogeneously regulated by zinc exposure. Astrocyte-induced central nervous system inflammation potentially integrated neurotoxicity and behavior. Integrated analyses of brain and hepatic transcriptional signatures showed that genes (and pathways) dysregulated by zinc were associated with sensory functions, circadian rhythm, glucose and lipid metabolism, and amyloid β-protein clearance. Our results showed that environment-related zinc contamination can be heterogeneously toxic to brain cells and can disturb coordination of brain-liver physiology. This may disrupt neurobehavior and cause a neurodegeneration-like syndrome in adult zebrafish.

Diabetes: Risk factor and translational therapeutic implications for Alzheimer's disease

Type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) commonly co-occur. T2DM increases the risk for AD by approximately twofold. Animal models provide one means of interrogating the relationship of T2DM to AD and investigating brain insulin resistance in the pathophysiology of AD. Animal models show that persistent hyperglycaemia results in chronic low-grade inflammation that may contribute to the development of neuroinflammation and accelerate the pathobiology of AD. Epidemiological studies suggest that patients with T2DM who received treatment with specific anti-diabetic agents have a decreased risk for the occurrence of AD and all-cause dementia. Agents such as metformin ameliorate T2DM and may have other important systemic effects that lower the risk of AD. Glucagon-like peptide 1 (GLP-1) agonists have been associated with a decreased risk for AD in patients with T2DM. Both insulin and non-insulin anti-diabetic treatments have been evaluated for the treatment of AD in clinical trials. In most cases, patients included in the trials have clinical features of AD but do not have T2DM. Many of the trials were conducted prior to the use of diagnostic biomarkers for AD. Trials have had a wide range of durations and population sizes. Many of the agents used to treat T2DM do not cross the blood brain barrier, and the effects are posited to occur via lowering of peripheral hyperglycaemia and reduction of peripheral and central inflammation. Clinical trials of anti-diabetic agents to treat AD are ongoing and will provide insight into the therapeutic utility of these agents.

Intranasal metformin treatment ameliorates cognitive functions via insulin signaling pathway in ICV-STZ-induced mice model of Alzheimer's disease

AIMS

The relationship between type 2 diabetes and Alzheimer's disease (AD) provides evidence that insulin and insulin sensitizers may be beneficial for the treatment of AD. The present study investigated the effect and mechanism of action of intranasal metformin treatment on impaired cognitive functions in an experimental mice model of AD.

MAIN METHODS

Intracerebroventricularly (ICV) streptozotocin (STZ)-injected mice were treated with intranasal or oral metformin for 4 weeks. Learning and memory functions were evaluated using Morris water maze. Metformin and Aβ42 concentrations were determined by liquid chromatography tandem mass spectrometry and ELISA respectively. The expressions of insulin receptor, Akt and their phosphorylated forms were determined in the hippocampi and cerebral cortices of mice.

KEY FINDINGS

ICV-STZ-induced AD mice displayed impaired learning and memory functions which were improved by metformin treatment. ICV-STZ injection or intranasal/oral metformin treatments had no effect on blood glucose concentrations. Intranasal treatment yielded higher concentration of metformin in the hippocampus and lower in the plasma compared to oral treatment. ICV-STZ injection and metformin treatments did not change amyloid β-42 concentration in the hippocampus of mice. In hippocampal and cortical tissues of ICV-STZ-induced AD mice, insulin receptor (IR) and Akt expressions were unchanged, while phosphorylated insulin receptor (pIR) and pAkt expressions decreased compared to control. Metformin treatments did not change IR and Akt expressions but increased pIR and pAkt expressions.

SIGNIFICANCE

The present study showed for the first time that intranasal metformin treatment improved the impaired cognitive functions through increasing insulin sensitivity in ICV-STZ-induced mice model of AD.

Brain insulin resistance and the therapeutic value of insulin and insulin-sensitizing drugs in Alzheimer's disease neuropathology

The incidence of Alzheimer's disease (AD) is significantly higher in people with diabetes. Insulin and insulin receptor (IR) signaling intermediates are expressed in the brain. Insulin exerts multiple function in the brain. The role of compromised IR signaling in AD pathogenesis and the therapeutic value of insulin attract broad attention. This review summarizes the collective insulin action in the brain related to key factors of AD pathogenesis, updates the key features of insulin resistance in the AD brain and assesses the therapeutic potential of insulin and insulin-sensitizing drugs. Insulin stimulates neural growth and survival, suppresses amyloidogenic processing of the amyloid precursor protein (AβPP) and inhibits the Tau phosphorylation kinase, glycogen synthase kinase 3β. Central nervous IR signaling regulates systemic metabolism and increases glucose availability to neurons. The expression of IR and its downstream effectors is reduced in AD brain tissues. Insulin and insulin-sensitizing drugs can improve cognitive function in AD patients and AD animal models. Systemic insulin delivery is less effective than intranasal insulin treatment. The penetrance of insulin-sensitizing drugs to the blood brain barrier is problematic and new brain-prone drugs need be developed. Insulin resistance manifested by the degradation and the altered phosphorylation of IR intermediates precedes overt AD syndrome. Type 3 diabetes as a pure form of brain insulin resistance without systemic insulin resistance is proposed as a causal factor in AD. Further research is needed for the identification of critical factors leading to impaired IR signaling and the development of new molecules to stimulate brain IR signaling.

Using nanotechnology to deliver biomolecules from nose to brain - peptides, proteins, monoclonal antibodies and RNA

There is a growing number of biomolecules, including peptides, proteins, monoclonal antibodies and RNA, that could be potentially used for the treatment of central nervous system (CNS) diseases. However, the realization of their potential is being hampered by the extraordinary difficulties these complex biomolecules have to reach the brain in therapeutically meaningful amounts. Nose-to-brain (N-to-B) delivery is now being investigated as a potential option for the direct transport of biomolecules from the nasal cavity to different brain areas. Here, we discuss how different technological approaches enhance this N-to-B transport, with emphasis on those that have shown a potential for clinical translation. We also analyse how the physicochemical properties of nanocarriers and their modification with cell-penetrating peptides (CPPs) and targeting ligands affect their efficacy as N-to-B carriers for biomolecules.

Modulation of Brain Hyperexcitability: Potential New Therapeutic Approaches in Alzheimer's Disease

People with Alzheimer's disease (AD) have significantly higher rates of subclinical and overt epileptiform activity. In animal models, oligomeric Aβ amyloid is able to induce neuronal hyperexcitability even in the early phases of the disease. Such aberrant activity subsequently leads to downstream accumulation of toxic proteins, and ultimately to further neurodegeneration and neuronal silencing mediated by concomitant tau accumulation. Several neurotransmitters participate in the initial hyperexcitable state, with increased synaptic glutamatergic tone and decreased GABAergic inhibition. These changes appear to activate excitotoxic pathways and, ultimately, cause reduced long-term potentiation, increased long-term depression, and increased GABAergic inhibitory remodelling at the network level. Brain hyperexcitability has therefore been identified as a potential target for therapeutic interventions aimed at enhancing cognition, and, possibly, disease modification in the longer term. Clinical trials are ongoing to evaluate the potential efficacy in targeting hyperexcitability in AD, with levetiracetam showing some encouraging effects. Newer compounds and techniques, such as gene editing via viral vectors or brain stimulation, also show promise. Diagnostic challenges include identifying best biomarkers for measuring sub-clinical epileptiform discharges. Determining the timing of any intervention is critical and future trials will need to carefully stratify participants with respect to the phase of disease pathology.

Shortcut Approaches to Substance Delivery into the Brain Based on Intranasal Administration Using Nanodelivery Strategies for Insulin

The direct delivery of central nervous system (CNS) drugs into the brain after administration is an ideal concept due to its effectiveness and non-toxicity. However, the blood-brain barrier (BBB) prevents drugs from penetrating the capillary endothelial cells, blocking their entry into the brain. Thus, alternative approaches must be developed. The nasal cavity directly leads from the olfactory epithelium to the brain through the cribriform plate of the skull bone. Nose-to-brain drug delivery could solve the BBB-related repulsion problem. Recently, it has been revealed that insulin improved Alzheimer's disease (AD)-related dementia. Several ongoing AD clinical trials investigate the use of intranasal insulin delivery. Related to the real trajectory, intranasal labeled-insulins demonstrated distribution into the brain not only along the olfactory nerve but also the trigeminal nerve. Nonetheless, intranasally administered insulin was delivered into the brain. Therefore, insulin conjugates with covalent or non-covalent cargos, such as AD or other CNS drugs, could potentially contribute to a promising strategy to cure CNS-related diseases. In this review, I will introduce the CNS drug delivery approach into the brain using nanodelivery strategies for insulin through transcellular routes based on receptor-mediated transcytosis or through paracellular routes based on escaping the tight junction at the olfactory epithelium.

Heat therapy: possible benefits for cognitive function and the aging brain

Alzheimer's disease (AD) is the most common neurodegenerative disease, yet there are no disease-modifying treatments available and there is no cure. It is becoming apparent that metabolic and vascular conditions such as type 2 diabetes (T2D) and hypertension promote the development and accumulation of Alzheimer's disease-related dementia pathologies. To this end, aerobic exercise, which is a common lifestyle intervention for both metabolic disease and hypertension, is shown to improve brain health during both healthy aging and dementia. However, noncompliance or other barriers to exercise response are common in exercise treatment paradigms. In addition, reduced intracellular proteostasis and mitochondrial function could contribute to the etiology of AD. Specifically, compromised chaperone systems [i.e., heat shock protein (HSP) systems] can contribute to protein aggregates (i.e., β-amyloid plaques and neurofibrillary tangles) and reduced mitochondrial quality control (i.e., mitophagy). Therefore, novel therapies that target whole body metabolism, the vasculature, and chaperone systems (like HSPs) are needed to effectively treat AD. This review focuses on the role of heat therapy in the treatment and prevention of AD. Heat therapy has been independently shown to reduce whole body insulin resistance, improve vascular function, activate interorgan cross talk via endocytic vesicles, and activate HSPs to improve mitochondrial function and proteostasis in a variety of tissues. Thus, heat therapy could offer immense clinical benefit to patients suffering from AD. Importantly, future studies in patients are needed to determine the safety and efficacy of heat therapy in preventing AD.

Altered Cerebellar-Cerebral Circuits in Patients With Type 2 Diabetes Mellitus

The role of the cerebellum in type 2 diabetes mellitus (T2DM) has been receiving increased attention. However, the functional connectivity (FC) between the cerebellar subregions and the cerebral cortex has not been investigated in T2DM. Therefore, the purpose of this study was to investigate cerebellar-cerebral FC and the relationship between FC and clinical/cognitive variables in patients with T2DM. A total of 34 patients with T2DM and 30 healthy controls were recruited for this study to receive a neuropsychological assessment and undergo resting-state FC. We selected four subregions of the cerebellum (bilateral lobules IX, right and left Crus I/II, and left lobule VI) as regions of interest (ROIs) to examine the differences in cerebellar-cerebral circuits in patients with T2DM compared to healthy controls. Correlation analysis was performed to examine the relationship between FC and clinical/cognitive variables in the patients. Compared to healthy controls, patients with T2DM showed significantly decreased cerebellar-cerebral FC in the default-mode network (DMN), executive control network (ECN), and visuospatial network (VSN). In the T2DM group, the FC between the left cerebellar lobule VI and the right precuneus was negatively correlated with the Trail Making Test A (TMT-A) score (r = −0.430, P = 0.013), after a Bonferroni correction. In conclusion, patients with T2DM have altered FC between the cerebellar subregions and the cerebral networks involved in cognitive and emotional processing. This suggests that a range of cerebellar-cerebral circuits may be involved in the neuropathology of T2DM cognitive dysfunction.

Insulin deficiency, but not resistance, exaggerates cognitive deficits in transgenic mice expressing human amyloid and tau proteins. Reversal by Exendin-4 treatment

Epidemiological studies have pointed at diabetes as a risk factor for Alzheimer's disease (AD) and this has been supported by several studies in animal models of both type 1 and type 2 diabetes. However, side-by-side comparison of the two types of diabetes is limited. We investigated the role of insulin deficiency and insulin resistance in the development of memory impairments and the effect of Exendin-4 (Ex4) treatment in a mouse model of AD. Three-4-month-old female wild type (WT) mice and mice overexpressing human tau and amyloid precursor protein (TAPP) were injected with streptozotocin (STZ) or fed a high-fat diet (HFD). A second study was performed in TAPP-STZ mice treated with Ex4, a long-lasting analog of GLP-1. Plasma and brain were collected at study termination for ELISA, Western blot, and immunohistochemistry analysis. Learning and memory deficits were impaired in TAPP transgenic mice compared with WT mice at the end of the study. Deficits were exaggerated by insulin deficiency in TAPP mice but 12 weeks of insulin resistance did not affect memory performances in either WT or TAPP mice. Levels of phosphorylated tau were increased in the brain of WT-STZ and TAPP-STZ mice but not in the brain of WT or TAPP mice on HFD. In the TAPP-STZ mice, treatment with Ex4 initiated after established cognitive deficits ameliorated learning, but not memory, impairments. This was accompanied by the reduction of amyloid β and phosphorylated tau expression. Theses studies support the role of Ex4 in AD, independently from its actions on diabetes.

Extending Arms of Insulin Resistance from Diabetes to Alzheimer's Disease: Identification of Potential Therapeutic Targets

BACKGROUND & OBJECTIVE

Type 3 diabetes (T3D) is chronic insulin resistant state of brain which shares pathology with sporadic Alzheimer's disease (sAD). Insulin signaling is a highly conserved pathway in the living systems that orchestrate cell growth, repair, maintenance, energy homeostasis and reproduction. Although insulin is primarily studied as a key molecule in diabetes mellitus, its role has recently been implicated in the development of Alzheimer's disease (AD). Severe complications in brain of diabetic patients and metabolically compromised status is evident in brain of AD patients. Underlying shared pathology of two disorders draws a trajectory from peripheral insulin resistance to insulin unresponsiveness in the central nervous system (CNS). As insulin has a pivotal role in AD, it is not an overreach to address diabetic condition in AD brain as T3D. Insulin signaling is indispensable to nervous system and it is vital for neuronal growth, repair, and maintenance of chemical milieu at synapses. Downstream mediators of insulin signaling pathway work as a regulatory hub for aggregation and clearance of unfolded proteins like Aβ and tau.

CONCLUSION

In this review, we discuss the regulatory roles of insulin as a pivotal molecule in brain with the understanding of defective insulin signaling as a key pathological mechanism in sAD. This article also highlights ongoing trials of targeting insulin signaling as a therapeutic manifestation to treat diabetic condition in brain.

Interlink Between Insulin Resistance and Neurodegeneration with an Update on Current Therapeutic Approaches

The interlink between diabetes mellitus and neurodegenerative diseases such as Alzheimer's Disease (AD) and Parkinson's Disease (PD) has been identified by several researchers. Patients with Type-2 Diabetes Mellitus (T2DM) are found to be affected with cognitive impairments leading to learning and memory deficit, while patients with Type-1 Diabetes Mellitus (T1DM) showed less severe levels of these impairments in the brain. This review aimed to discuss the connection between insulin with the pathophysiology of neurodegenerative diseases (AD and PD) and the current therapeutic approached mediated through insulin for management of neurodegenerative diseases. An extensive literature search was conducted using keywords "insulin"; "insulin resistance"; "Alzheimer's disease"; "Parkinson's disease" in public domains of Google scholar, PubMed, and ScienceDirect. Selected articles were used to construct this review. Studies have shown that impaired insulin signaling contributes to the accumulation of amyloid-β, neurofibrillary tangles, tau proteins and α-synuclein in the brain. Whereas, improvement in insulin signaling slows down the progression of cognitive decline. Various therapeutic approaches for altering the insulin function in the brain have been researched. Besides intranasal insulin, other therapeutics like PPAR-γ agonists, neurotrophins, stem cell therapy and insulin-like growth factor-1 are under investigation. Research has shown that insulin insensitivity in T2DM leads to neurodegeneration through mechanisms involving a variety of extracellular, membrane receptor, and intracellular signaling pathway disruptions. Some therapeutics, such as intranasal administration of insulin and neuroactive substances have shown promise but face problems related to genetic background, accessibility to the brain, and invasiveness of the procedures.

The Rho/Rac Guanine Nucleotide Exchange Factor Vav1 Regulates Hif-1α and Glut-1 Expression and Glucose Uptake in the Brain

Vav1 is a Rho/Rac (Ras-related C3 botulinum toxin substrate) guanine nucleotide exchange factor expressed in hematopoietic and endothelial cells that are involved in a wide range of cellular functions. It is also stabilized under hypoxic conditions when it regulates the accumulation of the transcription factor HIF (Hypoxia Inducible Factor)-1α, which activates the transcription of target genes to orchestrate a cellular response to low oxygen. One of the genes induced by HIF-1α is GLUT (Glucose Transporter)-1, which is the major glucose transporter expressed in vessels that supply energy to the brain. Here, we identify a role for Vav1 in providing glucose to the brain. We found that Vav1 deficiency downregulates HIF-1α and GLUT-1 levels in endothelial cells, including blood-brain barrier cells. This downregulation of GLUT-1, in turn, reduced glucose uptake to endothelial cells both in vitro and in vivo, and reduced glucose levels in the brain. Furthermore, endothelial cell-specific Vav1 knock-out in mice, which caused glucose uptake deficiency, also led to a learning delay in fear conditioning experiments. Our results suggest that Vav1 promotes learning by activating HIF-1α and GLUT-1 and thereby distributing glucose to the brain. We further demonstrate the importance of glucose transport by endothelial cells in brain functioning and reveal a potential new axis for targeting GLUT-1 deficiency syndromes and other related brain diseases.

Inhibitory Activity of Insulin on Aβ Aggregation Is Restricted Due to Binding Selectivity and Specificity to Polymorphic Aβ States

Clinical trials of intranasal insulin treatment for Alzheimer’s patients have shown cognitive and memory improvement, but the effect of insulin has shown a limitation. It was suggested that insulin molecule binds to Aβ aggregates and impedes Aβ aggregation. Yet, the specific interactions between insulin molecule and Aβ aggregates at atomic resolution are still elusive. Three main conclusions are observed in this work. First, insulin can interact across the fibril only to “U-shape” Aβ fibrils and not to “S-shape” Aβ fibrils. Therefore, insulin is not expected to influence the “S-shape” Aβ fibrils. Second, insulin disrupts β-strands along Aβ fibril-like oligomers via interaction with chain A, which is not a part of the recognition motif. It is suggested that insulin affects as an inhibitor of Aβ fibrillation, but it is limited due to the specificity of the polymorphic Aβ fibril-like oligomer. Third, the current work proposes that insulin promotes Aβ aggregation, when interacting along the fibril axis of Aβ fibril-like oligomer. The coaggregation could be initiated via the recognition motif. The lack of the interactions of insulin in the recognition motif impede the coaggregation of insulin and Aβ. The current work reports the specific binding domains between insulin molecule and polymorphic Aβ fibril-like oligomers. This research provides insights into the molecular mechanisms of the functional activity of insulin on Aβ aggregation that strongly depends on the particular polymorphic Aβ aggregates.

Obesity-related cognitive impairment: The role of endothelial dysfunction

Obesity is a global pandemic associated with macro- and microvascular endothelial dysfunction. Microvascular endothelial dysfunction has recently emerged as a significant risk factor for the development of cognitive impairment. In this review, we present evidence from clinical and preclinical studies supporting a role for obesity in cognitive impairment. Next, we discuss how obesity-related hyperinsulinemia/insulin resistance, systemic inflammation, and gut dysbiosis lead to cognitive impairment through induction of endothelial dysfunction and disruption of the blood brain barrier. Finally, we outline the potential clinical utility of dietary interventions, exercise, and bariatric surgery in circumventing the impacts of obesity on cognitive function.

BACE1 inhibitors: Current status and future directions in treating Alzheimer's disease

Alzheimer's disease (AD) is an irreversible, progressive neurodegenerative brain disorder with no current cure. One of the important therapeutic approaches of AD is the inhibition of β-site APP cleaving enzyme-1 (BACE1), which is involved in the rate-limiting step of the cleavage process of the amyloid precursor protein (APP) leading to the generation of the neurotoxic amyloid β (Aβ) protein after the γ-secretase completes its function. The produced insoluble Aβ aggregates lead to plaques deposition and neurodegeneration. BACE1 is, therefore, one of the attractive targets for the treatment of AD. This approach led to the development of potent BACE1 inhibitors, many of which were advanced to late stages in clinical trials. Nonetheless, the high failure rate of lead drug candidates targeting BACE1 brought to the forefront the need for finding new targets to uncover the mystery behind AD. In this review, we aim to discuss the most promising classes of BACE1 inhibitors with a description and analysis of their pharmacodynamic and pharmacokinetic parameters, with more focus on the lead drug candidates that reached late stages of clinical trials, such as MK8931, AZD-3293, JNJ-54861911, E2609, and CNP520. In addition, the manuscript discusses the safety concerns and insignificant physiological effects, which were highlighted for the most successful BACE1 inhibitors. Furthermore, the review demonstrates with increasing evidence that despite tremendous efforts and promising results conceived with BACE1 inhibitors, the latest studies suggest that their clinical use for treating Alzheimer's disease should be reconsidered. Finally, the review sheds light on alternative therapeutic options for targeting AD.

Prospective of managing impaired brain insulin signalling in late onset Alzheimers disease with excisting diabetic drugs

Late onset Alzheimer's disease (AD) is the most common cause of dementia among elderly. The exact cause of the disease is until now unknown and there is no complete cure for the disease. Growing evidence suggest that AD is a metabolic disorder associated with impairment in brain insulin signalling. These findings enriched the scope for the repurposing of diabetic drugs in AD management. Even though many of these drugs are moving in a positive direction in the ongoing clinical studies, the extent of the success has seen to influence by several properties of these drugs since they were originally designed to manage the peripheral insulin resistance. In depth understandings of these properties is hence highly significant to optimise the use of diabetic drugs in the clinical management of AD; which is the primary aim of the present review article.

Combination of Insulin with a GLP1 Agonist Is Associated with Better Memory and Normal Expression of Insulin Receptor Pathway Genes in a Mouse Model of Alzheimer's Disease

Disruption of brain insulin signaling may explain the higher Alzheimer's disease (AD) risk among type 2 diabetic (T2D) patients. There is evidence from in vitro and human postmortem studies that combination of insulin with hypoglycemic medications is neuroprotective and associated with less amyloid aggregation. We examined the effect of 8-month intranasal administration of insulin, exenatide (a GLP-1 agonist), combination therapy (insulin + exenatide) or saline, in wild-type (WT) and an AD-like mouse model (Tg2576). Mice were assessed for learning, gene expression of key mediators and effectors of the insulin receptor signaling pathway (IRSP-IRS1, AKT1, CTNNB1, INSR, IRS2, GSK3B, IGF1R, AKT3), and brain Amyloid Beta (Aβ) levels. In Tg2576 mice, combination therapy reduced expression of IRSP genes which was accompanied by better learning. Cortical Aβ levels were decreased by 15-30% in all groups compared to saline but this difference did not reach statistical significance. WT mice groups, with or without treatment, did not differ in any comparison. Disentangling the mechanisms underlying the potential beneficial effects of combination therapy on the IR pathway and AD-like behavior is warranted.

Role of simvastatin and insulin in memory protection in a rat model of diabetes mellitus and dementia

OBJECTIVES

The memory protective role of simvastatin and/or insulin, in a rat model of diabetes mellitus (DM) and dementia was examined.

METHODS

DM was induced by an intraperitoneal injection of streptozotocin. Diabetic rats were divided into untreated; insulin treated; simvastatin treated with 10 and 20 mg/kg/day; and combined insulin plus simvastatin treatment in the previous doses. Treatment started after blood glucose elevation and persisted for 6 weeks. Morris water maze and Y maze tests were held to detect behavioral changes. Serum glucose, cholesterol and insulin levels, the hippocampi insulin, amyloid beta (Aß) 1-42 and oxidative stress markers were measured.

RESULTS

Insulin increased the time spent in the target quadrant in the Morris water maze test and the percentage of alternations in the Y maze test, despite the mild improvements in brain parameters demonstrated by amyloid beta 1-42, malondialdehyde and reduced glutathione levels; while simvastatin in both doses improved brain parameters with no positive impact on behavioral tests. Insulin combined with simvastatin 20 mg/kg/day was effective in enhancing the behavioral tests and the measured brain parameters.

CONCLUSIONS

Treatment with insulin and simvastatin could provide a promising memory protective effect in diabetics.

Mammalian Target of Rapamycin at the Crossroad Between Alzheimer's Disease and Diabetes

Accumulating evidence suggests that Alzheimer's disease may manifest as a metabolic disorder with pathology and/or dysfunction in numerous tissues. Adults with Alzheimer's disease suffer with significantly more comorbidities than demographically matched Medicare beneficiaries (Zhao et al, BMC Health Serv Res 8:108, 2008b). Reciprocally, comorbid health conditions increase the risk of developing Alzheimer's disease (Haaksma et al, PLoS One 12(5):e0177044, 2017). Type 2 diabetes mellitus is especially notable as the disease shares many overlapping pathologies observed in patients with Alzheimer's disease, including hyperglycemia, hyperinsulinemia, insulin resistance, glucose intolerance, dyslipidemia, inflammation, and cognitive dysfunction, as described in Chap. 8 of this book (Yoshitake et al, Neurology 45(6):1161-1168, 1995; Leibson et al, Am J Epidemiol 145(4):301-308, 1997; Ott et al, Neurology 53(9):1937-1942, 1999; Voisin et al, Rev Med Interne 24(Suppl 3):288s-291s, 2003; Janson et al. Diabetes 53(2):474-481, 2004; Ristow M, J Mol Med (Berl) 82(8):510-529, 2004; Whitmer et al, BMJ 330(7504):1360, 2005, Curr Alzheimer Res 4(2):103-109, 2007; Ohara et al, Neurology 77(12):1126-1134, 2011). Although nondiabetic older adults also experience age-related cognitive decline, diabetes is uniquely associated with a twofold increased risk of Alzheimer's disease, as described in Chap. 2 of this book (Yoshitake et al, Neurology 45(6):1161-1168, 1995; Leibson et al, Am J Epidemiol 145(4):301-308, 1997; Ott et al. Neurology 53(9):1937-1942, 1999; Ohara et al, Neurology 77(12):1126-1134, 2011). Good glycemic control has been shown to improve cognitive status (Cukierman-et al, Diabetes Care 32(2):221-226, 2009), and the use of insulin sensitizers is correlated with a lower rate of cognitive decline in older adults (Morris JK, Burns JM, Curr Neurol Neurosci Rep 12(5):520-527, 2012). At the molecular level, the mechanistic/mammalian target of rapamycin (mTOR) plays a key role in maintaining energy homeostasis. Nutrient availability and cellular stress information, both extracellular and intracellular, are integrated and transduced through mTOR signaling pathways. Aberrant regulation of mTOR occurs in the brains of patients with Alzheimer's disease and in numerous tissues of individuals with type 2 diabetes (Mannaa et al, J Mol Med (Berl) 91(10):1167-1175, 2013). Moreover, modulating mTOR activity with a pharmacological inhibitor, rapamycin, provides wide-ranging health benefits, including healthy life span extension in numerous model organisms (Vellai et al, Nature 426(6967):620, 2003; Jia et al, Development 131(16):3897-3906, 2004; Kapahi et al, Curr Biol 14(10):885-890, 2004; Kaeberlein et al, Science 310(5751):1193-1196, 2005; Powers et al, Genes Dev 20(2):174-184, 2006; Harrison et al, Nature 460(7253):392-395, 2009; Selman et al, Science 326(5949):140-144, 2009; Sharp ZD, Strong R, J Gerontol A Biol Sci Med Sci 65(6):580-589, 2010), which underscores its importance to overall organismal health and longevity. In this chapter, we discuss the physiological role of mTOR signaling and the consequences of mTOR dysregulation in the brain and peripheral tissues, with emphasis on its relevance to the development of Alzheimer's disease and link to type 2 diabetes.

Antipsychotics, Metabolic Adverse Effects, and Cognitive Function in Schizophrenia

Cognitive impairment is a core symptom domain of schizophrenia. The effect of antipsychotics, the cornerstone of treatment in schizophrenia, on this domain is not fully clear. There is some evidence suggesting that antipsychotics may partially improve cognitive function, and that this improvement may vary depending on the specific cognitive domain. However, this research is confounded by various factors, such as age, duration/stage of illness, medication adherence, and extrapyramidal side effects that complicate the relationship between antipsychotics and cognitive improvement. Furthermore, antipsychotics-particularly the second generation, or "atypical" antipsychotics-can induce serious metabolic side effects, such as obesity, dyslipidemia and type 2 diabetes, illnesses which themselves have been linked to impairments in cognition. Thus, the inter-relationships between cognition and metabolic side effects are complex, and this review aims to examine them in the context of schizophrenia and antipsychotic treatment. The review also speculates on potential mechanisms underlying cognitive functioning and metabolic risk in schizophrenia. We conclude that the available literature examining the inter-section of antipsychotics, cognition, and metabolic effects in schizophrenia is sparse, but suggests a relationship between metabolic comorbidity and worse cognitive function in patients with schizophrenia. Further research is required to determine if there is a causal connection between the well-recognized metabolic adverse effects of antipsychotics and cognitive deficits over the course of the illness of schizophrenia, as well as, to determine underlying mechanisms. In addition, findings from this review highlight the importance of monitoring metabolic disturbances in parallel with cognition, as well as, the importance of interventions to minimize metabolic abnormalities for both physical and cognitive health.

The effects of insulin on the inflammatory activity of BV2 microglia

Microglia are the macrophages of the central nervous system (CNS), which function to monitor and maintain homeostasis. Microglial activation occurs after CNS injury, infection or disease. Prolonged microglial activation is detrimental to the CNS as they produce nitric oxide (NO), reactive oxygen species (ROS) and pro-inflammatory cytokines, resulting in neuronal cell dysfunction and death. Microglial activation is implicated in the neurological deficits following traumatic brain injury (TBI) and Alzheimer's disease. Intranasal insulin administration is a promising treatment of Alzheimer's disease and TBI. However, the exact effect of insulin on microglia is currently unclear. The goal of this study was therefore to examine the effect of insulin administration on activated microglia. The microglial cell line BV2 were exposed to a pro-inflammatory stimulus, lipopolysaccharide (LPS), followed by insulin administration. Outcome measures were conducted at 24 hours after treatment. In vitro assays quantified NO and ROS production. Western blot, immunocytochemistry and phagocytosis assay further examined the effect of insulin on microglial activity. Insulin treatment significantly reduced NO, ROS and TNFα production and increased phagocytic activity. Insulin treatment also significantly reduced iNOS expression, but had no significant effect on any other M1 or M2 macrophage polarization marker examined. These data suggest that insulin has very specific effects to reduce pro-inflammatory or chemoattractant properties of microglia, and this may be one mechanism by which insulin has beneficial effects in CNS injury or neurodegenerative conditions.

New Therapeutics to Modulate Mitochondrial Function in Neurodegenerative Disorders

BACKGROUND

Mitochondrial function and energy metabolism are impaired in neurodegenerative diseases. There is evidence for these functional declines both within the brain and systemically in Alzheimer's disease, Parkinson's disease, and Amyotrophic Lateral Sclerosis. Due to these observations, therapeutics targeted to alter mitochondrial function and energy pathways are increasingly studied in pre-clinical and clinical settings.

METHODS

The goal of this article was to review therapies with specific implications on mitochondrial energy metabolism published through May 2016 that have been tested for treatment of neurodegenerative diseases.

RESULTS

We discuss implications for mitochondrial dysfunction in neurodegenerative diseases and how this drives new therapeutic initiatives.

CONCLUSION

Thus far, treatments have achieved varying degrees of success. Further investigation into the mechanisms driving mitochondrial dysfunction and bioenergetic failure in neurodegenerative diseases is warranted.

Suppressor of Cytokine Signaling 3: Emerging Role Linking Central Insulin Resistance and Alzheimer's Disease

Currently, the etiology of Alzheimer’s disease (AD) is still elusive. Central insulin resistance has been determined to play an important role in the progress of AD. However, the mechanism underlying the development of disrupted insulin signaling pathways in AD is unclear. Suppressor of cytokine signaling 3 (SOCS3) is a member of the SOCS protein family that acts as a negative modulator of insulin signaling in sensitive tissues, such as hepatocytes and adipocytes. However, little is known about its role in neurological diseases. Recent evidence indicates that the level of SOCS3 is increased in the brains of individuals with AD, especially in areas with amyloid beta deposition, suggesting that SOCS3 may regulate the central insulin signaling pathways in AD. Here, we discuss the potential role of SOCS3 in AD and speculate that SOCS3 may be a promising therapeutic target for the treatment of AD.

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.

The serum metabolomics signature of type 2 diabetes is obscured in Alzheimer's disease

There is evidence for systemic metabolic impairment in Alzheimer's disease (AD), and type 2 diabetes (T2D) increases AD risk. Although studies analyzing blood metabolomics signatures have shown differences between cognitively healthy (CH) and AD subjects, these signatures have not been compared with individuals with T2D. We utilized untargeted analysis platforms (primary metabolism and complex lipids) to characterize the serum metabolome of 126 overnight-fasted elderly subjects classified into four groups based upon AD status (CH or AD) and T2D status [nondiabetic (ND) or T2D]. Cognitive diagnosis groups were a priori weighted equally with T2D subjects. We hypothesized that AD subjects would display a metabolic profile similar to cognitively normal elderly individuals with T2D. However, partial least squares-discriminant analysis (PLS-DA) modeling resulted in poor classification across the four groups (<50% classification accuracy of test subjects). Binary classification of AD vs. CH was poor, but binary classification of T2D vs. ND was good, providing >79.5% and >76.9% classification accuracy for held-out samples using primary metabolism and complex lipids, respectively. When modeling was limited to CH subjects, T2D discrimination improved for the primary metabolism platform (>89.5%) and remained accurate for complex lipids (>73% accuracy). Greater abundances of glucose, fatty acids (C20:2), and phosphatidylcholines and lower abundances of glycine, maleimide, octanol, and tryptophan, cholesterol esters, phosphatidylcholines, and sphingomyelins were identified in CH subjects with T2D relative to those without T2D. In contrast, T2D was not accurately discriminated within AD subjects. Results herein suggest that AD may obscure the typical metabolic phenotype of T2D.

Atorvastatin and insulin equally mitigate brain pathology in diabetic rats

Although insulin and atorvastatin have been shown to exert glycemic control and could improve brain function, the effects of atorvastatin or insulin as well as the combination of atorvastatin plus insulin on brain pathology in diabetes mellitus type 1 (T1DM) are unclear. Therefore, this study investigated the effect of atorvastatin, insulin or combined drugs on brain pathology in streptozotocin-induced diabetic rats. Thirty-six male rats were divided into two groups, a control group (n = 12) and a diabetic or experimental group (n = 24). Diabetic rats were further divided into four groups (n = 6/group) and the groups received either a vehicle (normal saline), atorvastatin (10 mg/kg/day), insulin (4 U/day) or a combination of the drugs for 4 weeks. The control group rats were divided into two groups (n = 6/group) to receive either just the vehicle or atorvastatin for 4 weeks. We found that streptozotocin-induced diabetic rats developed hyperglycemia, showing evidence of increased brain oxidative stress, impaired brain mitochondrial function, increased brain apoptosis, increased tau protein expression, increased phosphorylation of tau protein expression and amyloid beta levels, and decreased dendritic spine density. Although atorvastatin and insulin therapies led to an equal reduction in plasma glucose level in these diabetic rats, the combined drug therapy showed the greatest efficacy in decreasing plasma glucose level. Interestingly, atorvastatin, insulin and the combined drugs equally mitigated brain pathology. Our findings indicate that the combined drug therapy showed the greatest efficacy in improving metabolic parameters. However, atorvastatin, insulin and the combined drug therapy shared a similar efficacy in preventing brain damage in T1DM rats.

Erythrocytes as Potential Link between Diabetes and Alzheimer's Disease

Many studies support the existence of an association between type 2 diabetes (T2DM) and Alzheimer's disease (AD). In AD, in addition to brain, a number of peripheral tissues and cells are affected, including red blood cell (RBC) and because there are currently no reliable diagnostic biomarkers of AD in the blood, a gradually increasing attention has been given to the study of RBC's alterations. Recently it has been evidenced in diabetes, RBC alterations superimposable to the ones occurring in AD RBC. Furthermore, growing evidence suggests that oxidative stress plays a pivotal role in the development of RBC's alterations and vice versa. Once again this represents a further evidence of a shared pathway between AD and T2DM. The present review summarizes the two disorders, highlighting the role of RBC in the postulated common biochemical links, and suggests RBC as a possible target for clinical trials.

Brain glucose metabolism: Role of Wnt signaling in the metabolic impairment in Alzheimer's disease

The brain is an organ that has a high demand for glucose. In the brain, glucose is predominantly used in energy production, with almost 70% of the energy used by neurons. The importance of the energy requirement in neurons is clearly demonstrated by the fact that all neurodegenerative disorders exhibit a critical metabolic impairment that includes decreased glucose uptake/utilization and decreased mitochondrial activity, with a consequent diminution in ATP production. In fact, in Alzheimer's disease, the measurement of the general metabolic rate of the brain has been reported to be an accurate tool for diagnosis. Additionally, the administration of metabolic activators such as insulin/glucagon-like peptide 1 can improve memory/learning performance. Despite the importance of energy metabolism in the brain, little is known about the cellular pathways involved in the regulation of this process. Several reports postulate a role for Wnt signaling as a general metabolic regulator. Thus, in the present review, we discuss the antecedents that support the relationship between Wnt signaling and energy metabolism in the Alzheimer's disease.

Intranasal insulin treatment improves memory and learning in a rat amyloid-beta model of Alzheimer's disease

Recently, insulin has been used as a pro-cognitive agent for the potential treatment of Alzheimer's disease (AD), because of its ability to cross the brain-blood barrier (BBB) by a saturable transport system. This study has been designed to evaluate the effects of intranasal insulin regimen, as a bypass system of BBB, on spatial memory in amyloid-beta (Aβ) model of AD in rat. Unilateral infusion of Aβ_25-35 (10 nmol/2 µl/rat) into the lateral ventricular region of brain was used to produce a rat model of AD. After a 24-h recovery period, rats received insulin or vehicle via intraperitoneal or intranasal route (0.1, 0.2, and 0.3 IU) for 14 days. Memory function in rats was assessed by Morris water maze test, with 5 days of training and consequent probe test protocol. Different doses of intraperitoneal insulin did not have a significant effect on learning and memory in AD rats. However, intranasal insulin at doses of 0.2 and 0.3 IU improved the learning and memory in Aβ-received rats. In conclusion, intranasal insulin as a non-invasive strategy improves spatial learning and memory in AD model.

Perspectives for treating Alzheimer's disease: a review on promising pharmacological substances

CONTEXT AND OBJECTIVE

Dementia is a syndrome characterized by functional and cognitive decline. Alzheimer's disease (AD) is one of the most common causes of dementia and has high prevalence among the elderly. It is known that there is no drug capable of interfering with the course of the disease. Research on treatments for AD has been marked by the appearance of new drugs and their abandonment. This study aimed to describe drugs that have been studied with regard to treating AD and which are capable of influencing the course of the disease.

DESIGN AND SETTING

Narrative review on original articles published worldwide.

METHODS

A systematized search was conducted in the PubMed/MEDLINE, Cochrane Library/Cochrane and SciELO/Bireme databases. The descriptors "Molecular Mechanisms of Pharmacological Action" and "Drug Therapy" were each combined with the descriptor "Alzheimer disease". All of these can be found in MeSH and DeCS. These descriptors were used alone or in combination, and a filter specifying publication between January 2009 and October 2015 in English, Spanish or Portuguese was set.

RESULTS

6,888 articles were found, of which 37 were included in this review; 70.3% of the articles selected were of good quality with low or unclear risk of bias. 86 drugs were considered promising for AD treatment and these were classified into 20 pharmacological categories.

CONCLUSION

There are no drugs capable of influencing the course of AD such that treatments are safe and effective. However, immunomodulators stood out as promising, given their effectiveness and quality in the articles analyzed.

The impact of dementia on antidiabetic drug use in Medicare beneficiaries with diabetes: findings post-Medicare part D

AIM

The objectives of the study were to measure the utilization rates of antidiabetic drugs in diabetic patients with dementia and to assess the impact of dementia on antidiabetic drug use.

MATERIALS & METHODS

This study was a pooled cross-sectional study of the Medicare Current Beneficiaries Survey from 2006 to 2010.

RESULTS & CONCLUSION

Lower utilizations of biguanides, DPP-4 inhibitors and thiazolidinediones were observed in dementia patients. The likelihood of using antidiabetic drugs was about 30% (odds ratio: 0.69; 95% CI: 0.56-0.84) lower in dementia patients and 40% (odds ratio: 0.61; 95% CI: 0.44-0.84) lower in patients with Alzheimer's disease. Healthcare providers should be aware of underuse of antidiabetic drugs in patients with dementia.

Recent Developments in Understanding Brain Aging: Implications for Alzheimer's Disease and Vascular Cognitive Impairment

As the population of the Western world is aging, there is increasing awareness of age-related impairments in cognitive function and a rising interest in finding novel approaches to preserve cerebral health. A special collection of articles in The Journals of Gerontology: Biological Sciences and Medical Sciences brings together information of different aspects of brain aging, from latest developments in the field of neurodegenerative disorders to cerebral microvascular mechanisms of cognitive decline. It is emphasized that although the cellular changes that occur within aging neurons have been widely studied, more research is required as new signaling pathways are discovered that can potentially protect cells. New avenues for research targeting cellular senescence, epigenetics, and endocrine mechanisms of brain aging are also discussed. Based on the current literature it is clear that understanding brain aging and reducing risk for neurological disease with age requires searching for mechanisms and treatment options beyond the age-related changes in neuronal function. Thus, comprehensive approaches need to be developed that address the multiple, interrelated mechanisms of brain aging. Attention is brought to the importance of maintenance of cerebromicrovascular health, restoring neuroendocrine balance, and the pressing need for funding more innovative research into the interactions of neuronal, neuroendocrine, inflammatory and microvascular mechanisms of cognitive impairment, and Alzheimer's disease.

Cognitively impaired elderly exhibit insulin resistance and no memory improvement with infused insulin

Insulin resistance is a risk factor for Alzheimer's disease (AD), although its role in AD etiology is unclear. We assessed insulin resistance using fasting and insulin-stimulated measures in 51 elderly subjects with no dementia (ND; n = 37) and with cognitive impairment (CI; n = 14). CI subjects exhibited either mild CI or AD. Fasting insulin resistance was measured using the homeostatic model assessment of insulin resistance (HOMA-IR). Insulin-stimulated glucose disposal was assessed using the hyperinsulinemic-euglycemic clamp to calculate glucose disposal rate into lean mass, the primary site of insulin-stimulated glucose disposal. Because insulin crosses the blood-brain barrier, we also assessed whether insulin infusion would improve verbal episodic memory compared to baseline. Different but equivalent versions of cognitive tests were administered in counterbalanced order in the basal and insulin-stimulated state. Groups did not differ in age or body mass index. Cognitively impaired subjects exhibited greater insulin resistance as measured at fasting (HOMA-IR; ND: 1.09 [1.1] vs. CI: 2.01 [2.3], p = 0.028) and during the hyperinsulinemic clamp (glucose disposal rate into lean mass; ND: 9.9 (4.5) vs. AD 7.2 (3.2), p = 0.040). Cognitively impaired subjects also exhibited higher fasting insulin compared to ND subjects, (CI: 8.7 [7.8] vs. ND: 4.2 [3.8] μU/mL; p = 0.023) and higher fasting amylin (CI: 24.1 [39.1] vs. 8.37 [14.2]; p = 0.050) with no difference in fasting glucose. Insulin infusion elicited a detrimental effect on one test of verbal episodic memory (Free and Cued Selective Reminding Test) in both groups (p < 0.0001) and no change in performance on an additional task (delayed logical memory). In this study, although insulin resistance was observed in cognitively impaired subjects compared to ND controls, insulin infusion did not improve memory. Furthermore, a significant correlation between HOMA-IR and glucose disposal rate was present only in ND (p = 0.0002) but not in cognitively impaired (p = 0.884) subjects, indicating potentially important physiological differences between these cohorts.

Activation of mTOR: a culprit of Alzheimer's disease?

Alzheimer's disease (AD) is characterized by cognitive impairment in clinical presentation, and by β-amyloid (Aβ) production and the hyper-phosphorylation of tau in basic research. More highlights demonstrate that the activation of the mammalian target of rapamycin (mTOR) enhances Aβ generation and deposition by modulating amyloid precursor protein (APP) metabolism and upregulating β- and γ-secretases. mTOR, an inhibitor of autophagy, decreases Aβ clearance by scissoring autophagy function. mTOR regulates Aβ generation or Aβ clearance by regulating several key signaling pathways, including phosphoinositide 3-kinase (PI3-K)/protein kinase B (Akt), glycogen synthase kinase 3 [GSK-3], AMP-activated protein kinase (AMPK), and insulin/insulin-like growth factor 1 (IGF-1). The activation of mTOR is also a contributor to aberrant hyperphosphorylated tau. Rapamycin, the inhibitor of mTOR, may mitigate cognitive impairment and inhibit the pathologies associated with amyloid plaques and neurofibrillary tangles by promoting autophagy. Furthermore, the upstream and downstream components of mTOR signaling are involved in the pathogenesis and progression of AD. Hence, inhibiting the activation of mTOR may be an important therapeutic target for AD.

Association between insulin and executive functioning in alcohol dependence: a pilot study

Alcohol dependence is a disorder ascribable to multiple factors and leads to cognitive impairment. Given that insulin dysregulation can cause cognitive impairment, patients with alcohol dependence are likely to develop insulin dysregulation such as that in diabetes. The purposes of this study are to identify an association between cognitive functioning and insulin and to investigate insulin as the biomarker of cognitive functioning in alcohol-dependent patients. Serum insulin levels were measured and cognitive functions were assessed in 45 patients with chronic alcoholism. The Korean version of the Consortium to Establish a Registry for Alzheimer's Disease (CERAD-K), a battery of cognitive function tests, was used to assess cognitive functioning. Serum insulin levels were not significantly correlated with most CERAD-K scores, but there was a significant negative correlation with scores on the Trail Making Test B, which is designed to measure executive functioning. Lower serum insulin levels were associated with slower executive functioning responses on the Trail Making Test B, suggesting that executive functioning may be in proportion to serum insulin levels. Thus, in patients with alcohol dependence, insulin level is associated with cognitive functioning. In addition, the present findings suggest that insulin level is a potential biomarker for determining cognitive functioning.

The synergistic effects of HIV, diabetes, and aging on cognition: implications for practice and research

Thanks to highly active antiretroviral therapy, many people infected with HIV will likely live into old age. Although this is a welcome prognosis, new issues are emerging that may complicate the ability to successfully age in this clinical population. HIV and aging independently are related to cognitive impairments, so there are concerns that those aging with HIV may be more at risk of such cognitive impairments. Moreover, highly active antiretroviral therapy itself can create metabolic disorders, such as prediabetes and/or frank type 2 diabetes, which have also been linked to poorer cognitive functioning. Thus, concerns increase that, as people age with HIV and develop comorbid metabolic disorders that may lead to type 2 diabetes, they will be at triple risk of developing cognitive impairments that can impair everyday functioning and reduce quality of life. This article explores these issues and provides implications for practice and research.

Early adult to midlife cardiovascular risk factors and cognitive function

BACKGROUND

Studies have linked midlife and late-life cardiovascular risk factors (CVRFs) to cognitive function, yet little is known about CVRF exposure in early adulthood and subsequent cognitive function. In addition, most studies rely on single assessments of CVRFs, which may not accurately reflect long-term exposure. We sought to determine the association between cumulative exposure to CVRFs from early to middle adulthood and cognitive function at midlife.

METHODS AND RESULTS

In a prospective study of 3381 adults (age, 18-30 years at baseline) with 25 years of follow-up, we assessed cognitive function at year 25 (2010-2011) with the Digit Symbol Substitution Test, Stroop Test, and Rey Auditory Verbal Learning Test analyzed with standardized z scores. The primary predictor was 25-year cumulative exposure estimated by areas under the curve for resting systolic and diastolic blood pressures, fasting blood glucose, and total cholesterol. Higher cumulative systolic and diastolic blood pressures and fasting blood glucose were consistently associated with worse cognition on all 3 tests. These associations were significant primarily for exposures above recommended guidelines; cognitive test z scores were between 0.06 and 0.30 points less, on average, for each 1-SD increase in risk factor area under the curve after adjustment for age, race, sex, and education (P<0.05 for all). Fewer significant associations were observed for cholesterol.

CONCLUSIONS

Cumulative exposure to CVRFs from early to middle adulthood, especially above recommended guidelines, was associated with worse cognition in midlife. The meaning of this association and whether it warrants more aggressive treatment of CVRFs earlier in life require further investigation.