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

Association of late-life variability in hemoglobin A1C with postmortem neuropathologies

INTRODUCTION

To study the relationship of late-life hemoglobin A1C (A1C) with postmortem neuropathology in older adults with and without diabetes mellitus (DM).

METHODS

A total of 990 participants from five cohort studies of aging and dementia with at least two annually-collected A1C measures, who had autopsy. Neuropathologic evaluations documented cerebrovascular disease, Alzheimer's disease (AD), and other pathologies. To evaluate the association of A1C mean and variability (standard deviation [SD]) with neuropathology, we used a series of adjusted regression models.

RESULTS

Participants (mean age at death = 90.8 years; education = 15.8 years; 76% women) had six A1C measurements on average. Mean A1C was associated with greater odds of macroinfarcts (estimate = 0.14; p = 0.04) and subcortical infarcts (estimate = 0.16; p = 0.02). A1C variability was not associated with cerebrovascular pathology. A1C mean and variability were inversely associated with AD pathology.

DISCUSSION

The A1C average over time was associated with infarcts, and the A1C average and variability were inversely associated with AD pathology. Future studies should explore the underlying mechanisms linking A1C to dementia-related neuropathologies.

HIGHLIGHTS

Hemoglobin A1C (A1C), a measure of peripheral insulin resistance, is used to assess glycemic control. Higher A1C mean was associated with greater odds of macroscopic subcortical infarcts. A1C variability was not associated with cerebrovascular pathology. Both A1C mean and variability had inverse associations with AD pathology. None of the associations varied by diabetes mellitus status.

Exploring the global impact of obesity and diet on dementia burden: the role of national policies and sex differences

Obesity is a significant modifiable risk factor for dementia. This study aims to quantify the global impact of obesity on dementia burden and examine how national strategies for managing overweight/obesity and dietary factors influence dementia prevalence and mortality, with a focus on sex-specific differences. We used data from the Global Burden of Disease (GBD) and World Health Organization (WHO) to evaluate the association between obesity age-standardized prevalence rate (ASPR) and dementia age-standardized mortality rate (ASMR) and ASPR across 161 countries. A two-step multivariate analysis adjusted for socioeconomic and lifestyle factors was performed. Temporal trends in dementia were analyzed based on the presence of national obesity management strategies and varying dietary scores. A 1% increase in national obesity prevalence was associated with a 0.36% increase in dementia mortality (OR: 1.0036; 95% CI: 1.0028-1.0045) in males and 0.12% in females (OR: 1.0012; 95% CI: 1.0007-1.0018). A 1% increase in national obesity ASPR was associated with an increase in ASPR of dementia by 0.26% for males (OR: 1.0026, 95% CI: 1.0024-1.0028) and 0.05% for females (OR: 1.0005, 95% CI: 1.0004-1.0006). Males exhibited a higher susceptibility to obesity-related dementia. Countries with national obesity management strategies showed a significantly greater reduction in dementia mortality, particularly among females (P = 0.025). Higher dietary scores were associated with a more significant decrease in dementia prevalence across both sexes. Rising obesity prevalence is linked to increased dementia burden globally, with males being more vulnerable to this relationship. National management of overweight/obesity and healthier dietary habits may help mitigate the dementia burden, emphasizing the need for integrated public health interventions.

Insulin resistance, cognition, and functional brain network topology in older adults with obesity

Objective

Cross-sectional data from a sample of older adults with obesity was used to determine how peripheral and neuronal insulin resistance (IR) relate to executive function and functional brain network topology.

Methods

Older adults (n=71) with obesity but without type 2 diabetes were included. Peripheral IR was quantified by HOMA2-IR. Neuronal IR was quantified according to a proposed neuron-derived exosome-based method (NDE-IR). An executive function composite score, summed scores to the Auditory Verbal Learning Test (AVLT) trials 1-5, and functional brain networks generated from resting-state functional magnetic resonance imaging were outcomes in analyses. We used general linear models and a novel regression framework for brain network analysis to identify relationships between IR measures and brain-related outcomes.

Results

HOMA2-IR, but not NDE-IR, was negatively associated with executive function. Neither IR measure was associated with AVLT score. Peripheral IR was also related to hippocampal network topology in participants who had undergone functional neuroimaging. Neither peripheral nor neuronal IR were significantly related to network topology of the central executive network.

Conclusions

Cognitive and functional imaging effects were observed from HOMA2-IR, but not NDE-IR. The hippocampus may be particularly vulnerable to effects of peripheral IR.

Review of Associations of Diabetes and Insulin Resistance With Brain Health in Three Harmonised Cohort Studies of Ageing and Dementia

Diabetes increases the risk of dementia, and insulin resistance (IR) has emerged as a potential unifying feature. Here, we review published findings over the past 2 decades on the relation of diabetes and IR to brain health, including those related to cognition and neuropathology, in the Religious Orders Study, the Rush Memory and Aging Project, and the Minority Aging Research Study (ROS/MAP/MARS), three harmonised cohort studies of ageing and dementia at the Rush Alzheimer's Disease Center (RADC). A wide range of participant data, including information on medical conditions such as diabetes and neuropsychological tests, as well as other clinical and laboratory-based data collected annually. Neuropathology data are collected in participants who agree to autopsy at death. Recent studies have measured additional peripheral and brain IR data, including multi-omics. This review summarises findings from the RADC cohort studies that investigate the relation of diabetes and IR in older adults to cognition, neuropathology, omics in dementia, and other brain health measures. Examining the risk of clinically diagnosed dementia in older adults, our study found a 65% increased risk of Alzheimer's disease (AD) dementia in individuals with diabetes compared with those without. Regarding cognitive function, we have consistently observed associations of diabetes, as well as both peripheral and brain IR, with worse and declining performance in global cognition and specific cognitive domains, particularly semantic memory and perceptual speed. Studies utilising neuropathological data showed associations of diabetes and peripheral IR with brain infarcts, while brain IR measures, notably alpha serine/threonine-protein kinase1 (AKT1), were associated with both brain infarcts and AD pathology. Multi-omics studies suggested shared causal genes and pathways between diabetes and dementia. Recent epigenetic studies have revealed associations between IR and AD risk, along with distinct 5-hydroxymethylcytosine signatures in diabetes-associated AD. Furthermore, our studies have utilised other available data to investigate the impact of diabetes on neurological outcomes other than cognition and reported worsening of parkinsonian-like signs in diabetes. Recent studies have also explored risk factors for diabetes and have reported associations between lower literacy and decision-making abilities with elevated haemoglobin A1C levels, a peripheral IR measure. Overall, our findings, as summarised in this review, illustrate a range of mechanistic and other insights into the complex relationship of diabetes and IR with brain health. These findings may have important implications for future research on the ageing brain, including the prevention of cognitive decline and dementia in persons at risk for or with diabetes.

Role of Peripheral and Central Insulin Resistance in Neuropsychiatric Disorders

Insulin acts on different organs, including the brain, which helps it regulate energy metabolism. Insulin signaling plays an important role in the function of different cell types. In this review, we have summarized the key roles of insulin and insulin receptors in healthy brains and in different brain disorders. Insulin signaling, as well as insulin resistance (IR), is a major contributor in the regulation of mood, behavior, and cognition. Recent evidence showed that both peripheral and central insulin resistance play a role in the pathophysiology, clinical presentation, and management of neuropsychiatric disorders like Cognitive Impairment/Dementia, Depression, and Schizophrenia. Many human studies point out Insulin Resistance/Metabolic Syndrome can increase the risk of dementia especially Alzheimer's dementia (AD). IR has been shown to play a role in AD development but also in its progression. This review article discusses the pathophysiological pathways and mechanisms of insulin resistance in major neuropsychiatric disorders. The extent of insulin resistance can be quantified using IR biomarkers like insulin levels, HOMA-IR index, and Triglyceride glucose-body mass index (TyG-BMI) levels. IR has been shown to precede neurodegeneration. Human trials showed current treatment with certain antidiabetic drugs, as well as life style management, like weight loss and exercise for IR, have shown promise in the management of cognitive/neuropsychiatric disorders. This may pave the pathway to the development of new therapeutic approaches to these challenging disorders of dementia and psychiatric diseases. Recent clinical trials are showing some encouraging evidence for these pharmacological and nonpharmacological approaches for IR in psychiatric and cognitive disorders, even though more research is needed to apply this evidence into clinical practice. Early identification and management of IR may help as a strategy to potentially alter neuropsychiatric disorders onset as well as its progression.

From Plaques to Pathways in Alzheimer's Disease: The Mitochondrial-Neurovascular-Metabolic Hypothesis

Alzheimer's disease (AD) presents a public health challenge due to its progressive neurodegeneration, cognitive decline, and memory loss. The amyloid cascade hypothesis, which postulates that the accumulation of amyloid-beta (Aβ) peptides initiates a cascade leading to AD, has dominated research and therapeutic strategies. The failure of recent Aβ-targeted therapies to yield conclusive benefits necessitates further exploration of AD pathology. This review proposes the Mitochondrial-Neurovascular-Metabolic (MNM) hypothesis, which integrates mitochondrial dysfunction, impaired neurovascular regulation, and systemic metabolic disturbances as interrelated contributors to AD pathogenesis. Mitochondrial dysfunction, a hallmark of AD, leads to oxidative stress and bioenergetic failure. Concurrently, the breakdown of the blood-brain barrier (BBB) and impaired cerebral blood flow, which characterize neurovascular dysregulation, accelerate neurodegeneration. Metabolic disturbances such as glucose hypometabolism and insulin resistance further impair neuronal function and survival. This hypothesis highlights the interconnectedness of these pathways and suggests that therapeutic strategies targeting mitochondrial health, neurovascular integrity, and metabolic regulation may offer more effective interventions. The MNM hypothesis addresses these multifaceted aspects of AD, providing a comprehensive framework for understanding disease progression and developing novel therapeutic approaches. This approach paves the way for developing innovative therapeutic strategies that could significantly improve outcomes for millions affected worldwide.

Selecting a Brief Cognitive Screening Test Based on Patient Profile: It Is Never Too Early to Start

Introduction: Cognitive impairment, marked by a decline in memory and attention, is frequently underdiagnosed, complicating effective management. Cardiovascular risk factors (CVR) and anticholinergic burden (ACB) are significant contributors to dementia risk, with ACB often stemming from medications prescribed for neuropsychiatric disorders. This study evaluates cognitive profiles through three brief cognitive tests, analyzing the impact of CVR and ACB presence. Methods: This cross-sectional study was performed between 2019 and 2023 in community pharmacies and an outpatient clinic in Valencia, Spain. Eligible participants were patients with subjective memory complaints 50 years or older with clinical records of cardiovascular factors. Patients with conflicting information regarding diabetes diagnosis or not taking concomitant medications were excluded. Three brief cognitive tests (Memory Impairment Screening (MIS), Semantic Verbal Fluency Test, and SPMSQ) were assessed. CVR was calculated using the European SCORE2 table, and ACB was assessed using the CALS scale. Results: Among 172 patients with memory complaints and CVR factors, 60% failed at least one cognitive test. These patients were on significantly more medications and had higher blood pressure and HbA1c levels. An increase in CVR and ACB was associated with more failed tests. Additionally, elevated SCORE2 scores were associated with a greater failure rate on the MIS test, while patients with elevated ACB more frequently failed the SPMSQ test. Conclusions: Selecting an adequate brief cognitive test according to patients' characteristics offers an opportunity to screen patients who are probably cognitively impaired. Whereas the MIS test may be helpful for patients with cardiovascular risk, SPMSQ stands out among patients with significant ACB.

Brain Insulin Signaling is Associated with Late-Life Cognitive Decline

Type-2 diabetes is associated with an increased risk of dementia, and the underlying mechanism might involve abnormal insulin signaling in the brain. The objective of this study was to examine the association of postmortem brain insulin signaling with late-life cognitive decline. Among participants of Religious Orders Study, a community-based clinical-pathological cohort, 150 deceased and autopsied older individuals (75 with diabetes matched to 75 without by age at death, sex, and education) had postmortem brain insulin signaling measurements collected in the prefrontal cortex using ELISA and immunohistochemistry. By using adjusted linear mixed-effects models, we examined the association of postmortem brain insulin signaling with late-life cognitive function assessed longitudinally (mean follow-up duration = 9.4 years) using a battery of neuropsychological tests. We found that a higher level of serine/threonine-protein kinase (AKT) phosphorylation (pT308AKT1/total AKT1) was associated with a faster decline in global cognition (estimate = -0.023, p = 0.030), and three domains: episodic memory (estimate = -0.024, p = 0.032), working memory (estimate = -0.018, p = 0.012), and visuospatial abilities (estimate = -0.013, p = 0.027). The level of insulin receptor substrate-1 (IRS1) phosphorylation (pS307IRS1/total IRS1) was not associated with decline in global cognition or most cognitive domains, except for perceptual speed (estimate = 0.020, p = 0.020). The density of pS616IRS1-stained cells was not associated with decline in global cognition or any of the domains. In conclusion, these findings provide novel evidence for an association between brain insulin signaling and late-life cognitive decline. AKT phosphorylation is associated with a decline in global cognition and memory in particular, whereas IRS1 phosphorylation is associated with a decline in perceptual speed.

Bulk-RNA and single-nuclei RNA seq analyses reveal the role of lactate metabolism-related genes in Alzheimer's disease

Dysfunctional lactate metabolism in the brain has been implicated in neuroinflammation, Aβ deposition, and cell disturbance, all of which play a significant role in the pathogenesis of Alzheimer's disease (AD). In this study, we aimed to investigate the lactate metabolism-related genes (LMRGs) in AD via an integrated bulk RNA and single-nuclei RNA sequencing (snRNA-seq) analysis, with a specific focus on microglia. We obtained 26 HC and 24 AD snRNA-seq samples originated from human prefrontal cortex in Gene Expression Omnibus (GEO) database and collected 873 LMRGs from three databases, namely MSigDB, The Human Protein Atlas and GeneCards. Bulk RNA was analyzed with LMRG characteristics in AD by using Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), the protein-protein interaction (PPI), CytoHubba-MCC, Support Vector Machine (SVM) algorithms analyses. Then we conducted the Receiver Operating Characteristic (ROC) curve, correlation, and connection network analyses for biomarkers. Their differential expression validation was performed using AlzData database. The single-nuclei RNA analysis of microglia was applied to identify hub genes and pathways using cell-cell communication analysis and high dimensional Weighted Gene Co-Expression Network Analysis (hdWGCNA). Support Vector Machine (SVM) algorithm showed an AUC of 0.967, a sensitivity of 93.30% and a specificity of 100.00%. Our analysis identified biomarkers with LMRG characteristics, namely INSR, CDKL1, and PNISR. ROC analysis revealed that each of these biomarkers exhibited excellent diagnostic potential, as evidenced by their respective area under the curve (AUC) values: INSR (AUC: 0.679), CDKL1 (AUC: 0.788), and PNISR (AUC: 0.724). Correlation analysis showed that biomarkers exhibited a positive correlation with each other. Connection network illustrated their shared biological processes: aging, phosphorylation, metabolic process, and apoptosis. Cell-cell communication analysis revealed that GALECTIN signaling pathway was exclusively expressed in AD microglia, and only LGALS9 exhibited significant overexpression. HdWGCNA identified FTH1 as a hub gene enriched in ferroptosis and mineral absorption pathways within microglia. The roles of INSR, CDKL1, PNISR, LGALS9, and FTH1 should be taken into account to enhance our understanding of lactate metabolism in the context of AD.

Loss of Cldn5 -and increase in Irf7-in the hippocampus and cerebral cortex of diabetic mice at the early symptomatic stage

Analyzing changes in gene expression within specific brain regions of individuals with Type 2 Diabetes (T2DM) who do not exhibit significant cognitive deficits can yield valuable insights into the mechanisms underlying the progression towards a more severe phenotype. In this study, transcriptomic analysis of the cortex and hippocampus of mice with long-term T2DM revealed alterations in the expression of 28 genes in the cerebral cortex and 15 genes in the hippocampus. Among these genes, six displayed consistent changes in both the cortex and hippocampus: Interferon regulatory factor 7 (Irf7), Hypoxia-inducible factor 3 alpha (Hif-3α), period circadian clock 2 (Per2), xanthine dehydrogenase (Xdh), and Transforming growth factor β-stimulated clone 22/TSC22 (Tsc22d3) were upregulated, while Claudin-5 (Cldn5) was downregulated. Confirmation of these changes was achieved through RT-qPCR. At the protein level, CLDN5 and IRF7 exhibited similar alterations, with CLDN5 being downregulated and IRF7 being upregulated. In addition, the hippocampus and cortex of the T2DM mice showed decreased levels of IκBα, implying the involvement of NF-κB pathways as well. Taken together, these results suggest that the weakening of the blood-brain barrier and an abnormal inflammatory response via the Interferon 1 and NF-κB pathways underlie cognitive impairment in individuals with long-standing T2DM.

Brain responses to intermittent fasting and the healthy living diet in older adults

Diet may promote brain health in metabolically impaired older individuals. In an 8-week randomized clinical trial involving 40 cognitively intact older adults with insulin resistance, we examined the effects of 5:2 intermittent fasting and the healthy living diet on brain health. Although intermittent fasting induced greater weight loss, the two diets had comparable effects in improving insulin signaling biomarkers in neuron-derived extracellular vesicles, decreasing the brain-age-gap estimate (reflecting the pace of biological aging of the brain) on magnetic resonance imaging, reducing brain glucose on magnetic resonance spectroscopy, and improving blood biomarkers of carbohydrate and lipid metabolism, with minimal changes in cerebrospinal fluid biomarkers for Alzheimer's disease. Intermittent fasting and healthy living improved executive function and memory, with intermittent fasting benefiting more certain cognitive measures. In exploratory analyses, sex, body mass index, and apolipoprotein E and SLC16A7 genotypes modulated diet effects. The study provides a blueprint for assessing brain effects of dietary interventions and motivates further research on intermittent fasting and continuous diets for brain health optimization. For further information, please see ClinicalTrials.gov registration: NCT02460783.

Lower GLUT1 and unchanged MCT1 in Alzheimer's disease cerebrovasculature

The brain is a highly demanding organ, utilizing mainly glucose but also ketone bodies as sources of energy. Glucose transporter-1 (GLUT1) and monocarboxylates transporter-1 (MCT1) respectively transport glucose and ketone bodies across the blood-brain barrier. While reduced glucose uptake by the brain is one of the earliest signs of Alzheimer's disease (AD), no change in the uptake of ketone bodies has been evidenced yet. To probe for changes in GLUT1 and MCT1, we performed Western immunoblotting in microvessel extracts from the parietal cortex of 60 participants of the Religious Orders Study. Participants clinically diagnosed with AD had lower cerebrovascular levels of GLUT1, whereas MCT1 remained unchanged. GLUT1 reduction was associated with lower cognitive scores. No such association was found for MCT1. GLUT1 was inversely correlated with neuritic plaques and cerebrovascular β-secretase-derived fragment levels. No other significant associations were found between both transporters, markers of Aβ and tau pathologies, sex, age at death or apolipoprotein-ε4 genotype. These results suggest that, while a deficit of GLUT1 may underlie the reduced transport of glucose to the brain in AD, no such impairment occurs for MCT1. This study thus supports the exploration of ketone bodies as an alternative energy source for the aging brain.

Associations of renin-angiotensin system inhibitor use with brain insulin signaling and neuropathology

OBJECTIVE

To examine the associations of renin-angiotensin system (RAS) inhibitor use with postmortem brain insulin signaling and neuropathology.

METHODS

Among Religious Orders Study participants, 150 deceased and autopsied older individuals (75 with diabetes matched to 75 without by age at death, sex, and education) had measurements of insulin receptor substrate-1 (IRS-1) and RAC-alpha serine/threonine protein kinase (AKT1) collected in the prefrontal cortex using ELISA and immunohistochemistry. Alzheimer's disease (AD), brain infarcts, and cerebral vessel pathology data were assessed by systematic neuropathologic evaluations. RAS inhibitor use was determined based on visual inspection of medication containers during study visits. The associations of RAS inhibitor use with brain insulin signaling measures and neuropathology were examined using adjusted regression analyses.

RESULTS

Of the 90 RAS inhibitor users (54 with diabetes), 65 had used only angiotensin-converting enzyme inhibitors, 11 only angiotensin II receptor blockers, and 14 used both. RAS inhibitor use was associated with lower pT308AKT1/total AKT1, but not with pS307IRS-1/total IRS-1 or the density of cells stained positive for pS616 IRS-1. RAS inhibitor use was not associated with the level of global AD pathology or amyloid beta burden, but it was associated with a lower tau-neurofibrillary tangle density. Additionally, we found a significant interaction between diabetes and RAS inhibitors on tangle density. Furthermore, AKT1 phosphorylation partially mediated the association of RAS inhibitor use with tau tangle density. Lastly, RAS inhibitor use was associated with more atherosclerosis, but not with other cerebral blood vessel pathologies or cerebral infarcts.

INTERPRETATION

Late-life RAS inhibitor use may be associated with lower brain AKT1 phosphorylation and fewer neurofibrillary tangles.

Vascular Contributions to Healthy Aging and Dementia

Vascular pathologies are among the most common contributors to neurodegenerative changes across the spectrum of normal aging to dementia. Cerebral small vessel disease (SVD) encompasses a wide range of conditions affecting capillaries, small arteries, and arterioles, as well as perivascular spaces and fluid dynamics in the brain, playing a significant role in vascular contributions to cognitive impairment and dementia (VCID). These factors can accelerate the progression of SVD and neuronal degeneration. Since aging is the primary risk factor for Alzheimer's disease (AD) and AD-related dementias (ADRD), this Research Topic aims to gather recent research to better understand vascular contributions to healthy aging and age-related cognitive impairment. Other risk factors include diabetes, lifestyle factors, high cholesterol, vascular inflammation, and immune remodeling, all of which can accelerate cognitive dysfunction progression. This special issue includes a total of 21 articles comprising Reviews, Perspectives, and Original Research articles. The articles cover various technical and biological aspects related to recent progress in aging and dementia research. We aim to promote research exchange across different fields, including imaging, VCID, molecular biology, neuroinflammation, and immunology. Most papers in this special issue focus on understanding the disease mechanisms of AD/ADRD and developing new therapeutic strategies.

Comparing regional brain uptake of incretin receptor agonists after intranasal delivery in CD-1 mice and the APP/PS1 mouse model of Alzheimer's disease

Targeting brain insulin resistance (BIR) has become an attractive alternative to traditional therapeutic treatments for Alzheimer's disease (AD). Incretin receptor agonists (IRAs), targeting either or both of the glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptors, have proven to reverse BIR and improve cognition in mouse models of AD. We previously showed that many, but not all, IRAs can cross the blood-brain barrier (BBB) after intravenous (IV) delivery. Here we determined if widespread brain uptake of IRAs could be achieved by circumventing the BBB using intranasal (IN) delivery, which has the added advantage of minimizing adverse gastrointestinal effects of systemically delivered IRAs. Of the 5 radiolabeled IRAs tested (exenatide, dulaglutide, semaglutide, DA4-JC, and DA5-CH) in CD-1 mice, exenatide, dulaglutide, and DA4-JC were successfully distributed throughout the brain following IN delivery. We observed significant sex differences in uptake for DA4-JC. Dulaglutide and DA4-JC exhibited high uptake by the hippocampus and multiple neocortical areas. We further tested and found the presence of AD-associated Aβ pathology minimally affected uptake of dulaglutide and DA4-JC. Of the 5 tested IRAs, dulaglutide and DA4-JC are best capable of accessing brain regions most vulnerable in AD (neocortex and hippocampus) after IN administration. Future studies will need to be performed to determine if IN IRA delivery can reduce BIR in AD or animal models of that disorder.

Cannula Implantation Reduces the Severity of the Beta Amyloid Effect on Peroxidized Lipids and Glutathione Levels in the Brain of BALB/c Mice

Sporadic Alzheimer's disease (sAD) is the most common of neurodegenerative disorders. The lack of effective therapy indicates that the mechanisms of sAD development remain poorly understood. To investigate this pathology in animals, intracerebroventricular injection of β-amyloid peptide (Aβ) using a Hamilton syringe, either during stereotactic surgery or through a pre-implanted cannula, is used. In this study, we analyzed the effect of chronic cannula implantation on the severity of Aβ effects at the behavioral, histological, and biochemical levels. The results showed that the local damage to neural tissue caused by cannulation has no bearing on the effect of Aβ on animal behavior and the microglial parameters of the unilateral hippocampus two weeks after the Aβ administration. However, cannula implantation fundamentally modifies some biochemical markers of the oxidative stress that occurs in the brain tissue in response to Aβ administration. Thus, the presence of a cannula reduces the severity of the Aβ impact on the levels of peroxidized lipids and glutathione two- and 10-fold, respectively. It is important to note that the detected changes are chronic and systemic. This is known because the homogenate of the entire contralateral (in relation to the cannula implantation site) hemisphere was analyzed, and the analysis was performed two weeks after implantation. At the same time, cannulation does not affect the rate of reactive oxygen species production. The obtained data indicate that chronic implantation of a cannula into the brain of experimental animals fundamentally distorts some parameters of oxidative stress in the neural tissue, which are widely used to assess the severity of experimental Alzheimer's-type diseases.

Mind the Metabolic Gap: Bridging Migraine and Alzheimer's disease through Brain Insulin Resistance

Brain insulin resistance has recently been described as a metabolic abnormality of brain glucose homeostasis that has been proven to downregulate insulin receptors, both in astrocytes and neurons, triggering a reduction in glucose uptake and glycogen synthesis. This condition may generate a mismatch between brain's energy reserve and expenditure, ??mainly during high metabolic demand, which could be involved in the chronification of migraine and, in the long run, at least in certain subsets of patients, in the prodromic phase of Alzheimer's disease, along a putative metabolic physiopathological continuum. Indeed, the persistent disruption of glucose homeostasis and energy supply to neurons may eventually impair protein folding, an energy-requiring process, promoting pathological changes in Alzheimer's disease, such as amyloid-β deposition and tau hyperphosphorylation. Hopefully, the "neuroenergetic hypothesis" presented herein will provide further insight on there being a conceivable metabolic bridge between chronic migraine and Alzheimer's disease, elucidating novel potential targets for the prophylactic treatment of both diseases.

Cerebrospinal fluid soluble insulin receptor levels in Alzheimer's disease

INTRODUCTION

Brain insulin resistance and deficiency is a consistent feature of Alzheimer's disease (AD). Insulin resistance can be mediated by the surface expression of the insulin receptor (IR). Cleavage of the IR generates the soluble IR (sIR).

METHODS

We measured the levels of sIR present in cerebrospinal fluid (CSF) from individuals along the AD diagnostic spectrum from two cohorts: Seattle (n = 58) and the Consortium for the Early Identification of Alzheimer's Disease-Quebec (CIMA-Q; n = 61). We further investigated the brain cellular contribution for sIR using human cell lines.

RESULTS

CSF sIR levels were not statistically different in AD. CSF sIR and amyloid beta (Aβ)42 and Aβ40 levels significantly correlated as well as CSF sIR and cognition in the CIMA-Q cohort. Human neurons expressing the amyloid precursor protein "Swedish" mutation generated significantly greater sIR and human astrocytes were also able to release sIR in response to both an inflammatory and insulin stimulus.

DISCUSSION

These data support further investigation into the generation and role of sIR in AD.

Highlights

Cerebrospinal fluid (CSF) soluble insulin receptor (sIR) levels positively correlate with amyloid beta (Aβ)42 and Aβ40.CSF sIR levels negatively correlate with cognitive performance (Montreal Cognitive Assessment score).CSF sIR levels in humans remain similar across Alzheimer's disease diagnostic groups.Neurons derived from humans with the "Swedish" mutation in which Aβ42 is increased generate increased levels of sIR.Human astrocytes can also produce sIR and generation is stimulated by tumor necrosis factor α and insulin.

Insulin and the blood-brain barrier

The blood-brain barrier (BBB) predominantly regulates insulin transport into and levels within the brain. The BBB is also an important site of insulin binding and mediator of insulin receptor (INSR) signaling. The insulin transporter is separate from the INSR, highlighting the important, unique role of each protein in this structure. After a brief introduction on the structure of insulin and the INSR, we discuss the importance of insulin interactions at the BBB, the properties of the insulin transporter and the role of the BBB insulin transporter in various physiological conditions. We go on to further describe insulin BBB signaling and the impact not only within brain endothelial cells but also the cascade into other cell types within the brain. We close with future considerations to advance our knowledge about the importance of insulin at the BBB.

Glucose Transport and Utilization in the Hippocampus: From Neurophysiology to Diabetes-Related Development of Dementia

The association of diabetes with cognitive dysfunction has at least 60 years of history, which started with the observation that children with type 1 diabetes mellitus (T1D), who had recurrent episodes of hypoglycemia and consequently low glucose supply to the brain, showed a deficit of cognitive capacity. Later, the growing incidence of type 2 diabetes mellitus (T2D) and dementia in aged populations revealed their high association, in which a reduced neuronal glucose supply has also been considered as a key mechanism, despite hyperglycemia. Here, we discuss the role of glucose in neuronal functioning/preservation, and how peripheral blood glucose accesses the neuronal intracellular compartment, including the exquisite glucose flux across the blood-brain barrier (BBB) and the complex network of glucose transporters, in dementia-related areas such as the hippocampus. In addition, insulin resistance-induced abnormalities in the hippocampus of obese/T2D patients, such as inflammatory stress, oxidative stress, and mitochondrial stress, increased generation of advanced glycated end products and BBB dysfunction, as well as their association with dementia/Alzheimer's disease, are addressed. Finally, we discuss how these abnormalities are accompained by the reduction in the expression and translocation of the high capacity insulin-sensitive glucose transporter GLUT4 in hippocampal neurons, which leads to neurocytoglycopenia and eventually to cognitive dysfunction. This knowledge should further encourage investigations into the beneficial effects of promising therapeutic approaches which could improve central insulin sensitivity and GLUT4 expression, to fight diabetes-related cognitive dysfunctions.

Impact of insulin resistance on mild cognitive impairment in type 2 diabetes mellitus patients with non-alcoholic fatty liver disease

AIMS

Insulin resistance (IR) is a pivotal factor in the pathogenesis of type 2 diabetes mellitus (T2DM) and non-alcoholic fatty liver disease (NAFLD). Nevertheless, the impact of IR on cognitive dysfunction in T2DM patients with NAFLD remains inadequately understood. We aim to investigate the effect of IR on mild cognitive impairment (MCI) in T2DM individuals with NAFLD.

MATERIALS AND METHODS

143 T2DM individuals were categorized into Non-MCI and MCI groups, as well as Non-NAFLD and NAFLD groups. Clinical parameters and cognitive preference test outcomes were compared. Correlation and regression analyses were executed to explore the interconnections between IR and cognitive details across all T2DM patients, as well as within the subgroup of individuals with NAFLD.

RESULTS

In comparison to the Non-MCI group, the MCI group displayed elevated HOMA-IR levels. Similarly, the NAFLD group exhibited higher HOMA-IR levels compared to the Non-NAFLD group. Additionally, a higher prevalence of MCI was observed in the NAFLD group as opposed to the Non-NAFLD group. Notably, HOMA-IR levels were correlated with Verbal Fluency Test (VFT) and Trail Making Test-B (TMTB) scores, both related to executive functions. Elevated HOMA-IR emerged as a risk factor for MCI in the all patients. Intriguingly, increased HOMA-IR not only correlated with TMTB scores but also demonstrated an influence on TMTA scores, reflecting information processing speed function in patients with NAFLD.

CONCLUSION

IR emerges as a contributory factor to cognitive dysfunction in T2DM patients. Furthermore, it appears to underlie impaired executive function and information processing speed function in T2DM individuals with NAFLD.