Insulin, cognition, and dementia

Abnormal cerebral blood flow and brain function in type 2 diabetes mellitus

PURPOSE

Type 2 diabetes mellitus (T2DM) lead to impaired cerebral blood perfusion, which leads to changes in brain function and affects the cognitive function of patients. In this study, cerebral blood flow (CBF) was used to evaluate the effect of T2DM on cerebral perfusion, and functional connectivity (FC) analysis was further used to explore whether the FC between the abnormal CBF region and the whole brain was changed. In addition, amplitude of low-frequency fluctuation (ALFF) and degree centrality (DC) were used to investigate the changes in spontaneous activity and connectivity strength of the brain network.

METHODS

We recruited 40 T2DM patients and 55 healthy controls (HCs). They underwent 3D-T1WI, rs-fMRI, arterial spin labeling (ASL) sequence scans and a series of cognitive tests. Cognitive test scores and brain imaging indicators were compared between the two groups, and the relationships among laboratory indicators, cognitive test scores, and brain imaging indicators were explored in the T2DM group.

RESULTS

Compared to HCs, The CBF values of Calcarine_L and Precuneus_R in the T2DM group were lower. The DC value of Paracentral_Lobule_L and Precuneus_L, and the ALFF value of Hippocampus_L in the T2DM group were higher. In addition, the CBF values of Calcarine_L was negatively correlated with fasting insulin and HOMA_IR.

CONCLUSION

This study found that there were regions of cerebral hypoperfusion in T2DM patients, which are associated with insulin resistance. In addition, we found abnormally elevated brain activity and enhanced functional connectivity in T2DM patients, which we speculated was the compensatory mechanism of brain neural activity.

A high fat diet potentiates neonatal iron overload-induced memory impairments in rats

PURPOSE

The present study aimed at evaluating possible synergistic effects between two risk factors for cognitive decline and neurodegenerative disorders, i.e. iron overload and exposure to a hypercaloric/hyperlipidic diet, on cognition, insulin resistance, and hippocampal GLUT1, GLUT3, Insr mRNA expression, and AKT phosporylation.

METHODS

Male Wistar rats were treated with iron (30 mg/kg carbonyl iron) or vehicle (5% sorbitol in water) from 12 to 14th post-natal days. Iron-treated rats received a standard laboratory diet or a high fat diet from weaning to adulthood (9 months of age). Recognition and emotional memory, peripheral blood glucose and insulin levels were evaluated. Glucose transporters (GLUT 1 and GLUT3) and insulin signaling were analyzed in the hippocampus of rats.

RESULTS

Both iron overload and exposure to a high fat diet induced memory deficits. Remarkably, the association of iron with the high fat diet induced more severe cognitive deficits. Iron overload in the neonatal period induced higher insulin levels associated with significantly higher HOMA-IR, an index of insulin resistance. Long-term exposure to a high fat diet resulted in higher fasting glucose levels. Iron treatment induced changes in Insr and GLUT1 expression in the hippocampus. At the level of intracellular signaling, both iron treatment and the high fat diet decreased AKT phosphorylation.

CONCLUSION

The combination of iron overload with exposure to a high fat diet only led to synergistic deleterious effect on emotional memory, while the effects induced by iron and by the high fat diet on AKT phosphorylation were comparable. These findings indicate that there is, at least to some extent, an additive effect of iron combined with the diet. Further studies investigating the mechanisms associated to deleterious effects on cognition and susceptibility for the development of age-associated neurodegenerative disorders are warranted.

Medium-Chain Triglycerides (MCTs) for the Symptomatic Treatment of Dementia-Related Diseases: A Systematic Review

Pathomechanisms of dementias involve increasing damage to neuronal energy metabolism, resulting in degeneration-related insulin resistance and glucose hypometabolism. In this case, ketone bodies can provide an alternative energy source. Supplementation with medium-chain triglycerides (MCTs), which can induce ketogenesis, may alleviate brain energy deficits and improve neuronal function. This review aims to determine the effectiveness of MCT as a symptomatic treatment approach. The systematic literature search was conducted in April 2023 following the Cochrane Handbook and PRISMA guidelines. A total of 21 studies were included, comprising eight uncontrolled trials and 13 RCTs investigating the effects of MCT on Alzheimer's disease (AD) and mild cognitive impairment (MCI). A substantial increase in plasma ketone levels and brain metabolic rates was observed. Cognitive assessments showed only occasional or domain-specific performance improvements. The effects on functional abilities or psychological outcomes have been inadequately studied. Besides gastrointestinal side effects, no harmful effects were observed. However, the evidence was severely weakened by heterogeneous and poorly designed study protocols, bias, and conflicts of interest. In conclusion, the ketogenic properties of MCTs may have beneficial effects on brain metabolism in AD and MCI but do not always result in measurable clinical improvement. Current evidence is insufficient to recommend MCT as a comparable symptomatic treatment option.

Individual and dual trajectories of insomnia symptoms and body mass index before and after retirement and their associations with changes in subjective cognitive functioning

BACKGROUND

We examined individual and dual trajectories of insomnia symptoms and body mass index (BMI) before and after retirement, and their associations with changes in subjective cognitive functioning after retirement.

METHODS

We used the Helsinki Health Study's (n = 2360, 79% women, aged 40-60 at baseline, Finland) repeated surveys to identify the developmental patterns of insomnia symptoms and BMI (2000-2017) and changes in subjective cognitive functioning (2017-2022). We analysed the data using latent group-based dual trajectory modelling and logistic regression analysis.

RESULTS

Three latent groups were identified for insomnia symptoms (stable low, decreasing and increasing symptoms) and BMI (stable healthy weight, stable overweight and stable obesity). Insomnia symptoms were associated with declining subjective cognitive functioning and largely explained the effects in the dual models.

CONCLUSION

The association between dual trajectories of insomnia symptoms and BMI with subjective cognitive decline is dominated by insomnia symptoms.

Diabetes-Alzheimer's connection in older age: SGLT2 inhibitors as promising modulators of disease pathways

Late-onset Alzheimer's disease (LOAD) is the most frequent cause of dementia in older persons. Subjects affected by type 2 diabetes mellitus (T2DM) are at higher risk of vascular disease, cognitive decline, and dementia. LOAD has many characteristics shared with impaired insulin signaling pathways, and substantial evidence has demonstrated a pivotal role in dysregulated glucose metabolism in its pathogenesis. Recent studies have shown that some anti-diabetic drugs, other than regulating the metabolism of peripheral tissues, can also modulate the brain's metabolism, reduce inflammation, and have a direct neuroprotective effect. Sodium-glucose cotransporter-2 inhibitors (SGLT2i) are a newer class with many pleiotropic effects that may have strong neuroprotective potential. After a summary of the principal "anti-diabetic" drugs acting as suitable candidates in treating LOAD, this narrative review explored the potential role of SGLT2i on cognition from pre-clinical to clinical studies.

Positive association between weight-adjusted-waist index and dementia in the Chinese population with hypertension: a cross-sectional study

PURPOSE

The links between obesity and dementia remain equivocal. Therefore, this study aimed to explore the association between weight-adjusted waist index (WWI), a new anthropometric indicator reflecting obesity, and dementia in the Chinese population with hypertension.

METHODS

A total of 10,289 participants with hypertension were enrolled in this cross-sectional study, a subset of the China H-type hypertension registry study. WWI was calculated as waist circumference (WC) divided by the square root of bodyweight. Mini-mental state examination (MMSE) scale was performed to evaluate the cognitive function. According to educational background, different MMSE cut-off values were applied to define dementia: < 24 for participants with ≥ 7 years of education, < 20 for those with 1-6 years of education, and < 17 for illiterate participants. Multivariable linear regression and multivariable binary logistic regression analyses were conducted to assess the associations between WWI and MMSE and dementia, respectively.

RESULTS

Overall, the mean age was 63.7 ± 9.7 years, and 49.0% were males. Multivariate linear regression analyses showed that WWI was negatively associated with MMSE (β, -1.09; 95% confidence interval [CI]: -1.24, -0.94). Consistently, multivariable binary logistic regression analyses found a positive association between WWI and the risk of dementia (odds ratio [OR], 1.45; 95% CI: 1.35, 1.56). Compared with individuals in quartile 1 of WWI, the adjusted β and OR values of WWI for MMSE and dementia were -2.28 (95% CI: -2.62, -1.94) and 2.12 (95% CI: 1.81, 2.48), respectively. Results of smoothing curve fitting confirmed the linear association between WWI and MMSE and dementia. Subgroup analysis showed a stronger association between WWI and dementia in participants with hypertension with midday napping.

CONCLUSION

WWI was independently and positively associated with dementia among the population with hypertension, especially in those with midday napping. The data suggests that WWI may serve as a simple and effective tool for the assessment of the risk of dementia in clinical practice.

High fat diet allows food-predictive stimuli to energize action performance in the absence of hunger, without distorting insulin signaling on accumbal cholinergic interneurons

Obesity can disrupt how food-predictive stimuli control action performance and selection. These two forms of control recruit cholinergic interneurons (CIN) located in the nucleus accumbens core (NAcC) and shell (NAcS), respectively. Given that obesity is associated with insulin resistance in this region, we examined whether interfering with CIN insulin signaling disrupts how food-predictive stimuli control actions. To interfere with insulin signaling we used a high-fat diet (HFD) or genetic excision of insulin receptor (InsR) from cholinergic cells. HFD left intact the capacity of food-predictive stimuli to energize performance of an action earning food when mice were tested hungry. However, it allowed this energizing effect to persist when the mice were tested sated. This persistence was linked to NAcC CIN activity but was not associated with distorted CIN insulin signaling. Accordingly, InsR excision had no effect on how food-predicting stimuli control action performance. Next, we found that neither HFD nor InsR excision altered the capacity of food-predictive stimuli to guide action selection. Yet, this capacity was associated with changes in NAcS CIN activity. These results indicate that insulin signaling on accumbal CIN does not modulate how food-predictive stimuli control action performance and selection. However, they show that HFD allows food-predictive stimuli to energize performance of an action earning food in the absence of hunger.

Unlocking Modifiable Risk Factors for Alzheimer's Disease: Does the Oral Microbiome Hold Some of the Keys?

Advancing age is recognized as the primary risk factor for Alzheimer's disease (AD); however approximately one third of dementia cases are attributable to modifiable risk factors such as hypertension, diabetes, smoking, and obesity. Recent research also implicates oral health and the oral microbiome in AD risk and pathophysiology. The oral microbiome contributes to the cerebrovascular and neurodegenerative pathology of AD via the inflammatory, vascular, neurotoxic, and oxidative stress pathways of known modifiable risk factors. This review proposes a conceptual framework that integrates the emerging evidence regarding the oral microbiome with established modifiable risk factors. There are numerous mechanisms by which the oral microbiome may interact with AD pathophysiology. Microbiota have immunomodulatory functions, including the activation of systemic pro-inflammatory cytokines. This inflammation can affect the integrity of the blood-brain barrier, which in turn modulates translocation of bacteria and their metabolites to brain parenchyma. Amyloid-β is an antimicrobial peptide, a feature which may in part explain its accumulation. There are microbial interactions with cardiovascular health, glucose tolerance, physical activity, and sleep, suggesting that these modifiable lifestyle risk factors of dementia may have microbial contributors. There is mounting evidence to suggest the relevance of oral health practices and the microbiome to AD. The conceptual framework presented here additionally demonstrates the potential for the oral microbiome to comprise a mechanistic intermediary between some lifestyle risk factors and AD pathophysiology. Future clinical studies may identify specific oral microbial targets and the optimum oral health practices to reduce dementia risk.

Sex-specific associations between diabetes and dementia: the role of age at onset of disease, insulin use and complications

BACKGROUND

Whether the association of type 2 diabetes (T2DM) with dementia was differed by sex remains unclear, and the roles of age at onset of disease, insulin use and diabetes' complications in their association are unknown.

METHODS

This study analyzed data of 447 931 participants from the UK Biobank. We used Cox proportional hazards models to estimate sex-specific hazard ratios (HRs) and 95% confidence intervals (CI), and women-to-men ratio of HRs (RHR) for the association between T2DM and incident dementia [all-cause dementia, Alzheimer's disease (AD), and vascular dementia (VD)]. The roles of age at onset of disease, insulin use and diabetes' complications in their association were also analyzed.

RESULTS

Compared to people with no diabetes at all, people with T2DM had increased risk of all-cause dementia (HR 2.85, 95% CI 2.56-3.17). The HRs between T2DM and AD were higher in women than men, with an RHR (95%CI) of 1.56 (1.20, 2.02). There was a trend that people who experienced T2DM before age 55 had higher risk of VD than those who had T2DM after age 55. In addition, there was a trend that T2DM had higher effect on VD that occurred before age 75 years than events that occurred after age 75. Patients with T2DM using insulin had higher risk of all-cause dementia than those without insulin, with an RHR (95%CI) of 1.54 (1.00-2.37). People with complications had doubled risk of all-cause dementia, AD and VD.

CONCLUSIONS

Adopting a sex-sensitive strategy to address the risk of dementia in patients with T2DM is instrumental for a precision medicine approach. Meanwhile, it is warranted to consider patients' age at onset of T2DM, insulin use status and complications conditions.

Impaired Early Insulin Response to Glucose Load Predicts Episodic Memory Decline: A 10-Year Population-Based Cohort Follow-Up of 45-74-Year-Old Men and Women

BACKGROUND

Diabetes increases the risk for cognitive decline, but the mechanisms behind this association remain unknown. Impaired early insulin secretion in elderly men and insulin resistance, both of which are pathophysiological features of type 2 diabetes, have previously been linked to Alzheimer's disease.

OBJECTIVE

To examine if the early insulin response to oral glucose load predicts cognitive performance after 10 years in men and women aged 45-74 years.

METHODS

This study was based on a subpopulation of the Health 2000 Survey, a Finnish nationwide, population-based health examination study, and its follow-up, the Health 2011 Study. In total, 961 45-74-year-old individuals (mean age at baseline 55.6 years, 55.8% women) were examined. An oral glucose tolerance test was performed in 2001-2002, and early insulin response was defined as the ratio of the 30-min increment in insulin concentration to that of glucose concentration. Cognitive function was evaluated at baseline and follow-up with categorical verbal fluency, word-list learning, and word-list delayed recall. Statistical analyses were performed using multivariable linear models adjusted for age, sex, education, APOE&z.epsi;4 genotype, vascular risk factors including diabetes, and depressive symptoms.

RESULTS

A lower early insulin response to glucose load predicted lower performance (β: 0.21, p = 0.03) and greater decline (β: 0.19, p = 0.03) in the word-list delayed recall test. Baseline early insulin response did not predict verbal fluency or word-list learning (all p-values≥0.13).

CONCLUSION

Our results suggest that decreased early insulin secretion predicts episodic memory decline in middle-aged to elderly men and women.

Cardiovascular Disease Risk Burden, Cognitive Impairments and Incident Dementia among Community-Dwelling Middle-Aged and Older Adults: An 8-Year Longitudinal Follow-up Study

OBJECTIVES

To evaluate the associations between cardiovascular disease (CVD) risk burden (estimated by the World Health Organization (WHO) algorithm) and cognitive impairments (e.g., incident dementia, global and domain-specific impairments) among CVD-, dementia- and disability-free, community-dwelling middle-aged and older adults during an 8-year follow-up.

DESIGN

A community-based longitudinal cohort study.

SETTING

Yuanshan township in Yi-Lan County, Taiwan.

PARTICIPANTS

A total of 889 community-dwelling residents aged 50 years or older.

MEASUREMENTS

Age, sex, educational level, employment status, alcohol status, body mass index, physical activity, gait speed, depressive symptoms, WHO region-specific CVD risk scores (10-year CV risk, low: <10% vs. moderate-to-high: ≥ 10%), Chinese version of the Mini-Mental State Examination (MMSE), verbal memory by the delay-free recall in the Chinese Version Verbal Learning Test (CVVLT), language function by the Boston Naming Test and the category (animal) Verbal Fluency Test, visuospatial function by the Taylor Complex Figure Test, executive function by the digit backward and the Clock Drawing Test.

RESULTS

Compared to those with low CVD risk, middle-aged and older adults with moderate-to-high CVD risk were at greater risk for cognitive impairments with respect to the MMSE (adjusted odds ratio (aOR) 1.60 [95% confidence interval (CI) 1.19-2.15], P=0.002), verbal memory (aOR 1.97 [1.43-2.70], P< 0.001) and language (aOR 1.99 [1.46-2.70], P< 0.001), as well as incident dementia (aOR 2.40 [1.33-4.33], P=0.004). After adjusting for all covariates, CVD risk was not associated with other domains of cognitive impairment.

CONCLUSIONS

Among healthy, community-dwelling, middle-aged and older adults, those with moderate-to-high cardiovascular risk burden were significantly associated with incident dementia and global and domain-specific cognitive impairments (verbal memory and language), which suggests the existence of a relationship between early cognitive deficits and CVD risk burden. Further studies are needed to elucidate the pathophysiological mechanism of the link between CVD risk burden and cognitive impairment.

Prevailing Antagonistic Risks in Pleiotropic Associations with Alzheimer's Disease and Diabetes

BACKGROUND

The lack of efficient preventive interventions against Alzheimer's disease (AD) calls for identifying efficient modifiable risk factors for AD. As diabetes shares many pathological processes with AD, including accumulation of amyloid plaques and neurofibrillary tangles, insulin resistance, and impaired glucose metabolism, diabetes is thought to be a potentially modifiable risk factor for AD. Mounting evidence suggests that links between AD and diabetes may be more complex than previously believed.

OBJECTIVE

To examine the pleiotropic architecture of AD and diabetes mellitus (DM).

METHODS

Univariate and pleiotropic analyses were performed following the discovery-replication strategy using individual-level data from 10 large-scale studies.

RESULTS

We report a potentially novel pleiotropic NOTCH2 gene, with a minor allele of rs5025718 associated with increased risks of both AD and DM. We confirm previously identified antagonistic associations of the same variants with the risks of AD and DM in the HLA and APOE gene clusters. We show multiple antagonistic associations of the same variants with AD and DM in the HLA cluster, which were not explained by the lead SNP in this cluster. Although the ɛ2 and ɛ4 alleles played a major role in the antagonistic associations with AD and DM in the APOE cluster, we identified non-overlapping SNPs in this cluster, which were adversely and beneficially associated with AD and DM independently of the ɛ2 and ɛ4 alleles.

CONCLUSION

This study emphasizes differences and similarities in the heterogeneous genetic architectures of AD and DM, which may differentiate the pathogenic mechanisms of these diseases.

Inflammation moderates the effects of lifestyle modification on neurocognition among individuals with resistant hypertension

Individuals with resistant hypertension (RH) have the greatest risk of cerebrovascular disease and cognitive impairment among individuals with hypertension. Elevated levels of pro-inflammatory cytokines may represent a critical yet unexamined factor influencing the impact of healthy lifestyle changes on cognitive function. We explored the influence of inflammation on changes in cognition following lifestyle modification among individuals with RH participating in the TRIUMPH clinical trial. One hundred forty participants with RH completed a battery of neurocognitive tests along with the inflammatory marker C-reactive protein (hsCRP) and were subsequently randomized to an intensive 4-month lifestyle modification intervention or to education and physician advice control. Results indicated that the effects of lifestyle modification on Executive Function and Learning were moderated by pre-intervention hsCRP levels (P = .049), with treatment efficacy increasing across levels of baseline inflammation levels (low: d = 0.12; mild: d = 0.43; moderate: d = 0.81). We conclude that inflammatory profiles may help identify individuals more likely to improve executive functioning resulting from lifestyle modification.

Type 2 Diabetes Mellitus and Alzheimer's Disease: Shared Molecular Mechanisms and Potential Common Therapeutic Targets

The global prevalence of diabetes mellitus and Alzheimer's disease is increasing alarmingly with the aging of the population. Numerous epidemiological data suggest that there is a strong association between type 2 diabetes and an increased risk of dementia. These diseases are both degenerative and progressive and share common risk factors. The amyloid cascade plays a key role in the pathophysiology of Alzheimer's disease. The accumulation of amyloid beta peptides gradually leads to the hyperphosphorylation of tau proteins, which then form neurofibrillary tangles, resulting in neurodegeneration and cerebral atrophy. In Alzheimer's disease, apart from these processes, the alteration of glucose metabolism and insulin signaling in the brain seems to induce early neuronal loss and the impairment of synaptic plasticity, years before the clinical manifestation of the disease. The large amount of evidence on the existence of insulin resistance in the brain during Alzheimer's disease has led to the description of this disease as "type 3 diabetes". Available animal models have been valuable in the understanding of the relationships between type 2 diabetes and Alzheimer's disease, but to date, the mechanistical links are poorly understood. In this non-exhaustive review, we describe the main molecular mechanisms that may link these two diseases, with an emphasis on impaired insulin and IGF-1 signaling. We also focus on GSK3β and DYRK1A, markers of Alzheimer's disease, which are also closely associated with pancreatic β-cell dysfunction and type 2 diabetes, and thus may represent common therapeutic targets for both diseases.

Metformin inhibits cardiometabolic syndrome associated cognitive deficits in high fat diet rats

Objectives

Glucose intolerance and insulin resistance are hallmarks of metabolic syndrome and lead to Alzheimer's disease (AD). The purpose of this study is to elucidate the neuroprotective effect of metformin through insulin regulation with cardiometabolic and neurotransmitter metabolic enzyme regulation in high-fat, high-sucrose diet and streptozotocin (HFHS-STZ)-induced rats.

Methods

Male Wistar rats were treated with metformin (180 mg/kg and 360 mg/kg). STZ (35 mg/kg i.p.) injection was performed on the 14th day of 42 days of HFHS diet treatment. Brain neurotransmitter metabolic enzymes (acetylcholinesterase and monoamine oxidase) were determined along with sodium-potassium ATPase (Na+K+-ATPase). Plasma lipids and homeostasis model assessment of insulin resistance (HOMA-IR) was performed. Mean arterial blood pressure, heart rate and electrocardiogram (QT, QTc and RR intervals) were analysed with PowerLab.

Results

Metformin treatment significantly (p < 0.001) reduced the HOMA-IR index and decreased neurotransmitter metabolic enzymes such as AChE and MAO (p < 0.01 and p < 0.05). The lipid profile was significantly (p < 0.001) controlled with cardiometabolic functions.

Conclusions

Our investigation revealed that metformin has a remarkable role in regulating brain insulin, vascular system with monoaminergic metabolic enzymes and enhancing synaptic plasticity. Metformin may be a selective early therapeutic agent in metabolic syndrome associated with cognitive decline.

“Ferrocrinology”—Iron Is an Important Factor Involved in Gluco- and Lipocrinology

“Ferrocrinology” is the term used to describe the study of iron effects on the functioning of adipose tissue, which together with muscle tissue makes the largest endocrine organ in the human body. By impairing exercise capacity, reducing AMP-activated kinase activity, and enhancing insulin resistance, iron deficiency can lead to the development of obesity and type 2 diabetes mellitus. Due to impaired browning of white adipose tissue and reduced mitochondrial iron content in adipocytes, iron deficiency (ID) can cause dysfunction of brown adipose tissue. By reducing ketogenesis, aconitase activity, and total mitochondrial capacity, ID impairs muscle performance. Another important aspect is the effect of ID on the impairment of thermogenesis due to reduced binding of thyroid hormones to their nuclear receptors, with subsequently impaired utilization of norepinephrine in tissues, and impaired synthesis and distribution of cortisol, which all make the body’s reactivity to stress in ID more pronounced. Iron deficiency can lead to the development of the most common endocrinopathy, autoimmune thyroid disease. In this paper, we have discussed the role of iron in the cross-talk between glucocrinology, lipocrinology and myocrinology, with thyroid hormones acting as an active bystander.

Can Alzheimer's Disease Be Secondary to Type-2 Diabetes Mellitus?

Alzheimer's disease and insulin resistance are prevalent in older adults. Insulin's ability to effectively affect target tissues is diminished by IR. Hyperglycemia, higher blood pressure, elevated triglyceride levels, decreased HDL levels and central obesity are the outcomes of a condition, namely metabolic syndrome. Cognitive impairment and abnormalities of the brain have been linked to metabolic syndrome (MetS), a grouping of risk factors for type 2 diabetes mellitus. Type-2 diabetes mellitus and its relationship to other conditions have been investigated on the assorted extent in the pair of, human and animal subjects. First, it was shown that insulin receptors are present in the brain, namely the hippocampus. Most insulin is delivered to the brain by crossing the blood-brain barrier. Second, numerous research revealed that insulin impacts various neurotransmitters in a way that enhances memory and cognition. Thirdly, several pathological research has also shown that beta-amyloid plaques, hyperphosphorylated tau protein, and brain shrinkage, particularly in the hippocampus, are shared brain lesions between insulin and Alzheimer's disease. In light of this, type 2 diabetes mellitus may be viewed as a liability for dementia and Alzheimer's disease.

Review of the effects of polycystic ovary syndrome on Cognition: Looking beyond the androgen hypothesis

Polycystic-ovary syndrome (PCOS) is the most common endocrine disorder affecting women of reproductive age, and many features associated with PCOS - such as elevated androgens, insulin resistance and inflammation - are known to affect cognition. However, effects of PCOS on cognition are not well-understood. Here we review the current literature on PCOS and cognition, note the extent of PCOS symptomatology studied in relation to cognitive outcomes, and identify key research gaps and common methodological concerns. Findings indicate a pattern of worse performance across cognitive domains and brain measures in women with PCOS relative to non-PCOS controls, as well as a lack of evidence for the common assumption that women with PCOS will have higher performance on tasks with a demonstrated male-advantage due to high testosterone levels. We suggest strategies for moving beyond the focus on elevated androgens, in favor of research practices that account for the nuances and heterogeneity of PCOS symptoms.

The effects of a 6-week controlled, hypocaloric ketogenic diet, with and without exogenous ketone salts, on cognitive performance and mood states in overweight and obese adults

Background

Ketogenic diets are a commonly used weight loss method, but little is known how variations in sodium content and ketones influence cognition and mood during the early keto-adaptation period.

Objectives

To investigate the effects of an exogenous ketone salt (KS) as part of a hypocaloric KD on mood and cognitive outcomes in overweight and obese adults. A secondary objective was to evaluate changes in biochemical markers associated with inflammatory and cognitive responses.

Materials and methods

Adults who were overweight or obese participated in a 6-week controlled-feeding intervention comparing hypocaloric diets (∼75% of energy expenditure). KD groups received twice daily ketone salt (KD + KS; n = 12) or a flavor-matched placebo, free of minerals (KD + PL; n = 13). A separate group of age and BMI matched adults were later assigned to an isoenergetic low-fat diet (LFD; n = 12) as comparison to KD. Mood was assessed by shortened Profile of Mood States and Visual Analog Mood Scale surveys. Cognitive function was determined by the Automated Neuropsychological Assessment Metrics mental test battery.

Results

Both KD groups achieved nutritional ketosis. Fasting serum glucose decreased in both KD groups, whereas glucose was unaffected in the LFD. Insulin decreased at week 2 and remained lower in all groups. At week 2, depression scores in the KD + PL group were higher compared to KD + KS. Performance in the math processing and go/no-go cognitive tests were lower for KD + PL and LFD participants, respectively, compared to KD + KS. Serum leptin levels decreased for all groups throughout the study but were higher for KD + KS group at week 6. Serum TNF-α steadily increased for LFD participants, reaching significance at week 6.

Conclusion

During a short-term hypocaloric diet, no indication of a consistent decline in mood or cognitive function were seen in participants following either KD, despite KD + PL being relatively low in sodium. WK2 scores of “anger” and “depression” were higher in the LFD and KD + PL groups, suggesting that KS may attenuate negative mood parameters during the early intervention stages.

Can Iron Play a Crucial Role in Maintaining Cardiovascular Health in the 21st Century?

In the 21st century the heart is facing more and more challenges so it should be brave and iron to meet these challenges. We are living in the era of the COVID-19 pandemic, population aging, prevalent obesity, diabetes and autoimmune diseases, environmental pollution, mass migrations and new potential pandemic threats. In our article we showed sophisticated and complex regulations of iron metabolism. We discussed the impact of iron metabolism on heart diseases, treatment of heart failure, diabetes and obesity. We faced the problems of constant stress, climate change, environmental pollution, migrations and epidemics and showed that iron is really essential for heart metabolism in the 21st century.

Glucometabolic Changes Are Associated with Structural Gray Matter Alterations in Prodromal Dementia

BACKGROUND

Glucometabolic changes, such as high glycemic load (GL) diet and insulin resistance (IR), are potential risk factor of Alzheimer's disease (AD). Yet, the effect of these factors on brain alterations that contribute to AD pathology has not been clearly demonstrated.

OBJECTIVE

We aimed to assess the relationship of GL and IR with gray matter volumes involved in prodromal dementia.

METHODS

GL and Triglyceride-Glucose (TyG) index, an IR surrogate marker, were calculated in 497 participants who underwent magnetic resonance imaging (MRI). The gray matter volumes most related to prodromal dementia/mild cognitive impairment (diagnosed in 18/158 participants during the 7-year follow-up) were identified using a data-driven machine learning algorithm.

RESULTS

Higher GL diet was associated with reduced amygdala volume. The TyG index was negatively associated with the hippocampus, amygdala, and putamen volumes.

CONCLUSION

These results suggest that GL and IR are associated with lower gray matter volumes in brain regions involved in AD pathology.

Falling Short: The Contribution of Central Insulin Receptors to Gait Dysregulation in Brain Aging

Insulin resistance, which manifests as a reduction of insulin receptor signaling, is known to correlate with pathological changes in peripheral tissues as well as in the brain. Central insulin resistance has been associated with impaired cognitive performance, decreased neuronal health, and reduced brain metabolism; however, the mechanisms underlying central insulin resistance and its impact on brain regions outside of those associated with cognition remain unclear. Falls are a leading cause of both fatal and non-fatal injuries in the older population. Despite this, there is a paucity of work focused on age-dependent alterations in brain regions associated with ambulatory control or potential therapeutic approaches to target these processes. Here, we discuss age-dependent alterations in central modalities that may contribute to gait dysregulation, summarize current data supporting the role of insulin signaling in the brain, and highlight key findings that suggest insulin receptor sensitivity may be preserved in the aged brain. Finally, we present novel results showing that administration of insulin to the somatosensory cortex of aged animals can alter neuronal communication, cerebral blood flow, and the motivation to ambulate, emphasizing the need for further investigations of intranasal insulin as a clinical management strategy in the older population.

Pathophysiological mechanisms explaining the association between low skeletal muscle mass and cognitive function

Low skeletal muscle mass is associated with cognitive impairment and dementia in older adults. This review describes the possible underlying pathophysiological mechanisms: systemic inflammation, insulin metabolism, protein metabolism, and mitochondrial function. We hypothesize that the central tenet in this pathophysiology is the dysfunctional myokine secretion consequent to minimal physical activity. Myokines, such as fibronectin type III domain containing 5/irisin and cathepsin B, are released by physically active muscle and cross the blood–brain barrier. These myokines upregulate local neurotrophin expression such as brain-derived neurotrophic factor (BDNF) in the brain microenvironment. BDNF exerts anti-inflammatory effects that may be responsible for neuroprotection. Altered myokine secretion due to physical inactivity exacerbates inflammation and impairs muscle glucose metabolism, potentially affecting the transport of insulin across the blood–brain barrier. Our working model also suggests other underlying mechanisms. A negative systemic protein balance, commonly observed in older adults, contributes to low skeletal muscle mass and may also reflect deficient protein metabolism in brain tissues. As a result of age-related loss in skeletal muscle mass, decrease in the abundance of mitochondria and detriments in their function lead to a decrease in tissue oxidative capacity. Dysfunctional mitochondria in skeletal muscle and brain result in the excessive production of reactive oxygen species, which drives tissue oxidative stress and further perpetuates the dysfunction in mitochondria. Both oxidative stress and accumulation of mitochondrial DNA mutations due to aging drive cellular senescence. A targeted approach in the pathophysiology of low muscle mass and cognition could be to restore myokine balance by physical activity.

Contributive Role of Hyperglycemia and Hypoglycemia Towards the Development of Alzheimer's Disease

Alzheimer's disease (AD) is one of the causes of dementia that results from several infections/biological conditions leading to either cell disruption or loss of neuronal communication. Studies have documented the accumulation of two proteins, beta-amyloid (Aβ), which accumulates on the exteriors of neurons, and tau (Tau), which assembles at the interiors of brain cells and is chiefly liable for the progression of the disease. Several molecular and cellular pathways account for the accumulation of amyloid-β and the formation of neurofibrillary tangles, which are phosphorylated variants of Tau protein. Moreover, research has revealed a potential connection between AD and diabetes. It has also been demonstrated that both hypoglycemia and hyperglycemia have a significant role in the development of AD. In addition, SUMO (small ubiquitin-like modifier protein) plays a crucial role in the pathogenesis of AD. SUMOylation is the process by which modification of amyloid precursor protein (APP) and Tau takes place. Furthermore, Drosophila melanogaster has proven to be an efficient model organism in studies to establish the relationship between AD and variations in blood glucose levels. In addition, the review successfully identifies the common pathway that links the effects of fluctuations in glucose levels on AD pathogenesis and advancements.

Hepatic Encephalopathy and Melatonin

Hepatic encephalopathy (HE) is a severe metabolic syndrome linked with acute/chronic hepatic disorders. HE is also a pernicious neuropsychiatric complication associated with cognitive decline, coma, and death. Limited therapies are available to treat HE, which is formidable to oversee in the clinic. Thus, determining a novel therapeutic approach is essential. The pathogenesis of HE has not been well established. According to various scientific reports, neuropathological symptoms arise due to excessive accumulation of ammonia, which is transported to the brain via the blood–brain barrier (BBB), triggering oxidative stress and inflammation, and disturbing neuronal-glial functions. The treatment of HE involves eliminating hyperammonemia by enhancing the ammonia scavenging mechanism in systemic blood circulation. Melatonin is the sole endogenous hormone linked with HE. Melatonin as a neurohormone is a potent antioxidant that is primarily synthesized and released by the brain’s pineal gland. Several HE and liver cirrhosis clinical studies have demonstrated impaired synthesis, secretion of melatonin, and circadian patterns. Melatonin can cross the BBB and is involved in various neuroprotective actions on the HE brain. Hence, we aim to elucidate how HE impairs brain functions, and elucidate the precise molecular mechanism of melatonin that reverses the HE effects on the central nervous system.

Brain insulin resistance as a mechanistic mediator links peripheral metabolic disorders with declining cognition

BACKGROUND AND AIMS

Studies continue to investigate the underlying mechanism of the association between the increased risk of different types of cognitive decline and metabolic dysregulation. Brain insulin resistance (BIR) has been suggested to explain this association. The vital role of insulin in the body has been examined intensively and extensively; however, its role in the brain requires further investigation. Herein, we confined our focus to summarize the role of brain insulin signaling and the negative effect of dysmetabolism on insulin functioning in the brain.

METHODS

Published scientific manuscripts between 1998 and 2020 that discussed the effect of selected metabolic disorder conditions such as obesity, type 2 diabetes mellitus (T2DM), and high-fat diet (HFD) on brain functions were reviewed. The main keywords used were insulin resistance, brain insulin resistance, obesity, T2DM, and cognition.

RESULTS

Various metabolic disorders were linked to the increased risk of BIR, and was suggested to increase the probability of cognition impairment occurrence. Several proposed mechanisms explain this association among which insulin resistance and hyperinsulinemia were primary factors attributed to an increased risk of BIR among various metabolic disorders.

CONCLUSIONS

Understanding the trajectory of the association between metabolic disorders and alternation in cognition status could expand our vision of those overlapping conditions and pave the road to both treatment and preventative strategies for cognitive disorders.

Use of medium chain triglyceride (MCT) oil in subjects with Alzheimer's disease: A randomized, double-blind, placebo-controlled, crossover study, with an open-label extension

Introduction

Cerebral glucose and insulin metabolism is impaired in Alzheimer's disease (AD). Ketones provide alternative energy. Will medium chain triglyceride (MCT) oil, a nutritional source of ketones, impact cognition in AD?

Methods

This was a 6-month randomized, double-blind, placebo-controlled, crossover study, with 6-month open-label extension in probable AD subjects, on stable medications. MCT dose was 42 g/day, or maximum tolerated. Cognition was assessed with Mini-Mental State Examination (MMSE), Montreal Cognitive Assessment (MoCA), and Cognigram®.

Results

Twenty subjects, average age 72.6 years, 45% women, 70% university educated had baseline MMSE 22.6/30 (10-29); MoCA 15.6/30 (4-27); baseline Cognigram® Part 1: 65-106, Part 2: 48-107. Average MCT oil consumption was 1.8 tablespoons/day (25.2 g, 234 kcal). Eighty percent remained stable or improved. Longer MCT exposure and age > 73, resulted in higher final MMSE (P < .001) and Cognigram® 1 scores.

Discussion

This is the longest duration MCT AD study to date. Eighty percent had stabilization or improvement in cognition, and better response with 9-month continual MCT oil.

Reduced mitochondria membrane potential and lysosomal acidification are associated with decreased oligomeric Aβ degradation induced by hyperglycemia: A study of mixed glia cultures

Diabetes is a risk factor for Alzheimer’s disease (AD), a chronic neurodegenerative disease. We and others have shown prediabetes, including hyperglycemia and obesity induced by high fat and high sucrose diets, is associated with exacerbated amyloid beta (Aβ) accumulation and cognitive impairment in AD transgenic mice. However, whether hyperglycemia reduce glial clearance of oligomeric amyloid-β (oAβ), the most neurotoxic Aβ aggregate, remains unclear. Mixed glial cultures simulating the coexistence of astrocytes and microglia in the neural microenvironment were established to investigate glial clearance of oAβ under normoglycemia and chronic hyperglycemia. Ramified microglia and low IL-1β release were observed in mixed glia cultures. In contrast, amoeboid-like microglia and higher IL-1β release were observed in primary microglia cultures. APPswe/PS1dE9 transgenic mice are a commonly used AD mouse model. Microglia close to senile plaques in APPswe/PS1dE9 transgenic mice exposed to normoglycemia or chronic hyperglycemia exhibited an amoeboid-like morphology; other microglia were ramified. Therefore, mixed glia cultures reproduce the in vivo ramified microglial morphology. To investigate the impact of sustained high-glucose conditions on glial oAβ clearance, mixed glia were cultured in media containing 5.5 mM glucose (normal glucose, NG) or 25 mM glucose (high glucose, HG) for 16 days. Compared to NG, HG reduced the steady-state level of oAβ puncta internalized by microglia and astrocytes and decreased oAβ degradation kinetics. Furthermore, the lysosomal acidification and lysosomal hydrolysis activity of microglia and astrocytes were lower in HG with and without oAβ treatment than NG. Moreover, HG reduced mitochondrial membrane potential and ATP levels in mixed glia, which can lead to reduced lysosomal function. Overall, continuous high glucose reduces microglial and astrocytic ATP production and lysosome activity which may lead to decreased glial oAβ degradation. Our study reveals diabetes-induced hyperglycemia hinders glial oAβ clearance and contributes to oAβ accumulation in AD pathogenesis.

High fat diet-induced brain damaging effects through autophagy-mediated senescence, inflammation and apoptosis mitigated by ginsenoside F1-enhanced mixture

Background

Herbal medicines are popular approaches to capably prevent and treat obesity and its related diseases. Excessive exposure to dietary lipids causes oxidative stress and inflammation, which possibly induces cellular senescence and contribute the damaging effects in brain. The potential roles of selective enhanced ginsenoside in regulating high fat diet (HFD)-induced brain damage remain unknown.

Methods

The protection function of Ginsenoside F1-enhanced mixture (SGB121) was evaluated by in vivo and in vitro experiments. Human primary astrocytes and SH-SY5Y cells were treated with palmitic acid conjugated Bovine Serum Albumin, and the effects of SGB121 were determined by MTT and lipid uptake assays. For in vivo tests, C57BL/6J mice were fed with high fat diet for 3 months with or without SGB121 administration. Thereafter, immunohistochemistry, western blot, PCR and ELISA assays were conducted with brain tissues.

Results and conclusion

SGB121 selectively suppressed HFD-induced oxidative stress and cellular senescence in brain, and reduced subsequent inflammation responses manifested by abrogated secretion of IL-6, IL-1β and TNFα via NF-κB signaling pathway. Interestingly, SGB121 protects against HFD-induced damage by improving mitophagy and endoplasmic reticulum-stress associated autophagy flux and inhibiting apoptosis. In addition, SGB121 regulates lipid uptake and accumulation by FATP4 and PPARα. SGB121 significantly abates excessively phosphorylated tau protein in the cortex and GFAP activation in corpus callosum. Together, our results suggest that SGB121 is able to favor the resistance of brain to HFD-induced damage, therefore provide explicit evidence of the potential to be a functional food.

•

High fat diet induces oxidative stress and subsequent cellular senescence in mice brain.

•

High fat diet induces pathologies in cortex and GFAP activation in corpus callosum.

•

Ginsenoside F1-enhanced mixture ameliorates damaging effect by modulating autophagy flux and inflammation.

IRS1 expression in hippocampus is age-dependent and is required for mature spine maintenance and neuritogenesis

Insulin and insulin-like growth factor type I (IGF-1) play prominent roles in brain activity throughout the lifespan. Insulin/IGF1 signaling starts with the activation of the intracellular insulin receptor substrates (IRS). In this work, we performed a comparative study of IRS1 and IRS2, together with the IGF1 (IGF1R) and insulin (IR) receptor expression in the hippocampus and prefrontal cortex during development. We found that IRS1 and IRS2 expression is prominent during development and declines in the aged hippocampus, contrary to IR, which increases in adulthood and aging. In contrast, IGF1R expression is unaffected by age. Expression patterns are similar in the prefrontal cortex. Neurite development occurs postnatally in the rodent hippocampus and cortex, and it declines in the mature and aged brain and is influenced by trophic factors. In our previous work, we demonstrated that knockdown of IRS1 by shRNA impairs learning and reduces synaptic plasticity in a rat model, as measured by synaptophysin puncta in axons. In this study, we report that shIRS1 alters spine maturation in adult hilar hippocampal neurons. Lastly, to understand the role of IRS1 in neuronal neurite tree, we transfect shIRS1 into primary neuronal cultures and observed that shIRS1 reduced neurite branching and neurite length. Our results demonstrate that IRS1/2 and insulin/IGF1 receptors display different age-dependent expression profiles and that IRS1 is required for spine maturation, demonstrating a novel role for IRS1 in synaptic plasticity.

Nutrition, Physical Activity, and Other Lifestyle Factors in the Prevention of Cognitive Decline and Dementia

Multiple factors combined are currently recognized as contributors to cognitive decline. The main independent risk factor for cognitive impairment and dementia is advanced age followed by other determinants such as genetic, socioeconomic, and environmental factors, including nutrition and physical activity. In the next decades, a rise in dementia cases is expected due largely to the aging of the world population. There are no hitherto effective pharmaceutical therapies to treat age-associated cognitive impairment and dementia, which underscores the crucial role of prevention. A relationship among diet, physical activity, and other lifestyle factors with cognitive function has been intensively studied with mounting evidence supporting the role of these determinants in the development of cognitive decline and dementia, which is a chief cause of disability globally. Several dietary patterns, foods, and nutrients have been investigated in this regard, with some encouraging and other disappointing results. This review presents the current evidence for the effects of dietary patterns, dietary components, some supplements, physical activity, sleep patterns, and social engagement on the prevention or delay of the onset of age-related cognitive decline and dementia.

Empagliflozin nanoparticles attenuates type2 diabetes induced cognitive impairment via oxidative stress and inflammatory pathway in high fructose diet induced hyperglycemic mice

There is snowballing evidence that type 2 diabetes (T2D) predisposes to neuropathophysiological alterations including oxidative stress and triggered inflammatory responses in brain that eventually culminates into cognitive impairment.Accumulating evidences suggest that SGLT2 inhibitor can be a promising intervention for cognitive decline in T2DM. In the present paper, the potential effects of Empagliflozin (EMPA), a SGLT2 inhibitor, against T2D induced cognitive dysfunction have been explored. The effect of EMPA on array of inflammatory mediators including Interleukin-6(IL-6), Interleukin -1β (IL-1β), and Tumour necrosis factor-α(TNF-α)), neuronal proteins including glycogen synthase kinase-3β (GSK- 3β), Phosphorylated tau (p-tau), amyloid beta (Aβ) (1-40, 1-42) and altered oxidative parameters including SOD, catalase, TBARS was determined in the high fructose diet induced hyperglycaemic mice. The obtained results were compared with EMPA nanoparticles (Nps) formulated in our laboratory and found that EMPA Nps significantly showed reduced levels of inflammatory mediators and oxidative stress. Further, decrease in levels of p-tau, Aβ (1-40) and Aβ (1-42) were also observed with EMPA nanoparticles.Thus, the study has demonstrated that EMPA Nps could be a promising therapy to alleviate the progression of cognitive decline in T2D.

Porphyromonas gingivalis (W83) Infection Induces Alzheimer's Disease-Like Pathophysiology in Obese and Diabetic Mice

BACKGROUND

Periodontal disease(s) and metabolic illnesses negatively impact the quality of life and, eventually mental health.

OBJECTIVE

This study investigated the effect of Porphyromonas gingivalis (W83) oral infection on the development of Alzheimer's disease (AD) pathophysiology in a wild-type obese, diabetic (db/db) mouse model.

METHODS

The db/db mice were either orally infected with P. gingivalis and Fusobacterium nucleatum or sham infected for 16 weeks. The presence of amyloid-β (Aβ) and neurofibrillary tangles (NFTs) were assessed using a silver impregnation technique and subsequently by immunohistochemistry for tau and neuroinflammation. The mRNA abundance of a panel of 184 genes was performed using quantitative real-time PCR, and the differentially expressed genes were analyzed by Ingenuity Pathway Analysis.

RESULTS

While no Aβ plaques and NFTs were evident by silver impregnation, immunohistochemistry (glial cell markers) of the P. gingivalis-infected mice tissue sections exhibited neuroinflammation in the form of reactive microglia and astrocytes. Anti-tau immunopositivity, in addition to cells, was prominent in thickened axons of hippocampal CA neurons. The mRNA abundance of crucial genes in the insulin signaling pathway (INSR, IGF1, IRS, IDE, PIK3R, SGK1, GYS, GSK3B, AKT1) were upregulated, potentially exacerbating insulin resistance in the brain by P. gingivalis oral infection. Increased mRNA abundance of several kinases, membrane receptors, transcription factors, and pro-inflammatory mediators indicated hyperactivation of intracellular cascades with potential for tau phosphorylation and Aβ release in the same infection group.

CONCLUSION

P. gingivalis W83 infection of db/db mice provides a disease co-morbidity model with the potential to reproduce AD pathophysiology with induced periodontal disease.

Sex differences in risk factors for cognitive decline and dementia, including death as a competing risk, in individuals with diabetes: Results from the ADVANCE trial

AIM

To estimate the associations between risk factors and cognitive decline (CD)/dementia, and the sex differences in these risk factors in individuals with type 2 diabetes, while accounting for the competing risk of death.

MATERIALS AND METHODS

The Action in Diabetes and Vascular Disease: Preterax and Diamicron Modified Release Controlled Evaluation (ADVANCE) trial of 11,140 individuals with type 2 diabetes was used to estimate the odds of CD/dementia using multinomial logistic regression.

RESULTS

During a median 5-year follow-up, 1827 participants (43.2% women) had CD/dementia (1718 with CD only; 21 with dementia only; 88 with CD and dementia), and 929 (31.0% women) died without CD/dementia. Women had lower odds of CD/dementia than men (odds ratio [OR] [95% confidence interval], 0.88 [0.77, 1.00]); older age, higher total cholesterol, HbA1c, waist circumference, waist-to-height ratio, moderately increased albumin-creatinine ratio, stroke/transient ischaemic attack and retinal disease were each associated with greater odds of CD/dementia; higher years at education completion, baseline cognitive function, taller stature and current alcohol use were inversely associated. Higher waist circumference (women-to-men ratio of ORs [ROR], 1.05 [1.00, 1.10] per 5 cm) and presence of anxiety/depression (ROR, 1.28 [1.01, 1.63]) were associated with greater ORs for CD/dementia in women than men.

CONCLUSIONS

Several risk factors were associated with CD/dementia. Higher waist circumference and mental health symptoms were more strongly associated with CD/dementia in women than men. Further studies should examine the mechanisms that underlie these sex differences.

Investigation of common risk factors between polycystic ovary syndrome and Alzheimer's disease: a narrative review

BACKGROUND

The most common endocrine and metabolic disorders in premenopausal women is polycystic ovary syndrome (PCOS), characterized by hyperandrogenism, chronic anovulation, and/or ultrasound evidence of small ovarian cysts. Obesity and insulin resistance are also the main factors influencing the clinical manifestations of this syndrome. Alzheimer's disease (AD) is the most typical progressive neurodegenerative disorder of the brain, and recent studies suggest a relationship between endocrinal dysregulation and neuronal loss during AD pathology.

AIM

This study aimed to evaluate the common risk factors for Alzheimer's and PCOS based on previous studies. Knowing the common risk factors and eliminating them may prevent neurodegenerative Alzheimer's disease in the future.

METHOD

In this narrative review, international databases, including Google Scholar, Scopus, PubMed, and the Web of Science, were searched to retrieve the relevant studies. The relevant studies' summaries were categorized to discuss the possible pathways that may explain the association between Alzheimer's and PCOS signs/symptoms and complications.

RESULTS

According to our research, the factors involved in Alzheimer's and PCOS disorders may share some common risk factors. In patients with PCOS, increased LH to FSH ratio, decreased vitamin D, insulin resistance, and obesity are some of the most important factors that may increase the risk of Alzheimer's disease.

Insulin as a neuroendocrine hormone

The year 2021 marks 100 years since the discovery of insulin and this Special Issue of the Journal of Neuroendocrinology was conceived as a way to mark that historic breakthrough. The discovery of insulin and its subsequent use in the treatment of diabetes is one of the most striking success stories in biomedical research. From a neuroendocrinology perspective, the recognition that insulin also exerts widespread and varied actions in the brain is more recent, but potentially also of equal importance with relevance for conditions ranging from obesity to dementia. The reviews contained in this Special Issue were selected to cover the range of known actions of insulin in neuroendocrine function, and also to highlight areas where further understanding of insulin actions in the brain hold great promise for further improvements in human health.

A historical perspective on the interactions of insulin at the blood-brain barrier

Subsequent to the discovery of insulin in 1921, the role of insulin in the brain has been investigated throughly. The ability of insulin to act within the brain to regulate peripheral glucose levels helped evolve the research surrounding the ability of insulin to be transported into the brain. Investigations aiming to determine the transport of insulin into the brain from the circulation soon followed. Once it was established that insulin could enter the brain, the ability of insulin to bind brain microvessels and regulators of this process were determined. As technology advanced, quantitative measurements to specify the transport rate of insulin across the blood-brain barrier (BBB) and the impact of physiological conditions and diseases were the logical next steps. Lastly, with the advent of genetic mouse models and high-specificity antagonists, the specific role of the insulin receptor in mediating insulin transport could begin to be explored. In this review, we summarise the main findings throughout the decades regarding the interactions of insulin at the BBB.

Olfactory dysfunction predicts the development of dementia in older patients with type 2 diabetes

AIMS

Olfactory dysfunction is associated with the transition from normal cognition to dementia in persons without type 2 diabetes. This study aimed to investigate whether olfactory dysfunction could be an early marker of future dementia in older patients with type 2 diabetes.

METHODS

This exploratory study included 151 older Japanese outpatients with type 2 diabetes who did not have a diagnosis of probable dementia at baseline. A multivariate logistic regression model was used to determine whether Open Essence (OE) test score at baseline is associated with the development of probable dementia.

RESULTS

Over 3 years, approximately 9% of the study subjects developed probable dementia. Subjects with olfactory dysfunction at baseline developed probable dementia more frequently than those without. Multivariate logistic regression showed that lower OE test score, higher age, lower Mini-Mental State Examination (MMSE) score, higher total protein concentration, and more frequent use of a sulfonylurea are significantly associated with the development of probable dementia. Stepwise multivariate regression analysis demonstrated that change in OE test score over 3 years is significantly associated with change in MMSE score.

CONCLUSIONS

Our study suggested that olfactory dysfunction precedes the development of probable dementia in older patients with type 2 diabetes.

Intranasal insulin improves the structure-function of the brain mitochondrial ATP-sensitive Ca2+ activated potassium channel and respiratory chain activities under diabetic conditions

Although it is well established that diabetes impairs mitochondrial respiratory chain activity, little is known of the effects of intranasal insulin (INI) on the mitochondrial respiratory chain and structure-function of mitoBKCa channel in diabetes. We have investigated this mechanism in an STZ-induced early type 2 diabetic model. Single ATP-sensitive mitoBKCa channel activity was considered in diabetic and INI-treated rats using a channel incorporated into the bilayer lipid membrane. Because mitoBKCa channels have been involved in mitochondrial respiratory chain activity, a study was undertaken to investigate whether the NADH, complexes I and IV, mitochondrial ROS production, and ΔΨm are altered in an early diabetic model. In this work, we provide evidence for a significant decrease in channel open probability and conductance in diabetic rats. Evidence has been shown that BKCa channel β2 subunits induce a left shift in the BKCa channel voltage dependent curve in low Ca2+ conditions,; our results indicated a significant decrease in mitoBKCa β2 subunits using Western blot analysis. Importantly, INI treatment improved mitoBKCa channel behaviors and β2 subunits expression up to ~70%. We found that early diabetes decreased activities of complex I and IV and increased NADH, ROS production, and ΔΨm. Surprisingly, INI modified the mitochondrial respiratory chain, ROS production, and ΔΨm up to ~70%. Our results thus demonstrate an INI improvement in respiratory chain activity and ROS production in brain mitochondrial preparations coming from the STZ early diabetic rat model, an effect potentially linked to INI improvement in mitoBKCa channel activity and channel β2 subunit expression.

Bioactivity of natural biflavonoids in metabolism-related disease and cancer therapies

Natural biflavonoids, such as amentoflavone, bilobetin, ginkgetin, isoginkgetin, taiwaniaflavone, morelloflavone, delicaflavone, hinokiflavone, and other derivatives (~ 40 biflavonoids), are isolated from Selaginella sp., Ginkgo biloba, Garcinia sp., and several other species of plants. They are able to exert therapeutic benefits by regulating several proteins/enzymes (PPAR-γ, CCAAT/enhancer-binding protein α [C/EBPα], STAT5, pancreatic lipase, PTP1B, fatty acid synthase, α-glucosidase [AG]) and insulin signaling pathways (via PI3K-AKT), which are linked to metabolism, cell growth, and cell survival mechanisms. Deregulated insulin signaling can cause complications of obesity and diabetes, which can lead to cognitive disorders such as Alzheimer's, Parkinson's, and dementia; therefore, the therapeutic benefits of these biflavones in these areas are highlighted. Since biflavonoids have shown potential to regulate metabolism, growth- and survival-related protein/enzymes, their relation to tumor growth and metastasis of cancer associated with angiogenesis are highlighted. The translational role of biflavones in cancer with respect to the inhibition of metabolism-related processes/pathways, enzymes, or proteins, such as STAT3/SHP-1/PTEN, kinesins, tissue kallikreins, aromatase, estrogen, protein modifiers, antioxidant, autophagy, and apoptosis induction mechanisms, are discussed. Finally, considering their observed bioactivity potential, oral bioavailability studies of biflavones and related clinical trials are outlined.

Glycine and N-acetylcysteine (GlyNAC) supplementation in older adults improves glutathione deficiency, oxidative stress, mitochondrial dysfunction, inflammation, insulin resistance, endothelial dysfunction, genotoxicity, muscle strength, and cognition: Results of a pilot clinical trial

BACKGROUND

Oxidative stress (OxS) and mitochondrial dysfunction are implicated as causative factors for aging. Older adults (OAs) have an increased prevalence of elevated OxS, impaired mitochondrial fuel-oxidation (MFO), elevated inflammation, endothelial dysfunction, insulin resistance, cognitive decline, muscle weakness, and sarcopenia, but contributing mechanisms are unknown, and interventions are limited/lacking. We previously reported that inducing deficiency of the antioxidant tripeptide glutathione (GSH) in young mice results in mitochondrial dysfunction, and that supplementing GlyNAC (combination of glycine and N-acetylcysteine [NAC]) in aged mice improves naturally-occurring GSH deficiency, mitochondrial impairment, OxS, and insulin resistance. This pilot trial in OA was conducted to test the effect of GlyNAC supplementation and withdrawal on intracellular GSH concentrations, OxS, MFO, inflammation, endothelial function, genotoxicity, muscle and glucose metabolism, body composition, strength, and cognition.

METHODS

A 36-week open-label clinical trial was conducted in eight OAs and eight young adults (YAs). After all the participants underwent an initial (pre-supplementation) study, the YAs were released from the study. OAs were studied again after GlyNAC supplementation for 24 weeks, and GlyNAC withdrawal for 12 weeks. Measurements included red-blood cell (RBC) GSH, MFO; plasma biomarkers of OxS, inflammation, endothelial function, glucose, and insulin; gait-speed, grip-strength, 6-min walk test; cognitive tests; genomic-damage; glucose-production and muscle-protein breakdown rates; and body-composition.

RESULTS

GlyNAC supplementation for 24 weeks in OA corrected RBC-GSH deficiency, OxS, and mitochondrial dysfunction; and improved inflammation, endothelial dysfunction, insulin-resistance, genomic-damage, cognition, strength, gait-speed, and exercise capacity; and lowered body-fat and waist-circumference. However, benefits declined after stopping GlyNAC supplementation for 12 weeks.

CONCLUSIONS

GlyNAC supplementation for 24-weeks in OA was well tolerated and lowered OxS, corrected intracellular GSH deficiency and mitochondrial dysfunction, decreased inflammation, insulin-resistance and endothelial dysfunction, and genomic-damage, and improved strength, gait-speed, cognition, and body composition. Supplementing GlyNAC in aging humans could be a simple and viable method to promote health and warrants additional investigation.

Saroglitazar, a novel dual PPAR-α/γ agonist, reverses high fat-low protein diet-induced metabolic and cognitive aberrations in C57BL/6J male mice

AIMS

Insulin resistance (IR) has become one of the major causative factors for the pathogenesis of various metabolic and neurometabolic diseases. The sedentary lifestyle in association with the consumption of protein-deficient and high-calorie diet results in IR development. This study was aimed to evaluate the neuroprotective effects of Saroglitazar (SGZ), a dual peroxisome-proliferator activated receptor (PPAR-α/γ) in a high fat-low protein diet (HFLPD) fed mouse model of MetS and associated cognitive deficits.

METHODS

Adult male C57BL/6J mice were fed with HFLPD plus 15% oral fructose solution for 16 weeks. Starting at the 13th week, SGZ (5 & 10 mg/kg; p.o.) was administered along with HFLPD for four weeks, i.e., the 12th to 16th week of the study groups. Various physiological, serum metabolic, neurobehavioral, neuroinflammatory, and oxidative stress parameters were assessed. The brain histopathology and mRNA expression of diverse genes in specific brain regions were also estimated.

RESULTS

The treatment with SGZ at both doses have significantly reversed various HFLPD-induced metabolic and cognitive alterations by improving the glucose and lipid profile in the periphery in addition to the enhanced cerebral glucose homeostasis, BBB integrity, reduced oxidative stress, and neuroinflammation. Furthermore, the SGZ improved locomotion and memory retention while reducing the HFLPD-induced anxiety-like behaviors in the mice.

CONCLUSIONS

SGZ treatment showed significant metabo-neuroprotective effects in mice fed with HFLPD, possibly through peripherally mediated activation of PPAR-α/γ and insulin downstream signaling in the cortex and hippocampus.

The Association between Hepatic Encephalopathy and Diabetic Encephalopathy: The Brain-Liver Axis

Hepatic encephalopathy (HE) is one of the main consequences of liver disease and is observed in severe liver failure and cirrhosis. Recent studies have provided significant evidence that HE shows several neurological symptoms including depressive mood, cognitive dysfunction, impaired circadian rhythm, and attention deficits as well as motor disturbance. Liver disease is also a risk factor for the development of diabetes mellitus. Diabetic encephalopathy (DE) is characterized by cognitive dysfunction and motor impairment. Recent research investigated the relationship between metabolic changes and the pathogenesis of neurological disease, indicating the importance between metabolic organs and the brain. Given that a diverse number of metabolites and changes in the brain contribute to neurologic dysfunction, HE and DE are emerging types of neurologic disease. Here, we review significant evidence of the association between HE and DE, and summarise the common risk factors. This review may provide promising therapeutic information and help to design a future metabolic organ-related study in relation to HE and DE.

Sweet but Bitter: Focus on Fructose Impact on Brain Function in Rodent Models

Fructose consumption has drastically increased during the last decades due to the extensive commercial use of high-fructose corn syrup as a sweetener for beverages, snacks and baked goods. Fructose overconsumption is known to induce obesity, dyslipidemia, insulin resistance and inflammation, and its metabolism is considered partially responsible for its role in several metabolic diseases. Indeed, the primary metabolites and by-products of gut and hepatic fructolysis may impair the functions of extrahepatic tissues and organs. However, fructose itself causes an adenosine triphosphate (ATP) depletion that triggers inflammation and oxidative stress. Many studies have dealt with the effects of this sugar on various organs, while the impact of fructose on brain function is, to date, less explored, despite the relevance of this issue. Notably, fructose transporters and fructose metabolizing enzymes are present in brain cells. In addition, it has emerged that fructose consumption, even in the short term, can adversely influence brain health by promoting neuroinflammation, brain mitochondrial dysfunction and oxidative stress, as well as insulin resistance. Fructose influence on synaptic plasticity and cognition, with a major impact on critical regions for learning and memory, was also reported. In this review, we discuss emerging data about fructose effects on brain health in rodent models, with special reference to the regulation of food intake, inflammation, mitochondrial function and oxidative stress, insulin signaling and cognitive function.

Exploiting Common Aspects of Obesity and Alzheimer's Disease

Alzheimer’s disease (AD) is an example of age-related dementia, and there are still no known preventive or curative measures for this disease. Obesity and associated metabolic changes are widely accepted as risk factors of age-related cognitive decline. Insulin is the prime mediator of metabolic homeostasis, which is impaired in obesity, and this impairment potentiates amyloid-β (Aβ) accumulation and the formation of neurofibrillary tangles (NFTs). Obesity is also linked with functional and morphological alterations in brain mitochondria leading to brain insulin resistance (IR) and memory deficits associated with AD. Also, increased peripheral inflammation and oxidative stress due to obesity are the main drivers that increase an individual’s susceptibility to cognitive deficits, thus doubling the risk of AD. This enhanced risk of AD is alarming in the context of a rapidly increasing global incidence of obesity and overweight in the general population. In this review, we summarize the risk factors that link obesity with AD and emphasize the point that the treatment and management of obesity may also provide a way to prevent AD.