High-fat diet aggravates amyloid-beta and tau pathologies in the 3xTg-AD mouse model

Acarbose ameliorates Western diet-induced metabolic and cognitive impairments in the 3xTg mouse model of Alzheimer's disease

Age is the greatest risk factor for Alzheimer's disease (AD) as well as for other disorders that increase the risk of AD such as diabetes and obesity. There is growing interest in determining if interventions that promote metabolic health can prevent or delay AD. Acarbose is an anti-diabetic drug that not only improves glucose homeostasis, but also extends the lifespan of wild-type mice. Here, we test the hypothesis that acarbose will not only preserve metabolic health, but also slow or prevent AD pathology and cognitive deficits in 3xTg mice, a model of AD, fed either a Control diet or a high-fat, high-sucrose Western diet (WD). We find that acarbose decreases the body weight and adiposity of WD-fed 3xTg mice, increasing energy expenditure while also stimulating food consumption, and improves glycemic control. Both male and female WD-fed 3xTg mice have worsened cognitive deficits than Control-fed mice, and these deficits are ameliorated by acarbose treatment. Molecular and histological analysis of tau and amyloid pathology identified sex-specific effects of acarbose which are uncoupled from the dramatic improvements in cognition in females, suggesting that the benefits of acarbose on AD may be largely driven by improved metabolic health. In conclusion, our results suggest that acarbose may be a promising intervention to prevent, delay, or even treat AD, especially in individuals consuming a WD.

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

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

Age-dependent impact of streptozotocin on metabolic endpoints and Alzheimer's disease pathologies in 3xTg-AD mice

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease with a complex origin, thought to involve a combination of genetic, biological and environmental factors. Insulin dysfunction has emerged as a potential factor contributing to AD pathogenesis, particularly in individuals with diabetes, and among those with insulin deficiency or undergoing insulin therapy. The intraperitoneal administration of streptozotocin (STZ) is widely used in rodent models to explore the impact of insulin deficiency on AD pathology, although prior research predominantly focused on young animals, with no comparative analysis across different age groups. Our study aimed to fill this gap by analyzing the impact of insulin dysfunction in 7 and 23 months 3xTg-AD mice, that exhibit both amyloid and tau pathologies. Our objective was to elucidate the age-specific consequences of insulin deficiency on AD pathology. STZ administration led to insulin deficiency in the younger mice, resulting in an increase in cortical amyloid-β (Aβ) and tau aggregation, while tau phosphorylation was not significantly affected. Conversely, older mice displayed an unexpected resilience to the peripheral metabolic impact of STZ, while exhibiting an increase in both tau phosphorylation and aggregation without significantly affecting amyloid pathology. These changes were paralleled with alterations in signaling pathways involving tau kinases and phosphatases. Several markers of blood-brain barrier (BBB) integrity declined with age in 3xTg-AD mice, which might have facilitated a direct neurotoxic effect of STZ in older mice. Overall, our research confirms the influence of insulin signaling dysfunction on AD pathology, but also advises careful interpretation of data related to STZ-induced effects in older animals.

Time-dependent effects of high-fat diet on cognition and cerebral insulin signaling: Window for recovery and potential therapeutic target

While high-fat diet (HFD)-induced obesity is a major threat to global public health, the effect of HFD on cognition and insulin signaling during ageing remains controversial. The aim of this study was to characterize the dynamic alterations in cognition and cerebral insulin signaling during 6-month HFD consumption, and to investigate the potential therapeutic target and optimal timing to rescue obesity-related cognitive deficits. In the present study, impaired memory retention induced by 2-month HFD was recovered after 4 months on HFD. Prolonged (6-month) HFD did not further enhance tau hyperphosphorylation and β-amyloid deposition, which was consistent with the alleviation of memory retention. In brain insulin signaling, 2-month HFD increased IRS-1 and p-IRS-1(Ser307)/IRS-1, while decreasing pAKT(Ser473)/AKT, PI3K and mTOR; 4-month HFD decreased IRS-1 and pAKT(Ser473)/AKT, while increasing AKT; 6-month HFD increased IRS-1, pAKT(Ser473)/AKT, and mTOR, while decreasing p-IRS-1(Ser307)/IRS-1, PI3K and AKT. Notably, bioinformatic analysis revealed a rhythmic process presented only in 4-month HFD group, with Srebf1 emerging as a link between circadian rhythms and insulin signaling pathway. These results suggest that prolonged HFD prevents further cognitive decline and the progression of Alzheimer's disease (AD)-related pathologies during ageing. Moreover, there may be a window for recovery, in which Srebf1 acts as a self-recovery switch to address obesity-related cognitive disorders in elders.

Acarbose ameliorates Western diet-induced metabolic and cognitive impairments in the 3xTg mouse model of Alzheimer's disease

Age is the greatest risk factor for Alzheimer's disease (AD) as well as for other disorders that increase the risk of AD such as diabetes and obesity. There is growing interest in determining if interventions that promote metabolic health can prevent or delay AD. Acarbose is an anti-diabetic drug that not only improves glucose homeostasis, but also extends the lifespan of wild-type mice. Here, we test the hypothesis that acarbose will not only preserve metabolic health, but also slow or prevent AD pathology and cognitive deficits in 3xTg mice, a model of AD, fed either a Control diet or a high-fat, high-sucrose Western diet (WD). We find that acarbose decreases the body weight and adiposity of WD-fed 3xTg mice, increasing energy expenditure while also stimulating food consumption, and improves glycemic control. Both male and female WD-fed 3xTg mice have worsened cognitive deficits than Control-fed mice, and these deficits are ameliorated by acarbose treatment. Molecular and histological analysis of tau and amyloid pathology identified sex-specific effects of acarbose which are uncoupled from the dramatic improvements in cognition, suggesting that the benefits of acarbose on AD are largely driven by improved metabolic health. In conclusion, our results suggest that acarbose may be a promising intervention to prevent, delay, or even treat AD, especially in individuals consuming a Western diet.

Effect of astaxanthin in type-2 diabetes -induced APPxhQC transgenic and NTG mice

OBJECTIVES

Aggregation and misfolding of amyloid beta (Aβ) and tau proteins, suggested to arise from post-translational modification processes, are thought to be the main cause of Alzheimer's disease (AD). Additionally, a plethora of evidence exists that links metabolic dysfunctions such as obesity, type 2 diabetes (T2D), and dyslipidemia to the pathogenesis of AD. We thus investigated the combinatory effect of T2D and human glutaminyl cyclase activity (pyroglutamylation), on the pathology of AD and whether astaxanthin (ASX) treatment ameliorates accompanying pathophysiological manifestations.

METHODS

Male transgenic AD mice, APPxhQC, expressing human APP751 with the Swedish and the London mutation and human glutaminyl cyclase (hQC) enzyme and their non-transgenic (NTG) littermates were used. Both APPxhQC and NTG mice were allocated to 3 groups, control, T2D-control, and T2D-ASX. Mice were fed control or high fat diet ± ASX for 13 weeks starting at an age of 11-12 months. High fat diet fed mice were further treated with streptozocin for T2D induction. Effects of genotype, T2D induction, and ASX treatment were evaluated by analysing glycemic readouts, lipid concentration, Aβ deposition, hippocampus-dependent cognitive function and nutrient sensing using immunosorbent assay, ELISA-based assays, western blotting, immunofluorescence staining, and behavioral testing via Morris water maze (MWM), respectively.

RESULTS

APPxhQC mice presented a higher glucose sensitivity compared to NTG mice. T2D-induced brain dysfunction was more severe in NTG compared to the APPxhQC mice. T2D induction impaired memory functions while increasing hepatic LC3B, ABCA1, and p65 levels in NTG mice. T2D induction resulted in a progressive shift of Aβ from the soluble to insoluble form in APPxhQC mice. ASX treatment reversed T2D-induced memory dysfunction in NTG mice and in parallel increased hepatic pAKT while decreasing p65 and increasing cerebral p-S6rp and p65 levels. ASX treatment reduced soluble Aβ38 and Aβ40 and insoluble Aβ40 levels in T2D-induced APPxhQC mice.

CONCLUSIONS

We demonstrate that T2D induction in APPxhQC mice poses additional risk for AD pathology as seen by increased Aβ deposition. Although ASX treatment reduced Aβ expression in T2D-induced APPxhQC mice and rescued T2D-induced memory impairment in NTG mice, ASX treatment alone may not be effective in cases of T2D comorbidity and AD.

The Role of miR-137 in Neurodegenerative Disorders

Neurodegenerative diseases affect an increasing part of the population of modern societies, burdening healthcare systems and causing immense suffering at the personal level. The pathogenesis of several of these disorders involves dysregulation of gene expression, which depends on several molecular processes ranging from transcription to protein stability. microRNAs (miRNAs) are short non-coding RNA molecules that modulate gene expression by suppressing the translation of partially complementary mRNAs. miR-137 is a conserved, neuronally enriched miRNA that is implicated in neurodegeneration. Here, we review the current body of knowledge about the role that miR-137 plays in five prominent neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and multiple sclerosis. The presented data indicate that, rather than having a general neuroprotective role, miR-137 modulates the pathology of distinct disorders differently.

Western diet increases brain metabolism and adaptive immune responses in a mouse model of amyloidosis

Diet-induced increase in body weight is a growing health concern worldwide. Often accompanied by a low-grade metabolic inflammation that changes systemic functions, diet-induced alterations may contribute to neurodegenerative disorder progression as well. This study aims to non-invasively investigate diet-induced metabolic and inflammatory effects in the brain of an APPPS1 mouse model of Alzheimer's disease. [18F]FDG, [18F]FTHA, and [18F]GE-180 were used for in vivo PET imaging in wild-type and APPPS1 mice. Ex vivo flow cytometry and histology in brains complemented the in vivo findings. 1H- magnetic resonance spectroscopy in the liver, plasma metabolomics and flow cytometry of the white adipose tissue were used to confirm metaflammatory condition in the periphery. We found disrupted glucose and fatty acid metabolism after Western diet consumption, with only small regional changes in glial-dependent neuroinflammation in the brains of APPPS1 mice. Further ex vivo investigations revealed cytotoxic T cell involvement in the brains of Western diet-fed mice and a disrupted plasma metabolome. 1H-magentic resonance spectroscopy and immunological results revealed diet-dependent inflammatory-like misbalance in livers and fatty tissue. Our multimodal imaging study highlights the role of the brain-liver-fat axis and the adaptive immune system in the disruption of brain homeostasis in amyloid models of Alzheimer's disease.

High-Fat Diets in Animal Models of Alzheimer's Disease: How Can Eating Too Much Fat Increase Alzheimer's Disease Risk?

High dietary intake of saturated fatty acids is a suspected risk factor for neurodegenerative diseases, including Alzheimer's disease (AD). To decipher the causal link behind these associations, high-fat diets (HFD) have been repeatedly investigated in animal models. Preclinical studies allow full control over dietary composition, avoiding ethical concerns in clinical trials. The goal of the present article is to provide a narrative review of reports on HFD in animal models of AD. Eligibility criteria included mouse models of AD fed a HFD defined as > 35% of fat/weight and western diets containing > 1% cholesterol or > 15% sugar. MEDLINE and Embase databases were searched from 1946 to August 2022, and 32 preclinical studies were included in the review. HFD-induced obesity and metabolic disturbances such as insulin resistance and glucose intolerance have been replicated in most studies, but with methodological variability. Most studies have found an aggravating effect of HFD on brain Aβ pathology, whereas tau pathology has been much less studied, and results are more equivocal. While most reports show HFD-induced impairment on cognitive behavior, confounding factors may blur their interpretation. In summary, despite conflicting results, exposing rodents to diets highly enriched in saturated fat induces not only metabolic defects, but also cognitive impairment often accompanied by aggravated neuropathological markers, most notably Aβ burden. Although there are important variations between methods, particularly the lack of diet characterization, these studies collectively suggest that excessive intake of saturated fat should be avoided in order to lower the incidence of AD.

Interaction Between a High-Fat Diet and Tau Pathology in Mice: Implications for Alzheimer's Disease

BACKGROUND

Obesity is a modifiable risk factor for Alzheimer's disease (AD). However, its relation with tau pathology (i.e., aberrant tau protein behavior in tauopathies such as AD) has been inconclusive.

OBJECTIVE

This study investigated the interaction between a high-fat diet (HFD) and tau pathology in adult male mice.

METHODS

Transgenic mice overexpressing human P301S Tau (those with the pathology) and wild-type (WT) littermates were subjected to behavioral tests, functional magnetic resonance imaging (fMRI), diffusion tensor imaging (DTI), and western blotting analysis to investigate the effects of prolonged HFD versus regular diet during adulthood.

RESULTS

HFD increased body weight in both WT and P301S mice but had minimal effect on blood glucose levels. The brain response to HFD was tau genotype-specific. WT mice exhibited decreased recognition memory and enhanced network connectivity in fMRI, while P301S mice exhibited white matter tract disorganization in DTI as the sole significant finding. The reduction of insulin receptor β, insulin downstream signaling, neuronal nuclear protein, CD68-positive phagocytic activity, and myelin basic protein level were confined to the cortex of WT mice. In contrast to P301S mice, WT mice showed significant changes in the tau protein and its phosphorylation levels along with increased soluble neurofilament light levels in the hippocampus.

CONCLUSIONS

HFD-induced brain dysfunction and pathological changes were blunted in mice with the pathology and more profound in healthy mice. Our findings highlight the need to consider this interaction between obesity and tau pathology when tailoring treatment strategies for AD and other tauopathies.

The complex relationship between obesity and neurodegenerative diseases: an updated review

Obesity is a global epidemic, affecting roughly 30% of the world's population and predicted to rise. This disease results from genetic, behavioral, societal, and environmental factors, leading to excessive fat accumulation, due to insufficient energy expenditure. The adipose tissue, once seen as a simple storage depot, is now recognized as a complex organ with various functions, including hormone regulation and modulation of metabolism, inflammation, and homeostasis. Obesity is associated with a low-grade inflammatory state and has been linked to neurodegenerative diseases like multiple sclerosis (MS), Alzheimer's (AD), and Parkinson's (PD). Mechanistically, reduced adipose expandability leads to hypertrophic adipocytes, triggering inflammation, insulin and leptin resistance, blood-brain barrier disruption, altered brain metabolism, neuronal inflammation, brain atrophy, and cognitive decline. Obesity impacts neurodegenerative disorders through shared underlying mechanisms, underscoring its potential as a modifiable risk factor for these diseases. Nevertheless, further research is needed to fully grasp the intricate connections between obesity and neurodegeneration. Collaborative efforts in this field hold promise for innovative strategies to address this complex relationship and develop effective prevention and treatment methods, which also includes specific diets and physical activities, ultimately improving quality of life and health.

High-fat diet negatively affects brain markers, cognitive behaviors, and noncognitive behaviors in the rTg4510 tau mouse model

Alzheimer's disease (AD) drastically impacts cognitive and noncognitive behaviors in both humans and animal models. Two hallmark proteins in AD, amyloid-β plaques and tau neurofibrillary tangles, accumulate in regions of the brain critical for learning and memory, including the hippocampus. Poor dietary choices have been shown to exacerbate cognitive deficits seen in AD. In this study, we assessed the effects of a high-fat diet (HFD - 60 kcal% fat) on cognitive & noncognitive behaviors as well as on brain markers in the rTg4510 tau mouse model. While all mice learned the Morris Water Maze (MWM) task, it was noted that on the last day of acquisition female tau mice had a significantly higher latency to find the platform than male tau mice (p < 0.01). Mice given the HFD spent significantly less time in the target quadrant than those given a control diet (CD) (p < 0.05). Tau mice showed impaired burrowing (p < 0.05) and nesting behaviors (p < 0.001) compared to WT mice and HFD administration worsened burrowing in tau mice. Tau mice exhibited greater levels of glial fibrillary acidic protein (GFAP) (p < 0.05) and significantly less hippocampal cell density than WT mice (p < 0.001). We observed trends of HFD mice having greater levels of GFAP and greater average tangle size than CD mice. These results highlight the importance of dietary choices, especially in older populations more susceptible to AD and its effects.

Higher angiotensin-converting enzyme 2 (ACE2) levels in the brain of individuals with Alzheimer's disease

Cognitive decline due to Alzheimer's disease (AD) is frequent in the geriatric population, which has been disproportionately affected by the COVID-19 pandemic. In this study, we investigated the levels of angiotensin-converting enzyme 2 (ACE2), a regulator of the renin-angiotensin system and the main entry receptor of SARS-CoV-2 in host cells, in postmortem parietal cortex samples from two independent AD cohorts, totalling 142 persons. Higher concentrations of ACE2 protein (p < 0.01) and mRNA (p < 0.01) were found in individuals with a neuropathological diagnosis of AD compared to age-matched healthy control subjects. Brain levels of soluble ACE2 were inversely associated with cognitive scores (p = 0.02) and markers of pericytes (PDGFRβ, p = 0.02 and ANPEP, p = 0.007), but positively correlated with concentrations of soluble amyloid-β peptides (Aβ) (p = 0.01) and insoluble phospho-tau (S396/404, p = 0.002). However, no significant differences in ACE2 were observed in the 3xTg-AD mouse model of tau and Aβ neuropathology. Results from immunofluorescence and Western blots showed that ACE2 protein is predominantly localized in microvessels in the mouse brain whereas it is more frequently found in neurons in the human brain. The present data suggest that higher levels of soluble ACE2 in the human brain may contribute to AD, but their role in CNS infection by SARS-CoV-2 remains unclear.

Sex differences of microglia in the healthy brain from embryonic development to adulthood and across lifestyle influences

Microglia, the central nervous system innate immune cells, play a critical role in maintaining a homeostatic environment in the brain throughout life. These cells exhibit an impressive range of functions and characteristics that help to ensure proper functioning of the brain. Notably, microglia can present differences in their genetic and physical traits, which can be influenced by a range of factors, including age, environmental exposures, disease, and sex. Remarkably, microglia have been found to express receptors for sex hormones, suggesting that these hormones may play a role in modulating microglial behavior and potentially contribute to sex differences. Additionally, sex-chromosomal factors were shown to impact microglial genetics and functioning. In this review, we will examine how microglial responses in homeostasis are impacted by their interaction with sex hormones and sex chromosomes. Specifically, our investigation will focus on examining this interaction from embryonic development to adulthood, and the influence of lifestyle elements on various microglial features, including density and distribution, morphology, transcriptome, and proteome.

Feeding High-Fat Diet Accelerates Development of Peripheral and Central Insulin Resistance and Inflammation and Worsens AD-like Pathology in APP/PS1 Mice

Alzheimer's disease (AD) is a progressive brain disorder characterized by extracellular amyloid-β (Aβ) plaques, intracellular neurofibrillary tangles formed by hyperphosphorylated Tau protein and neuroinflammation. Previous research has shown that obesity and type 2 diabetes mellitus, underlined by insulin resistance (IR), are risk factors for neurodegenerative disorders. In this study, obesity-induced peripheral and central IR and inflammation were studied in relation to AD-like pathology in the brains and periphery of APP/PS1 mice, a model of Aβ pathology, fed a high-fat diet (HFD). APP/PS1 mice and their wild-type controls fed either a standard diet or HFD were characterized at the ages of 3, 6 and 10 months by metabolic parameters related to obesity via mass spectroscopy, nuclear magnetic resonance, immunoblotting and immunohistochemistry to quantify how obesity affected AD pathology. The HFD induced substantial peripheral IR leading to central IR. APP/PS1-fed HFD mice had more pronounced IR, glucose intolerance and liver steatosis than their WT controls. The HFD worsened Aβ pathology in the hippocampi of APP/PS1 mice and significantly supported both peripheral and central inflammation. This study reveals a deleterious effect of obesity-related mild peripheral inflammation and prediabetes on the development of Aβ and Tau pathology and neuroinflammation in APP/PS1 mice.

Obesity, Cardiovascular and Neurodegenerative Diseases: Potential Common Mechanisms

The worldwide increase in the incidence of obesity and cardiovascular and neurodegenerative diseases, e.g. Alzheimer's disease, is related to many factors, including an unhealthy lifestyle and aging populations. However, the interconnection between these diseases is not entirely clear, and it is unknown whether common mechanisms underlie these conditions. Moreover, there are currently no fully effective therapies for obesity and neurodegeneration. While there has been extensive research in preclinical models addressing these issues, the experimental findings have not been translated to the clinic. Another challenge relates to the time of onset of individual diseases, which may not be easily identified, since there are no specific indicators or biomarkers that define disease onset. Hence knowing when to commence preventive treatment is unclear. This is especially pertinent in neurodegenerative diseases, where the onset of the disease may be subtle and occur decades before the signs and symptoms manifest. In metabolic and cardiovascular disorders, the risk may occur in-utero, in line with the concept of fetal programming. This review provides a brief overview of the link between obesity, cardiovascular and neurodegenerative diseases and discusses potential common mechanisms including the role of the gut microbiome.

Prenatal alcohol exposure causes persistent microglial activation and age- and sex- specific effects on cognition and metabolic outcomes in an Alzheimer's Disease mouse model

Background: Prenatal alcohol exposure (PAE) causes behavioral deficits and increases risk of metabolic diseases. Alzheimer's Disease (AD) is a neurodegenerative disease that has a higher risk in adults with metabolic diseases. Both present with persistent neuroinflammation.Objectives: We tested whether PAE exacerbates AD-related cognitive decline in a mouse model (3xTg-AD; presenilin/amyloid precursor protein/tau), and assessed associations among cognition, metabolic impairment, and microglial reactivity.Methods: Alcohol-exposed (ALC) pregnant 3xTg-AD mice received 3 g/kg alcohol from embryonic day 8.5-17.5. We evaluated recognition memory and associative memory (fear conditioning) in 8-10 males and females per group at 3 months of age (3mo), 7mo, and 11mo, then assessed glucose tolerance, body composition, and hippocampal microglial activation at 12mo.Results: ALC females had higher body weights than controls from 5mo (p < .0001). Controls showed improved recognition memory at 11mo compared with 3mo (p = .007); this was not seen in ALC mice. Older animals froze more during fear conditioning than younger, and ALC mice were hyper-responsive to the fear-related cue (p = .017). Fasting blood glucose was lower in ALC males and higher in ALC females than controls. Positive associations occurred between glucose and fear-related context (p = .04) and adiposity and fear-related cue (p = .0002) in ALC animals. Hippocampal microglial activation was higher in ALC than controls (p < .0001); this trended to correlate with recognition memory.Conclusions: ALC animals showed age-related cognitive impairments that did not interact with AD risk but did correlate with metabolic dysfunction and somewhat with microglial activation. Thus, metabolic disorders may be a therapeutic target for people with FASDs.

Cerebrospinal Fluid Metabolomics: Pilot Study of Using Metabolomics to Assess Diet and Metabolic Interventions in Alzheimer's Disease and Mild Cognitive Impairment

Brain glucose hypometabolism is an early sign of Alzheimer's disease (AD), and interventions which offset this deficit, such as ketogenic diets, show promise as AD therapeutics. Conversely, high-fat feeding may exacerbate AD risk. We analyzed the metabolomic profile of cerebrospinal fluid (CSF) in a pilot study of older adults who underwent saline and triglyceride (TG) infusions. Older adults (12 cognitively normal (CN), age 65.3 ± 8.1, and 9 with cognitive impairment (CI), age 70.9 ± 8.6) underwent a 5 h TG or saline infusion on different days using a random crossover design; CSF was collected at the end of infusion. Aqueous metabolites were measured using a targeted mass spectroscopy (MS) platform focusing on 215 metabolites from over 35 different metabolic pathways. Data were analyzed using MetaboAnalyst 4.0 and SAS. Of the 215 targeted metabolites, 99 were detectable in CSF. Only one metabolite significantly differed by treatment: the ketone body 3-hydroxybutyrate (HBA). Post hoc analyses showed that HBA levels were associated with age and markers of metabolic syndrome and demonstrated different correlation patterns for the two treatments. When analyzed by cognitive diagnosis group, TG-induced increases in HBA were over 3 times higher for those with cognitive impairment (change score CN +9.8 uM ± 8.3, CI +32.4 ± 7.4, p = 0.0191). Interestingly, individuals with cognitive impairment had higher HBA levels after TG infusion than those with normal cognition. These results suggest that interventions that increase plasma ketones may lead to higher brain ketones in groups at risk for AD and should be confirmed in larger intervention studies.

Eicosapentaenoic Acid Protects against Metabolic Impairments in the APPswe/PS1dE9 Alzheimer's Disease Mouse Model

BACKGROUND

Alzheimer's disease (AD) is an age-related neurodegenerative disease characterized by amyloid-β (Aβ) plaques. Systemic inflammation and obesity may exacerbate AD pathogenesis. We previously reported anti-inflammatory and anti-obesity effects of EPA in mice.

OBJECTIVES

We aimed to determine whether EPA reduces obesity-associated metabolic dysfunctions and Aβ accumulation in AD amyloidogenic mice.

METHODS

Two-mo-old APPswe/PS1dE9 transgenic (TG) mice and non-TG littermates were randomly assigned to low fat (LF; 10% kcal fat), high fat (HF; 45% kcal fat), or EPA (36 g/kg)-supplemented HF diets. Body composition, glucose tolerance, and energy expenditure were measured, and serum and brain metabolic markers were tested 38 wk postintervention. Outcomes were statistically analyzed via 3-factor ANOVA, modeling genotype, sex, and diet interactions.

RESULTS

HF-fed males gained more weight than females (Δ = 61 mg; P < 0.001). Compared with LF, HF increased body weights of wild-type (WT) males (Δ = 31 mg; P < 0.001). EPA reduced HF-induced weight gain in WT males (Δ = 24 mg; P = 0.054) but not in females. HF mice showed decreased glucose clearance and respiratory energy compared with LF-fed groups (Δ = -1.31 g/dL; P < 0.001), with no significant effects of EPA. However, EPA conferred metabolic improvements by decreasing serum leptin and insulin (Δ = -2.51 g/mL and Δ = -0.694 ng/mL, respectively compared with HF, P ≤ 0.05) and increasing adiponectin (Δ = 21.6 ng/mL; P < 0.001). As we expected, TG mice expressed higher serum and brain Aβ than WT mice (Δ = 0.131 ng/mL; P < 0.001 and Δ = 0.56%; P < 0.01, respectively), and EPA reduced serum Aβ1-40 in TG males compared with HF (Δ = 0.053 ng/mL; P ≤ 0.05).

CONCLUSIONS

To our knowledge, this is the first report that EPA reduces serum Aβ1-40 in obese AD male mice, warranting further investigations into tissue-specific mechanisms of EPA in AD.

Assessment of Lab4P Probiotic Effects on Cognition in 3xTg-AD Alzheimer's Disease Model Mice and the SH-SY5Y Neuronal Cell Line

Aging and metabolic syndrome are associated with neurodegenerative pathologies including Alzheimer's disease (AD) and there is growing interest in the prophylactic potential of probiotic bacteria in this area. In this study, we assessed the neuroprotective potential of the Lab4P probiotic consortium in both age and metabolically challenged 3xTg-AD mice and in human SH-SY5Y cell culture models of neurodegeneration. In mice, supplementation prevented disease-associated deteriorations in novel object recognition, hippocampal neurone spine density (particularly thin spines) and mRNA expression in hippocampal tissue implying an anti-inflammatory impact of the probiotic, more notably in the metabolically challenged setting. In differentiated human SH-SY5Y neurones challenged with β-Amyloid, probiotic metabolites elicited a neuroprotective capability. Taken together, the results highlight Lab4P as a potential neuroprotective agent and provide compelling support for additional studies in animal models of other neurodegenerative conditions and human studies.

Dietary High-Fat Promotes Cognitive Impairment by Suppressing Mitophagy

Dietary habits contribute to the characteristics of Alzheimer's disease (AD) and cognitive impairment, which are partly induced by the accumulation of hyperphosphorylated Tau, a microtubule-associated protein. In mice, a fat-rich diet facilitates cognitive dysfunction. However, the mechanism by which dietary fat damages the brain remains unclear. In this study, 13-month-old C57BL/6 mice were fed a normal or high-fat diet (HFD) for 6 months. Neuro-2a cells were incubated with the normal medium or palmitic acid (200 μM). Spatial memory was assessed utilizing a behavioral test. Further, western blotting and immunofluorescence techniques were used to determine the levels of mitophagy-related proteins. The synaptic morphology and phosphorylation of Tau proteins were also evaluated. Administration of HFD decreased the expression of synaptophysin and brain-derived neurotrophic factor expression, leading to significant damage to neurons. Tau protein hyperphosphorylation was detected at different loci both in vivo and in vitro. Significantly impaired learning and memory abilities, accompanied by impaired mitophagy-related processes, were observed in mice fed with HFD as compared to mice fed with normal food. In conclusion, high fatty-acid intake hinders mitophagy and upregulates Tau protein phosphorylation, including age-related synaptic dysfunction, which leads to cognitive decline.

Higher Angiotensin I Converting Enzyme 2 (ACE2) levels in the brain of individuals with Alzheimer's disease

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a major cause of death in the elderly. Cognitive decline due to Alzheimer's disease (AD) is frequent in the geriatric population disproportionately affected by the COVID-19 pandemic. Interestingly, central nervous system (CNS) manifestations have been reported in SARS-CoV-2-infected patients. In this study, we investigated the levels of Angiotensin I Converting Enzyme 2 (ACE2), the main entry receptor of SARS-COV-2 in cells, in postmortem parietal cortex samples from two independent AD cohorts, totalling 142 persons. Higher concentrations of ACE2 protein and mRNA were found in individuals with a neuropathological diagnosis of AD compared to age-matched healthy control subjects. Brain levels of soluble ACE2 were inversely associated with cognitive scores (p = 0.02), markers of pericytes (PDGFRβ, p=0.02 and ANPEP, p = 0.007) and caveolin1 (p = 0.03), but positively correlated with soluble amyloid-β peptides (Aβ) concentrations (p = 0.01) and insoluble phospho- tau (S396/404, p = 0.002). No significant differences in ACE2 were observed in the 3xTgAD mouse model of tau and Aβ neuropathology. Results from immunofluorescence and Western blots showed that ACE2 protein is mainly localized in neurons in the human brain but predominantly in microvessels in the mouse brain. The present data show that an AD diagnosis is associated with higher levels of soluble ACE2 in the human brain, which might contribute to a higher risk of CNS SARS-CoV-2 infection.

Type 2 Diabetes and Alzheimer's Disease: The Emerging Role of Cellular Lipotoxicity

Type 2 diabetes (T2D) and Alzheimer's diseases (AD) represent major health issues that have reached alarming levels in the last decades. Although growing evidence demonstrates that AD is a significant comorbidity of T2D, and there is a ~1.4-2-fold increase in the risk of developing AD among T2D patients, the involvement of possible common triggers in the pathogenesis of these two diseases remains largely unknown. Of note, recent mechanistic insights suggest that lipotoxicity could represent the missing ring in the pathogenetic mechanisms linking T2D to AD. Indeed, obesity, which represents the main cause of lipotoxicity, has been recognized as a major risk factor for both pathological conditions. Lipotoxicity can lead to inflammation, insulin resistance, oxidative stress, ceramide and amyloid accumulation, endoplasmic reticulum stress, ferroptosis, and autophagy, which are shared biological events in the pathogenesis of T2D and AD. In the current review, we try to provide a critical and comprehensive view of the common molecular pathways activated by lipotoxicity in T2D and AD, attempting to summarize how these mechanisms can drive future research and open the way to new therapeutic perspectives.

Cerebrovascular insulin receptors are defective in Alzheimer's disease

Central response to insulin is suspected to be defective in Alzheimer's disease. As most insulin is secreted in the bloodstream by the pancreas, its capacity to regulate brain functions must, at least partly, be mediated through the cerebral vasculature. However, how insulin interacts with the blood-brain barrier and whether alterations of this interaction could contribute to Alzheimer's disease pathophysiology both remain poorly defined. Here, we show that human and murine cerebral insulin receptors (INSRs), particularly the long isoform INSRα-B, are concentrated in microvessels rather than in the parenchyma. Vascular concentrations of INSRα-B were lower in the parietal cortex of subjects diagnosed with Alzheimer's disease, positively correlating with cognitive scores, leading to a shift towards a higher INSRα-A/B ratio, consistent with cerebrovascular insulin resistance in the Alzheimer's disease brain. Vascular INSRα was inversely correlated with amyloid-β plaques and β-site APP cleaving enzyme 1, but positively correlated with insulin-degrading enzyme, neprilysin and P-glycoprotein. Using brain cerebral intracarotid perfusion, we found that the transport rate of insulin across the blood-brain barrier remained very low (<0.03 µl/g·s) and was not inhibited by an insulin receptor antagonist. However, intracarotid perfusion of insulin induced the phosphorylation of INSRβ that was restricted to microvessels. Such an activation of vascular insulin receptor was blunted in 3xTg-AD mice, suggesting that Alzheimer's disease neuropathology induces insulin resistance at the level of the blood-brain barrier. Overall, the present data in post-mortem Alzheimer's disease brains and an animal model of Alzheimer's disease indicate that defects in the insulin receptor localized at the blood-brain barrier strongly contribute to brain insulin resistance in Alzheimer's disease, in association with β-amyloid pathology.

Obesity-induced neuroinflammation and cognitive impairment in young adult versus middle-aged mice

BACKGROUND

Obesity rates are increasing worldwide. Obesity leads to many complications, including predisposing individuals to the development of cognitive impairment as they age. Immune dysregulation, including inflammaging (e.g., increased circulating cytokines) and immunosenescence (declining immune system function), commonly occur in obesity and aging and may impact cognitive impairment. As such, immune system changes across the lifespan may impact the effects of obesity on neuroinflammation and associated cognitive impairment. However, the role of age in obesity-induced neuroinflammation and cognitive impairment is unclear. To further define this putative relationship, the current study examined metabolic and inflammatory profiles, along with cognitive changes using a high-fat diet (HFD) mouse model of obesity.

RESULTS

First, HFD promoted age-related changes in hippocampal gene expression. Given this early HFD-induced aging phenotype, we fed HFD to young adult and middle-aged mice to determine the effect of age on inflammatory responses, metabolic profile, and cognitive function. As anticipated, HFD caused a dysmetabolic phenotype in both age groups. However, older age exacerbated HFD cognitive and neuroinflammatory changes, with a bi-directional regulation of hippocampal inflammatory gene expression.

CONCLUSIONS

Collectively, these data indicate that HFD promotes an early aging phenotype in the brain, which is suggestive of inflammaging and immunosenescence. Furthermore, age significantly compounded the impact of HFD on cognitive outcomes and on the regulation of neuroinflammatory programs in the brain.

Curcumin-Nicotinate Attenuates Hippocampal Synaptogenesis Dysfunction in Hyperlipidemia Rats by the BDNF/TrkB/CREB Pathway: Involving Idol/LDLR Signaling to Eliminate Aβ Deposition

Hyperlipidemia has been demonstrated to evoke Alzheimer disease (AD) pathologies such as Amyloid-β (Aβ) deposition and synaptogenesis dysfunction in the hippocampus. Curcumin gives protection against anti-amyloid properties and synaptogenesis dysfunction. Curcumin-Nicotinate (CurTn), a new type of curcumin derivative, ameliorates cognitive impairment by rescuing autophagic flux in the CA1 hippocampus of diabetic rats. However, whether Curtn possesses an antagonistic effect on AD-related pathologies in the hippocampus induced by hyperlipidemia remains ill-defined. The present study aims to investigate whether CurTn alleviates synaptogenesis dysfunction by promoting the activation of brain-derived neurotrophic factor (BDNF)/tyrosine kinase receptor B (TrkB)/cAMP-response element binding protein (CREB) signaling and whether the underlying fundamental mechanism involves the elimination of Aβ deposition due to Idol/low-density lipoprotein receptor (LDLR) signaling in the hippocampus of high-fat diet (HFD)-induced hyperlipidemia rats. The results demonstrated that CurTn not only improved synaptogenesis dysfunction in the hippocampus of HFD rats, as evidenced by the increases in the expressions of synapse-related proteins postsynaptic density protein 95 (PSD-95), synapsin-1, and Glutamate receptor 1 (GluR1), but also activated BDNF/TrkB/CREB signaling, as evidenced by the elevation of the expressions of BDNF, pTrkB, and CREB. Moreover, CurTn modulated the Idol/LDLR pathway in the hippocampus of HFD rats, as evidenced by the decreased expression of Idol and the increased expression of LDLR. Furthermore, CurTn eliminated the deposition of Aβ, as evidenced by the reduction in the content of Aβ40 and Aβ42. These results reveal that CurTn may attenuate synaptogenesis dysfunction by activating BDNF/TrkB/CREB signaling, as the possible result of the modulation of Idol/LDLR signaling to eliminate Aβ deposition in the hippocampus of HFD rats.

Mitigation of Memory Impairment with Fermented Fucoidan and λ-Carrageenan Supplementation through Modulating the Gut Microbiota and Their Metagenome Function in Hippocampal Amyloid-β Infused Rats

Attenuating acetylcholinesterase and insulin/insulin-like growth factor-1 signaling in the hippocampus is associated with Alzheimer’s disease (AD) development. Fucoidan and carrageenan are brown and red algae, respectively, with potent antibacterial, anti-inflammatory, antioxidant and antiviral activities. This study examined how low-molecular-weight (MW) and high-MW fucoidan and λ-carrageenan would improve memory impairment in Alzheimer’s disease-induced rats caused by an infusion of toxic amyloid-β(Aβ). Fucoidan and λ-carrageenan were dissected into low-MW by Luteolibacter algae and Pseudoalteromonas carrageenovora. Rats receiving an Aβ(25–35) infusion in the CA1 region of the hippocampus were fed dextrin (AD-Con), 1% high-MW fucoidan (AD-F-H), 1% low-MW fucoidan (AD-F-L), 1% high-MW λ-carrageenan (AD-C-H), and 1% low-MW λ-carrageenan (AD-C-L) for six weeks. Rats to receive saline infusion (Normal-Con) had an AD-Con diet. The AD-F-L group showed an improved memory function, which manifested as an enhanced Y-maze spontaneous alternation test, water maze, and passive avoidance tests, similar to the Normal-Con group. AD-F-L also potentiated hippocampal insulin signaling and increased the expression of ciliary neurotrophic factor (CNTF) and brain-derived neurotrophic factor (BDNF) in the hippocampus. AD-C-L improved the memory function mainly by increasing the BDNF content. AD-F-H and AD-C-H did not improve the memory function. Compared to AD-Con, the ascending order of AD-C-H, AD-F-H, AD-C-L, and AD-F-L increased insulin signaling by enhancing the pSTAT3→pAkt→pGSK-3β pathway. AD-F-L improved glucose tolerance the most. Compared to AD-CON, the AD-F-L treatment increased the serum acetate concentrations and compensated for the defect of cerebral glucose metabolism. AD-Con increased Clostridium, Terrisporobacter and Sporofaciens compared to Normal-Con, and AD-F-L and AD-C-L increased Akkermentia. In conclusion, AD-F-L and AD-C-L alleviated the memory function in the rats with induced AD symptoms by modulating.

The Impact of Probiotic Supplementation on Cognitive, Pathological and Metabolic Markers in a Transgenic Mouse Model of Alzheimer's Disease

Brain degenerative disorders such as Alzheimer’s disease (AD) can be exacerbated by aberrant metabolism. Supplementation with probiotic bacteria is emerging as a promising preventative strategy for both neurodegeneration and metabolic syndrome. In this study, we assess the impact of the Lab4b probiotic consortium on (i) cognitive and pathological markers of AD progression and (ii) metabolic status in 3xTg-AD mice subjected to metabolic challenge with a high fat diet. The group receiving the probiotic performed better in the novel object recognition test and displayed higher hippocampal neuronal spine density than the control group at the end of the 12 weeks intervention period. These changes were accompanied by differences in localised (brain) and systemic anti-inflammatory responses that favoured the Probiotic group together with the prevention of diet induced weight gain and hypercholesterolaemia and the modulation of liver function. Compositional differences between the faecal microbiotas of the study groups included a lower Firmicutes:Bacteroidetes ratio and less numbers of viable yeast in the Probiotic group compared to the Control. The results illustrate the potential of the Lab4b probiotic as a neuroprotective agent and encourage further studies with human participants.

High-fat diet exacerbates cognitive decline in mouse models of Alzheimer's disease and mixed dementia in a sex-dependent manner

BACKGROUND

Approximately 70% of Alzheimer's disease (AD) patients have co-morbid vascular contributions to cognitive impairment and dementia (VCID); this highly prevalent overlap of dementia subtypes is known as mixed dementia (MxD). AD is more prevalent in women, while VCID is slightly more prevalent in men. Sex differences in risk factors may contribute to sex differences in dementia subtypes. Unlike metabolically healthy women, diabetic women are more likely to develop VCID than diabetic men. Prediabetes is 3× more prevalent than diabetes and is linked to earlier onset of dementia in women, but not men. How prediabetes influences underlying pathology and cognitive outcomes across different dementia subtypes is unknown. To fill this gap in knowledge, we investigated the impact of diet-induced prediabetes and biological sex on cognitive function and neuropathology in mouse models of AD and MxD.

METHODS

Male and female 3xTg-AD mice received a sham (AD model) or unilateral common carotid artery occlusion surgery to induce chronic cerebral hypoperfusion (MxD model). Mice were fed a control or high fat (HF; 60% fat) diet from 3 to 7 months of age. In both sexes, HF diet elicited a prediabetic phenotype (impaired glucose tolerance) and weight gain.

RESULTS

In females, but not males, metabolic consequences of a HF diet were more severe in AD or MxD mice compared to WT. In both sexes, HF-fed AD or MxD mice displayed deficits in spatial memory in the Morris water maze (MWM). In females, but not males, HF-fed AD and MxD mice also displayed impaired spatial learning in the MWM. In females, but not males, AD or MxD caused deficits in activities of daily living, regardless of diet. Astrogliosis was more severe in AD and MxD females compared to males. Further, AD/MxD females had more amyloid beta plaques and hippocampal levels of insoluble amyloid beta 40 and 42 than AD/MxD males. In females, but not males, more severe glucose intolerance (prediabetes) was correlated with increased hippocampal microgliosis.

CONCLUSIONS

High-fat diet had a wider array of metabolic, cognitive, and neuropathological consequences in AD and MxD females compared to males. These findings shed light on potential underlying mechanisms by which prediabetes may lead to earlier dementia onset in women.

The Effect of Gut Microbe Dysbiosis on the Pathogenesis of Alzheimer's Disease (AD) and related conditions

It has been hypothesized that the shift in gut microbiota composition, known as gut microbe dysbiosis, may be correlated with the onset of Alzheimer's disease (AD), which is the most common cause of dementia characterized by a gradual deterioration in cognitive function associated with the development of amyloid-beta (Aβ) plaques. The gut microbiota dysbiosis induces the release of significant amounts of amyloids, lipopolysaccharides, and neurotoxins, which might play a role in modulating signaling pathways and immune activation, leading to the production of proinflammatory cytokines related to the pathogenesis of AD. The dysbiosis of gut microbe is associated with various diseases such as type 2 diabetes, obesity, hypertension, and some neuropsychiatric disorders like depression, anxiety, and stress. It is conceivable that these diseases trigger the onset of AD. Thus, modifying the gut microbiota composition with probiotic and prebiotic supplementation can reduce depression and anxiety symptoms, lower stress reactivity, and improve memory. This narrative review aimed to examine the possible role of gut microbe dysbiosis in AD's pathogenesis.

Effects of estradiol supplementation on the brain transcriptome of old rhesus macaques maintained on an obesogenic diet

Obesity, the cessation of ovarian steroids with menopause, and age are risk factors for mood disorders, dementia, and Alzheimer's disease (AD). However, immediate hormone therapy (HT) after menopause may have beneficial effects in different brain regions involved in memory and cognition. To more closely replicate the age, endocrine, and metabolic environment of obese postmenopausal women, either on or off HT, middle-aged female rhesus macaques were ovariectomized/hysterectomized (OvH) and maintained on a high-fat, high-sugar, obesogenic Western-style diet (WSD) for 30 months; half of the animals received HT immediately after OvH and half served as placebo controls. RNAseq of the occipital (OC) and prefrontal cortex (PFC), hippocampus (HIP), and amygdala (AMG) identified 293, 379, 505, and 4993 differentially expressed genes (DEGs), respectively. Pathway enrichment analysis identified an activation of neuroinflammation in OC and HIP, but an inhibition in the AMG with HT. Synaptogenesis, circadian rhythm, mitochondrial dysfunction, mTOR, glutamate, serotonin, GABA, dopamine, epinephrine/norepinephrine, glucocorticoid receptor signaling, neuronal NOS, and amyloid processing were exclusively enriched in AMG. As compared to the placebo control group, most of these signaling pathways are downregulated after HT, suggesting a protective effect of HT in OvH females under a WSD. Overall, our results suggest that a chronic obesogenic diet may induce a wide range of alterations in multiple signaling pathways that are linked to age-associated brain pathology and dementia. In these individuals, HT seems to have a protective effect against neuroinflammation, amyloid beta depositions, and tau tangle formation.

Palmitic Acid and Oleic Acid Differently Modulate TLR2-Mediated Inflammatory Responses in Microglia and Macrophages

The relationship between systemic immunity and neuroinflammation is widely recognised. Infiltration of peripheral immune cells to the CNS during certain chronic inflammatory states contributes significantly to neuropathology. Obesity and its co-morbidities are primary risk factors for neuroinflammatory and neurodegenerative conditions, including Alzheimer’s disease (AD). Dietary fats are among the most proinflammatory components of the obesogenic diet and play a prominent role in the low-grade systemic inflammation associated with the obese state. Saturated fatty acid (SFA) is largely implicated in the negative consequences of obesity, while the health benefits of monounsaturated fatty acid (MUFA) are widely acknowledged. The current study sought to explore whether SFA and MUFA differently modulate inflammatory responses in the brain, compared with peripheral immune cells. Moreover, we assessed the neuroinflammatory impact of high-fat-induced obesity and hypothesised that a MUFA-rich diet might mitigate inflammation despite obesogenic conditions. Toll-like receptor (TLR)2 mediates the inflammation associated with both obesity and AD. Using the TLR2 agonist lipoteichoic acid (LTA), we report that pre-exposure to either palmitic acid (PA) or oleic acid (OA) attenuated cytokine secretion from microglia, but heightened sensitivity to nitric oxide (NO) production. The reduction in cytokine secretion was mirrored in LTA-stimulated macrophages following exposure to PA only, while effects on NO were restricted to OA, highlighting important cell-specific differences. An obesogenic diet over 12 weeks did not induce prominent inflammatory changes in either cortex or hippocampus, irrespective of fat composition. However, we reveal a clear disparity in the effects of MUFA under obesogenic and non-obesogenic conditions.

Metabolic determinants of Alzheimer's disease: A focus on thermoregulation

Alzheimer's disease (AD) is a complex age-related neurodegenerative disease, associated with central and peripheral metabolic anomalies, such as impaired glucose utilization and insulin resistance. These observations led to a considerable interest not only in lifestyle-related interventions, but also in repurposing insulin and other anti-diabetic drugs to prevent or treat dementia. Body temperature is the oldest known metabolic readout and mechanisms underlying its maintenance fail in the elderly, when the incidence of AD rises. This raises the possibility that an age-associated thermoregulatory deficit contributes to energy failure underlying AD pathogenesis. Brown adipose tissue (BAT) plays a central role in thermogenesis and maintenance of body temperature. In recent years, the modulation of BAT activity has been increasingly demonstrated to regulate energy expenditure, insulin sensitivity and glucose utilization, which could also provide benefits for AD. Here, we review the evidence linking thermoregulation, BAT and insulin-related metabolic defects with AD, and we propose mechanisms through which correcting thermoregulatory impairments could slow the progression and delay the onset of AD.

Phytosterols: Potential Metabolic Modulators in Neurodegenerative Diseases

Phytosterols constitute a class of natural products that are an important component of diet and have vast applications in foods, cosmetics, and herbal medicines. With many and diverse isolated structures in nature, they exhibit a broad range of biological and pharmacological activities. Among over 200 types of phytosterols, stigmasterol and β-sitosterol were ubiquitous in many plant species, exhibiting important aspects of activities related to neurodegenerative diseases. Hence, this mini-review presented an overview of the reported studies on selected phytosterols related to neurodegenerative diseases. It covered the major phytosterols based on biosynthetic considerations, including other phytosterols with significant in vitro and in vivo biological activities.

The Role of Immune Cells in Oxi-Inflamm-Aging

Aging is the result of the deterioration of the homeostatic systems (nervous, endocrine, and immune systems), which preserve the organism's health. We propose that the age-related impairment of these systems is due to the establishment of a chronic oxidative stress situation that leads to low-grade chronic inflammation throughout the immune system's activity. It is known that the immune system weakens with age, which increases morbidity and mortality. In this context, we describe how the function of immune cells can be used as an indicator of the rate of aging of an individual. In addition to this passive role as a marker, we describe how the immune system can work as a driver of aging by amplifying the oxidative-inflammatory stress associated with aging (oxi-inflamm-aging) and inducing senescence in far tissue cells. Further supporting our theory, we discuss how certain lifestyle conditions (such as social environment, nutrition, or exercise) can have an impact on longevity by affecting the oxidative and inflammatory state of immune cells, regulating immunosenescence and its contribution to oxi-inflamm-aging.

From obesity to Alzheimer's disease through insulin resistance

Alzheimer's disease is one of the most frequent forms of dementia. It is a progressive neurodegenerative disease, characterized by presence of amyloid plaques and neurofibrillary tangles in the brain. Obesity is regarded as abnormal fat accumulation with deleterious impact on human health. There is full scientific evidence that obesity and the metabolic comorbidities (e.g., insulin resistance, hyperglycaemia, and type 2 diabetes) are related to Alzheimer's disease and likely in the causative pathway. Numerous studies have identified several overlapping neurodegenerative mechanisms, including oxidative stress, mitochondrial dysfunction, and inflammation. In this review, we present how obesity and the associated lipotoxicity as well as chronic inflammation initiate a state of insulin resistance that in turn, may have a role in causing the characteristic cerebral alterations of AD. In particular, we focus on the molecular mechanisms linking the obesity-induced impairment in insulin signalling to the upregulation of Aβ aggregation, tau hyper-phosphorylation, inflammation, oxidative stress and mitochondrial dysfunction.

Short-term high-fat diet favors the appearances of apoptosis and gliosis by activation of ERK1/2/p38MAPK pathways in brain

High-fat diet (HFD) has been associated with neuroinflammation and apoptosis in distinct brain regions. To explore the effect of short-term (7, 14 and 21 days) high-fat overfeeding on apoptosis, inflammatory signaling proteins, APP changes and glial cell activities in cerebral cortex and cerebellum. Mice were fed with HFD for different lengths (up to 21 days) and after each time body weights of mice was tested, then the apoptotic proteins, IL-1β, APP, BACE1and MAPKs, Akt and NF-κB signaling activity were evaluated by western blots. Results demonstrate that short period of high-fat overnutrition significantly promotes apoptosis, APP expression at day 21 of cerebral cortex and at day 7 of cerebellum compared to chow diet. In addition, increased GFAP+astrocytes, Iba-1+microglia and IL-1β 30 were observed in cerebral cortex after 21 days HFD, but no changes for 7 days overfeeding of cerebellum. Serendipitously, ERK1/2 pathway was activated both in cerebral cortex and cerebellum for different time course of HFD. Furthermore, increased phospho-p38 MAPK level was observed in cerebellum only. In consistent with in vivo results, SH-SY5Y cells treatment with cholesterol (50 μM, 100 μM) for 48 h culture in vitro demonstrated that pro-apoptotic proteins were enhanced as well. In brief, short-term HFD consumption increases sensitivity to apoptosis, APP and IL-1β production as well as gliosis in cerebral cortex and cerebellum, which may be related to enhancement of ERK1/2 and p38 MAPK activation.

The brain as an insulin-sensitive metabolic organ

BACKGROUND

The brain was once thought of as an insulin-insensitive organ. We now know that the insulin receptor is present throughout the brain and serves important functions in whole-body metabolism and brain function. Brain insulin signaling is involved not only in brain homeostatic processes but also neuropathological processes such as cognitive decline and Alzheimer's disease.

SCOPE OF REVIEW

In this review, we provide an overview of insulin signaling within the brain and the metabolic impact of brain insulin resistance and discuss Alzheimer's disease, one of the neurologic diseases most closely associated with brain insulin resistance.

MAJOR CONCLUSIONS

While brain insulin signaling plays only a small role in central nervous system glucose regulation, it has a significant impact on the brain's metabolic health. Normal insulin signaling is important for mitochondrial functioning and normal food intake. Brain insulin resistance contributes to obesity and may also play an important role in neurodegeneration.

The effect of photoperiod and high fat diet on the cognitive response in photoperiod-sensitive F344 rats

In many species, seasonal changes in day length (photoperiod) have profound effects on physiology and behavior. In humans, these include cognitive function and mood. Here we investigated the effect of photoperiod and high fat diets on cognitive deficits, as measured by novel object recognition, in the photoperiod-sensitive F344 rat, which exhibits marked natural changes in growth, body weight and food intake in response to photoperiod. 32 male juvenile F344 rats were housed in either long or short photoperiod and fed either a high fat or nutrient-matched chow diet. Rats were tested in the novel object recognition test before photoperiod and diet intervention and re-tested 28 days after intervention. In both tests during the acquisition trials there was no significant difference in exploration levels of the left and right objects in the groups. Before intervention, all groups showed a significant increase in exploration of the novel object compared to the familiar object. However, following the photoperiod and diet interventions the retention trial revealed that only rats in the long photoperiod-chow group explored the novel object significantly more than the familiar object, whereas all other groups showed no significant preference. These results suggest that changing rats to short photoperiod impairs their memory regardless of diet. The cognitive performance of rats on long photoperiod-chow remained intact, whereas the high fat diet in the long photoperiod group induced a memory impairment. In conclusion, our study suggests that photoperiod and high fat diet have an impact on object recognition in photoperiod-sensitive F344 rats.

High-fat diet increases gliosis and immediate early gene expression in APOE3 mice, but not APOE4 mice

Background

APOE4 is the strongest genetic risk factor for Alzheimer’s disease (AD), and obesity is a strong environmental risk factor for AD. These factors result in multiple central nervous system (CNS) disturbances and significantly increase chances of AD. Since over 20% of the US population carry the APOE4 allele and over 40% are obese, it is important to understand how these risk factors interact to affect neurons and glia in the CNS.

Methods

We fed male and female APOE3 and APOE4 knock-in mice a high-fat diet (HFD-45% kcal fat) or a "control" diet (CD-10% kcal fat) for 12 weeks beginning at 6 months of age. At the end of the 12 weeks, brains were collected and analyzed for gliosis, neuroinflammatory genes, and neuronal integrity.

Results

APOE3 mice on HFD, but not APOE4 mice, experienced increases in gliosis as measured by GFAP and Iba1 immunostaining. APOE4 mice on HFD showed a stronger increase in the expression of Adora2a than APOE3 mice. Finally, APOE3 mice on HFD, but not APOE4 mice, also showed increased neuronal expression of immediate early genes cFos and Arc.

Conclusions

These findings demonstrate that APOE genotype and obesity interact in their effects on important processes particularly related to inflammation and neuronal plasticity in the CNS. During the early stages of obesity, the APOE3 genotype modulates a response to HFD while the APOE4 genotype does not. This supports a model where early dysregulation of inflammation in APOE4 brains could predispose to CNS damages from various insults and later result in the increased CNS damage normally associated with the APOE4 genotype.

High-fat diet alters stress behavior, inflammatory parameters and gut microbiota in Tg APP mice in a sex-specific manner

Long-term high-fat diet (HFD) consumption commonly leads to obesity, a major health concern of western societies and a risk factor for Alzheimer's disease (AD). Both conditions present glial activation and inflammation and show sex differences in their incidence, clinical manifestation, and disease course. HFD intake has an important impact on gut microbiota, the bacteria present in the gut, and microbiota dysbiosis is associated with inflammation and certain mental disorders such as anxiety. In this study, we have analyzed the effects of a prolonged (18 weeks, starting at 7 months of age) HFD on male and female mice, both wild type (WT) and TgAPP mice, a model for AD, investigating the behavioral profile, gut microbiota composition and inflammatory/phagocytosis-related gene expression in hippocampus. In the open-field test, no overt differences in motor activity were observed between male and female or WT and TgAPP mice on a low-fat diet (LFD). However, HFD induced anxiety, as judged by decreased motor activity and increased time in the margins in the open-field, and a trend towards increased immobility time in the tail suspension test, with increased defecation. Intriguingly, female TgAPP mice on HFD showed less immobility and defecation compared to female WT mice on HFD. HFD induced dysbiosis of gut microbiota, resulting in reduced microbiota diversity and abundance compared with LFD fed mice, with some significant differences due to sex and little effect of genotype. Gene expression of pro-inflammatory/phagocytic markers in the hippocampus were not different between male and female WT mice, and in TgAPP mice of both sexes, some cytokines (IL-6 and IFNγ) were higher than in WT mice on LFD, more so in female TgAPP (IL-6). HFD induced few alterations in mRNA expression of inflammatory/phagocytosis-related genes in male mice, whether WT (IL-1β, MHCII), or TgAPP (IL-6). However, in female TgAPP, altered gene expression returned towards control levels following prolonged HFD (IL-6, IL-12β, TNFα, CD36, IRAK4, PYRY6). In summary, we demonstrate that HFD induces anxiogenic symptoms, marked alterations in gut microbiota, and increased expression of inflammatory genes, except for female TgAPP that appear to be resistant to the diet effects. Lifestyle interventions should be introduced to prevent AD onset or exacerbation by reducing inflammation and its associated symptoms; however, our results suggest that the eventual goal of developing prevention and treatment strategies should take sex into consideration.

Western diet as a trigger of Alzheimer's disease: From metabolic syndrome and systemic inflammation to neuroinflammation and neurodegeneration

An excess of saturated fatty acids and simple sugars in the diet is a known environmental risk factor of Alzheimer's disease (AD) but the holistic view of the interacting processes through which such diet may contribute to AD pathogenesis is missing. We addressed this need through extensive analysis of published studies investigating the effects of western diet (WD) on AD development in humans and laboratory animals. We reviewed WD-induced systemic alterations comprising metabolic changes, induction of obesity and adipose tissue inflammation, gut microbiota dysbiosis and acceleration of systemic low-grade inflammation. Next we provide an overview of the evidence demonstrating that WD-associated systemic alterations drive impairment of the blood-brain barrier (BBB) and development of neuroinflammation paralleled by accumulation of toxic amyloid. Later these changes are followed by dysfunction of synaptic transmission, neurodegeneration and finally memory and cognitive impairment. We conclude that WD can trigger AD by acceleration of inflammaging, and that BBB impairment induced by metabolic and systemic inflammation play the central role in this process. Moreover, the concurrence of neuroinflammation and Aβ dyshomeostasis, which by reciprocal interactions drive the vicious cycle of neurodegeneration, contradicts Aβ as the primary trigger of AD. Given that in 2019 the World Health Organization recommended focusing on modifiable risk factors in AD prevention, this overview of the sequential, complex pathomechanisms initiated by WD, which can lead from peripheral disturbances to neurodegeneration, can support future prevention strategies.

New Horizon: Exercise and a Focus on Tissue-Brain Crosstalk

The world population is aging, leading to increased rates of neurodegenerative disorders. Exercise has countless health benefits and has consistently been shown to improve brain health and cognitive function. The purpose of this review is to provide an overview of exercise-induced adaptations in the brain with a focus on crosstalk between peripheral tissues and the brain. We highlight recent investigations into exercise-induced circulating factors, or exerkines, including irisin, cathepsin B, GPLD1, and ketones and the mechanisms mediating their effects in the brain.

The Impact of Obesity on Microglial Function: Immune, Metabolic and Endocrine Perspectives

Increased life expectancy in combination with modern life style and high prevalence of obesity are important risk factors for development of neurodegenerative diseases. Neuroinflammation is a feature of neurodegenerative diseases, and microglia, the innate immune cells of the brain, are central players in it. The present review discusses the effects of obesity, chronic peripheral inflammation and obesity-associated metabolic and endocrine perturbations, including insulin resistance, dyslipidemia and increased glucocorticoid levels, on microglial function.

Repurposing beta-3 adrenergic receptor agonists for Alzheimer's disease: beneficial effects in a mouse model

Background

Old age, the most important risk factor for Alzheimer’s disease (AD), is associated with thermoregulatory deficits. Brown adipose tissue (BAT) is the main thermogenic driver in mammals and its stimulation, through β3 adrenergic receptor (β3AR) agonists or cold acclimation, counteracts metabolic deficits in rodents and humans. Studies in animal models show that AD neuropathology leads to thermoregulatory deficits, and cold-induced tau hyperphosphorylation is prevented by BAT stimulation through cold acclimation. Since metabolic disorders and AD share strong pathogenic links, we hypothesized that BAT stimulation through a β3AR agonist could exert benefits in AD as well.

Methods

CL-316,243, a specific β3AR agonist, was administered to the triple transgenic mouse model of AD (3xTg-AD) and non-transgenic controls from 15 to 16 months of age at a dose of 1 mg/kg/day i.p.

Results

Here, we show that β3AR agonist administration decreased body weight and improved peripheral glucose metabolism and BAT thermogenesis in both non-transgenic and 3xTg-AD mice. One-month treatment with a β3AR agonist increased recognition index by 19% in 16-month-old 3xTg-AD mice compared to pre-treatment (14-month-old). Locomotion, anxiety, and tau pathology were not modified. Finally, insoluble Aβ42/Aβ40 ratio was decreased by 27% in the hippocampus of CL-316,243-injected 3xTg-AD mice.

Conclusions

Overall, our results indicate that β3AR stimulation reverses memory deficits and shifts downward the insoluble Aβ42/Aβ40 ratio in 16-month-old 3xTg-AD mice. As β3AR agonists are being clinically developed for metabolic disorders, repurposing them in AD could be a valuable therapeutic strategy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-021-00842-3.

Western Diet Induces Impairment of Liver-Brain Axis Accelerating Neuroinflammation and Amyloid Pathology in Alzheimer's Disease

Alzheimer's disease (AD) is an aging-dependent, irreversible neurodegenerative disorder and the most common cause of dementia. The prevailing AD hypothesis points to the central role of altered cleavage of amyloid precursor protein (APP) and formation of toxic amyloid-β (Aβ) deposits in the brain. The lack of efficient AD treatments stems from incomplete knowledge on AD causes and environmental risk factors. The role of lifestyle factors, including diet, in neurological diseases is now beginning to attract considerable attention. One of them is western diet (WD), which can lead to many serious diseases that develop with age. The aim of the study was to investigate whether WD-derived systemic disturbances may accelerate the brain neuroinflammation and amyloidogenesis at the early stages of AD development. To verify this hypothesis, transgenic mice expressing human APP with AD-causing mutations (APPswe) were fed with WD from the 3rd month of age. These mice were compared to APPswe mice, in which short-term high-grade inflammation was induced by injection of lipopolysaccharide (LPS) and to untreated APPswe mice. All experimental subgroups of animals were subsequently analyzed at 4-, 8-, and 12-months of age. APPswe mice at 4- and 8-months-old represent earlier pre-plaque stages of AD, while 12-month-old animals represent later stages of AD, with visible amyloid pathology. Already short time of WD feeding induced in 4-month-old animals such brain neuroinflammation events as enhanced astrogliosis, to a level comparable to that induced by the administration of pro-inflammatory LPS, and microglia activation in 8-month-old mice. Also, WD feeding accelerated increased Aβ production, observed already in 8-month-old animals. These brain changes corresponded to diet-induced metabolic disorders, including increased cholesterol level in 4-months of age, and advanced hypercholesterolemia and fatty liver disease in 8-month-old mice. These results indicate that the westernized pattern of nourishment is an important modifiable risk factor of AD development, and that a healthy, balanced, diet may be one of the most efficient AD prevention methods.

Can Natural Products Exert Neuroprotection without Crossing the Blood-Brain Barrier?

The scope of evidence on the neuroprotective impact of natural products has been greatly extended in recent years. However, a key question that remains to be answered is whether natural products act directly on targets located in the central nervous system (CNS), or whether they act indirectly through other mechanisms in the periphery. While molecules utilized for brain diseases are typically bestowed with a capacity to cross the blood–brain barrier, it has been recently uncovered that peripheral metabolism impacts brain functions, including cognition. The gut–microbiota–brain axis is receiving increasing attention as another indirect pathway for orally administered compounds to act on the CNS. In this review, we will briefly explore these possibilities focusing on two classes of natural products: omega-3 polyunsaturated fatty acids (n-3 PUFAs) from marine sources and polyphenols from plants. The former will be used as an example of a natural product with relatively high brain bioavailability but with tightly regulated transport and metabolism, and the latter as an example of natural compounds with low brain bioavailability, yet with a growing amount of preclinical and clinical evidence of efficacy. In conclusion, it is proposed that bioavailability data should be sought early in the development of natural products to help identifying relevant mechanisms and potential impact on prevalent CNS disorders, such as Alzheimer’s disease.

Dietary restrictions modulate the gut microbiota: Implications for health and disease

The health benefits of carefully restricting the energy intake in a strategic manner whilst avoiding malnutrition are widely discussed. In the recent years, the great impact of the gut microbiota on its host has been clarified more and more. Since the gut microbiota produces a number of metabolites and molecules that can affect host metabolism, modulating it with dietary restriction can influence the health and the progression of disease of its host on various levels. This review comprises 15 studies investigating the effect of different variants of fasting and caloric restriction on the gastrointestinal microbiome and its metabolites. The data suggest that changing the gut microbiota composition by dietary restriction has the potential to positively influence the progression of several diseases such as obesity, diabetes, neurological diseases or inflammatory bowel disease. Finally, the relevance of the findings for clinical practice is evaluated and approaches for future research are proposed.