Brain Insulin Research Articles

DEP-1 is a brain insulin receptor phosphatase that prevents the simultaneous activation of counteracting metabolic pathways

A healthy metabolism relies on precise regulation of anabolic and catabolic pathways. While insulin deficiency impairs anabolism, insulin resistance in obesity causes metabolic dysfunction, especially via altered brain insulin receptor (IR) activity. Density-enhanced phosphatase 1 (DEP-1) negatively modulates the IR in peripheral tissues. Our study shows that DEP-1 is an insulin-regulated gene, dysregulated in obesity, and uncovers its role in brain insulin signaling, impacting both anabolic and catabolic pathways. Neuro-2a cells lacking DEP-1 demonstrated heightened IR phosphorylation upon acute insulin stimulation. This coincided with simultaneous AMP-activated protein kinase (AMPK) activation, which governs catabolic pathways, due to increased phospholipase C-gamma 1 signaling. These opposing pathways in male DEP-1 forebrain-specific knockout mice resulted in elevated lipolysis in white adipose tissue and fat oxidation in brown adipose tissue, with enhanced sympathetic activation and β-adrenergic receptor expression. In conclusion, DEP-1 deficiency causes the simultaneous activation of IR and AMPK signaling in the brain, with enhanced sympathetic activity in adipose tissues.

Biochemical investigation and in silico analysis of the therapeutic efficacy of Ipriflavone through Tet-1 Surface-Modified-PLGA nanoparticles in Streptozotocin-Induced Alzheimer’s like Disease: Reduced oxidative damage and etiological Descriptors

Ipriflavone (IPRI), an isoflavone derivative, is clinically used to prevent postmenopausal bone loss in addition to its antioxidant and cognitive benefits. However, its poor aqueous solubility retained its bioavailability. New strategies have been developed to improve the bioavailability and solubility of neurological medications to enhance their potency and limit adverse effects. This study aimed to prepare targeted IPRI-poly-lactic-co-glycolic acid (PLGA) nanoparticles coupled with Tet-1 peptide to increase the therapeutic potency of IPRI in a rat model of Alzheimer’s disease (AD). Streptozotocin (STZ) exacerbates Alzheimer-related alterations by promoting central insulin resistance resulted from defective signaling pathways related to neuroinflammation and neurotoxicity. Bilateral intracerebroventricular (icv) injection of STZ was used to introduce the AD model. Icv-STZ injection significantly affected brain insulin, oxidative stress, inflammatory, and apoptotic indicators and caused behavioral abnormalities. STZ promoted the formation of amyloid β42 (Aβ42) by increasing BACE1 and reducing ADAM10 and ADAM17 expression levels. STZ also triggered the accumulation of neurofibrillary tangles and synaptic dysfunction, which are crucial for neurological impairments. Icv-STZ injection showed evident degenerative changes in the pyramidal cell layer and significantly reduced the count of viable cells in both CA1 and prefrontal cortex, indicating increased neuronal cell death. IPRI successfully ameliorated cognitive dysfunction by improving the phosphorylated forms of cAMP-response element-binding protein (pCREB) and extracellular signal-regulated kinase 1/2 (pERK1/2) related to synaptic plasticity. Targeted IPRI nanoparticles exceeded free IPRI potential in reducing oxidative stress, acetylcholinesterase/monoamine oxidase activities, Tau phosphorylation, and Aβ42 levels revealing less degenerative changes and increased viable neuron counts. IPRI-targeted nanoparticles improved the neuroprotective potential of free IPRI, making this strategy applicable to treat many neurodegenerative diseases. Finally, the in silico study predicted its ability to cross the BBB and to bind various protein targets in the brain.

Inhibition of zDHHC7-driven protein S-palmitoylation prevents cognitive deficits in an experimental model of Alzheimer’s disease

Protein post-translational modifications (PTM) play a crucial role in the modulation of synaptic function and their alterations are involved in the onset and progression of neurodegenerative disorders. S-palmitoylation is a PTM catalyzed by zinc finger DHHC domain containing (zDHHC) S-acyltransferases that affects both localization and activity of proteins regulating synaptic plasticity and amyloid-β (Aβ) metabolism. Here, we found significant increases of both zDHHC7 expression and protein S-palmitoylation in hippocampi of both 3×Tg-AD mice and post-mortem Alzheimer's disease (AD) patients. Chronic intranasal administration of the S-palmitoylation inhibitor 2-bromopalmitate counteracted synaptic plasticity and cognitive deficits, reduced the Aβ deposition in the hippocampus and extended the lifespan of both male and female 3×Tg-AD mice. Moreover, hippocampal silencing of zDHHC7 prevented the onset of cognitive deficits in the same experimental model. We also identified a FoxO1-mediated epigenetic mechanism inducing zDHHC7 expression, which was triggered by brain insulin resistance in 3×Tg-AD mice. Finally, in hippocampi of AD patients S-palmitoylation levels of Beta-Secretase 1 were associated with Aβ 1 to 42 load and they inversely correlated with Mini Mental State Examination scores. Our data reveal a key role of both zDHHC7 overexpression and protein hyperpalmitoylation in the onset and progression of AD-related alterations of synaptic plasticity and memory.

Metformin effectively alleviates the symptoms of Alzheimer in rats by lowering amyloid β deposition and enhancing the insulin signal

Alzheimer’s disease (AD) exhibits distinct biochemical and histopathological attributes, encompassing cellular, neuronal, and oxidative impairment. There is also an abnormal buildup, misfolding and clumping of amyloid β (Aβ). The present study aimed to explore the influence of the antihyperglycemic agent metformin on rats with AD-like symptoms, while also elucidating the intricate relationship between insulin resistance and AD. The rats were categorized into five groups: a control group, a saline-administered group, a metformin-treated group, AD-model rats, and AD-rats treated with a 200 mg/kg dose of metformin. Cognitive impairment was rated using the classical labyrinth test. Moreover, serum biochemical parameters, encompassing glucose levels, Homeostatic Model Assessment for Insulin Resistance (HOMA-IR), Glycated hemoglobin (HbA1c), lipid profile, kidney, and liver function, were evaluated. Additionally, oxidative, antioxidant, and neurotransmitter parameters were measured in hippocampus tissues. Also, the Aβ and insulin receptor substrate 2 (IRS-2) were measured by immunoblotting. Besides hippocampal histopathology, glial fibrillary acidic protein (GFAP) and calretinin immunoreactivity were monitored. The study findings disclosed deficits in memory and learning capabilities among AD rats. Furthermore, AD-afflicted rats exhibited heightened glucose levels, elevated HOMA-IR and HbA1c values, alongside compromised liver, and kidney functions. Additionally, an upsurge in oxidative stress coincided with a notable reduction in the antioxidant system and neurotransmitters activities. The levels of Aβ deposition increased, while IRS-2 expression subsided, accompanied by alterations in the hippocampal structure and neuronal damage. These changes were paralleled by an intensification in GFAP reactivity and a detracting in calretinin reactivity. Metformin was altogether able to move forward cognitive execution by means of bringing down oxidative stress and Aβ conglomeration. Furthermore, metformin was able to improve neurotransmitters and insulin signals. AD, glucose impairment, and brain insulin resistance are completely interlinked, and future AD medications may be inspired by diabetic medication.

Research progress on the association of insulin resistance with type 2 diabetes mellitus and Alzheimer's disease.

Type 2 diabetes mellitus (T2DM) is a metabolic disorder that is characterized by insulin resistance and hyperglycemia. It is also known to be a risk factor for Alzheimer's disease (AD). Insulin plays a crucial role in regulating the body's metabolism and is responsible for activating the Phosphoinotide-3-Kinase (PI3K)/Protein Kinase B (Akt) signaling pathway. This pathway is activated when insulin binds to the insulin receptor on nerve cells, and it helps regulate the metabolism of glucose and lipids. Dysfunction in the insulin signaling pathway can lead to a decrease in brain insulin levels and insulin sensitivity, thereby inducing disruptions in insulin signal transduction and leading to disorders in brain energy metabolism. Moreover, these dysfunctions also contribute to the accumulation of β-amyloid (Aβ) deposition and the hyperphosphorylation of Tau protein, both of which are characteristic features of AD. Therefore, this article focuses on insulin resistance to reveal the complex mechanism between brain insulin resistance and AD occurrence in T2DM. On this basis, this article further summarizes the biological effects and mechanisms of antidiabetic drugs on the two diseases, aiming to provide new ideas for the discovery of drugs for the treatment of T2DM combined with AD.

Aqueous extract of Spirulina exerts neuroprotection in an experimental model of Alzheimer sporadic disease in mice induced by Streptozotocin.

Alzheimer's disease (AD) is a progressive neurodegenerative disease that causes gradual memory loss and cognitive impairment. Intracerebroventricular injections of streptozotocin (ICV-STZ) have been used as an experimental model of sporadic Alzheimer's disease (SAD) because they produce deficits in brain insulin signaling, oxidative stress, neuroinflammation, and neurodegeneration, resulting in cognitive decline and memory impairment. Spirulina platensis (SPI) is a nutraceutical with anti-inflammatory, antioxidant, and neuroprotective properties. The objective of this work was to study the effects of SPI on cognitive deficits and neuronal damage in mice submitted to the experimental model of SAD induced by ICV-STZ. Male Swiss mice (25-35g) received ICV-STZ (3mg/Kg) bilaterally on days 1 and 3, SPI (50 and 100mg/Kg, o.p.) or vehicle (saline) was administered 2h after the second surgery, and once a day for 16 days. SPI treatment prevented working, episodic, spatial, and aversive memory deficits. Locomotor activity was not altered. ICV-STZ caused an increase in MDA, nitrite, and superoxide anion, while decreasing GSH. SPI treatment protected against GSH increase in the prefrontal cortex and hippocampus, and inhibited AChE activity in the prefrontal cortex. SPI prevented astrogliosis and microgliosis induced by ICV-STZ. These findings highlight the therapeutic potential of SPI for the treatment of SAD, indicating that its neuroprotective action is linked to antioxidant, anti-inflammatory, and AChE inhibitory activity.

Effects of age and dietary methionine restriction on cognitive and behavioural phenotypes in the rTg4510 mouse model of frontotemporal dementia

Metabolic disorders such as diabetes and obesity are linked to neurodegenerative diseases, with evidence of lower brain glucose metabolism and insulin resistance in dementia patients. Dietary methionine restriction (MR) is a nutritional intervention that enhances insulin sensitivity and delays ageing-associated metabolic alterations, however, its impact on neurodegenerative diseases is not fully understood. Here, we examined the behavioural and metabolic phenotypes of a murine tauopathy model (rTg4510), which overexpresses human P301L mutated tau, at 6 and 12 months of age, assessing the impact of an 8-week dietary MR in the older group. While rTg4510 mice displayed progressive behavioural and motor impairments at both ages, MR led to significant benefits in the 12-month-old cohort, improving motor coordination, short-term memory, and social recognition. These effects were accompanied by increased glycolysis markers and FGF21R1 levels in the hippocampus, alongside unaltered glucose metabolism/adiposity. Overall, our results reveal the impact of MR on an FTD-mouse model, suggesting this as a potential therapeutic intervention to delay and/or improve the progression of tau-related disease.

A DISEASE-BASED PERSPECTIVE OF THE RELATIONSHIP BETWEEN NEUROINFLAMMATION AND IMPAIRED GLUCOSE METABOLISM

Neuroinflammation is a significant contributor to the pathogenesis of several central nervous system disorders including Alzheimer's Disease, Parkinson's Disease, Huntington's Disease, and amyotrophic lateral sclerosis. Neuroinflammation is the immune response of the central nervous system against central or peripheral abnormalities disturbed by foreign agents, molecules, metabolic activities, or various diseases. Astrocytes and microglia activation are the main activators of neuroinflammation. The polarization changes of these defender cells have some key roles in bodily metabolism as much as neuronal behavior. The blood-brain barrier is known as the first defender of brain parenchyma. Neuroinflammation disrupts blood-brain barrier integrity and may cause blood-brain barrier breakdown. Glucose is the main energy source of brain and glucose uptake is achieved through the blood-brain barrier. Altered glucose metabolism may have detrimental effects on brain functions and may cause brain disorders. Also, it has been suggested that neuroinflammation may have crucial roles in glucose metabolism. The distribution of the blood-brain barrier in vascular endothelial cells of neurons, astrocytes, and microglia contributes to the transport of glucose to the cells of brain. Microglia and astrocyte polarization are suggested as the two main underlying mechanisms in neuroinflammation. It’s obviously determined that neuroinflammation-caused neurodegenerative diseases are tightly linked with the brain insulin resistance and disrupted cerebral and peripheral glucose metabolism. However, there is lacking knowledge about glucose metabolism deficiencies and microglia/astrocyte polarization. Herein this review, we summarized the neuroinflammation and glucose metabolism with the most common neurological diseases and the possible effects of microglia/astrocyte polarization on glucose metabolism.

Conquering Insulin Network Dysfunctions in Alzheimer's Disease: Where Are We Today?

Functional impairments in the brain's insulin and insulin-like growth factor (IGF) signal transduction networks are recognized mediators of dysregulated energy metabolism, a major driver of the Alzheimer's disease (AD) neurodegeneration cascade. AD-associated insulin-deficient and insulin-resistant states mimic those of diabetes mellitus and affect all cell types in the brain. Besides accounting for abundant amyloid-β and hyperphosphorylated tau lesions in AD, insulin/IGF pathway dysfunctions cause cortical atrophy, loss of synaptic plasticity, white matter myelin/oligodendrocyte degeneration, astrocyte and microglial neuroinflammation and oxidative stress, deficits in energy metabolism, mitochondrial dysfunction, and microvascular disease. These same neuropathological processes have been linked to cognitive impairment in type 2 diabetes mellitus, Parkinson's disease, and vascular dementia. Strategies to address metabolic mediators of cognitive impairment have been borrowed from diabetes and other insulin-resistant diseases and leveraged on preclinical AD model data. The repurposing of diabetes drugs led to clinical trials with intranasal insulin, followed by insulin sensitizers including metformin and peroxisome-proliferator-activated receptor agonists, and then incretin mimetics primarily targeting GLP-1 receptors. In addition, other glucose-lowering agents have been tested for their efficacy in preventing cognitive declines. The strengths and limitations of these approaches are discussed. The main conclusion of this review is that we have now arrived at a stage in which it is time to address long-term deficits in trophic factor availability and receptor responsiveness, signaling abnormalities that extend beyond insulin and include IGFs and interconnected pathways, and the need for multi-pronged rather than single-pronged therapeutic targeting to remediate AD and other forms of neurodegeneration.

Therapies Using Anti-Diabetics to Prevent and Manage Progression of Parkinson's Disease

<p>Currently, a significant proportion of older individuals have demonstrated connections between Parkinson's disease (PD) and type 2 diabetes mellitus (T2DM). Emerging evidence from cohort studies, clinical data, and preclinical meta-analyses reveals that T2DM patients have an increased risk of developing PD, often accompanied by more severe disease progression. These two chronic conditions are believed to share common pathways. The pathways that likely connect the neurodegenerative process of PD include autophagy, mitochondrial dysfunction, dysregulated insulin signaling, inflammation, and insulin resistance. Preclinical PD models highlight the importance of cerebral insulin signaling, which confers numerous neuroprotective benefits. Several ongoing phase II and phase III trials are being conducted in PD-affected populations. These trials reflect the potential of existing anti-diabetic medications to improve PD motor symptoms and halt the progression of neurodegeneration. Brain insulin resistance plays a crucial role in the pathophysiology of PD. The insulin signaling pathway represents a promising novel target for disease modification in both diabetic and non-diabetic PD. Incretin receptors in the brain have emerged as the most promising targets for repurposing existing drugs in PD treatment. Targeting these receptors mitigates inflammation, apoptosis, toxic protein aggregation, long-term potentiation, autophagy, and insulin signaling failure. </p><p> Presently, various novel anti-diabetic targets are available that can regulate the insulin signaling pathway in dopaminergic neurons, suggesting improvements in cognitive function and significant neuroprotective effects in PD. Throughout neurodegenerative processes, insulin and incretins exert therapeutic effects in multiple manners. Preclinical studies have demonstrated that glucagon-- like peptide 1 (GLP-1) receptor agonists reduce neuroinflammation, tau phosphorylation, and amyloid deposition while increasing synaptic function and enhancing memory formation. Incretin mimetics may act through the restoration of insulin signaling pathways, inducing further neuroprotective effects. </p><p> This review examined the reported shared cellular and molecular pathologies between PD and T2DM, and explored the repurposing of anti-diabetic drugs for the prevention of PD progression through the regulation of insulin signaling pathways.</p>

Cellular Profile of Subfornical Organ Insulin Receptors in Mice.

Brain insulin receptor signaling is strongly implicated in cardiovascular and metabolic physiological regulation. In particular, we recently demonstrated that insulin receptors within the subfornical organ (SFO) play a tonic role in cardiovascular and metabolic regulation in mice. The SFO is a forebrain sensory circumventricular organ that regulates cardiometabolic homeostasis due to its direct exposure to the circulation and thus its ability to sense circulating factors, such as insulin. Previous work has demonstrated broad distribution of insulin receptor-expressing cells throughout the entire SFO, indirectly indicating insulin receptor expression in multiple cell types. Based on this, we sought to determine the cellular phenotypes that express insulin receptors within the SFO by combining immunohistochemistry with genetically modified reporter mouse models. Interestingly, SFO neurons, including both excitatory and inhibitory types, were the dominant cell site for insulin receptor expression, although a weak degree of insulin receptor expression was also detected in astrocytes. Moreover, SFO angiotensin type 1a receptor neurons also expressed insulin receptors. Collectively, these anatomical findings indicate the existence of potentially complex cellular networks within the SFO through which insulin signaling can influence physiology and further point to the SFO as a possible brain site for crosstalk between angiotensin-II and insulin.

Bridging brain insulin resistance to Alzheimer’s pathogenesis

Emerging evidence links type 2 diabetes mellitus (T2DM) and Alzheimer’s disease (AD), with brain insulin resistance (BIR) as a key factor. In a recent study, Lanzillotta et al. reveal that reduced biliverdin reductase-A (BVR-A) impairs glycogen synthase kinase 3β (GSK3β) phosphorylation, causing mitochondrial dysfunction and exacerbating brain insulin resistance in the progression of both T2DM and AD.

Association Between Type 2 Diabetes Mellitus and Alzheimer's Disease: Common Molecular Mechanism and Therapeutic Targets.

Diabetes mellitus (DM) and Alzheimer's disease (AD) rates are rising, mirroring the global trend of an aging population. Numerous epidemiological studies have shown that those with Type 2 diabetes (T2DM) have an increased risk of developing dementia. These degenerative and progressive diseases share some risk factors. To a large extent, the amyloid cascade is responsible for AD development. Neurofibrillary tangles induce neurodegeneration and brain atrophy; this chain reaction begins with hyperphosphorylation of tau proteins caused by progressive amyloid beta (Aβ) accumulation. In addition to these processes, it seems that alterations in brain glucose metabolism and insulin signalling lead to cell death and reduced synaptic plasticity in AD, before the onset of symptoms, which may be years away. Due to the substantial evidence linking insulin resistance in the brain with AD, researchers have coined the name "Type 3 diabetes" to characterize the condition. We still know little about the processes involved, even though current animal models have helped illuminate the links between T2DM and AD. This brief overview discusses insulin and IGF-1 signalling disorders and the primary molecular pathways that may connect them. The presence of GSK-3β in AD is intriguing. These proteins' association with T2DM and pancreatic β-cell failure suggests they might be therapeutic targets for both disorders.

The Association of Brain Insulin Resistance with Anesthesia/Surgery-Induced Cognitive Deterioration Is Female-Specific in 5XFAD Transgenic Mice.

Our previous studies indicated that anesthesia/surgery could aggravate cognitive impairment and tau pathology in female 5XFAD transgenic (Tg) mice. However, it is unknown whether there are sex differences in the susceptibility of developing postoperative cognitive dysfunction in 5XFAD Tg mice. In this study, we aim to determine whether anesthesia/surgery can have different effects on female and male 5XFAD Tg mice, and to explore the underpinning mechanisms. The mice received abdominal surgery under isoflurane anesthesia. Morris water maze was used to assess the cognitive function. Hippocampal levels of p-tau (AT8), p-IRS1 (Ser612), IRS1, p-GSK3β (Tyr216), and p-GSK3β (Ser9) at postoperative day 1 were evaluated by western blot assays. Anesthesia/surgery exaggerated cognitive impairment and tau pathology in female, but not male 5XFAD Tg mice. The anesthesia/surgery led to elevated hippocampus protein levels of p-IRS1 (Ser612)/IRS1 ratio and p-GSK3β (Tyr216) and reduced hippocampus protein levels of p-GSK3β (Ser9) in female, but not male 5XFAD Tg mice. This study demonstrated that female 5XFAD Tg mice were more susceptible to anesthesia/surgery-induced cognitive deterioration and tau pathology aggravation, potentially due to female-specific brain insulin resistance.

Intracerebroventricular insulin injection acutely normalizes the augmented exercise pressor reflex in male rats with type 2 diabetes mellitus.

The exercise pressor reflex (EPR) is exaggerated in type 2 diabetes mellitus (T2DM), but the underlying central nervous system aberrations have not been fully delineated. Stimulation of muscle afferents within working skeletal muscle activates the EPR, by sending information to neurons in the brainstem, where it is integrated and results in reflexively increased mean arterial pressure (MAP) and sympathetic nerve activity. Brain insulin is known to regulate neural activity within the brainstem. We hypothesize that brain insulin injection in T2DM rats attenuates the augmented EPR, and that T2DM is associated with decreased brain insulin. Using male Sprague-Dawley rats, T2DM and control rats were generated via an induction protocol with two low doses of streptozotocin (35 and 25mg/kg, i.p.) in combination with a 14-23-week high-fat diet or saline injections and a low-fat diet, respectively. After decerebration, MAP and renal sympathetic nerve activity (RSNA) were evaluated during EPR stimulation, evoked by electrically induced muscle contraction via ventral root stimulation, before and after (1 and 2h post) intracerebroventricular (i.c.v.) insulin microinjections (500mU, 50nl). i.c.v. insulin decreased peak MAP (ΔMAP Pre (36±14mmHg) vs. 1h (21±14mmHg) vs. 2h (11±6mmHg), P<0.05) and RSNA (ΔRSNA Pre (107.5±40%), vs. 1h (75.4±46%) vs. 2h (51±35%), P<0.05) responses in T2DM, but not controls. In T2DM rats, cerebrospinal fluid insulin was decreased (0.41±0.19 vs. 0.11±0.05ng/ml, control (n=14) vs. T2DM (n=4), P<0.01). The results demonstrated that insulin injections into the brain normalized the augmented EPR in brain hypoinsulinaemic T2DM rats, indicating that the EPR can be regulated by brain insulin. KEY POINTS: The reflexive increase in blood pressure and sympathetic nerve activity mediated by the autonomic nervous system during muscle contractions is also known as the exercise pressor reflex. The exercise pressor reflex is dangerously augmented in type 2 diabetes, in both rats and humans. In type 2 diabetic rats both cerebrospinal fluid insulin and phosphoinositide 3-kinase signalling within cardiovascular brainstem neurons decrease in parallel. Brain insulin injections decrease the magnitude of the reflexive pressor and sympathetic responses to hindlimb muscle contraction in type 2 diabetic rats. Partial correction of low insulin within the central nervous system in type 2 diabetes may treat aberrant exercise pressor reflex function.

Effects of a high-fat diet on cognition and brain distribution of intranasal insulin in E3 and E4 male and female mice

There are genetic and environmental risk factors that contribute to the development of cognitive decline in Alzheimer’s disease (AD). Some of these include the genetic predisposition of the apolipoprotein E4 genotype, consuming a high-fat diet (HFD), and the female sex. Brain insulin receptor resistance and deficiency have also been shown to be associated with AD and cognitive impairment. Intranasal (INL) insulin enhances cognition in AD, but the response varies due to genotype, diet, and sex. We investigated here the combination of these risk factors in a humanized mouse model, expressing E3 or E4, following a HFD in males and females on cognitive performance and the brain distribution of insulin following INL delivery. The HFD had a negative effect on survival in male mice only, requiring sex to be collapsed. We found many genotype, diet, and genotype x diet effects in anxiety-related tasks. We further found beneficial effects of INL insulin in our memory tests, with the most important findings showing a beneficial effect of INL insulin in mice on a HFD. We found insulin distribution throughout the brain after INL delivery was largely unaffected by diet and genotype, indicating these susceptible groups can still receive adequate levels of insulin following INL delivery. Our findings support the involvement of brain insulin signaling in cognition and highlight continuing efforts investigating mechanisms resulting from treatment with INL insulin.

Effects of chronic treatment with metformin on brain glucose hypometabolism and central insulin actions in transgenic mice with tauopathy

Brain glucose hypometabolism and insulin alterations are common features of many neurological diseases. Herein we sought to corroborate the brain glucose hypometabolism that develops with ageing in 12-months old Tau-VLW transgenic mice, a model of tauopathy, as well as to determine whether this model showed signs of altered peripheral glucose metabolism. Our results demonstrated that 12-old months Tau mice exhibited brain glucose hypometabolism as well as basal hyperglycemia, impaired glucose tolerance, hyperinsulinemia, and signs of insulin resistance. Then, we further studied the effect of chronic metformin treatment (9 months) in Tau-VLW mice from 9 to 18 months of age. Longitudinal PET neuroimaging studies revealed that chronic metformin altered the temporal profile in the progression of brain glucose hypometabolism associated with ageing. Besides, metformin altered the content and/or phosphorylation of key components of the insulin signal transduction pathway in the frontal cortex leading to significant changes in the content of the active forms. Thus, metformin increased the expression of pAKT-Y474 while reducing pmTOR-S2448 and pGSK3β. These changes might be related, at least partially, to a slow progression of ageing, neurological damage, and cognitive decline. Metformin also improved the peripheral glucose tolerance and the ability of the Tau-VLW mice to maintain their body weight through ageing. Altogether our study shows that the tau-VLW mice could be a useful model to study the potential interrelationship between tauopathy and central and peripheral glucose metabolism alterations. More importantly our results suggest that chronic metformin treatment may have direct beneficial central effects by post-transcriptional modulation of key components of the insulin signal transduction pathway.

Impact of Serotonin Transporter Absence on Brain Insulin Receptor Expression, Plasma Metabolome Changes, and ADHD-like Behavior in Mice fed a Western Diet.

The impaired function of the serotonin transporter (SERT) in humans has been linked to a higher risk of obesity and type 2 diabetes, especially as people age. Consuming a "Western diet" (WD), which is high in saturated fats, cholesterol, and sugars, can induce metabolic syndrome. Previous research indicated that mice carrying a targeted inactivation of the Sert gene (knockout, KO) and fed a WD display significant metabolic disturbances and behaviors reminiscent of ADHD. These abnormalities might be mediated via a dysfunction in insulin receptor (IR) signaling, which is also associated with adult ADHD. However, the impact of Sert deficiency on IR signaling and systemic metabolic changes has not been thoroughly explored. In this study, we conducted a detailed analysis of locomotor behavior in wild-type (WT) and KO mice fed a WD or control diet. We investigated changes in the blood metabolome and examined, via PCR, the expression of insulin receptor A and B isoforms and key regulators of their function in the brain. Twelve-month-old KO mice and their WT littermates were fed a WD for three weeks. Nuclear magnetic resonance spectroscopy analysis of plasma samples showed that KO mice on a WD had higher levels of lipids and lipoproteins and lower levels of glucose, lactate, alanine, valine, and isoleucine compared to other groups. SERT-KO mice on the control diet exhibited increased brain levels of both IR A and B isoforms, accompanied by a modest increase in the negative regulator ENPP. The KO mice also displayed anxiety-like behavior and reduced exploratory activity in an open field test. However, when the KO animals were fed a WD, the aberrant expression levels of IR isoforms in the KO mice and locomotor behavior were ameliorated indicating a complex interaction between genetic and dietary factors that might contribute to ADHD-like symptoms. Overall, our findings suggest that the lack of Sert leads to a unique metabolic phenotype in aged mice, characterized by dysregulated IR-related pathways. These changes are exacerbated by WD in the blood metabolome and are associated with behavioral abnormalities.

Interaction between the Neuroprotective and Hyperglycemia Mitigation Effects of Walnut-Derived Peptide LVRL via the Wnt3a/β-Catenin/GSK-3β Pathway in a Type 2 Diabetes Mellitus Model.

The term type 3 diabetes mellitus (T3DM) has been considered for Alzheimer's disease (AD) due to the common molecular and cellular characteristics found between type 2 diabetes mellitus (T2DM) and cognitive deficits. However, the specific mechanism of T3DM remains elusive, especially the neuroprotective effects of dietary components in hyperglycemic individuals. In this study, a peptide, Leu-Val-Arg-Leu (LVRL), found in walnuts significantly improved memory decline in streptozotocin (STZ)- and high-fat-diet (HFD)-stimulated T2DM mouse models (p < 0.05). The LVRL peptide also mitigated hyperglycemia, enhanced synaptic plasticity, and ameliorated mitochondrial dysfunction, as demonstrated by Morris water maze tests, immunoblotting, immunofluorescence, immunohistochemistry, transmission electron microscopy, and cellular staining. A Wnt3a inhibitor, DKK1, was subsequently used to verify the possible role of the Wnt3a/β-Catenin/GSK-3β pathway in glucose-induced insulin resistance in PC12 cells. In vitro LVRL treatment dramatically modulated the protein expression of p-Tau (Ser404), Synapsin-1, and PSD95, elevated the insulin level, increased glucose consumption, and relieved the mitochondrial membrane potential, and MitoSOX (p < 0.05). These data suggested that peptides like LVRL could modulate the relationship between brain insulin and altered cognition status via the Wnt3a/β-Catenin/GSK-3β pathway.

1-Deoxynojirimycin attenuates pathological markers of Alzheimer's disease in the invitro model of neuronal insulin resistance.

Insulin resistance, the hallmark of type 2 diabetes mellitus (T2DM), has emerged as a pathological feature in Alzheimer's disease (AD). Given the shared role of insulin resistance in T2DM and AD, repurposing peripheral insulin sensitizers is a promising strategy to preserve neuronal insulin sensitivity and prevent AD. 1-Deoxynojirimycin (DNJ), a bioactive iminosugar, exhibited insulin-sensitizing effects in metabolic tissues and was detected in brain tissue post-oral intake. However, its impact on brain and neuronal insulin signaling has not been described. Here, we investigated the effect of DNJ treatment on insulin signaling and AD markers in insulin-resistant human SK-N-SH neuroblastoma, a cellular model of neuronal insulin resistance. Our findings show that DNJ increased the expression of insulin signaling genes and the phosphorylation status of key molecules implicated in insulin resistance (Y1146-pIRβ, S473-pAKT, S9-GSK3B) while also elevating the expression of glucose transporters Glut3 and Glut4, resulting in higher glucose uptake upon insulin stimuli. DNJ appeared to mitigate the insulin resistance-driven increase in phosphorylated tau and Aβ1-42 levels by promoting insulin-induced phosphorylation of GSK3B (a major tau kinase) and enhancing mRNA expression of the insulin-degrading enzyme (IDE) pivotal for insulin and Aβ clearance. Overall, our study unveils probable mechanisms underlying the potential benefits of DNJ for AD, wherein DNJ attenuates tau and amyloid pathologies by reversing neuronal insulin resistance. This provides a scientific basis for expanding the use of DNJ-containing products for neuroprotective purposes and prompts further research into compounds with similar mechanisms of action.