Long-term diet-induced obesity does not lead to learning and memory impairment in adult mice

Metabolic clues to memory loss: High-fat diets and brain-adipose crosstalk in zebrafish

Obesity is a growing public health concern that significantly impacts cognitive functions, including memory. This research explores how a high-fat diet affects short-term memory, employing the novel object recognition (NOR) test and NMR-based metabolomics to elucidate metabolic alterations in the brain and adipose tissue. The zebrafish were divided into two groups: one receiving a standard diet (SD) and the other a high-fat diet (HFD). Body mass index (BMI) was assessed every two weeks for a period of eight weeks. The NOR test was used to determine the discrimination index (DI) for evaluating the short-term memory of the SD and HFD groups. NMR spectroscopy was employed to investigate the metabolites in brain and adipose tissues, and multivariate data analysis was conducted to discover significant metabolic alterations. The high-fat diet (HFD) resulted in a significant increase in body mass index (BMI) (p < 0.0001) compared to the standard diet (SD) group from week 4 to week 8. A significant reduction in the discrimination index (24.95 %) in the HFD group against the SD group suggests a decline in memory performance among HFD subjects. NMR-based metabolomics of adipose tissue revealed that linoleic acid and caprylic acid were consistently found to exhibit increased levels in the HFD group across all assessments, whereas lauric acid, ALA, EPA, and DHA were consistently present at elevated levels in the adipose tissue of the SD group. NMR-based metabolomics of the brain identified GABA, taurine, and histamine as the key metabolites distinguishing the HFD from the SD group in female zebrafish. For male zebrafish brains, taurine, phenylalanine, and tryptophan were identified as the most significant metabolites for differentiating between HFD and SD. These metabolites demonstrated a notable decrease in the HFD group relative to the SD group. The results of this study align with those of previously reported studies in rodents and humans, indicating that memory impairment associated with obesity may stem from neuroinflammation and changes in synaptic plasticity. This research provides insights into the molecular changes in adipose tissue and the brain that occur when individuals receive a high-fat diet (HFD), which may enhance our understanding of the link between obesity and memory impairment, ultimately leading to a better comprehension of the disease.

Sex-dependent effects of a high-fat diet on the hypothalamic response in mice

Sex differences in rodent models of diet-induced obesity are still poorly documented, particularly regarding how central mechanisms vary between sexes in response to an obesogenic diet. Here, we wanted to determine whether obese phenotype and hypothalamic response to a high-fat diet (HFD) differed between male and female C57Bl/6J mice. Mice were exposed to either a 60% HFD or a standard diet first for both a long- (14 weeks) and shorter-periods of time (3, 7, 14 and 28 days). Analysis of the expression profile of key neuronal, glial and inflammatory hypothalamic markers was performed using RT-qPCR. In addition, astrocytic and microglial morphology was examined in the arcuate nucleus. Monitoring of body weight and composition revealed that body weight and fat mass gain appeared earlier and was more pronounced in male mice. After 14 weeks of HFD exposure, normalized increase of body weight reached similar levels between male and female mice. Overall, both sexes under HFD displayed a decrease of orexigenic neuropeptides expression while an increase in Pomc gene expression was observed only in female mice. In addition, changes in the expression of hypothalamic inflammatory markers were relatively modest. We also reported that the glial cell markers expression and morphology were affected by HFD in a sex- and time dependent manner, suggesting a more pronounced glial cell activation in female mice. Taken together, these data show that male and female mice responded differently to HFD exposure, both on short- and long-term and suggest that a strong inflammatory hypothalamic profile is not systematically present in diet-induced obesity models. Nevertheless, in addition to these present data, the underlying mechanisms should be deciphered in further investigations.

Nitric oxide donors rescue metabolic and mitochondrial dysfunction in obese Alzheimer's model

Reduced nitric oxide (NO) bioavailability is a pathological link between obesity and Alzheimer's disease (AD). Obesity-associated metabolic and mitochondrial bioenergetic dysfunction are key drivers of AD pathology. The hypothalamus is a critical brain region during the development of obesity and dysfunction is an area implicated in the development of AD. NO is an essential mediator of blood flow and mitochondrial bioenergetic function, but the role of NO in obesity-AD is not entirely clear. We investigated diet-induced obesity in female APPswe/PS1dE9 (APP) mouse model of AD, which we treated with two different NO donors (sodium nitrite or L-citrulline). After 26 weeks of a high-fat diet, female APP mice had higher adiposity, insulin resistance, and mitochondrial dysfunction (hypothalamus) than non-transgenic littermate (wild type) controls. Treatment with either sodium nitrite or L-citrulline did not reduce adiposity but improved whole-body energy expenditure, substrate oxidation, and insulin sensitivity. Notably, both NO donors restored hypothalamic mitochondrial respiration in APP mice. Our findings suggest that NO is an essential mediator of whole-body metabolism and hypothalamic mitochondrial function, which are severely impacted by the dual insults of obesity and AD pathology.

Structural characterization of polysaccharide from the peel of Trichosanthes kirilowii Maxim and its anti-hyperlipidemia activity by regulating gut microbiota and inhibiting cholesterol absorption

The peel of Trichosanthes kirilowii Maxim, is considered one of the primary sources for Trichosanthis pericarpium in traditional Chinese medicine, exhibiting lipid-lowering properties. The impact on hyperlipidemia mice of the crude polysaccharide from the peel of T. Kirilowii (TRP) was investigated in this study. The findings revealed that TRP exhibited a significant improvement in hepatic lipid deposition. Moreover, it significantly decreased serum levels of TC, TG, and LDL-C, while concurrently increasing HDL-C. 16S rRNA amplicon sequencing technique revealed that TRP group exhibited an increased relative abundance of Actinobacteria, a down-regulated relative abundance of Ruminiclostridium, and an up-regulated relative abundance of Ileibacterium. Therefore, TRP might play a role in anti-hyperlipidemia through regulation of the intestinal milieu and enhancement of microbial equilibrium. Consequently, targeted fractionation of TRP resulted in the isolation of a homogeneous acidic polysaccharide termed TRP-1. The TRP-1 polysaccharide, with an average molecular weight of 1.00 × 104 Da, and was primarily composed of Rha, GlcA, GalA, Glc, Gal and Ara. TRP-1 possessed a backbone consisting of alternating connections between → 6)-α-Galp-(1 → 4)-α-Rhap-(1 → 6)-α-Galp-(2 → 6)-β-Galp-(1 → 6)-α-Galp-(2 → 6)-β-Galp-(1 → units and branched chain containing → 6)-α-Glcp-(1→, 2,4)-β-Glcp-(1, and → 4)-α-GlapA-(1→. Both TRP and TRP-1 exhibited significant disruption of cholesterol micelles, highlighting their potential as lipid-lowering agents that effectively inhibit cholesterol absorption pathways.

Excision of the endothelial blood-brain barrier insulin receptor does not alter spatial cognition in mice fed either a chow or high-fat diet

Insulin is transported across the blood-brain barrier (BBB) endothelium to regulate aspects of metabolism and cognition. Brain insulin resistance often results from high-fat diet (HFD) consumption and is thought to contribute to spatial cognition deficits. To target BBB insulin function, we used Cre-LoxP genetic excision of the insulin receptor (InsR) from endothelial cells in adult male mice. We hypothesized that this excision would impair spatial cognition, and that high-fat diet consumption would exacerbate these effects. Excision of the endothelial InsR did not impair performance in two spatial cognition tasks, the Y-Maze and Morris Water Maze, in tests held both before and after 14 weeks of access to high-fat (or chow control) diet. The HFD increased body weight gain and induced glucose intolerance but did not impair spatial cognition. Endothelial InsR excision tended to increase body weight and reduce sensitivity to peripheral insulin, but these metabolic effects were not associated with impairments to spatial cognition and did not interact with HFD exposure. Instead, all mice showed intact spatial cognitive performance regardless of whether they had been fed chow or a HFD, and whether the InsR had been excised or not. Overall, the results indicate that loss of the endothelial InsR does not impact spatial cognition, which is in line with pharmacological evidence that other mechanisms at the BBB facilitate insulin transport and allow it to exert its pro-cognitive effects.

Sex-dependent effects of carbohydrate source and quantity on caspase-1 activity in the mouse central nervous system

BACKGROUND

Mounting evidence links glucose intolerance and diabetes as aspects of metabolic dysregulation that are associated with an increased risk of developing dementia. Inflammation and inflammasome activation have emerged as a potential link between these disparate pathologies. As diet is a key factor in both the development of metabolic disorders and inflammation, we hypothesize that long term changes in dietary factors can influence nervous system function by regulating inflammasome activity and that this phenotype would be sex-dependent, as sex hormones are known to regulate metabolism and immune processes.

METHODS

5-week-old male and female transgenic mice expressing a caspase-1 bioluminescent reporter underwent cranial window surgeries and were fed control (65% complex carbohydrates, 15% fat), high glycemic index (65% carbohydrates from sucrose, 15% fat), or ketogenic (1% complex carbohydrates, 79% fat) diet from 6 to 26 weeks of age. Glucose regulation was assessed with a glucose tolerance test following a 4-h morning fast. Bioluminescence in the brain was quantified using IVIS in vivo imaging. Blood cytokine levels were measured using cytokine bead array. 16S ribosomal RNA gene amplicon sequencing of mouse feces was performed to assess alterations in the gut microbiome. Behavior associated with these dietary changes was also evaluated.

RESULTS

The ketogenic diet caused weight gain and glucose intolerance in both male and female mice. In male mice, the high glycemic diet led to increased caspase-1 biosensor activation over the course of the study, while in females the ketogenic diet drove an increase in biosensor activation compared to their respective controls. These changes correlated with an increase in inflammatory cytokines present in the serum of test mice and the emergence of anxiety-like behavior. The microbiome composition differed significantly between diets; however no significant link between diet, glucose tolerance, or caspase-1 signal was established.

CONCLUSIONS

Our findings suggest that diet composition, specifically the source and quantity of carbohydrates, has sex-specific effects on inflammasome activation in the central nervous system and behavior. This phenotype manifested as increased anxiety in male mice, and future studies are needed to determine if this phenotype is linked to alterations in microbiome composition.

The effects of orlistat on oxidative stress, recognition memory, spatial memory and hippocampal tissue in experimentally induced obesity in rats

This study investigates the impact of orlistat on oxidative stress, spatial memory, recognition memory, and hippocampal tissue in obese rats. The study groups were divided into control, high fat diet-induced obese (HFDIO), HFDIO+orlistat (HFDIO+ORL) groups, each consisting of 8 animals. While control fed with standart diet, HFDIO and HFDIO+ORL fed with high-fat diets for 8 weeks to induce obesity. Then, ORL treated 10 mg/kg for 7 weeks, while control and HFDIO get water. At 16th week, novel object recognition (NOR) and Morris water maze (MWM) tests were performed. TNF-alpha, IL-1beta levels in hippocampal tissue, and total/native thiol/disulphide levels in serum were measured. TNF-alpha level of HFDIO was higher than control, while lower in HFDIO+ORL compared to HFDIO as like IL-1beta level. On the contrary, serum total thiol level was lower in HFDIO than control and higher in HFDIO+ORL compared to the HFDIO, while disulphide level was opposite of the total thiol levels. While recognition index was higher in HFDIO+ORL, in MWM, latency of finding platform in HFDIO was higher than control and latency of HFDIO+ORL was very similar to control in 2-4 days. The HFDIO group demonstrated decrease in time spent in platform zone compared to control, whereas time spent of the HFDIO+ORL was higher than HFDIO. Our study demonstrates that orlistat administration exerts beneficial effects on oxidative stress, spatial memory, recognition memory, and hippocampal tissue in obese rats. It shows that orlistat may have potential therapeutic implications for obesity-related cognitive impairments and hippocampal dysfunction.

Sex differences in response to obesity and caloric restriction on cognition and hippocampal measures of autophagic-lysosomal transcripts and signaling pathways

BACKGROUND

Obesity rates in the U.S. continue to increase, with nearly 50% of the population being either obese or morbidly obese. Obesity, along with female sex, are leading risk factors for sporadic Alzheimer's Disease (AD) necessitating the need to better understand how these variables impact cellular function independent of age or genetic mutations. Animal and clinical studies both indicate that autophagy-lysosomal pathway (ALP) dysfunction is among the earliest known cellular systems to become perturbed in AD, preceding cognitive decline, yet little is known about how obesity and sex affects these cellular functions in the hippocampus, a brain region uniquely susceptible to the negative effects of obesity. We hypothesized that obesity would negatively affect key markers of ALP in the hippocampus, effects would vary based on sex, and that caloric restriction would counteract obesity effects.

METHODS

Female and male mice were placed on an obesogenic diet for 10 months, at which point half were switched to caloric restriction for three months, followed by cognitive testing in the Morris watermaze. Hippocampus was analyzed by western blot and qPCR.

RESULTS

Cognitive function in female mice responded differently to caloric restriction based on whether they were on a normal or obesogenic diet; male cognition was only mildly affected by caloric restriction and not obesity. Significant male-specific changes occurred in cellular markers of autophagy, including obesity increasing pAkt, Slc38a9, and Atg12, while caloric restriction reduced pRPS6 and increased Atg7. In contrast females experienced changes due to diet/caloric restriction predominately in lysosomal markers including increased TFE3, FLCN, FNIP2, and pAMPK.

CONCLUSIONS

Results support that hippocampal ALP is a target of obesity and that sex shapes molecular responses, while providing insight into how dietary manipulations affect learning and memory based on sex.

High-fat diet feeding triggers a regenerative response in the adult zebrafish brain

Non-alcoholic fatty liver disease (NAFLD) includes a range of liver conditions ranging from excess fat accumulation to liver failure. NAFLD is strongly associated with high-fat diet (HFD) consumption that constitutes a metabolic risk factor. While HFD has been elucidated concerning its several systemic effects, there is little information about its influence on the brain at the molecular level. Here, by using a high-fat diet (HFD)-feeding of adult zebrafish, we first reveal that excess fat uptake results in weight gain and fatty liver. Prolonged exposure to HFD induces a significant increase in the expression of pro-inflammation, apoptosis, and proliferation markers in the liver and brain tissues. Immunofluorescence analyses of the brain tissues disclose stimulation of apoptosis and widespread activation of glial cell response. Moreover, glial activation is accompanied by an initial decrease in the number of neurons and their subsequent replacement in the olfactory bulb and the telencephalon. Long-term consumption of HFD causes activation of Wnt/β-catenin signaling in the brain tissues. Finally, fish fed an HFD induces anxiety, and aggressiveness and increases locomotor activity. Thus, HFD feeding leads to a non-traumatic brain injury and stimulates a regenerative response. The activation mechanisms of a regeneration response in the brain can be exploited to fight obesity and recover from non-traumatic injuries.

Long-Term High-Fat Diet Decreases Renal Insulin-Degrading Enzyme Expression and Function by Inhibiting the PPARγ Pathway

SCOPE

Long-term high-fat diet (HFD) causes insulin resistance, which is a primary etiological factor in the development of obesity and type 2 diabetes mellitus. Impaired insulin clearance is not only a consequence but also a cause of insulin resistance. The kidney is a major site of insulin clearance, where the insulin-degrading enzyme (IDE) plays a vital role in the proximal tubule. Thus, the study investigates the role of renal IDE in the regulation of insulin resistance in HFD-induced obese mice.

METHODS AND RESULTS

Twenty four-weeks of HFD in C57BL/6 mice causes insulin resistance and impaires insulin clearance, accompanied by a decrease in renal IDE expression and activity. Palmitic acid decreases IDE mRNA and protein expressions in HK-2 cells. RNA-Seq analysis found that the PPAR pathway is involved. 24-weeks of HFD decreases renal PPARγ, but not PPARα or PPARβ/δ mRNA expression. The inhibition of IDE expression by palmitic acid is prevented by the PPARγ agonist rosiglitazone. The amount of PPARγ bound to the promoters of IDE is decreased in palmitic acid-treated cells. Rosiglitazone improves insulin clearance and insulin resistance and increases renal IDE expression in HFD fed-mice.

CONCLUSION

Long-term HFD decreases renal IDE expression and activity, and causes insulin resistance, which involves PPARγ.

Partial MCT1 invalidation protects against diet-induced non-alcoholic fatty liver disease and the associated brain dysfunction

BACKGROUND & AIMS

Non-alcoholic fatty liver disease (NAFLD) has been associated with mild cerebral dysfunction and cognitive decline, although the exact pathophysiological mechanism remains ambiguous. Using a diet-induced model of NAFLD and monocarboxylate transporter-1 (Mct1^+/-) haploinsufficient mice, which resist high-fat diet-induced hepatic steatosis, we investigated the hypothesis that NAFLD leads to an encephalopathy by altering cognition, behaviour, and cerebral physiology. We also proposed that global MCT1 downregulation offers cerebral protection.

METHODS

Behavioural tests were performed in mice following 16 weeks of control diet (normal chow) or high-fat diet with high fructose/glucose in water. Tissue oxygenation, cerebrovascular reactivity, and cerebral blood volume were monitored under anaesthesia by multispectral optoacoustic tomography and optical fluorescence. Cortical mitochondrial oxygen consumption and respiratory capacities were measured using ex vivo high-resolution respirometry. Microglial and astrocytic changes were evaluated by immunofluorescence and 3D reconstructions. Body composition was assessed using EchoMRI, and liver steatosis was confirmed by histology.

RESULTS

NAFLD concomitant with obesity is associated with anxiety- and depression-related behaviour. Low-grade brain tissue hypoxia was observed, likely attributed to the low-grade brain inflammation and decreased cerebral blood volume. It is also accompanied by microglial and astrocytic morphological and metabolic alterations (higher oxygen consumption), suggesting the early stages of an obesogenic diet-induced encephalopathy. Mct1 haploinsufficient mice, despite fat accumulation in adipose tissue, were protected from NAFLD and associated cerebral alterations.

CONCLUSIONS

This study provides evidence of compromised brain health in obesity and NAFLD, emphasising the importance of the liver-brain axis. The protective effect of Mct1 haploinsufficiency points to this protein as a novel therapeutic target for preventing and/or treating NAFLD and the associated brain dysfunction.

IMPACT AND IMPLICATIONS

This study is focused on unravelling the pathophysiological mechanism by which cerebral dysfunction and cognitive decline occurs during NAFLD and exploring the potential of monocarboxylate transporter-1 (MCT1) as a novel preventive or therapeutic target. Our findings point to NAFLD as a serious health risk and its adverse impact on the brain as a potential global health system and economic burden. These results highlight the utility of Mct1 transgenic mice as a model for NAFLD and associated brain dysfunction and call for systematic screening by physicians for early signs of psychological symptoms, and an awareness by individuals at risk of these potential neurological effects. This study is expected to bring attention to the need for early diagnosis and treatment of NAFLD, while having a direct impact on policies worldwide regarding the health risk associated with NAFLD, and its prevention and treatment.

Feeding the Brain: Effect of Nutrients on Cognition, Synaptic Function, and AMPA Receptors

In recent decades, traditional eating habits have been replaced by a more globalized diet, rich in saturated fatty acids and simple sugars. Extensive evidence shows that these dietary factors contribute to cognitive health impairment as well as increase the incidence of metabolic diseases such as obesity and diabetes. However, how these nutrients modulate synaptic function and neuroplasticity is poorly understood. We review the Western, ketogenic, and paleolithic diets for their effects on cognition and correlations with synaptic changes, focusing mainly (but not exclusively) on animal model studies aimed at tracing molecular alterations that may contribute to impaired human cognition. We observe that memory and learning deficits mediated by high-fat/high-sugar diets, even over short exposure times, are associated with reduced arborization, widened synaptic cleft, narrowed post-synaptic zone, and decreased activity-dependent synaptic plasticity in the hippocampus, and also observe that these alterations correlate with deregulation of the AMPA-type glutamate ionotropic receptors (AMPARs) that are crucial to neuroplasticity. Furthermore, we explored which diet-mediated mechanisms modulate synaptic AMPARs and whether certain supplements or nutritional interventions could reverse deleterious effects, contributing to improved learning and memory in older people and patients with Alzheimer’s disease.

Overnutrition Induced Cognitive Impairment: Insulin Resistance, Gut-Brain Axis, and Neuroinflammation

Overnutrition-related obesity has become a worldwide epidemic, and its prevalence is expected to steadily rise in the future. It is widely recognized that obesity exerts negative impacts on metabolic disorders such as type 2 diabetes mellitus (T2DM) and cardiovascular diseases. However, relatively fewer reports exist on the impairment of brain structure and function, in the form of memory and executive dysfunction, as well as neurogenerative diseases. Emerging evidence indicates that besides obesity, overnutrition diets independently induce cognitive impairments via multiple mechanisms. In this study, we reviewed the clinical and preclinical literature about the detrimental effects of obesity or high-nutrition diets on cognitive performance and cerebral structure. We mainly focused on the role of brain insulin resistance (IR), microbiota-gut-brain axis, and neuroinflammation. We concluded that before the onset of obesity, short-term exposure to high-nutrition diets already blunted central responses to insulin, altered gut microbiome composition, and activated inflammatory mediators. Overnutrition is linked with the changes in protein expression in brain insulin signaling, leading to pathological features in the brain. Microbiome alteration, bacterial endotoxin release, and gut barrier hyperpermeability also occur to trigger mental and neuronal diseases. In addition, obesity or high-nutrition diets cause chronic and low-grade systematic inflammation, which eventually spreads from the peripheral tissue to the central nervous system (CNS). Altogether, a large number of unknown but potential routes interact and contribute to obesity or diet-induced cognitive impairment. The challenge for future research is to identify effective interventions involving dietary shifts and personalized therapy targeting the underlying mechanisms to prevent and improve cognition deficits.

The Obese Brain: Mechanisms of Systemic and Local Inflammation, and Interventions to Reverse the Cognitive Deficit

Overweight and obesity are now considered a worldwide pandemic and a growing public health problem with severe economic and social consequences. Adipose tissue is an organ with neuroimmune-endocrine functions, which participates in homeostasis. So, adipocyte hypertrophy and hyperplasia induce a state of chronic inflammation that causes changes in the brain and induce neuroinflammation. Studies with obese animal models and obese patients have shown a relationship between diet and cognitive decline, especially working memory and learning deficiencies. Here we analyze how obesity-related peripheral inflammation can affect central nervous system physiology, generating neuroinflammation. Given that the blood-brain barrier is an interface between the periphery and the central nervous system, its altered physiology in obesity may mediate the consequences on various cognitive processes. Finally, several interventions, and the use of natural compounds and exercise to prevent the adverse effects of obesity in the brain are also discussed.

Plant Sterol-Poor Diet Is Associated with Pro-Inflammatory Lipid Mediators in the Murine Brain

Plant sterols (PSs) cannot be synthesized in mammals and are exclusively diet-derived. PSs cross the blood-brain barrier and may have anti-neuroinflammatory effects. Obesity is linked to lower intestinal uptake and blood levels of PSs, but its effects in terms of neuroinflammation—if any—remain unknown. We investigated the effect of high-fat diet-induced obesity on PSs in the brain and the effects of the PSs campesterol and β-sitosterol on in vitro microglia activation. Sterols (cholesterol, precursors, PSs) and polyunsaturated fatty acid-derived lipid mediators were measured in the food, blood, liver and brain of C57BL/6J mice. Under a PSs-poor high-fat diet, PSs levels decreased in the blood, liver and brain (>50%). This effect was reversible after 2 weeks upon changing back to a chow diet. Inflammatory thromboxane B2 and prostaglandin D2 were inversely correlated to campesterol and β-sitosterol levels in all brain regions. PSs content was determined post mortem in human cortex samples as well. In vitro, PSs accumulate in lipid rafts isolated from SIM-A9 microglia cell membranes. In summary, PSs levels in the blood, liver and brain were associated directly with PSs food content and inversely with BMI. PSs dampen pro-inflammatory lipid mediators in the brain. The identification of PSs in the human cortex in comparable concentration ranges implies the relevance of our findings for humans.