Dietary Marine Hydrolysate Improves Memory Performance and Social Behavior through Gut Microbiota Remodeling during Aging

Aging is characterized by a decline in social behavior and cognitive functions leading to a decrease in life quality. In a previous study, we show that a fish hydrolysate supplementation prevents age-related decline in spatial short-term memory and long-term memory and anxiety-like behavior and improves the stress response in aged mice. The aim of this study was to determine the effects of a fish hydrolysate enriched with EPA/DHA or not on the cognitive ability and social interaction during aging and the biological mechanisms involved. We showed for the first time that a fish hydrolysate enriched with EPA/DHA or not improved memory performance and preference for social novelty that were diminished by aging. These changes were associated with the modulation of the gut microbiota, normalization of corticosterone, and modulation of the expression of genes involved in the mitochondrial respiratory chain, circadian clock, neuroprotection, and antioxidant activity. Thus, these changes may contribute to the observed improvements in social behavior and memory and reinforced the innovative character of fish hydrolysate in the prevention of age-related impairments.

Effects of a polyphenol-rich grape and blueberry extract (Memophenol™) on cognitive function in older adults with mild cognitive impairment: A randomized, double-blind, placebo-controlled study

Background

Polyphenols are naturally occurring organic compounds found in plants. Research suggests that their intake reduces the risk of cognitive decline and related dementias. Grapes and blueberries are polyphenol-rich foods that have attracted attention for their potential cognitive-enhancing effects.

Purpose

Examine the effects of supplementation with a standardized and patented polyphenol-rich grape and blueberry extract (Memophenol™) on cognitive function in older adults with mild cognitive impairment.

Study design

Two-arm, 6 month, parallel-group, randomized, double-blind, placebo-controlled trial.

Methods

One hundred and forty-three volunteers aged 60 to 80 years with mild cognitive impairment were supplemented with either 150 mg of Memophenol™, twice daily or a placebo. Outcome measures included computer-based cognitive tasks, the Behavior Rating Inventory of Executive Function (BRIEF-A), the Cognitive Failures Questionnaire, and the CASP-19.

Results

Compared to the placebo, Memophenol™ supplementation was associated with greater improvements in the speed of information processing (p = 0.020), visuospatial learning (p = 0.012), and the BRIEF-A global score (p = 0.046). However, there were no other statistically significant between-group differences in the performance of other assessed cognitive tests or self-report questionnaires. Memophenol™ supplementation was well-tolerated with no reports of significant adverse reactions.

Conclusion

The promising results from this trial suggest that 6-months of supplementation with Memophenol™ may improve aspects of cognitive function in adults with mild cognitive impairment. Further research will be important to expand on the current findings and identify the potential mechanisms of action associated with the intake of this polyphenol-rich extract.

Dietary Fish Hydrolysate Improves Memory Performance Through Microglial Signature Remodeling During Aging

Brain aging is characterized by a chronic low-grade inflammation, which significantly impairs cognitive function. Microglial cells, the immunocompetent cells of the brain, present a different phenotype, switching from a homeostatic signature (M0) to a more reactive phenotype called “MGnD” (microglial neurodegenerative phenotype), leading to a high production of pro-inflammatory cytokines. Furthermore, microglial cells can be activated by age-induced gut dysbiosis through the vagus nerve or the modulation of the peripheral immune system. Nutrients, in particular n-3 long chain polyunsaturated fatty acids (LC-PUFAs) and low molecular weight peptides, display powerful immunomodulatory properties, and can thus prevent age-related cognitive decline. The objective of this study was to investigate the effects of n-3 LC-PUFAs and low molecular weight peptides contained in a marine by-product-derived hydrolysate on microglial phenotypes and intestinal permeability and their consequences on cognition in mice. We demonstrated that the hydrolysate supplementation for 8 weeks prevented short- and long-term memory decline during aging. These observations were linked to the modulation of microglial signature. Indeed, the hydrolysate supplementation promoted homeostatic microglial phenotype by increasing TGF-β1 expression and stimulated phagocytosis by increasing Clec7a expression. Moreover, the hydrolysate supplementation promoted anti-inflammatory intestinal pathway and tended to prevent intestinal permeability alteration occurring during aging. Therefore, the fish hydrolysate appears as an interesting candidate to prevent cognitive decline during aging.

Role of Retinoid X Receptors (RXRs) and dietary vitamin A in Alzheimer's disease: Evidence from clinicopathological and preclinical studies

BACKGROUND

Vitamin A (VitA), via its active metabolite retinoic acid (RA), is critical for the maintenance of memory function with advancing age. Although its role in Alzheimer's disease (AD) is not well understood, data suggest that impaired brain VitA signaling is associated with the accumulation of β-amyloid peptides (Aβ), and could thus contribute to the onset of AD.

METHODS

We evaluated the protective action of a six-month-long dietary VitA-supplementation (20 IU/g), starting at 8 months of age, on the memory and the neuropathology of the 3xTg-AD mouse model of AD (n = 11-14/group; including 4-6 females and 7-8 males). We also measured protein levels of Retinoic Acid Receptor β (RARβ) and Retinoid X Receptor γ (RXRγ) in homogenates from the inferior parietal cortex of 60 participants of the Religious Orders study (ROS) divided in three groups: no cognitive impairment (NCI) (n = 20), mild cognitive impairment (MCI) (n = 20) and AD (n = 20).

RESULTS

The VitA-enriched diet preserved spatial memory of 3xTg-AD mice in the Y maze. VitA-supplementation affected hippocampal RXR expression in an opposite way according to sex by tending to increase in males and decrease in females their mRNA expression. VitA-enriched diet also reduced the amount of hippocampal Aβ₄₀ and Aβ₄₂, as well as the phosphorylation of tau protein at sites Ser396/Ser404 (PHF-1) in males. VitA-supplementation had no effect on tau phosphorylation in females but worsened their hippocampal Aβ load. However, the expression of Rxr-β in the hippocampus was negatively correlated with the amount of both soluble and insoluble Aβ in both males and females. Western immunoblotting in the human cortical samples of the ROS study did not reveal differences in RARβ levels. However, it evidenced a switch from a 60-kDa-RXRγ to a 55-kDa-RXRγ in AD, correlating with ante mortem cognitive decline and the accumulation of neuritic plaques in the brain cortex.

CONCLUSION

Our data suggest that (i) an altered expression of RXRs receptors is a contributor to β-amyloid pathology in both humans and 3xTg-AD mice, (ii) a chronic exposure of 3xTg-AD mice to a VitA-enriched diet may be protective in males, but not in females.

Dietary fish hydrolysate supplementation containing n-3 LC-PUFAs and peptides prevents short-term memory and stress response deficits in aged mice

Brain aging is characterized by a decline in cognitive functions, which can lead to the development of neurodegenerative pathologies. Age-related spatial learning and memory deficits are associated with a chronic low-grade inflammation. Anxiety disorders and stress response alterations, occurring for a part of the elderly, have also been linked to an increased neuroinflammation and thus, an accelerated cognitive decline. Nutrition is an innovative strategy to prevent age-related cognitive impairments. Among the nutrients, n-3 long chain polyunsaturated fatty acids (LC-PUFAs) and low molecular weight peptides from proteins, especially those from marine resources, are good candidates for their immunomodulatory, anxiolytic and neuroprotective properties. The aim of this study is to determine the combined effect of n-3 LC-PUFAs and low molecular weight peptides on cognitive functions, and their mechanism of action. We are the first to show that a dietary supplementation with a fish hydrolysate containing n-3 LC-PUFAs and low molecular weight peptides prevented the age-related spatial short-term memory deficits and modulated navigation strategies adopted during spatial learning. In addition, the fish hydrolysate displayed anxiolytic activities with the reduction of anxiety-like behaviour in aged mice, restored the plasmatic corticosterone levels similar to adult animals following an acute stress and modulated the hypothalamic stress response. These effects on behaviour can be explained by the immunomodulatory and neuroprotective properties of the fish hydrolysate that limited microgliosis in vivo, decreased LPS-induced expression of pro-inflammatory cytokines and increased the expression of growth factors such as BDNF and NGF in vitro. Thus, n-3 LC-PUFAs and low molecular weight peptides contained in the fish hydrolysate can play an important role in the limitation of neuroinflammation and stress response alterations during aging and represent a potential strategy for the prevention of age-related cognitive decline.

Dietary vitamin A supplementation prevents early obesogenic diet-induced microbiota, neuronal and cognitive alterations

BACKGROUND

Early consumption of obesogenic diets, rich in saturated fat and added sugar, is associated with a plethora of biological dysfunctions, at both peripheral and brain levels. Obesity is also linked to decreased vitamin A bioavailability, an essential molecule for brain plasticity and memory function.

METHODS

Here we investigated in mice whether dietary vitamin A supplementation (VAS) could prevent some of the metabolic, microbiota, neuronal and cognitive alterations induced by obesogenic, high-fat and high-sugar diet (HFSD) exposure from weaning to adulthood, i.e. covering periadolescent period.

RESULTS

As expected, VAS was effective in enhancing peripheral vitamin A levels as well as hippocampal retinoic acid levels, the active metabolite of vitamin A, regardless of the diet. VAS attenuated HFSD-induced excessive weight gain, without affecting metabolic changes, and prevented alterations of gut microbiota α-diversity. In HFSD-fed mice, VAS prevented recognition memory deficits but had no effect on aversive memory enhancement. Interestingly, VAS alleviated both HFSD-induced higher neuronal activation and lower glucocorticoid receptor phosphorylation in the hippocampus after training.

CONCLUSION

Dietary VAS was protective against the deleterious effects of early obesogenic diet consumption on hippocampal function, possibly through modulation of the gut-brain axis.

Chronic Supplementation with a Mix of Salvia officinalis and Salvia lavandulaefolia Improves Morris Water Maze Learning in Normal Adult C57Bl/6J Mice

Background: Two different species of sage, Salvia officinalis and Salvia lavandulaefolia, have demonstrated activities in cognitive function during preclinical and clinical studies related to impaired health situations or single administration. Different memory processes have been described to be significantly and positively impacted. Objective: Our objective is to explore the potential of these Salvia, and their additional activities, in healthy situations, and during prolonged administration, on memory and subsequent mechanisms of action related to putative effects. Design: This mouse study has implicated four investigational arms dedicated to control, Salvia officinalis aqueous extract, Salvia lavandulaefolia-encapsulated essential oil and a mix thereof (Cognivia™) for 2 weeks of administration. Cognitive functions have been assessed throughout Y-maze and Morris water maze models. The impact of supplementation on lipid peroxidation, oxidative stress, neurogenesis, neuronal activity, neurotrophins, neurotrophin receptors, CaM kinase II and glucocorticoid receptors has been assessed via post-interventional tissue collection. Results: All Salvia groups had a significant effect on Y-maze markers on day 1 of administration. Only the mix of two Salvia species demonstrated significant improvements in Morris water maze markers at the end of administration. Considering all biological and histological markers, we did not observe any significant effect of S. officinalis, S. lavandulaefolia and a mix of Salvia supplementation on lipid peroxidation, oxidative stress and neuronal plasticity (neurogenesis, neuronal activity, neurotrophins). Interestingly, CaM kinase II protein expression is significantly increased in animals supplemented with Salvia. Conclusion: The activities of Salvia alone after one intake have been confirmed; however, a particular combination of different types of Salvia have been shown to improve memory and present specific synergistic effects after chronic administration in healthy mice.

Polyphenols From Grape and Blueberry Improve Episodic Memory in Healthy Elderly with Lower Level of Memory Performance: A Bicentric Double-Blind, Randomized, Placebo-Controlled Clinical Study

Polyphenols are promising nutritional bioactives exhibiting beneficial effect on age-related cognitive decline. This study evaluated the effect of a polyphenol-rich extract from grape and blueberry (PEGB) on memory of healthy elderly subjects (60-70 years-old). A bicentric, randomized, double-blind, placebo-controlled trial was conducted with 215 volunteers receiving 600 mg/day of PEGB (containing 258 mg flavonoids) or a placebo for 6 months. The primary outcome was the CANTAB Paired Associate Learning (PAL), a visuospatial learning and episodic memory test. Secondary outcomes included verbal episodic and recognition memory (VRM) and working memory (SSP). There was no significant effect of PEGB on the PAL on the whole cohort. Yet, PEGB supplementation improved VRM-free recall. Stratifying the cohort in quartiles based on PAL at baseline revealed a subgroup with advanced cognitive decline (decliners) who responded positively to the PEGB. In this group, PEGB consumption was also associated with a better VRM-delayed recognition. In addition to a lower polyphenol consumption, the urine metabolomic profile of decliners revealed that they excreted more metabolites. Urinary concentrations of specific flavan-3-ols metabolites were associated, at the end of the intervention, with the memory improvements. Our study demonstrates that PEGB improves age-related episodic memory decline in individuals with the highest cognitive impairments.

n-3 Polyunsaturated Fatty Acids and Their Derivates Reduce Neuroinflammation during Aging

: Aging is associated to cognitive decline, which can lead to loss of life quality, personal suffering, and ultimately neurodegenerative diseases. Neuroinflammation is one of the mechanisms explaining the loss of cognitive functions. Indeed, aging is associated to the activation of inflammatory signaling pathways, which can be targeted by specific nutrients with anti-inflammatory effects. Dietary n-3 polyunsaturated fatty acids (PUFAs) are particularly attractive as they are present in the brain, possess immunomodulatory properties, and are precursors of lipid derivates named specialized pro-resolving mediators (SPM). SPMs are crucially involved in the resolution of inflammation that is modified during aging, resulting in chronic inflammation. In this review, we first examine the effect of aging on neuroinflammation and then evaluate the potential beneficial effect of n-3 PUFA as precursors of bioactive derivates, particularly during aging, on the resolution of inflammation. Lastly, we highlight evidence supporting a role of n-3 PUFA during aging.

Vitamin A deficiency impairs contextual fear memory in rats: Abnormalities in the glucocorticoid pathway

Vitamin A and its active metabolite, retinoic acid (RA), play a key role in the maintenance of cognitive functions in the adult brain. Depletion of RA using the vitamin A deficiency (VAD) model in Wistar rats leads to spatial memory deficits in relation to elevated intrahippocampal basal corticosterone (CORT) levels and increased hippocampal 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) activity. All of these effects are normalised by vitamin A supplementation. However, it is unknown whether vitamin A status also modulates contextual fear conditioning (CFC) in a glucocorticoid-associated fear memory task dependent on the functional integrity of the hippocampus. In the present study, we investigated the impact of VAD and vitamin A supplementation in adult male rats on fear memory processing, plasma CORT levels, hippocampal retinoid receptors and 11β-HSD1 expression following a novelty-induced stress. We also examined whether vitamin A supplementation or a single injection of UE2316, a selective 11β-HSD1 inhibitor, known to modulate local glucocorticoid levels, had any beneficial effects on contextual fear memory and biochemical parameters in VAD rats. We provide evidence that VAD rats exhibit a decreased fear conditioning response during training with a poor contextual fear memory 24 hours later. These VAD-induced cognitive impairments are associated with elevated plasma CORT levels under basal conditions, as well as following a stressful event, with saturated CORT release, altered hippocampal retinoid receptors and 11β-HSD1 expression. Vitamin A supplementation normalises VAD-induced fear conditioning training deficits and all biochemical effects, although it cannot prevent fear memory deficits. Moreover, a single injection of UE2316 not only impairs contextual fear memory, but also reduces plasma CORT levels, regardless of the vitamin A status and decreases slightly hippocampal 11β-HSD1 activity in VAD rats following stress. The present study highlights the importance of vitamin A status with respect to modulating fear memory conditioning in relation to plasma CORT levels and hippocampal 11β-HSD1.

Reduction of acute mild stress corticosterone response and changes in stress-responsive gene expression in male Balb/c mice after repeated administration of a Rhodiola rosea L. root extract

Rhodiola rosea L. (R. rosea) is an adaptogenic plant increasing body resistance to stress. Its efficacy has been evidenced mainly in chronic stress models, data concerning its effect in acute stress and underlying mechanisms being scarce. The objective was to investigate the effect of repeated doses of a R. rosea hydroethanolic root extract (HRE) on hypothalamic pituitary adrenal response in a murine model of acute mild stress and also the mechanisms involved. Stress response was measured in Balb/c mice having received by gavage HRE (5 g/kg) or vehicle daily for 2 weeks before being submitted to an acute mild stress protocol (open-field test then elevated plus maze). Corticosterone was measured in plasma from mandibular vein blood drawn before and 30, 60, and 90 min after initiation of the stress protocol. Mice were sacrificed at 90 min, and the hippocampus, prefrontal cortex, and amygdala were excised for high-frequency RT-PCR gene expression analysis. At 30 min after acute mild stress induction, corticosterone level in mice having received the HRE was lower than in control mice and comparable to that in nonstressed mice in the HRE group. HRE administration induced brain structure-dependent changes in expression of several stress-responsive genes implicated in neuronal structure, HPA axis activation, and circadian rhythm. In the acute mild stress model used, R. rosea HRE decreased corticosterone level and increased expression of stress-responsive genes, especially in the hippocampus and prefrontal cortex. These findings suggest that R. rosea HRE could be of value for modulating reactivity to acute mild stress.

Dietary n-3 long chain PUFA supplementation promotes a pro-resolving oxylipin profile in the brain

The brain is highly enriched in long chain polyunsaturated fatty acids (LC-PUFAs) that display immunomodulatory properties in the brain. At the periphery, the modulation of inflammation by LC-PUFAs occurs through lipid mediators called oxylipins which have anti-inflammatory and pro-resolving activities when derived from n-3 LC-PUFAs and pro-inflammatory activities when derived from n-6 LC-PUFAs. However, whether a diet rich in LC-PUFAs modulates oxylipins and neuroinflammation in the brain has been poorly investigated. In this study, the effect of a dietary n-3 LC-PUFA supplementation on oxylipin profile and neuroinflammation in the brain was analyzed. Mice were given diets deficient or supplemented in n-3 LC-PUFAs for a 2-month period starting at post-natal day 21, followed by a peripheral administration of lipopolysaccharide (LPS) at adulthood. We first showed that dietary n-3 LC-PUFA supplementation induced n-3 LC-PUFA enrichment in the hippocampus and subsequently an increase in n-3 PUFA-derived oxylipins and a decrease in n-6 PUFA-derived oxylipins. In response to LPS, n-3 LC-PUFA deficient mice presented a pro-inflammatory oxylipin profile whereas n-3 LC-PUFA supplemented mice displayed an anti-inflammatory oxylipin profile in the hippocampus. Accordingly, the expression of cyclooxygenase-2 and 5-lipoxygenase, the enzymes implicated in pro- and anti-inflammatory oxylipin synthesis, was induced by LPS in both diets. In addition, LPS-induced pro-inflammatory cytokine increase was reduced by dietary n-3 LC-PUFA supplementation. These results indicate that brain n-3 LC-PUFAs increase by dietary means and promote the synthesis of anti-inflammatory derived bioactive oxylipins. As neuroinflammation plays a key role in all brain injuries and many neurodegenerative disorders, the present data suggest that dietary habits may be an important regulator of brain cytokine production in these contexts.

Dietary omega-3 deficiency exacerbates inflammation and reveals spatial memory deficits in mice exposed to lipopolysaccharide during gestation

Maternal immune activation (MIA) is a common environmental insult on the developing brain and represents a risk factor for neurodevelopmental disorders. Animal models of in utero inflammation further revealed a causal link between maternal inflammatory activation during pregnancy and behavioural impairment relevant to neurodevelopmental disorders in the offspring. Accumulating evidence point out that proinflammatory cytokines produced both in the maternal and fetal compartments are responsible for social, cognitive and emotional behavioral deficits in the offspring. Polyunsaturated fatty acids (PUFAs) are essential fatty acids with potent immunomodulatory activities. PUFAs and their bioactive derivatives can promote or inhibit many aspects of the immune and inflammatory response. PUFAs of the n-3 series ('n-3 PUFAs', also known as omega-3) exhibit anti-inflammatory/pro-resolution properties and promote immune functions, while PUFAs of the n-6 series ('n-6 PUFAs' or omega-6) favor pro-inflammatory responses. The present study aimed at providing insight into the effects of n-3 PUFAs on the consequences of MIA on brain development. We hypothesized that a reduction in n-3 PUFAs exacerbates both maternal and fetal inflammatory responses to MIA and later-life defects in memory in the offspring. Based on a lipopolysaccharide (LPS) model of MIA (LPS injection at embryonic day 17), we showed that n-3 PUFA deficiency 1) alters fatty acid composition of the fetal and adult offspring brain; 2) exacerbates maternal and fetal inflammatory processes with no significant alteration of microglia phenotype, and 3) induces spatial memory deficits in the adult offspring. We also showed a strong negative correlation between brain content in n-3 PUFA and cytokine production in MIA-exposed fetuses. Overall, our study is the first to address the deleterious effects of n-3 PUFA deficiency on brain lipid composition, inflammation and memory performances in MIA-exposed animals and indicates that it should be considered as a potent environmental risk factor for the apparition of neurodevelopmental disorders.

Maternal n-3 polyunsaturated fatty acid dietary supply modulates microglia lipid content in the offspring

The brain is highly enriched in long chain polyunsaturated fatty acids (LC-PUFAs) that are esterified into phospholipids, the major components of cell membranes. They accumulate during the perinatal period when the brain is rapidly developing. Hence, the levels of LC-PUFAs in the brains of the offspring greatly depend on maternal dietary intake. Perinatal n-3 PUFA consumption has been suggested to modulate the activity of microglial cells, the brain's innate immune cells which contribute to the shaping of neuronal network during development. However, the impact of maternal n-3 PUFA intake on microglial lipid composition in the offspring has never been studied. To investigate the impact of maternal dietary n-3 PUFA supply on microglia lipid composition, pregnant mice were fed with n-3 PUFA deficient, n-3 PUFA balanced or n-3 PUFA supplemented diets during gestation and lactation. At weaning, microglia were isolated from the pup's brains to analyze their fatty acid composition and phospholipid class levels. We here report that post-natal microglial cells displayed a distinctive lipid profile as they contained high levels of eicosapentaenoic acid (EPA), more EPA than docosahexaenoic acid (DHA) and large amount of phosphatidylinositol (PI) / phosphatidylserine (PS). Maternal n-3 PUFA supply increased DHA levels and decreased n-6 docosapentaenoic acid (DPA) levels whereas the PI/PS membrane content was inversely correlated to the quantity of PUFAs in the diet. These results raise the possibility of modulating microglial lipid profile and their subsequent activity in the developing brain.

Retinoic acid increases glucocorticoid receptor phosphorylation via cyclin-dependent kinase 5

Glucocorticoid receptor (GR) function is modulated by phosphorylation. As retinoic acid (RA) can activate some cytoplasmic kinases able to phosphorylate GR, we investigated whether RA could modulate GR phosphorylation in neuronal cells in a context of long-term glucocorticoid exposure. A 4-day treatment of dexamethasone (Dex) plus RA, showed that RA potentiated the (Dex)-induced phosphorylation on GR Serine 220 (_pSer220GR) in the nucleus of a hippocampal HT22 cell line. This treatment increased the cytoplasmic ratio of p35/p25 proteins, which are major CDK5 cofactors. Roscovitine, a pharmacological CDK5 inhibitor, or a siRNA against CDK5 prevented RA potentiation of GR phosphorylation. Furthermore, roscovitine counter-acted the effect of RA on GR sensitive target proteins such as BDNF or tissue-transglutaminase. These data help understanding the interaction between RA- and glucocorticoid-signalling pathways, both of which have strong influences on the adult brain.

Diabetes and Insulin Injection Modalities: Effects on Hepatic and Hippocampal Expression of 11β-Hydroxysteroid Dehydrogenase Type 1 in Juvenile Diabetic Male Rats

BACKGROUND

Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis is often encountered in diabetes, leading to several clinical complications. Our recent results showing an elevated tetrahydrocortisol/tetrahydrocorticosterone ratio in morning urine of diabetic children compared to that of controls suggest an increased nocturnal activity of 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) in the former.

QUESTION

We hypothesized that these observations could be explained by a reduced inhibition of hepatic 11β-HSD1 activity by exogenous insulin owing to its subcutaneous (SC) administration and absence of first hepatic passage. Additionally, we hypothesized that hippocampal 11β-HSD1 activity might also be impaired by diabetes.

METHODS

We therefore measured HPA axis activity and 11β-HSD1 expression and activity in liver and hippocampus in streptozotocin-induced diabetic juvenile rats treated with SC or intraperitoneal (IP) insulin.

RESULTS

Plasma corticosterone levels were elevated in untreated diabetic rats during the resting phase and restored by both types of insulin treatment. The mRNA expression and activity of 11β-HSD1 were increased in the untreated diabetic group in liver. Although diabetes was controlled equally whatever the route of insulin administration, liver 11β-HSD1 gene expression and activity was decreased only in the IP group, suggesting that a first hepatic pass is needed for 11β-HSD1 hepatic inhibition. In hippocampus, 11β-HSD1 activity was elevated in the untreated diabetic group but restored by both types of insulin treatment. Thus, these data extend our findings in diabetic children by showing impairment of hippocampal 11β-HSD1 in diabetes and by demonstrating that IP is preferable to SC insulin administration to restore 11β-HSD1 activity in liver.

Resolvin D1 and E1 promote resolution of inflammation in microglial cells in vitro

Sustained inflammation in the brain together with microglia activation can lead to neuronal damage. Hence limiting brain inflammation and activation of microglia is a real therapeutic strategy for inflammatory disease. Resolvin D1 (RvD1) and resolvin E1 (RvE1) derived from n-3 long chain polyunsaturated fatty acids are promising therapeutic compounds since they actively turn off the systemic inflammatory response. We thus evaluated the anti-inflammatory activities of RvD1 and RvE1 in microglia cells in vitro. BV2 cells were pre-incubated with RvD1 or RvE1 before lipopolysaccharide (LPS) treatment. RvD1 and RvE1 both decreased LPS-induced proinflammatory cytokines (TNF-α, IL-6 and IL-1β) gene expression, suggesting their proresolutive activity in microglia. However, the mechanisms involved are distinct as RvE1 regulates NFκB signaling pathway and RvD1 regulates miRNAs expression. Overall, our findings support that pro-resolving lipids are involved in the resolution of brain inflammation and can be considered as promising therapeutic agents for brain inflammation.

EPA/DHA and Vitamin A Supplementation Improves Spatial Memory and Alleviates the Age-related Decrease in Hippocampal RXRγ and Kinase Expression in Rats

Studies suggest that eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and vitamin A are critical to delay aged-related cognitive decline. These nutrients regulate gene expression in the brain by binding to nuclear receptors such as the retinoid X receptors (RXRs) and the retinoic acid receptors (RARs). Moreover, EPA/DHA and retinoids activate notably kinase signaling pathways such as AKT or MAPK, which includes ERK1/2. This suggests that these nutrients may modulate brain function in a similar way. Therefore, we investigated in middle-aged rats the behavioral and molecular effects of supplementations with EPA/DHA and vitamin A alone or combined. 18-month-old rats exhibited reference and working memory deficits in the Morris water maze, associated with a decrease in serum vitamin A and hippocampal EPA/DHA contents. RARα, RXRβ, and RXRγ mRNA expression and CAMKII, AKT, ERK1/2 expression were decreased in the hippocampus of middle-aged rats. A combined EPA/DHA and vitamin A supplementation had a beneficial additive effect on reference memory but not in working memory in middle-aged rats, associated with an alleviation of the age-related decrease in RXRγ, CAMKII, AKT, and ERK1 expression in the hippocampus. This study provides a new combined nutritional strategy to delay brain aging.

Dietary Polyphenol Supplementation Prevents Alterations of Spatial Navigation in Middle-Aged Mice

Spatial learning and memory deficits associated with hippocampal synaptic plasticity impairments are commonly observed during aging. Besides, the beneficial role of dietary polyphenols has been suggested as potential functional food candidates to prevent this memory decline. Indeed, polyphenols could potentiate the signaling pathways of synaptic plasticity underlying learning and memory. In this study, spatial learning deficits of middle-aged mice were first highlighted and characterized according to their navigation patterns in the Morris water maze task. An eight-week polyphenol-enriched diet, containing a polyphenol-rich extract from grape and blueberry (PEGB; from the Neurophenols Consortium) with high contents of flavonoids, stilbenes and phenolic acids, was then successful in reversing these age-induced effects. The use of spatial strategies was indeed delayed with aging whereas a polyphenol supplementation could promote the occurrence of spatial strategies. These behavioral results were associated with neurobiological changes: while the expression of hippocampal calmodulin kinase II (CaMKII) mRNA levels was reduced in middle-aged animals, the polyphenol-enriched diet could rescue them. Besides, an increased expression of nerve growth neurotrophic factor (NGF) mRNA levels was also observed in supplemented adult and middle-aged mice. Thus these data suggest that supplementation with polyphenols could be an efficient nutritional way to prevent age-induced cognitive decline.

Retinoids and glucocorticoids have opposite effects on actin cytoskeleton rearrangement in hippocampal HT22 cells

A chronic excess of glucocorticoids elicits deleterious effects in the hippocampus. Conversely, retinoic acid plays a major role in aging brain plasticity. As synaptic plasticity depends on mechanisms related to cell morphology, we investigated the involvement of retinoic acid and glucocorticoids in the remodelling of the HT22 neurons actin cytoskeleton. Cells exhibited a significantly more elongated shape with retinoic acid and a rounder shape with dexamethasone; retinoic acid reversed the effects of dexamethasone. Actin expression and abundance were unchanged by retinoic acid or dexamethasone but F-actin organization was dramatically modified. Indeed, retinoic acid and dexamethasone increased (70 ± 7% and 176 ± 5%) cortical actin while retinoic acid suppressed the effect of dexamethasone (90 ± 6%). Retinoic acid decreased (-22 ± 9%) and dexamethasone increased (134 ± 16%) actin stress fibres. Retinoic acid also suppressed the effect of dexamethasone (-21 ± 7%). Spectrin is a key protein in the actin network remodelling. Its abundance was decreased by retinoic acid and increased by dexamethasone (-21 ± 11% and 52 ± 10%). However, retinoic acid did not modify the effect of dexamethasone (48 ± 7%). Calpain activity on spectrin was increased by retinoic acid and decreased by dexamethasone (26 ± 14% and -57 ± 5%); retinoic acid mildly but significantly modified the effect of dexamethasone (-44 ± 7%). The calpain inhibitor calpeptin suppressed the effects of retinoic acid and dexamethasone on cell shape and actin stress fibres remodelling but did not modify the effects on cortical actin. Retinoic acid and dexamethasone have a dramatic but mainly opposite effect on actin cytoskeleton remodelling. These effects originate, at least partly, from calpain activity.

Role of corticosteroid binding globulin in emotional reactivity sex differences in mice

Sex differences exist for stress reactivity as well as for the prevalence of depression, which is more frequent in women of reproductive age and often precipitated by stressful events. In animals, the differential effect of stress on male's and female's emotional behavior has been well documented. Crosstalk between the gonadal and stress hormones, in particular between estrogens and glucocorticoids, underlie these sex differences on stress vulnerability. We have previously shown that corticosteroid binding globulin (CBG) deficiency in a mouse model (Cbg k.o.) leads, in males, to an increased despair-like behavior caused by suboptimal corticosterone stress response. Because CBG displays a sexual dimorphism and is regulated by estrogens, we have now investigated whether it plays a role in the sex differences observed for emotional reactivity in mice. By analyzing Cbg k.o. and wild-type (WT) animals of both sexes, we detected sex differences in despair-like behavior in WT mice but not in Cbg k.o. animals. We showed through ovariectomy and estradiol (E2) replacement that E2 levels explain the sex differences found in WT animals. However, the manipulation of E2 levels did not affect the emotional behavior of Cbg k.o. females. As Cbg k.o. males, Cbg k.o. females have markedly reduced corticosterone levels across the circadian cycle and also after stress. Plasma free corticosterone levels in Cbg k.o. mice measured immediately after stress were blunted in both sexes compared to WT mice. A trend for higher mean levels of ACTH in Cbg k.o. mice was found for both sexes. The turnover of a corticosterone bolus was increased in Cbg k.o. Finally, the glucocorticoid-regulated immediate early gene early growth response 1 (Egr1) showed a blunted mRNA expression in the hippocampus of Cbg k.o. mutants while mineralocorticoid and glucocorticoid receptors presented sex differences but equivalent mRNA expression between genotypes. Thus, in our experimental conditions, sex differences for despair-like behavior in WT mice are explained by estrogens levels. Also, in both sexes, the presence of CBG is required to attain optimal glucocorticoid concentrations and normal emotional reactivity, although in females this is apparent only under low E2 concentrations. These findings suggest a complex interaction of CBG and E2 on emotional reactivity in females.

Stress and glucocorticoid regulation of NR4A genes in mice

The NR4A nuclear receptors subgroup, comprising Nur77 (NR4A1), Nurr1 (NR4A2), and Nor1 (NR4A3), are orphan receptors induced by a variety of signals, including stress. These receptors are described as early response genes and in vitro studies have shown that they take part in regulation of the hypothalamic-pituitary-adrenal (HPA) axis, the major stress-responsive neuroendocrine system. This study analyzes further the interweaving of NR4A receptors with the HPA axis at rest and after a restraint stress in vivo in mice. We show that each NR4A member has a similar mRNA expression pattern and low levels of expression at rest except, in particular in hippocampus for Nurr1 and in adrenals for Nur77. After restraint stress, mRNA expression of each NR4A is markedly induced in adrenals and pituitary and significantly in hypothalamus. In higher cerebral regions, such as cortex, hippocampus, and amygdala, induction of NR4A mRNA elicited by stress was very moderate or undetected. The influence of glucocorticoids on NR4A mRNA expression was analyzed by comparing wild-type and Cbg k.o. mice used as a model of glucocorticoid hyposignaling. Nur77 mRNA and protein expression and a downstream Nur77 target gene were found to be affected in the hypothalamus and pituitary of the Cbg k.o. mice but not in hippocampus and cortex. These results further support a physiological role of NR4A orphan receptors in the glucocorticoid response to stress.

Retinoic acid modulates intrahippocampal levels of corticosterone in middle-aged mice: consequences on hippocampal plasticity and contextual memory

It is now established that vitamin A and its derivatives, retinoic acid (RA), are required for cognitive functions in adulthood. RA hyposignaling and hyperactivity of glucocorticoid (GC) pathway appear concomitantly during aging and would contribute to the deterioration of hippocampal synaptic plasticity and functions. Furthermore, recent data have evidenced counteracting effects of retinoids on GC signaling pathway. In the present study, we addressed the following issue: whether the stimulation of RA pathway could modulate intrahippocampal corticosterone (CORT) levels in middle-aged mice and thereby impact on hippocampal plasticity and cognitive functions. We firstly investigated the effects of vitamin A supplementation and RA treatment in middle-aged mice, on contextual serial discrimination task, a paradigm which allows the detection of early signs of age-related hippocampal-dependent memory dysfunction. We then measured intrahippocampal CORT concentrations by microdialysis before and after a novelty-induced stress. Our results show that both RA treatment and vitamin A supplementation improve “episodic-like” memory in middle-aged mice but RA treatment appears to be more efficient. Moreover, we show that the beneficial effect of RA on memory is associated to an increase in hippocampal PSD-95 expression. In addition, intrahippocampal CORT levels are reduced after novelty-induced stress in RA-treated animals. This effect cannot be related to a modulation of hippocampal 11β-HSD1 expression. Interestingly, RA treatment induces a modulation of RA receptors RARα and RARβ expression in middle-aged mice, a finding which has been correlated with the amplitude of intrahippocampal CORT levels after novelty-induced stress. Taken together, our results suggest that the preventive action of RA treatment on age-related memory deficits in middle-aged mice could be, at least in part, due to an inhibitory effect of retinoids on GC activity.

Vitamin A status regulates glucocorticoid availability in Wistar rats: consequences on cognitive functions and hippocampal neurogenesis?

A disruption of the vitamin A signaling pathway has been involved in age-related memory decline and hippocampal plasticity alterations. Using vitamin A deficiency (VAD), a nutritional model leading to a hyposignaling of the retinoid pathway, we have recently demonstrated that retinoic acid (RA), the active metabolite of vitamin A, is efficient to reverse VAD-induced spatial memory deficits and adult hippocampal neurogenesis alterations. Besides, excess of glucocorticoids (GCs) occurring with aging is known to strongly inhibit hippocampal plasticity and functions and few studies report on the counteracting effects of RA signaling pathway on GCs action. Here, we have addressed whether the modulation of brain GCs availability could be one of the biological mechanisms involved in the effects of vitamin A status on hippocampal plasticity and functions. Thus, we have studied the effects of a vitamin A-free diet for 14 weeks and a 4-week vitamin A supplementation on plasma and hippocampal corticosterone (CORT) levels in Wistar rats. We have also investigated corticosteroid binding globulin (CBG) binding capacity and 11beta-Hydrosteroid Dehydrogenase type 1 (11β-HSD1) activity, both important modulators of CORT availability at the peripheral and hippocampal levels respectively. Interestingly, we show that the vitamin A status regulates levels of free plasma CORT and hippocampal CORT levels, by acting through a regulation of CBG binding capacity and 11β-HSD1 activity. Moreover, our results suggest that increased CORT levels in VAD rats could have some deleterious consequences on spatial memory, anxiety-like behavior and adult hippocampal neurogenesis whereas these effects could be corrected by a vitamin A supplementation. Thus, the modulation of GCs availability by vitamin A status is an important biological mechanism that should be taken into account in order to prevent age-related cognitive decline and hippocampal plasticity alterations.

Vitamin A regulates hypothalamic-pituitary-adrenal axis status in LOU/C rats

The aim of this study was to explore the involvement of retinoids in the hypoactivity and hyporeactivity to stress of the hypothalamic-pituitary-adrenal (HPA) axis in LOU/C rats. We measured the effects of vitamin A deficiency administered or not with retinoic acid (RA) on plasma corticosterone in standard conditions and in response to restraint stress and on hypothalamic and hippocampal expression of corticosteroid receptors, corticotropin-releasing hormone and 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) in LOU/C rats. Interestingly, under control conditions, we measured a higher plasma concentration of retinol in LOU/C than in Wistar rats, which could contribute to the lower basal activity of the HPA axis in LOU/C rats. Vitamin A deficiency induced an increased HPA axis activity in LOU/C rats, normalized by RA administration. Compared with LOU/C control rats, vitamin A-deficient rats showed a delayed and heightened corticosterone response to restraint stress. The expression of corticosteroid receptors was strongly decreased by vitamin A deficiency in the hippocampus, which could contribute to a less efficient feedback by corticosterone on HPA axis tone. The expression of 11β-HSD1 was increased by vitamin A deficiency in the hypothalamus (+62.5%) as in the hippocampus (+104.7%), which could lead to a higher production of corticosterone locally and contribute to alteration of the hippocampus. RA supplementation treatment restored corticosterone concentrations and 11β-HSD1 expression to control levels. The high vitamin A status of LOU/C rats could contribute to their low HPA axis activity/reactivity and to a protective effect against 11β-HSD1-mediated deleterious action on cognitive performances during ageing.

Retinoids and glucocorticoids target common genes in hippocampal HT22 cells

Vitamin A metabolite retinoic acid (RA) plays a major role in the aging adult brain plasticity. Conversely, chronic excess of glucocorticoids (GC) elicits some deleterious effects in the hippocampus. We questioned here the involvement of RA and GC in the expression of target proteins in hippocampal neurons. We investigated proteins involved either in the signaling pathways [RA receptor β (RARβ) and glucocorticoid receptor (GR)] or in neuron differentiation and plasticity [tissue transglutaminase 2 (tTG) and brain-derived neurotrophic factor (BDNF)] in a hippocampal cell line, HT22. We applied RA and/or dexamethasone (Dex) as activators of the pathways and investigated mRNA and protein expression of their receptors and of tTG and BDNF as well as tTG activity and BDNF secretion. Our results confirm the involvement of RA- and GC-dependent pathways and their interaction in our neuronal cell model. First, both pathways regulate the transcription and expression of own and reciprocal receptors: RA and Dex increased RARβ and decreased GR expressions. Second, Dex reduces the expression of tTG when associated with RA despite stimulating its expression when used alone. Importantly, when they are combined, RA counteracts the deleterious effect of glucocorticoids on BDNF regulation and thus may improve neuronal plasticity under stress conditions. In conclusion, GC and RA both interact through regulations of the two receptors, RARβ and GR. Furthermore, they both act, synergistically or oppositely, on other target proteins critical for neuronal plasticity, tTG and BDNF.

Adult-onset hypothyroidism induces the amyloidogenic pathway of amyloid precursor protein processing in the rat hippocampus

Thyroid dysfunction and dementia are conditions that become more prevalent with advancing age. Localised hypothyroidism of the central nervous system has been sugested in some patients with Alzheimer's disease. We investigated the consequence of adult-onset hypothyroidism on beta-amyloid precursor protein (APP) degrading pathways in rats treated with propylthiouracyl over a period of 5 weeks. We evaluated the amount of 3,5,3'-triiodothyronine nuclear receptors (TRalpha1 and TRbeta) and the expression of some APP processing indicators (i.e. APP, ADAM 10, BACE and PS1). The activity of secretases and Abeta peptides has been also quantified. The results obtained show that hypoactivity of the thyroid signalling pathway in the hippocampus induced an increase in the APP770-751/APP695 ratio accompanied by a modification in the amyloidogenic pathway for APP processing, leading to an increased amount of Abeta peptides. In this area of the brain, modification in the non-amyloidogenic pathway of APP processing characterised by alpha-secretase activity was only approximately 10% in hypothyroid rats compared to control rats. We suggest that hypothyroidism, which becomes more prevalent with advancing age, increased the vulnerability to the formation of amyloid deposits.

Plasma retinol and association with socio-demographic and dietary characteristics of free-living older persons: the Bordeaux sample of the three-city study

The objective was to describe retinol plasma concentration and its association with socio-demographic characteristics and dietary habits in French older persons. The study population consisted of 1664 subjects aged 65 + from Bordeaux (France), included in the Three-City cohort. Retinol plasma concentration was determined in fasting blood samples. Dietary assessment was performed by a food frequency questionnaire allowing estimation of weekly intake of dietary sources of vitamin A or provitamin A. The weekly number of glasses of alcohol was also recorded. Age, sex, marital status, educational and income levels, body-mass index (BMI), and smoking were registered. Cross-sectional analysis of the association between plasma retinol and socio-demographic characteristics and dietary habits was performed by multilinear regression. Mean plasma retinol was close to the homeostatically regulated concentration of 2.0 micromol/L but ranged from 0.35 to 6.77 micromol/L. It was higher in women and divorced or separated individuals, and increased with income but not with age or educational level. Plasma retinol was positively and independently associated with the frequency of offal consumption and to the number of glasses of alcohol consumed per week. These results allow targeting older individuals who are at risk of either excessive or deficient vitamin A status and who should benefit from dietary counseling.

RARgamma and TRbeta expressions are decreased in PBMC and SWAT of obese subjects in weight gain

In order to evaluate the expression of nuclear receptors at the peripheral level in obese subjects, messenger RNA (mRNA) levels of different isoforms of retinoic acid receptor (RAR), triiodothyronine (TR), and peroxisome proliferator-activated receptor (PPAR) were determined and compared in peripheral mononuclear blood cells (PBMC) and subcutaneous white adipose tissue (SWAT). Twelve lean subjects and 68 obese subjects divided into weight gain (WG), weight-stable (WS), and weight loss (WL) groups were studied. Nuclear receptor mRNA levels were assessed in PBMC and SWAT using a quantitative real-time reverse transcription polymerase chain reaction method. mRNA levels of RARgamma were significantly lower in PBMC of obese subjects (WG -19%, WS -30%, and WL -24.7%) as in SWAT of WG (-50%). Lower mRNA levels of TRbeta were observed in PBMC and SWAT of WG (-50.7% and -28%, respectively) just as for TRalpha in PBMC of WG (-19%). In contrast, retinoid X receptors alpha (RXRalpha) and RARalpha mRNA levels were higher in PBMC of obese subjects (+53% and +54.5% in WG, +56% and +67% in WS, and +68% and +49.7% in WL, respectively), while expression of RXRalpha was lower in SWAT of WG (-24.5%). As for PPARgamma, its mRNA level was significantly higher in PBMC of WG subjects (+34%) while its expression was not modified in SWAT, contrary to the PPARgamma2 isoform which was significantly higher. These data show that in both adipose tissue and blood compartment of obese subjects, expressions of RARgamma and TRbeta were downregulated. Thus, we suggest that the expression in PBMC of obese subjects may constitute new cellular indicators of nuclear receptor retinoid and thyroid status.

Vitamin A deficiency in rats induces anatomic and metabolic changes comparable with those of neurodegenerative disorders

Anatomic and metabolic changes in central nervous system induced by 14 wk of vitamin A deprivation (VAD) were monitored and quantified in rats. In vivo brain magnetic resonance imaging (4.7T) was performed at 5, 7, 9, 11, and 14 wk of each diet after weaning in the following: 1) VAD group; 2) control pair-fed group; and 3) control group that consumed the diet ad libitum (1.15 microg retinol/g diet). After 14 wk, high-resolution magic angle spinning proton NMR spectroscopy (11.7T) was performed on small samples of cortex, hippocampus, and striatum. Serum retinol concentrations remained stable and cerebral volume (CV) increased as a linear function of body weight in the ad libitum group (R(2) = 0.78; P = 0.047) and pair-fed controls (R(2) = 0.78; P = 0.046). In VAD rats, retinol decreased from the onset of deprivation (2.2 +/- 0.14 micromol/L) to reach 0.3 +/- 0.13 micromol/L at wk 5, followed by a stopping of body weight gain from wk 7. In VAD rats, the CV decreased from wk 5 and reached a value 11% lower than that of the control group (P < 0.001) at wk 14 and was correlated with retinol status (R(2) = 0.99; P = 0.002). The VAD hippocampal volume decreased beginning at wk 9 and was 22% lower than that of the control group at wk 14 (P < 0.001). Compared with the control, VAD led to lower N acetyl aspartate:creatine+phosphocreatine (Cr) in cortex (-36%), striatum (-22%), and hippocampus (-19%) and higher myoinositol:Cr in cortex (+127%) and striatum (+150%). VAD induced anatomic and metabolic changes comparable to those associated with neurodegenerative disorders. By wk 7 of deprivation, the slowing in cerebral growth that correlated with the retinol level could be considered as a predictive marker of brain disorders, confirmed by metabolic data from VAD rats after 14 wk.

Retinoic acid restores adult hippocampal neurogenesis and reverses spatial memory deficit in vitamin A deprived rats

A dysfunction of retinoid hippocampal signaling pathway has been involved in the appearance of affective and cognitive disorders. However, the underlying neurobiological mechanisms remain unknown. Hippocampal granule neurons are generated throughout life and are involved in emotion and memory. Here, we investigated the effects of vitamin A deficiency (VAD) on neurogenesis and memory and the ability of retinoic acid (RA) treatment to prevent VAD-induced impairments. Adult retinoid-deficient rats were generated by a vitamin A-free diet from weaning in order to allow a normal development. The effects of VAD and/or RA administration were examined on hippocampal neurogenesis, retinoid target genes such as neurotrophin receptors and spatial reference memory measured in the water maze. Long-term VAD decreased neurogenesis and led to memory deficits. More importantly, these effects were reversed by 4 weeks of RA treatment. These beneficial effects may be in part related to an up-regulation of retinoid-mediated molecular events, such as the expression of the neurotrophin receptor TrkA. We have demonstrated for the first time that the effect of vitamin A deficient diet on the level of hippoccampal neurogenesis is reversible and that RA treatment is important for the maintenance of the hippocampal plasticity and function.

Age-related effects of ethanol consumption on triiodothyronine and retinoic acid nuclear receptors, neurogranin and neuromodulin expression levels in mouse brain

The effects of ethanol consumption and ageing were investigated on the expression levels of retinoic acid (RA) and triiodothyronine (T3) nuclear receptors (RAR, RXR and TR) and of associated target genes involved in synaptic plasticity, neurogranin (RC3) and neuromodulin (GAP-43) in mice brain. For this purpose, C57BL/6 adult and aged mice were subjected to 5-month ethanol consumption and the mRNA expression of RAR, RXR, TR, RC3 and GAP-43 was measured using a real-time RT-PCR method. GAP-43 and RC3 protein levels also were measured by Western blot. Results showed that 12% ethanol consumption in adult mice (11 months) induced an increase in RARbeta, RXRbetagamma and TRalphabeta mRNA level in the brain with only an increase in RC3 expression. The same ethanol consumption in aged mice (22 months) reversed the age-related hypo-expression in brain RARbeta, TRalphabeta and target genes RC3 and GAP-43. Compared with our previous behavioral data showing that ethanol is able to partially suppress a selective age-related cognitive deficit, these results suggest that the ethanol-induced increase in RA and T3 nuclear receptors expression could be one of the mechanisms involved in the normalization of synaptic plasticity-associated gene expression altered in aging brain.

Retinoic acid normalizes nuclear receptor mediated hypo-expression of proteins involved in β-amyloid deposits in the cerebral cortex of vitamin A deprived rats

Recent data have revealed that disruption of vitamin A signaling observed in Alzheimer's disease (AD) leads to a deposition of beta-amyloid (Abeta). The aim of this study was to precise the role of vitamin A and its nuclear receptors (RAR) in the processes leading to the Abeta deposits. Thus, the effect of vitamin A depletion and subsequent administration of retinoic acid (RA, the active metabolite of vitamin A) on the expression of RARbeta, and of proteins involved in amyloidogenic pathway, e.g., amyloid precursor protein (APP), beta-secretase enzyme (BACE), and APP carboxy-terminal fragment (APP-CTF) was examined in the whole brain, hippocampus, striatum, and cerebral cortex of rats. Rats fed a vitamin A-deprived diet for 13 weeks exhibited decreased amount of RARbeta, APP695, BACE, and of APP-CTF in the whole brain and in the cerebral cortex. Administration of RA is able to restore all expression. The results suggest that fine regulation of vitamin A mediated gene expression seems fundamental for the regulation of APP processing.

A high-fat diet generates alterations in nuclear receptor expression: prevention by vitamin A and links with cyclooxygenase-2 and beta-catenin

Epidemiologic studies suggest that intake of high energy from fat, inducing overweight, increases the risk of cancer development and promotes colon carcinogenesis. It is therefore important to understand which parameters are affected early on by a high-fat diet in order to devise and improve protective nutritional strategies. We investigated the effect of high energy/fat intake on colon mucosa of male Wistar rats induced by a single 1,2-dimethylhydrazine (DMH) injection. Aberrant crypt foci (ACF) were numbered and modifications in cyclooxygenase-2 (COX-2) and beta-catenin levels assessed. Peroxisome proliferator- and retinoic acid-activated receptors (PPAR and RAR, RXR) are key transcription factors regulating gene expression in response to nutrient-activated signals. A short-term study was designed to evaluate whether alterations in mRNA expression of nuclear receptors can be detected at the beginning of the weight gain phase induced by an appetizing hyperlipidic diet (HLD). HLD consumption induced early downregulation of PPARgamma (-33.1%) and RARbeta (-53.1%) mRNA expression concomitant with an increase in levels of COX-2 (+45.5%) and beta-catenin (+84.56%) and in the number of ACF (191.56 +/- 88.60 vs. 21.14 +/- 11.64, p < 0.05). These findings suggest that HLD increases ACF occurrence, possibly through alterations in the mRNA expression profile of nuclear receptors. Moreover, the use HLD rich in retinyl esters or supplemented with all-trans retinoic acid led to a reduction in the number of ACF. Vitamin A also prevented HLD-induced alterations and the increase in levels of COX-2 and beta-catenin. The present observations show a protective role for vitamin A against disturbances associated with HLD exposure in induced colon carcinogenesis.

Decreased expression of retinoid nuclear receptor (RAR alpha and RAR gamma) mRNA determined by real-time quantitative RT-PCR in peripheral blood mononuclear cells of hypothyroid patients

In vivo assessment of the cellular impact of thyroid hormones on target tissues might be of help for physiological studies and to evaluate the consequences of various diseases of the thyroid gland in humans. Given the tenuous relationship between retinoid and tri-iodothyronine (T3) status and that retinoids have also intracellular roles, the aim of this study was to determine the effect of hypothyroidism on the expression of T3 nuclear receptors (TR) and retinoic acid nuclear receptors (RAR, RXR) in human peripheral blood mononuclear cells (PBMC). Using real time RT-PCR, we quantified the relative amount of mRNA of the thyroid (TR alpha and TR beta) and retinoid (RAR alpha, RAR gamma, and RXR alpha) nuclear receptors in PBMC of euthyroid (n = 22) compared with hypothyroid (n = 22) subjects. Classical plasma parameters (free T3 (FT3), free thyroxine (T4) (FT4), thyroid-stimulating hormone (TSH), retinol (ROH), retinol-binding protein (RBP) and transthyretin (TTR)) were also measured. In hypothyroid subjects, the concentration of TSH was elevated, and dramatically low T3 and T4 concentrations were associated with a decrease in the expression of TR beta. Expression of RAR alpha and RAR gamma significantly decreased in hypothyroid versus control subjects, while an increased concentration of ROH was emphasised by hypothyroidism. These results first indicated that primary hypothyroidism induces hypoactivation of the retinoid nuclear pathway in PBMC, which was not predicted by the plasma ROH level. Further investigations will be necessary to evaluate these parameters in very small changes in thyroid hormone production such as mild (subclinical) hypothyroidism.

Differential effect of retinoic acid and triiodothyronine on the age-related hypo-expression of neurogranin in rat

Given the important role of retinoids and thyroid hormone for optimal brain functioning and the tenuous relationship between retinoic acid (RA) and triiodothyronine (T3) signalings, we compared the effects of RA or T3 administrations on RA and T3 nuclear receptors (RAR, RXR and TR) and on their target genes, neuromodulin (GAP43) and neurogranin (RC3) in 24-month-old rats. Quantitative real time PCR and western blot analysis allowed us to verify that retinoid and thyroid signalings and GAP43 and RC3 expression are affected by age. By in situ hybridization we observed a decreased expression of RC3 in hippocampus, striatum and cerebral cortex. RARbeta, RXRbeta/gamma and GAP43 were up-regulated by RA as well as T3 treatment. The abundance of TRalpha/beta mRNA and RC3 expression were only increased by T3 administration in the whole brain. This up-regulator effect of T3 on RC3 was only observed in the striatum. During aging, T3 become a limiting factor alone able to correct the age-related concomitant hypo-activation of retinoid and thyroid signalings and alterations of synaptic plasticity.

Aging affects the retinoic acid and the triiodothyronine nuclear receptor mRNA expression in human peripheral blood mononuclear cells

BACKGROUND

Inadequate retinoid status has often been described as occurring with aging. Moreover, subclinical hypothyroid status has also been evoked in the elderly. Several studies performed in animals have described the crucial incidence of age-related hypo-functioning of retinoid and thyroid signalling pathways, particularly in the brain.

OBJECTIVE

The aim of the present study was to clarify whether aging modifies retinoid and thyroid signalling in humans.

METHODS

Using real-time RT-PCR the relative amount of mRNA of the retinoid (RARalpha, RARgamma and RXRalpha) and thyroid (TRalpha and TRbeta) nuclear receptors in peripheral blood mononuclear cells (PBMC) of young (24-57 years old, n = 22) compared with elderly (69-90 years old, n = 24) healthy subjects was quantitated. Classical plasma parameters used to characterize the retinoid and thyroid status - retinol (ROH), retinol-binding protein (RBP), free triiodothyronine (FT3) and thyroxine (FT4), thyroid-stimulating hormone (TSH) and transthyretin (TTR) - were also assessed.

RESULTS

RARgamma expression was significantly decreased in elderly versus young subjects while no modification of the retinoid-related plasma parameters ROH and RBP were emphasized by aging. Concerning thyroid criteria, the elderly exhibited an increase in TSH concentration (+39%) without significant modifications of FT3 and FT4, which indicated an age-related sub-clinical hypothyroidism. Concurrently, the amount of TR mRNA (alpha as well as beta subtypes) was significantly decreased in the elderly.

CONCLUSION

These data constitute the first evidence of an age-related hypo-activation of the retinoid and thyroid nuclear pathways in PBMC. Further study of the possible association between the expression of the retinoid and thyroid nuclear receptors and age-related cognitive alterations in humans would be interesting.

Vitamin A prevents high fat diet-induced ACF development and modifies the pattern of expression of peroxisome proliferator and retinoic acid receptor m-RNA

Some dietary compounds, among them fats, are modulators of colon cancer risk. This study reports the modulating effects of n-6, with or without vitamin A, on promotion of colon preneoplasic lesions induced by 1,2-dimethylhydrazine (DMH) and on the expression of nuclear receptors (PPARgamma, RXRalpha, and RARbeta). One group of male Fisher rats was fed a basic diet (5% safflower oil) and two groups were fed a high-fat diet (HFD, 25% safflower oil). Of these, one was supplemented with 200 IU vitamin A for 5 mo. The safflower oil contained polyunsaturated fatty acids, mainly linoleic acid (73%). The data showed an increasing effect of safflower oil-enriched diet on aberrant crypt foci occurrence and multiplicity. This effect was impaired by vitamin A supplementation. In addition, an HFD-related up-regulation of PPARgamma and a concomitant down-regulation of RARbeta mRNA expression were observed with or without chemical initiation and were prevented by vitamin A. Moreover, when treated with DMH, HFD rats exhibited a dramatically decreased expression of RXRalpha mRNA (-49%). It was hypothesized that HFD, leading to hyperexpression of PPARgamma, would produce an alteration of retinoic acid signaling and, in this way, create a background modulating colon cancer risk.

Retinoic acid reverses the PTU related decrease in neurogranin level in mice brain

Recent data have shown that fine regulation of retinoid mediated gene expression is fundamentally important for optimal brain functioning in aged mice. Nevertheless, alteration of the thyroid hormone signalling pathway may be a limiting factor, which impedes retinoic acid (RA) from exerting its modulating effect. Mild hypothyroidism is often described in the elderly. Thus, in the present study, it was of interest to determine if RA exerts its neurological modulating effect in mild hypothyroidism. To obtain further insight into this question, mice were submitted to a low propylthiouracyl (PTU) drink (0.05%) in order to slightly reduce the serum level of triiodothyronine (T3). A quantitative evaluation of RA nuclear receptors (RAR, RXR), T3 nuclear receptor (TR) and of neurogranin (RC3, a RA target gene which codes for a protein considered as a good marker of synaptic plasticity) in PTU treated mice injected with vehicle or RA or T3 was carried out. The PTU-related decrease in expression of RAR, RXR and RC3 was restored following RA or T3 administration, as observed in aged mice. The amount of TR mRNA, which was not affected in PTU treated mice, was increased only after T3 treatment as observed in overt hypothyroidism. These results suggest that neurobiological alterations observed in aged mice are probably related to RA and T3 signalling pathway modifications associated, in part, with mild changes in thyroid function.

Expression of neurogranin and neuromodulin is affected in the striatum of vitamin A-deprived rats

Our previous data showed that vitamin A deficiency (VAD) induces, in whole brain, a reduced amount of mRNA for brain retinoic acid (RA) and triiodothyronine (T3) nuclear receptors (i.e., RAR, RXR, and TR, respectively), which is accompanied by reduced amounts of mRNA and protein of neurogranin (RC3, a neuronal protein involved in synaptic plasticity) as well as selective behavioral impairment. Given the important role of retinoids for optimal brain functioning, the effects of vitamin A depletion and subsequent administration of RA or T3 on the mRNA levels of RA and T3 nuclear receptors and on two target genes' (RC3 and neuromodulin or GAP43) mRNA and protein levels were examined in the hippocampus, striatum, and cerebral cortex. A quantitative real-time polymerase chain reaction (PCR), in situ hybridization, and Western blot analysis demonstrated that the striatal region is the brain site where both RA and T3 signaling pathways are most affected by VAD. Indeed, rats fed a vitamin A-free diet for 10 weeks exhibited decreased expression of RAR, RXR, TR, RC3, and GAP43 in the striatum. The administration of T3 to these vitamin A-deprived rats reversed the reduction in mRNA levels of RA and T3 nuclear receptors and in mRNA and protein levels of target genes in this region. These data suggest that modifications that appear preferentially in the striatum, a region highly sensitive to vitamin A bioavailability, may contribute to neurobiological alterations and the spatial learning impairment that occurs in vitamin A-deprived animals.

Vitamin A deficiency and relational memory deficit in adult mice: relationships with changes in brain retinoid signalling

Vitamin A and its derivatives, the retinoids, have recently been reported to be implicated in the synaptic plasticity of the hippocampus and in cognitive functions. Acting via transcription factors, retinoids can regulate gene expression via their nuclear receptors [retinoic acid receptors (RARs) and retinoid X receptors (RXRs)]. We recently showed that a moderate (about 30%) hypoexpression of brain (and hippocampal) retinoid signalling, like that naturally occurring in the aged brain of mice, might be related to a selective relational memory deficit. To further assess this hypothesis, the present study investigated the effects of Vitamin A deprivation of varying duration both on the brain expression of retinoid receptors (RARbeta and RXRbeta/gamma) and two associated target genes [tissue-type transglutaminase (tTG) and neurogranin, (RC3)], and on radial maze discrimination learning using young adult mice as subjects. We observed that irrespective of its duration (i.e. 31 or 39 weeks), Vitamin A deprivation resulted in a significant reduction (25-30%) in the expression of brain RARbeta, RXRbeta/gamma and tTG mRNAs. Conversely, only the 39-week condition was found to induce a significant decrease in brain RC3 mRNAs contents and a selective relational memory impairment. Finally, daily administration of retinoic acid (RA) failed to reverse the 39-week Vitamin A deficiency (VAD)-related cognitive deficit and to fully normalise the associated brain retinoid hyposignalling. In particular, there was no evidence for an up-regulating effect of RA on whole brain (and hippocampal) RC3 mRNAs of the 39-week-depleted mice. The results show that post-natal VAD may induce a selective memory impairment and give further support to the hypothesis that the fine regulation of retinoid-mediated gene expression is important for optimal brain functioning and higher cognition.

Hyperlipidic diets induce early alterations of the vitamin A signalling pathway in rat colonic mucosa

OBJECTIVE

Dietary factors can be associated with colorectal cancer. Fatty acids modulate gene expression in various tissues, mediated by activation of the peroxisome proliferator activated receptor: PPAR. Vitamin A signalling is mediated by retinoic acid (RA) receptors (RAR) and retinoid X receptors (RXR). The steroid nuclear receptors PPAR, RAR, RXR, are DNA-binding proteins and they induce gene transcription upon activation by specific ligands and interacting with distinct promoter sequences in the target genes. The aim of this study was to investigate the impact of hyperlipidic diets on the expression of PPARg, RXRa and RARb mRNA in rat colon.

METHODS

Rats were fed during 4 weeks with the following diets: a cafeteria diet where 60% of the energy was supplied as lipids and a high fat diet (HFD) represented by 25% of a safflower oil (w/w) rich in polyunsaturated fatty acids, mainly n-6. Nuclear receptors mRNA were quantified by realtime RT-PCR with TaqMan probe process or SYBRGreen I chemical.

RESULTS

The cafeteria diet and the HFD induced a significant decrease in RARb mRNA: -36% (p<0.02) and -64% (P<0.001) respectively. Simultaneously, an increased expression of PPARg mRNA was observed for cafeteria diet +35% (P<0.05) and for HFD +45% (P<0.05). The level of RXRa mRNA was significantly increased for cafeteria diet: +53% (P<0.0002), while no significant difference in RXRa mRNA was observed in colonic mucosa rats whose fed with the 25% HFD.

CONCLUSIONS

These results showed that an hyperlipidic diet could induce early modifications in the pattern of expression of nuclear receptors in rat colon. Many mechanisms could be probably involved but one hypothesis is that a modification of the balance between the nuclear receptors, resulting from an increased expression of PPARg, could induce a decreased expression of RARb in rat colon.

Triiodothyronine administration reverses vitamin A deficiency-related hypo-expression of retinoic acid and triiodothyronine nuclear receptors and of neurogranin in rat brain

Recent studies have revealed that retinoids play an important role in the adult central nervous system and cognitive functions. Previous investigations in mice have shown that vitamin A deficiency (VAD) generates a hypo-expression of retinoic acid (RA, the active metabolite of vitamin A) receptors and of neurogranin (RC3, a neuronal protein involved in synaptic plasticity) and a concomitant selective behavioural impairment. Knowing that RC3 is both a triiodothyronine (T3) and a RA target gene, and in consideration of the relationships between the signalling pathways of retinoids and thyroid hormones, the involvement of T3 on RA signalling functionality in VAD was investigated. Thus, the effects of vitamin A depletion and subsequent administration with RA and/or T3 on the expression of RA nuclear receptors (RAR, RXR), T3 nuclear receptor (TR) and on RC3 in the brain were examined. Rats fed a vitamin A-deficient diet for 10 weeks exhibited a decreased expression of RAR, RXR and TR mRNA and of RC3 mRNA and proteins. RA administration to these vitamin A-deficient rats reversed only the RA hypo-signalling in the brain. Interestingly, T3 is able to restore its own brain signalling simultaneously with that of vitamin A and the hypo-expression of RC3. These results obtained in vivo revealed that one of the consequences of VAD is a dysfunction in the thyroid signalling pathway in the brain. This seems of crucial importance since the down regulation of RC3 observed in the depleted rats was corrected only by T3.