Jean-Christophe Delpech

Long term adequate n-3 polyunsaturated fatty acid diet protects from depressive-like behavior but not from working memory disruption and brain cytokine expression in aged mice.

Converging epidemiological studies suggest that dietary essential n-3 polyunsaturated fatty acid (PUFA) are likely to be involved in the pathogenesis of mood and cognitive disorders linked to aging. The question arises as to whether the decreased prevalence of these symptoms in the elderly with high n-3 PUFA consumption is also associated with improved central inflammation, i.e. cytokine activation, in the brain. To answer this, we measured memory performance and emotional behavior as well as cytokine synthesis and PUFA level in the spleen and the cortex of adult and aged mice submitted to a diet with an adequate supply of n-3 PUFA in form of α-linolenic acid (α-LNA) or a n-3 deficient diet. Our results show that docosahexaenoic acid (DHA), the main n-3 PUFA in the brain, was higher in the spleen and cortex of n-3 adequate mice relative to n-3 deficient mice and this difference was maintained throughout life. Interestingly, high level of brain DHA was associated with a decrease in depressive-like symptoms throughout aging. On the opposite, spatial memory was maintained in adult but not in aged n-3 adequate mice relative to n-3 deficient mice. Furthermore, increased interleukin-6 (IL-6) and decreased IL-10 expression were found in the cortex of aged mice independently of the diets. All together, our results suggest that n-3 PUFA dietary supply in the form of α-LNA is sufficient to protect from deficits in emotional behavior but not from memory disruption and brain proinflammatory cytokine expression linked to age.

Nutritional n-3 PUFAs deficiency during perinatal periods alters brain innate immune system and neuronal plasticity-associated genes

Low dietary intake of the n-3 polyunsaturated fatty acids (PUFAs) is a causative factor of neurodevelopmental disorders. However the mechanisms linking n-3 PUFAs low dietary intake and neurodevelopmental disorders are poorly understood. Microglia, known mainly for their immune function in the injured or infected brain, have recently been demonstrated to play a pivotal role in regulating maturation of neuronal circuits during normal brain development. Disruption of this role during the perinatal period therefore could significantly contribute to psychopathologies with a neurodevelopmental neurodevelopmental component. N-3 PUFAs, essential lipids and key structural components of neuronal membrane phospholipids, are highly incorporated in cell membranes during the gestation and lactation phase. We previously showed that in a context of perinatal n-3 PUFAs deficiency, accretion of these latter is decreased and this is correlated to an alteration of endotoxin-induced inflammatory response. We thus postulated that dietary n-3 PUFAs imbalance alters the activity of microglia in the developing brain, leading to abnormal formation of neuronal networks. We first confirmed that mice fed with a n-3 PUFAs deficient diet displayed decreased n-3 PUFAs levels in the brain at post-natal days (PND)0 and PND21. We then demonstrated that n-3 PUFAs deficiency altered microglia phenotype and motility in the post-natal developing brain. This was paralleled by an increase in pro-inflammatory cytokines expression at PND21 and to modification of neuronal plasticity-related genes expression. Overall, our findings show for the first time that a dietary n-3 PUFAs deficiency from the first day of gestation leads to the development of a pro-inflammatory condition in the central nervous system that may contribute to neurodevelopmental alterations.

Transgenic Increase in n-3/n-6 Fatty Acid Ratio Protects Against Cognitive Deficits Induced by an Immune Challenge through Decrease of Neuroinflammation

Polyunsaturated fatty acids (PUFAs) display immunomodulatory properties in the brain, n-3 PUFAs being able to reduce inflammation whereas n-6 PUFAs are more pro-inflammatory. It has been extensively demonstrated that exposure to a peripheral immune challenge leads to the production and release of inflammatory mediators in the brain in association with cognitive deficits. The question arises whether n-3 PUFA supplementation could downregulate the brain inflammatory response and subsequent cognitive alterations. In this study, we used a genetically modified mouse line carrying the fat-1 gene from the roundworm Caenorhabditis elegans, encoding an n-3 PUFA desaturase that catalyzes conversion of n-6 into n-3 PUFA. Consequently, these mice display endogenously elevated n-3 PUFA tissue contents. Fat-1 mice or wild-type (WT) littermates were injected peripherally with lipopolysaccharide (LPS), a bacterial endotoxin, to induce an inflammatory episode. Our results showed that LPS altered differently the phenotype of microglia and the expression of cytokines and chemokines in Fat-1 and WT mice. In Fat-1 mice, pro-inflammatory factors synthesis was lowered compared with WT mice, whereas anti-inflammatory mechanisms were favored 24 h after LPS treatment. Moreover, LPS injection impaired spatial memory in WT mice, whereas interestingly, the Fat-1 mice showed normal cognitive performances. All together, these data suggest that the central n-3 PUFA increase observed in Fat-1 mice modulated the brain innate immune system activity, leading to the protection of animals against LPS-induced pro-inflammatory cytokine production and subsequent spatial memory alteration.

Early life stress perturbs the maturation of microglia in the developing hippocampus

Children exposed to abuse or neglect show abnormal hippocampal development and similar findings have been reported in rodent models. Using brief daily separation (BDS), a mouse model of early life stress, we previously showed that exposure to BDS impairs hippocampal function in adulthood and perturbs synaptic maturation, synaptic pruning, axonal growth and myelination in the developing hippocampus. Given that microglia are involved in these developmental processes, we tested whether BDS impairs microglial activity in the hippocampus of 14 (during BDS) and 28-day old mice (one week after BDS). We found that BDS increased the density and altered the morphology of microglia in the hippocampus of 14-day old pups, effects that were no longer present on postnatal day (PND) 28. Despite the normal cell number and morphology seen at PND28, the molecular signature of hippocampal microglia, assessed using the NanoString immune panel, was altered at both ages. We showed that during normal hippocampal development, microglia undergo significant changes between PND14 and PND28, including reduced cell density, decreased ex vivo phagocytic activity, and an increase in the expression of genes involved in inflammation and cell migration. However, microglia harvested from the hippocampus of 28-day old BDS mice showed an increase in phagocytic activity and reduced expression of genes that normally increase across development. Promoter analysis indicated that alteration in the transcriptional activity of PU.1, Creb1, Sp1, and RelA accounted for most of the transcriptional changes seen during normal microglia development and for most of the BDS-induced changes at PND14 and PND28. These findings are the first to demonstrate that early life stress dysregulates microglial function in the developing hippocampus and to identify key transcription factors that are likely to mediate these changes.

Dietary n-3 PUFAs deficiency increases vulnerability to inflammation-induced spatial memory impairment

Dietary n-3 polyunsaturated fatty acids (PUFAs) are critical components of inflammatory response and memory impairment. However, the mechanisms underlying the sensitizing effects of low n-3 PUFAs in the brain for the development of memory impairment following inflammation are still poorly understood. In this study, we examined how a 2-month n-3 PUFAs deficiency from pre-puberty to adulthood could increase vulnerability to the effect of inflammatory event on spatial memory in mice. Mice were given diets balanced or deficient in n-3 PUFAs for a 2-month period starting at post-natal day 21, followed by a peripheral administration of lipopolysaccharide (LPS), a bacterial endotoxin, at adulthood. We first showed that spatial memory performance was altered after LPS challenge only in n-3 PUFA-deficient mice that displayed lower n-3/n-6 PUFA ratio in the hippocampus. Importantly, long-term depression (LTD), but not long-term potentiation (LTP) was impaired in the hippocampus of LPS-treated n-3 PUFA-deficient mice. Proinflammatory cytokine levels were increased in the plasma of both n-3 PUFA-deficient and n-3 PUFA-balanced mice. However, only n-3 PUFA-balanced mice showed an increase in cytokine expression in the hippocampus in response to LPS. In addition, n-3 PUFA-deficient mice displayed higher glucocorticoid levels in response to LPS as compared with n-3 PUFA-balanced mice. These results indicate a role for n-3 PUFA imbalance in the sensitization of the hippocampal synaptic plasticity to inflammatory stimuli, which is likely to contribute to spatial memory impairment.

Dynamic cross-talk between microglia and peripheral monocytes underlies stress-induced neuroinflammation and behavioral consequences.

Psychological stress promotes the development and recurrence of anxiety and depressive behavioral symptoms. Basic and clinical research indicates that stress exposure can influence the neurobiology of mental health disorders through dysregulation of neuroimmune systems. Consistent with this idea several studies show that repeated stress exposure causes microglia activation and recruitment of peripheral monocytes to the brain contributing to development of anxiety- and depressive-like behavior. Further studies show that stress-induced re-distribution of peripheral monocytes leads to stress-sensitized neuroimmune responses and recurrent anxiety-like behavior. These stress-associated immune changes are important because brain resident and peripheral immune cells contribute to physiological processes that support neuroplasticity. Thus, perturbations in neuroimmune function can lead to impaired neuronal responses and synaptic plasticity deficits that underlie behavioral symptoms of mental health disorders. In this review we discuss recent advances in neuroimmune regulation of behavior and summarize studies showing that stress-induced microglia activation and monocyte trafficking in the brain contribute to the neurobiology of mental health disorders.

Microglial Activation Enhances Associative Taste Memory through Purinergic Modulation of Glutamatergic Neurotransmission

The cerebral innate immune system is able to modulate brain functioning and cognitive processes. During activation of the cerebral innate immune system, inflammatory factors produced by microglia, such as cytokines and adenosine triphosphate (ATP), have been directly linked to modulation of glutamatergic system on one hand and learning and memory functions on the other hand. However, the cellular mechanisms by which microglial activation modulates cognitive processes are still unclear. Here, we used taste memory tasks, highly dependent on glutamatergic transmission in the insular cortex, to investigate the behavioral and cellular impacts of an inflammation restricted to this cortical area in rats. We first show that intrainsular infusion of the endotoxin lipopolysaccharide induces a local inflammation and increases glutamatergic AMPA, but not NMDA, receptor expression at the synaptic level. This cortical inflammation also enhances associative, but not incidental, taste memory through increase of glutamatergic AMPA receptor trafficking. Moreover, we demonstrate that ATP, but not proinflammatory cytokines, is responsible for inflammation-induced enhancement of both associative taste memory and AMPA receptor expression in insular cortex. In conclusion, we propose that inflammation restricted to the insular cortex enhances associative taste memory through a purinergic-dependent increase of glutamatergic AMPA receptor expression at the synapse.

Mechanisms Involved in Dual Vasopressin/Apelin Neuron Dysfunction during Aging

Normal aging is associated with vasopressin neuron adaptation, but little is known about its effects on the release of apelin, an aquaretic peptide colocalized with vasopressin. We found that plasma vasopressin concentrations were higher and plasma apelin concentrations lower in aged rats than in younger adults. The response of AVP/apelin neurons to osmotic challenge was impaired in aged rats. The overactivity of vasopressin neurons was sustained partly by the increased expression of Transient receptor potential vanilloid2 (Trpv2), because central Trpv blocker injection reversed the age-induced increase in plasma vasopressin concentration without modifying plasma apelin concentration. The morphofunctional plasticity of the supraoptic nucleus neuron-astrocyte network normally observed during chronic dehydration in adults appeared to be impaired in aged rats as well. IL-6 overproduction by astrocytes and low-grade microglial neuroinflammation may contribute to the modification of neuronal functioning during aging. Indeed, central treatment with antibodies against IL-6 decreased plasma vasopressin levels and increased plasma apelin concentration toward the values observed in younger adults. Conversely, minocycline treatment (inhibiting microglial metabolism) did not affect plasma vasopressin concentration, but increased plasma apelin concentration toward control values for younger adults. This study is the first to demonstrate dual vasopressin/apelin adaptation mediated by inflammatory molecules and neuronal Trpv2, during aging.

N-3 polyunsaturated fatty acid and neuroinflammation in aging and Alzheimer's disease

The innate immune system of the brain is mainly composed of microglial cells, which play a key role in the maintenance of synapses and the protection of neurons against noxious agents or lesions owing to their phagocytic activity. In the healthy brain, microglia are highly motile and strongly interact with neurons either by physical contact, induction of oxidative stress or through specific mediators, such as chemokines and cytokines. In response to inflammatory insult however, microglial cells get activated and produce inflammatory cytokines. The action of cytokines on specific receptors expressed in the brain triggers the development of sickness behavior and altered cognitive and emotional processes. The effects are acute and reversible as normal behavior is restored once the synthesis of inflammatory brain cytokines returns to baseline after a few hours. However, in pathological situations, these cytokines may reach toxic levels and have irreversible consequences such as neuronal death, as observed in neurodegenerative disorders such as Alzheimers disease. Omega-3 (n-3) polyunsaturated fatty acids (PUFAs) are essential nutrients and fundamental components of neuronal and glial cell membranes. They accumulate in the brain during the perinatal period in a dietary supply-dependent fashion. Their brain levels may diminish with age, but can be increased by diets enriched in n-3 PUFAs. Changes in the immune profile have been associated with n-3 PUFAs intake in humans and animal models. Therefore, the increasing exposure of the population to diets low in n-3 PUFAs could contribute to the deleterious effects of the chronic activation of microglia in the brain.

Amygdala hyper-connectivity in a mouse model of unpredictable early life stress

Childhood maltreatment is associated with a wide range of psychopathologies including anxiety that emerge in childhood and in many cases persist in adulthood. Increased amygdala activation in response to threat and abnormal amygdala connectivity with frontolimbic brain regions, such as the hippocampus and the prefrontal cortex, are some of the most consistent findings seen in individuals exposed to childhood maltreatment. The underlying mechanisms responsible for these changes are difficult to study in humans but can be elucidated using animal models of early-life stress. Such studies are especially powerful in the mouse where precise control of the genetic background and the stress paradigm can be coupled with resting-state fMRI (rsfMRI) to map abnormal connectivity in circuits that regulate anxiety. To address this issue we first compared the effects of two models of early-life stress, limited bedding (LB) and unpredictable postnatal stress (UPS), on anxiety-like behavior in juvenile and adult mice. We found that UPS, but not LB, causes a robust increase in anxiety in juvenile and adult male mice. Next, we used rsfMRI to compare frontolimbic connectivity in control and UPS adult male mice. We found increased amygdala–prefrontal cortex and amygdala–hippocampus connectivity in UPS. The strength of the amygdala–hippocampal and amygdala–prefrontal cortex connectivity was highly correlated with anxiety-like behavior in the open-field test and elevated plus maze. These findings are the first to link hyperconnectivity in frontolimbic circuits and increased anxiety in a mouse model of early-life stress, allowing for more mechanistic understanding of parallel findings in humans.