Effects of maternal immune activation on neurodevelopmental changes in the mouse: Potential role of disrupted microglial fractalkine signaling

Inflammation during pregnancy can disturb brain development and lead to neurodevelopmental disorders like autism spectrum disorder (ASD). Abnormal synaptic formation and neuronal connections are critical features of these disorders, and microglia—better known as the resident immune cells of the brain, have been shown to be essential for synaptic pruning, the developmental shaping of connections between neurons. This sculpting task during brain development involves several microglia immune signals to eliminate superfluous synaptic spines including microglial fractalkine receptor (CX3CR1). As an immune cell with a role in brain development, we sought to understand whether an acute activation of the maternal immune system is associated with aberrant microglia pruning function and altered neurodevelopment.We used lipopolysaccharide, a bacterial immunogen, to induce immune activation during late gestation in mice. We assessed the resulting inflammatory response and in both male and female offspring, then, quantified the number of spines in Golgi-stained neurons of the hippocampus at the peak of synaptic pruning and measured mRNA changes by RT-qPCR for known microglia-relevant pruning genes (Cx3cr1, C1q and C3). We further assessed behavior characteristic of neurodevelopmental disorders like ASD and to identify critical mediators we investigated microglia changes in number and phenotype. We found a male-specific higher spine density in cells of the pup’s hippocampus that was paralleled by a significant reduction in the expression of CX3CR1 mRNA. We further observed behavioral changes relevant to ASD in all the offspring and determined that while microglia numbers remain intact, these cells undergo morphological changes typical of a more activated phenotype. Lastly, there was a male-specific decrease in microglial receptor CX3CR1 in the offspring, which to rescue would require a tool to overexpress it in microglia. We therefore constructed plasmids with microglia promoter regions for the Cx3cr1 gene to adapt a novel red-light activated gene switch.A CX3CR1 gene variant in humans, that results in disrupted fractalkine signaling, has been recently associated with an increased risk of developing neurodevelopmental disorders like schizophrenia and ASD. We show that an acute immune insult during late gestation can potentially disrupt fractalkine signaling by reducing the expression of CX3CR1 and appears to induce similar neurodevelopmental characteristics and behavioral deficits as those observed in the genetic Cx3cr1 knock-out a mouse ASD model. Our work helps position microglia fractalkine signaling as a relevant target underlying environmental risk factors leading to neurodevelopmental disorders.L'inflammation pendant la grossesse peut perturber le développement cérébral et conduire à des troubles du développement neurologique tels que les troubles du spectre de l’autisme (TSA). Ces troubles sont essentiellement caractérisés par des connexions neuronales et formation synaptiques anormales. La microglie, cellule immunitaire résidente du cerveau, est essentielle pour l'élagage synaptique ainsi que la formation des connexions entre neurones. Au cours du développement cérébral, cette tâche de modelage implique plusieurs signaux immunitaires de la microglie qui permettent l’élimination des épines synaptiques superflues, notamment le récepteur microglial de la fractalkine (CX3CR1). Considérant les propriétés immunitaires et le rôle dans le développement cérébral des microglies, nous avons cherché à comprendre si une activation aiguë du système immunitaire de la mère affecte l'élagage synaptique par la microglie et est associée à une altération du développement neurologique.Nous avons utilisé le lipopolysaccharide, un immunogène bactérien, pour induire une activation immunitaire en fin de gestation chez la souris et évaluer la réponse inflammatoire résultante chez la progéniture mâle et femelle. Nous avons également quantifié les épines dans les neurones de l'hippocampe en utilisant une coloration Golgi au pic de l'élagage synaptique et mesuré les changements d'ARNm par RT-qPCR pour des gènes microgliaux impliqués dans l'élagage synaptique (CX3CR1, C1q et C3). Enfin, nous avons évalué certains aspects du comportement qui s’apparentent à certains troubles du développement ; et dans le but d'identifier des médiateurs critiques, nous avons mesuré le nombre et cherché à identifier le phénotype de la microglie.Nous avons trouvé une augmentation du nombre d'épines ainsi qu'une réduction significative de l’expression de l’ARNm de CX3CR1 dans les cellules de l’hippocampe des progénitures mâle. Nous avons en outre observé des modifications comportementales chez tous les descendants et avons déterminé que, même si le nombre de microglies demeure intact, ces cellules subissent des modifications morphologiques typiques d'un phénotype plus activé. Enfin, nous avons mesuré une diminution du récepteur microglial CX3CR1 chez la progéniture mâle spécifiquement, motivant le développement d'un outil permettant sa surexpression dans la microglie. Nous avons donc construit des plasmides avec des régions du promoteur de la microglie pour le gène Cx3cr1 afin d'adapter un nouveau commutateur génique activé par la lumière rouge.Une variante du gène CX3CR1, entraînant une perturbation de la signalisation de la fractalkine, a récemment été identifiée chez l’homme et associée à un risque accru de troubles du développement cérébral tels que la schizophrénie et l’autisme. Nous avons démontré qu'une activation immunitaire aiguë en fin de gestation peut potentiellement perturber la signalisation de la fractalkine en réduisant l'expression de CX3CR1. Cette procédure semble également induire des caractéristiques neurodéveloppementales et des déficits comportementaux similaires à ceux observés dans le modèle génétique d'invalidation du gène Cx3cr1, un modèle murin de trouble du spectre de l’autisme. Ainsi, notre travail présente la signalisation de la fractalkine dans la microglie comme un des facteurs de risque environnementaux menant à des troubles du développement cérébral

Microglial and peripheral immune priming is partially sexually dimorphic in adolescent mouse offspring exposed to maternal high-fat diet

Background: Maternal nutrition is critical for proper fetal development. While increased nutrient intake is essential during pregnancy, an excessive consumption of certain nutrients, like fat, can lead to long-lasting detrimental consequences on the offspring. Animal work investigating the consequences of maternal high-fat diet (mHFD) revealed in the offspring a maternal immune activation (MIA) phenotype associated with increased inflammatory signals. This inflammation was proposed as one of the mechanisms causing neuronal circuit dysfunction, notably in the hippocampus, by altering the brain-resident macrophages—microglia. However, the understanding of mechanisms linking inflammation and microglial activities to pathological brain development remains limited. We hypothesized that mHFD-induced inflammation could prime microglia by altering their specific gene expression signature, population density, and/or functions. Methods: We used an integrative approach combining molecular (i.e., multiplex-ELISA, rt-qPCR) and cellular (i.e., histochemistry, electron microscopy) techniques to investigate the effects of mHFD (saturated and unsaturated fats) vs control diet on inflammatory priming, as well as microglial transcriptomic signature, density, distribution, morphology, and ultrastructure in mice. These analyses were performed on the mothers and/or their adolescent offspring at postnatal day 30. Results: Our study revealed that mHFD results in MIA defined by increased circulating levels of interleukin (IL)-6 in the mothers. This phenotype was associated with an exacerbated inflammatory response to peripheral lipopolysaccharide in mHFD-exposed offspring of both sexes. Microglial morphology was also altered, and there were increased microglial interactions with astrocytes in the hippocampus CA1 of mHFD-exposed male offspring, as well as decreased microglia-associated extracellular space pockets in the same region of mHFD-exposed offspring of the two sexes. A decreased mRNA expression of the inflammatory-regulating cytokine Tgfb1 and microglial receptors Tmem119, Trem2, and Cx3cr1 was additionally measured in the hippocampus of mHFD-exposed offspring, especially in males. Conclusions: Here, we described how dietary habits during pregnancy and nurturing, particularly the consumption of an enriched fat diet, can influence peripheral immune priming in the offspring. We also found that microglia are affected in terms of gene expression signature, morphology, and interactions with the hippocampal parenchyma, in a partially sexually dimorphic manner, which may contribute to the adverse neurodevelopmental outcomes on the offspring.Medicine, Faculty ofNon UBCBiochemistry and Molecular Biology, Department ofReviewedFacult

Impact of prebiotics on metabolic and behavioral alterations in a mouse model of metabolic syndrome.

Mounting evidence shows that the gut microbiota, an important player within the gut-brain communication axis, can affect metabolism, inflammation, brain function and behavior. Interestingly, gut microbiota composition is known to be altered in patients with metabolic syndrome (MetS), who also often display neuropsychiatric symptoms. The use of prebiotics, which beneficially alters the microbiota, may therefore be a promising way to potentially improve physical and mental health in MetS patients. This hypothesis was tested in a mouse model of MetS, namely the obese and type-2 diabetic db/db mice, which display emotional and cognitive alterations associated with changes in gut microbiota composition and hippocampal inflammation compared to their lean db/+ littermates. We assessed the impact of chronic administration (8 weeks) of prebiotics (oligofructose) on both metabolic (body weight, food intake, glucose homeostasis) and behavioral (increased anxiety-like behavior and impaired spatial memory) alterations characterizing db/db mice, as well as related neurobiological correlates, with particular attention to neuroinflammatory processes. Prebiotic administration improved excessive food intake and glycemic dysregulations (glucose tolerance and insulin resistance) in db/db mice. This was accompanied by an increase of plasma anti-inflammatory cytokine IL-10 levels and hypothalamic mRNA expression of the anorexigenic cytokine IL-1β, whereas unbalanced mRNA expression of hypothalamic orexigenic (NPY) and anorexigenic (CART, POMC) peptides was unchanged. We also detected signs of improved blood-brain-barrier integrity in the hypothalamus of oligofructose-treated db/db mice (normalized expression of tight junction proteins ZO-1 and occludin). On the contrary, prebiotic administration did not improve behavioral alterations and associated reduction of hippocampal neurogenesis displayed by db/db mice, despite normalization of increased hippocampal IL-6 mRNA expression. Of note, we found a relationship between the effect of treatment on dentate gyrus neurons and spatial memory. These findings may prove valuable for introducing novel approaches to treat some of the comorbidities associated with MetS

Maternal high-fat diet modifies myelin organization, microglial interactions, and results in social memory and sensorimotor gating deficits in adolescent mouse offspring

Prenatal exposure to maternal high-fat diet (mHFD) acts as a risk factor for various neurodevelopmental alterations in the progeny. Recent studies in mice revealed that mHFD results in both neuroinflammation and hypomyelination in the exposed offspring. Microglia, the brain-resident macrophages, play crucial roles during brain development, notably by modulating oligodendrocyte populations and performing phagocytosis of myelin sheaths. Previously, we reported that mHFD modifies microglial phenotype (i.e., morphology, interactions with their microenvironment, transcripts) in the hippocampus of male and female offspring. In the current study, we further explored whether mHFD may induce myelination changes among the hippocampal-corpus callosum-prefrontal cortex pathway, and result in behavioral outcomes in adolescent offspring of the two sexes. To this end, female mice were fed with control chow or HFD for 4 weeks before mating, during gestation, and until weaning of their litter. Histological and ultrastructural analyses revealed an increased density of myelin associated with a reduced area of cytosolic myelin channels in the corpus callosum of mHFD-exposed male compared to female offspring. Transcripts of myelination-associated genes including Igf1 –a growth factor released by microglia– were also lower, specifically in the hippocampus (without changes in the prefrontal cortex) of adolescent male mouse offspring. These changes in myelin were not related to an altered density, distribution, or maturation of oligodendrocytes, instead we found that microglia within the corpus callosum of mHFD-exposed offspring showed reduced numbers of mature lysosomes and increased synaptic contacts, suggesting microglial implication in the modified myelination. At the behavioral level, both male and female mHFD-exposed adolescent offspring presented loss of social memory and sensorimotor gating deficits. These results together highlight the importance of studying oligodendrocyte-microglia crosstalk and its involvement in the long-term brain alterations that result from prenatal mHFD in offspring across sexes

Prehistoria, frontera del conocimiento: Homenaje a L. Gerardo Vega Toscano

Prehistoria, frontera del conocimiento: Homenaje a L. Gerardo Vega Toscan