Deficient neuron-microglia signaling results in impaired functional brain connectivity and social behavior

C1q and SRPX2 regulate microglia mediated synapse elimination during early development in the visual thalamus but not the visual cortex

The classical complement cascade mediates synapse elimination in the visual thalamus during early brain development. However, whether the primary visual cortex also undergoes complement-mediated synapse elimination during early visual system development remains unknown. Here, we examined microglia-mediated synapse elimination in the visual thalamus and the primary visual cortex of early postnatal C1q and SRPX2 knockout mice. In the lateral geniculate nucleus, deletion of C1q caused a persistent decrease in synapse elimination and microglial synapse engulfment, while deletion of SRPX2 caused a transient increase in the same readouts. In the C1q-SRPX2 double knockout mice, the C1q knockout phenotypes were dominant over the SRPX2 knockout phenotypes, a result which is consistent with SRPX2 being an inhibitor of C1q. We found that genetic deletion of either C1q or SRPX2 did not affect synapse elimination or microglial engulfment of synapses in layer 4 of the primary visual cortex in early brain development. Together, these results show that the classical complement pathway regulates microglia-mediated synapse elimination in the visual thalamus but not the visual cortex during early development of the central nervous system.

Phagocytic astrocytes: Emerging from the shadows of microglia

Elimination of dead or live cells take place in both a healthy and diseased central nervous system (CNS). Dying or dead cells are quickly cleared by phagocytosis for the maintenance of a healthy CNS or for recovery after injury. Live cells or parts thereof, such as the synapses and myelin, are appropriately eliminated by phagocytosis to maintain or refine neural networks during development and adulthood. Microglia, the specific population of resident macrophages in the CNS, are classically considered as primary phagocytes; however, astrocytes have also been highlighted as phagocytes in the last decade. Phagocytic targets and receptors are reported to be mostly common between astrocytes and microglia, which raises the question of how astrocytic phagocytosis differs from microglial phagocytosis, and how these two phagocytic systems cooperate. In this review, we address the consequences of astrocytic phagocytosis, particularly focusing on these elusive points.

Reconfiguration of Brain Network Dynamics in Autism Spectrum Disorder Based on Hidden Markov Model

Autism spectrum disorder (ASD) is a group of complex neurodevelopment disorders characterized by altered brain connectivity. However, the majority of neuroimaging studies for ASD focus on the static pattern of brain function and largely neglect brain activity dynamics, which might provide deeper insight into the underlying mechanism of brain functions for ASD. Therefore, we proposed a framework with Hidden Markov Model (HMM) analysis for resting-state functional MRI (fMRI) from a large multicenter dataset of 507 male subjects. Specifically, the 507 subjects included 209 subjects with ASD and 298 well-matched health controls across 14 sites from the Autism Brain Imaging Data Exchange (ABIDE). Based on the HMM, we can identify the recurring brain function networks over time across ASD and healthy controls (HCs). Then we assessed the dynamical configuration of the whole-brain networks and further analyzed the community structure of transitions across the brain states. Based on the 19 HMM states, we found that the global temporal statistics of the specific HMM states (including fractional occupancies and lifetimes) were significantly altered in ASD compared to HCs. These specific HMM states were characterized by the activation pattern of default mode network (DMN), sensory processing networks [including visual network, auditory network, and sensory and motor network (SMN)]. Meanwhile, we also find that the specific modules of transitions between states were closely related to ASD. Our findings indicate the temporal reconfiguration of the brain network in ASD and provide novel insights into the dynamics of the whole-brain networks for ASD.

Vagus nerve stimulation modulates hippocampal inflammation caused by continuous stress in rats

Background

Previous studies have shown that vagus nerve stimulation (VNS) can attenuate inflammatory responses in peripheral tissues and also improve some neurological disorders and cognitive function in the brain. However, it is not clear how VNS is involved in neuropathological processes in brain tissues. Here, we investigated the regulatory effects of VNS on the production of proinflammatory cytokines in the hippocampus of an animal model of continuous stress (CS).

Methods

CS was induced by placing rats in cages immersed with water, and acute or chronic electrical stimulation was applied to the cervical vagus nerve of CS animals. Protein levels in the gastric and hippocampal tissues were measured by western blotting and protein signals analyzed by immunofluorescence staining. von Frey test and forced swimming test were performed to assess pain sensitivity and depressive-like behavior in rats, respectively.

Results

Levels of TNF-α, IL-1β, and IL-6 in the gastric and hippocampal tissues were significantly increased in CS animals compared to the untreated control and downregulated by acute VNS (aVNS). Iba-1-labeled microglial cells in the hippocampus of CS animals revealed morphological features of activated inflammatory cells and then changed to a normal shape by VNS. VNS elevated hippocampal expression of α7 nicotinic acetylcholine receptors (α7 nAChR) in CS animals, and pharmacological blockade of α7 nAChR increased the production of TNF-α, IL-1β, and IL-6, thus suppressing cholinergic anti-inflammatory activity that was mediated by VNS. Chronic VNS (cVNS) down-regulated the hippocampal production of active form of caspase 3 and 5-HT1A receptors and also decreased levels of TNF-α, IL-1β, and IL-6 in the gastric and hippocampal tissues of CS animals. Pain sensitivity and depressive-like behavior, which were increased by CS, were improved by cVNS.

Conclusions

Our data suggest that VNS may be involved in modulating pathophysiological processes caused by CS in the brain.

Gut Barrier Dysfunction and Type 2 Immunity: Implications for Compulsive Behavior

To date, much of the focus of gut-brain axis research has been on gut microbiota regulation of anxiety and stress-related behaviors. Much less attention has been directed to potential connections between gut microbiota and compulsive behavior. Here, we discuss a potential link between gut barrier dysfunction and compulsive behavior that is mediated through "type 2" rather than "type 1" inflammation. We examine connections between compulsive behavior and type 2 inflammation in Tourette syndrome, obsessive-compulsive disorder, autism, addiction, and post-traumatic stress disorder. Next, we discuss potential connections between gut barrier dysfunction, type 2 inflammation, and compulsive behavior. We posit a potential mechanism whereby gut barrier dysfunction-associated type 2 inflammation may drive compulsive behavior through histamine regulation of dopamine neurotransmission. Finally, we discuss the possibility of exploiting the greater accessibility of the gut relative to the brain in identifying targets to treat compulsive behavior disorders.

Systemic inflammation during the first year of life is associated with brain functional connectivity and future cognitive outcomes

The first years of life are a sensitive period of rapid neural and immune system development vulnerable to the impact of adverse experiences. Several studies support inflammation as a consequence of various adversities and an exposure negatively associated with developmental outcomes. The mechanism by which systemic inflammation may affect brain development and later cognitive outcomes remains unclear. In this longitudinal cohort study, we examine the associations between recurrent systemic inflammation, defined as C-reactive protein elevation on ≥ 2 of 4 measurements across the first year of life, electroencephalography (EEG) functional connectivity (FC) at 36 months, and composite cognitive outcomes at 3, 4, and 5 years among 122 children living in a limited-resource setting in Dhaka, Bangladesh. Recurrent systemic inflammation during the first year of life is significantly negatively associated with cognitive outcomes at 3, 4, and 5 years, after accounting for stunting and family care indicators (a measure of stimulation in the home environment). Recurrent systemic inflammation is significantly positively associated with parietal-occipital FC in the Beta band at 36 months, which in turn is significantly negatively associated with composite cognitive scores at 3 and 4 years. However, FC does not mediate the relationship between recurrent systemic inflammation and cognitive outcomes.

Mice lacking 5-lipoxygenase display motor deficits associated with cortical and hippocampal synapse abnormalities

Neural-immune interactions are related to the synapse plasticity and other dynamic processes in the nervous system. The absence or dysfunction of cellular/molecular elements from the immune system lead to impairments in the central and peripheral nervous system with behavior consequences such as cognitive, sensory, and locomotor deficits as well as social disabilities and anxiety disturbances. Cellular interactions between immune cells such as macrophages, microglia, and neutrophils with glial or neuronal cells have been of increasing interest over the last years. However, little is known about the role of immune-derived soluble factors in the context of homeostasis of the nervous system. Leukotrienes (LTs) are lipid mediators derived from the oxidation of arachidonic acid by 5-lipoxygenase (5-LO), and are classically involved in inflammation, allergies, and asthma. Here, we demonstrated that adult mice lacking 5-LO (5-LO^-/-) showed motor deficits in rotarod test and increased repetitive behavior (marble burying test). These behavioral changes are accompanied by increased levels of synapse proteins (PSD95 and synaptophysin) at the motor cortex and hippocampus, but not with BDNF alterations. No changes in microglial cell density or morphology were seen in the brains of 5-LO^-/- mice. Furthermore, expression of fractalkine receptor CX3CR1 was increased and of its ligand CX3CL1 was decreased in the cortex of 5-LO^-/- mice. Here we provide evidence for the involvement of 5-LO products structuring synapses network with motor behavior consequences. We suggest that the absence of 5-LO products lead to modified microglial/neuron interaction, reducing microglial pruning.

Investigating EEG-based functional connectivity patterns for multimodal emotion recognition

Objective. Previous studies on emotion recognition from electroencephalography (EEG) mainly rely on single-channel-based feature extraction methods, which ignore the functional connectivity between brain regions. Hence, in this paper, we propose a novel emotion-relevant critical subnetwork selection algorithm and investigate three EEG functional connectivity network features: strength, clustering coefficient, and eigenvector centrality. Approach. After constructing the brain networks by the correlations between pairs of EEG signals, we calculated critical subnetworks through the average of brain network matrices with the same emotion label to eliminate the weak associations. Then, three network features were conveyed to a multimodal emotion recognition model using deep canonical correlation analysis along with eye movement features. The discrimination ability of the EEG connectivity features in emotion recognition is evaluated on three public datasets: SEED, SEED-V, and DEAP. Main results. The experimental results reveal that the strength feature outperforms the state-of-the-art features based on single-channel analysis. The classification accuracies of multimodal emotion recognition are 95.08 ± 6.42 % on the SEED dataset, 84.51 ± 5.11 % on the SEED-V dataset, and 85.34 ± 2.90 % and 86.61 ± 3.76 % for arousal and valence on the DEAP dataset, respectively, which all achieved the best performance. In addition, the brain networks constructed with 18 channels achieve comparable performance with that of the 62-channel network and enable easier setups in real scenarios. Significance. The EEG functional connectivity networks combined with emotion-relevant critical subnetworks selection algorithm we proposed is a successful exploration to excavate the information between channels.

Sex differences in immune gene expression in the brain of a small shorebird

Males and females often exhibit differences in behaviour, life histories, and ecology, many of which are typically reflected in their brains. Neuronal protection and maintenance include complex processes led by the microglia, which also interacts with metabolites such as hormones or immune components. Despite increasing interest in sex-specific brain function in laboratory animals, the significance of sex-specific immune activation in the brain of wild animals along with the variables that could affect it is widely lacking. Here, we use the Kentish plover (Charadrius alexandrinus) to study sex differences in expression of immune genes in the brain of adult males and females, in two wild populations breeding in contrasting habitats: a coastal sea-level population and a high-altitude inland population in China. Our analysis yielded 379 genes associated with immune function. We show a significant male-biased immune gene upregulation. Immune gene expression in the brain did not differ in upregulation between the coastal and inland populations. We discuss the role of dosage compensation in our findings and their evolutionary significance mediated by sex-specific survival and neuronal deterioration. Similar expression profiles in the coastal and inland populations suggest comparable genetic control by the microglia and possible similarities in pathogen pressures between habitats. We call for further studies on gene expression of males and females in wild population to understand the implications of immune function for life-histories and demography in natural systems.

Cx3Cr1-Cre induction leads to microglial activation and IFN-1 signaling caused by DNA damage in early postnatal brain

Cx3cr1^CreER-driven Cre recombinase (Cre) is a widely used genetic tool for enabling gene manipulation in microglia and macrophages. However, an in-depth analysis of the possible detrimental effects of Cre activity in microglia, surprisingly, remains missing. Here, we demonstrate an age-dependent sensitivity of microglia to Cx3cr1-Cre toxicity, wherein Cre induction, specifically in early postnatal microglia, is detrimental to microglial development, proliferation, and function. Tamoxifen (TAM)-induced Cre activity leads to microglial activation, type 1 interferon (IFN-1) signaling, and increased phagocytosis, causing aberrant synaptic pruning during the early postnatal period and anxious behavior at later age. The detrimental effects of Cre induction are caused by DNA-damage-induced toxicity in microglia and are limited to the early postnatal period, showing no detrimental effects in adult microglia. Thus, our study reveals an age-dependent vulnerability of microglia to Cre activity, thereby highlighting age dependency of Cre action, which could be especially applicable in the broader context of environment-responsive cell types.

Microglia: Synaptic modulator in autism spectrum disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by variable impairment of social communication and repetitive behaviors, highly restricted interests, and/or sensory behaviors beginning early in life. Many individuals with ASD have dysfunction of microglia, which may be closely related to neuroinflammation, making microglia play an important role in the pathogenesis of ASD. Mounting evidence indicates that microglia, the resident immune cells of the brain, are required for proper brain function, especially in the maintenance of neuronal circuitry and control of behavior. Dysfunction of microglia will ultimately affect the neural function in a variety of ways, including the formation of synapses and alteration of excitatory-inhibitory balance. In this review, we provide an overview of how microglia actively interact with neurons in physiological conditions and modulate the fate and functions of synapses. We put a spotlight on the multi-dimensional neurodevelopmental roles of microglia, especially in the essential influence of synapses, and discuss how microglia are currently thought to influence ASD progression.

The relationship between pesticide exposure during critical neurodevelopment and autism spectrum disorder: A narrative review

Agricultural pesticides have been one of the most extensively used compounds throughout the world. The main sources of contamination for humans are dietary intake and occupational exposure. The impairments caused by agricultural pesticide exposure have been a significant global public health problem. Recent studies have shown that low-level agricultural pesticide exposure during the critical period of neurodevelopment (pregnancy and lactation) is closely related to autism spectrum disorder (ASD). Inhibition of acetylcholinesterase, gut microbiota, neural dendrite morphology, synaptic function, and glial cells are targets for the effects of pesticides during nervous system development. In the present review, we summarize the associations between several highly used and frequently studied pesticides (e.g., glyphosate, chlorpyrifos, pyrethroids, and avermectins) and ASD. We also discusse future epidemiological and toxicological research directions on the relationship between pesticides and ASD.

Prenatal Exposure to Air Pollution and Autism Spectrum Disorder: Sensitive Windows of Exposure and Sex Differences

BACKGROUND

Studies have shown that air pollution exposures during pregnancy are associated with an increased risk of autism spectrum disorder (ASD) in children, and the risk appears to be greater for boys. However, studies assessing gestational windows of susceptibility have been mostly limited by trimesters.

OBJECTIVE

We identified sensitive windows of exposure to regional air pollution and risk of ASD and examined sex differences in a large birth cohort.

METHODS

This population-based retrospective cohort study included 294,937 mother-child pairs with singleton deliveries in Kaiser Permanente Southern California (KPSC) hospitals from 2001 to 2014. Children were followed using electronic medical records until clinical ASD diagnosis, non-KPSC membership, death, or 31 December 2019, whichever came first. Weekly mean fine particulate matter [PM with an aerodynamic diameter of ≤2.5μm (PM2.5)], nitrogen dioxide (NO2), and ozone (O3) pregnancy exposures were estimated using spatiotemporal prediction models. Cox proportional hazard models with distributed lags were used to estimate weekly pollutant exposure associations with ASD risk for the entire cohort, and separately for boys and for girls. Models were adjusted for child sex (for full cohort), maternal race/ethnicity, maternal age at delivery, parity, maternal education, maternal comorbidities, medical center, census tract median household income, birth year, and season.

RESULTS

There were 5,694 ASD diagnoses (4,636 boys, 1,058 girls). Sensitive PM2.5 exposure windows associated with ASD were found early in pregnancy, statistically significant throughout the first two trimesters [1-27 wk of gestation, cumulative hazard ratio (HR)=1.14 [95% confidence interval (CI): 1.06, 1.23] per interquartile range (IQR) (7.4-μg/m3) increase]. O3 exposure during 34-37 wk of gestation was associated with increased risk [HR=1.06 (95% CI: 1.01, 1.11) per IQR (17.4 ppb) increase] but with reduced risk during 20-28 wk of gestation [HR=0.93 (95% CI: 0.89, 0.98)]. No associations were observed with NO2. Sex-stratified early gestational PM2.5 associations were stronger among boys [boys HR=1.16 (95% CI: 1.08, 1.26); girls HR=1.06 (95% CI: 0.89, 1.26)]. O3 associations in later gestation were observed only in boys [boys HR=1.10 (95% CI: 1.04, 1.16); girls HR=0.94 (95% CI: 0.84, 1.05)].

CONCLUSIONS

Exposures to PM2.5 in the first two gestational trimesters were associated with increased ASD risk in children, with stronger associations observed for boys. The role of O3 exposure on ASD risk merits further investigation. https://doi.org/10.1289/EHP9509.

Crosstalk between Microglia and Neurons in Neurotrauma: An Overview of the Underlying Mechanisms

Microglia are the resident immune cells of the brain and play a crucial role in housekeeping and maintaining homeostasis of the brain microenvironment. Upon injury or disease, microglial cells become activated, at least partly, via signals initiated by injured neurons. Activated microglia, thereby, contribute to both neuroprotection and neuroinflammation. However, sustained microglial activation initiates a chronic neuroinflammatory response which can disturb neuronal health and disrupt communications between neurons and microglia. Thus, microglia-neuron crosstalk is critical in a healthy brain as well as during states of injury or disease. As most studies focus on how neurons and microglia act in isolation during neurotrauma, there is a need to understand the interplay between these cells in brain pathophysiology. This review highlights how neurons and microglia reciprocally communicate under physiological conditions and during brain injury and disease. Furthermore, the modes of microglia-neuron communication are exposed, focusing on cell-contact dependent signaling and communication by the secretion of soluble factors like cytokines and growth factors. In addition, it has been discussed that how microglia-neuron interactions could exert either beneficial neurotrophic effects or pathologic proinflammatory responses. We further explore how aberrations in microglia-neuron crosstalk may be involved in central nervous system (CNS) anomalies, namely traumatic brain injury (TBI), neurodegeneration, and ischemic stroke. A clear understanding of how the microglia-neuron crosstalk contributes to the pathogenesis of brain pathologies may offer novel therapeutic avenues of brain trauma treatment.

Comparisons of Serum Interleukin-8 Levels in Major Depressive Patients With Drug-Free Versus SSRIs Versus Healthy Controls

OBJECTIVE

The interleukin-8 (IL-8) has been reported to play an important role in depression, which might be modulated by the selective serotonin reuptake inhibitors (SSRIs). Thus, the aim of this study was to investigate serum IL-8 levels, depressive symptom, and their associations in drug-free MDD patients, MDD patients with SSRIs, and healthy controls (HCs).

METHODS

Fifty-seven drug-free MDD patients (male/female = 35/22, mean age: 39.24 years), 30 MDD patients with SSRIs (male/female = 11/19, mean age: 39.73 years), and 101 HCs (male/female = 52/49, mean age: 37.38 years) were recruited in this cross-sectional study. Serum IL-8 levels and depressive symptom were assessed using the Flow Cytometer and Hamilton Depression Scale (HAMD). The analysis of variance was used for the comparison between groups. The relationship between serum log₁₀ IL-8 levels and HAMD score was analyzed by Pearson correlation.

RESULTS

Serum log₁₀IL-8 levels were lower in all patients than HCs after controlling for covariates (F = 4.86, p = 0.03). There was significant difference in serum Log₁₀IL-8 levels among three groups after controlling for covariates (F = 14.63, p < 0.001). Serum Log₁₀IL-8 levels in drug-free patients were lower compared to HCs (F = 19.38, p < 0.001) or patients with SSRIs (F = 21.89, p < 0.001) after controlling for covariates. However, there was not difference in serum log₁₀IL-8 levels between patients with SSRIs and HCs after controlling for covariates. Moreover, serum Log₁₀IL-8 levels were negatively correlated with HAMD score in all patients (r = -0.37, p = 0.02). Also, serum Log₁₀IL-8 levels were negatively correlated with HAMD score in drug-free patients (r = -0.74, p = 0.01), but not in patients with SSRIs.

CONCLUSION

Our data supported that the decline in serum IL-8 levels was association with depression. Moreover, the SSRIs might modulate increased serum IL-8 levels of depression.

Antibiotic-induced gut dysbiosis leads to activation of microglia and impairment of cholinergic gamma oscillations in the hippocampus

Antibiotics are widely applied for the treatment of bacterial infections, but their long-term use may lead to gut flora dysbiosis and detrimental effects on brain physiology, behavior as well as cognitive performance. Still, a striking lack of knowledge exists concerning electrophysiological correlates of antibiotic-induced changes in gut microbiota and behavior. Here, we investigated changes in the synaptic transmission and plasticity together with behaviorally-relevant network activities from the hippocampus of antibiotic-treated mice. Prolonged antibiotic treatment led to a reduction of myeloid cell pools in bone marrow, circulation and those surveilling the brain. Circulating Ly6C^hi inflammatory monocytes adopted a proinflammatory phenotype with increased expression of CD40 and MHC II. In the central nervous system, microglia displayed a subtle activated phenotype with elevated CD40 and MHC II expression, increased IL-6 and TNF production as well as with an increased number of Iba1 + cells in the hippocampal CA3 and CA1 subregions. Concomitantly, we detected a substantial reduction in the synaptic transmission in the hippocampal CA1 after antibiotic treatment. In line, carbachol-induced cholinergic gamma oscillation were reduced upon antibiotic treatment while the incidence of hippocampal sharp waves was elevated. These alterations were associated with the global changes in the expression of neurotrophin nerve growth factor and inducible nitric oxide synthase, both of which have been shown to influence cholinergic system in the hippocampus. Overall, our study demonstrates that antibiotic-induced dysbiosis of the gut microbiome and subsequent alteration of the immune cell function are associated with reduced synaptic transmission and gamma oscillations in the hippocampus, a brain region that is critically involved in mediation of innate and cognitive behavior.

Compromised fractalkine signaling delays microglial occupancy of emerging modules in the multisensory midbrain

Microglial cells (MGCs) are highly dynamic and have been implicated in shaping discrete neural maps in several unimodal systems. MGCs respond to numerous cues in their microenvironment, including the neuronally expressed chemokine, fractalkine (CX3CL1), via interactions with its corresponding fractalkine receptor (CX3CR1). The present study examines microglial and CX3CL1 patterns with regard to the emerging modular-extramodular matrix organization within the lateral cortex of the inferior colliculus (LCIC). The LCIC is a multisensory shell region of the midbrain inferior colliculus where discrete compartments receive modality-specific connections. Somatosensory inputs terminate within modular confines, while auditory inputs target the surrounding matrix. Glutamic acid decarboxylase (GAD) is an established marker of LCIC modules in developing mouse. During early postnatal development, multimodal LCIC afferents segregate into discrete, neurochemically defined compartments. Here, we analyzed neonatal GAD67-GFP (GFP is defined as green fluorescent protein) and CX3CR1-GFP mice to assess: (1) whether MGCs are recruited to distinct LCIC compartments known to be undergoing active circuit assembly, and (2) if such behaviors are fractalkine signaling-dependent. MGCs colonize the nascent LCIC by birth and increase in density until postnatal day 12 (P12). At the peak critical period (P4-P8), MGCs conspicuously border emerging LCIC modules, prior to their subsequent invasion by P12. CX3CL1 expression becomes distinctly modular at P12, in keeping with the notion of fractalkine-mediated recruitment of microglia to modular centers. In CX3CR1^GFP/GFP mice with compromised fractalkine signaling, microglial recruitment into modules is delayed. Taken together, these results suggest a potential role for microglia and fractalkine signaling in sculpting multisensory LCIC maps during an early critical period.

Sickness and the social brain: How the immune system regulates behavior across species

Many instances of sickness critically involve the immune system. The immune system talks to the brain in a bi-directional loop. This discourse affords the immune system immense control, such that it can influence behavior and optimize recovery from illness. These behavioral responses to infection are called sickness behaviors and can manifest in many ways, including changes in mood, motivation, or energy. Fascinatingly, most of these changes are conserved across species, and most organisms demonstrate some form of sickness behaviors. One of the most interesting sickness behaviors, and not immediately obvious, is altered sociability. Here, we discuss how the immune system impacts social behavior, by examining the brain regions and immune mediators involved in this process. We first outline how social behavior changes in response to infection in various species. Next, we explore which brain regions control social behavior and their evolutionary origins. Finally, we describe which immune mediators establish the link between illness and social behavior, in the context of both normal development and infection. Overall, we hope to make clear the striking similarities between the mechanisms that facilitate changes in sociability in derived and ancestral vertebrate, as well as invertebrate, species.

SARS-CoV-2-associated cytokine storm during pregnancy as a possible risk factor for neuropsychiatric disorder development in post-pandemic infants

A strong association between perinatal viral infections and neurodevelopmental disorders has been established. Both the direct contact of the virus with the developing brain and the strong maternal immune response originated by viral infections can impair proper neurodevelopment. Coronavirus disease 2019 (COVID-19), caused by the highly-infectious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is currently responsible for a large global outbreak and is a major public health issue. While initial studies focused on the viral impact on the respiratory system, increasing evidence suggest that SARS-CoV-2 infects other organs and tissues including the mature brain. While studies continue to determine the neuropathology associated to COVID-19, the consequences of SARS-CoV-2 infection to the developing brain remain largely unexplored. The present review discusses evidence suggesting that SARS-CoV-2 infection may have persistent effects on the course of pregnancy and on brain development. Studies have shown that several proinflammatory mediators which are increased in the SARS-CoV-2-associated cytokine storm, are also modified in other viral infections known to increase the risk of neurodevelopmental disorders. In this sense, further studies should assess the genuine effects of SARS-CoV-2 infection during pregnancy and delivery along with an extended follow-up of the offspring, including neurocognitive, neuroimaging, and electrophysiological examination. It also remains to be determined whether and by which mechanisms SARS-CoV-2 intrauterine and early life infection could lead to an increased risk of developing neuropsychiatric disorders, such as autism (ASD) and schizophrenia (SZ), in the offspring.

Lyl-1 regulates primitive macrophages and microglia development

During ontogeny, macrophage populations emerge in the Yolk Sac (YS) via two distinct progenitor waves, prior to hematopoietic stem cell development. Macrophage progenitors from the primitive/"early EMP" and transient-definitive/"late EMP" waves both contribute to various resident primitive macrophage populations in the developing embryonic organs. Identifying factors that modulates early stages of macrophage progenitor development may lead to a better understanding of defective function of specific resident macrophage subsets. Here we show that YS primitive macrophage progenitors express Lyl-1, a bHLH transcription factor related to SCL/Tal-1. Transcriptomic analysis of YS macrophage progenitors indicate that primitive macrophage progenitors present at embryonic day 9 are clearly distinct from those present at later stages. Disruption of Lyl-1 basic helix-loop-helix domain leads initially to an increased emergence of primitive macrophage progenitors, and later to their defective differentiation. These defects are associated with a disrupted expression of gene sets related to embryonic patterning and neurodevelopment. Lyl-1-deficiency also induce a reduced production of mature macrophages/microglia in the early brain, as well as a transient reduction of the microglia pool at midgestation and in the newborn. We thus identify Lyl-1 as a critical regulator of primitive macrophages and microglia development, which disruption may impair resident-macrophage function during organogenesis.

Ion Channel Dysfunction and Neuroinflammation in Migraine and Depression

Migraine and major depression are debilitating disorders with high lifetime prevalence rates. Interestingly these disorders are highly comorbid and show significant heritability, suggesting shared pathophysiological mechanisms. Non-homeostatic function of ion channels and neuroinflammation may be common mechanisms underlying both disorders: The excitation-inhibition balance of microcircuits and their modulation by monoaminergic systems, which depend on the expression and function of membrane located K⁺, Na⁺, and Ca⁺² channels, have been reported to be disturbed in both depression and migraine. Ion channels and energy supply to synapses not only change excitability of neurons but can also mediate the induction and maintenance of inflammatory signaling implicated in the pathophysiology of both disorders. In this respect, Pannexin-1 and P2X7 large-pore ion channel receptors can induce inflammasome formation that triggers release of pro-inflammatory mediators from the cell. Here, the role of ion channels involved in the regulation of excitation-inhibition balance, synaptic energy homeostasis as well as inflammatory signaling in migraine and depression will be reviewed.

Human microglia states are conserved across experimental models and regulate neural stem cell responses in chimeric organoids

Microglia are resident macrophages in the brain that emerge in early development and respond to the local environment by altering their molecular and phenotypic states. Fundamental questions about microglia diversity and function during development remain unanswered because we lack experimental strategies to interrogate their interactions with other cell types and responses to perturbations ex vivo. We compared human microglia states across culture models, including cultured primary and pluripotent stem cell-derived microglia. We developed a "report card" of gene expression signatures across these distinct models to facilitate characterization of their responses across experimental models, perturbations, and disease conditions. Xenotransplantation of human microglia into cerebral organoids allowed us to characterize key transcriptional programs of developing microglia in vitro and reveal that microglia induce transcriptional changes in neural stem cells and decrease interferon signaling response genes. Microglia additionally accelerate the emergence of synchronized oscillatory network activity in brain organoids by modulating synaptic density.

Glia-Driven Brain Circuit Refinement Is Altered by Early-Life Adversity: Behavioral Outcomes

Early-life adversity (ELA), often clinically referred to as "adverse childhood experiences (ACE)," is the exposure to stress-inducing events in childhood that can result in poor health outcomes. ELA negatively affects neurodevelopment in children and adolescents resulting in several behavioral deficits and increasing the risk of developing a myriad of neuropsychiatric disorders later in life. The neurobiological mechanisms by which ELA alters neurodevelopment in childhood have been the focus of numerous reviews. However, a comprehensive review of the mechanisms affecting adolescent neurodevelopment (i.e., synaptic pruning and myelination) is lacking. Synaptic pruning and myelination are glia-driven processes that are imperative for brain circuit refinement during the transition from adolescence to adulthood. Failure to optimize brain circuitry between key brain structures involved in learning and memory, such as the hippocampus and prefrontal cortex, leads to the emergence of maladaptive behaviors including increased anxiety or reduced executive function. As such, we review preclinical and clinical literature to explore the immediate and lasting effects of ELA on brain circuit development and refinement. Finally, we describe a number of therapeutic interventions best-suited to support adolescent neurodevelopment in children with a history of ELA.

Physical abuse during childhood predicts IL-2R levels in adult panic disorder patients

BACKGROUND

Childhood maltreatment confers higher risk of adulthood mental disease. However, the biological mechanism mediating this association remains largely unknown, with evidence suggesting dysregulation of the immune system as a possible biological mediator. The present paper conducted a case-control study to establish whether early-life adversity contributes to potentially pathogenic pro-inflammatory phenotypes in adult Panic Disorder (PD) individuals.

METHODS

Eighty-four PD patients and seventy-eight matched healthy controls were assessed for peripheral serum levels of interleukin (IL)-2R, IL-1β, IL-10, and for specific trauma types through child trauma questionnaire evaluation.

RESULTS

Analyses for specific types of trauma (sexual, physical or emotional abuse or neglect) revealed that these impact differentially the single inflammatory markers, and a significant association between physical abuse and the inflammatory marker IL-2R in PD patients was observed (β = 0.40, p = 0.013). The analysis demonstrates that childhood trauma contributes to a proinflammatory state in adulthood, with specific inflammatory profiles counting on the specific type of traumatic event.

CONCLUSION

This study is unique in providing inflammatory biomarkers evidence of distinct biological modifications in PD in the presence or absence of exposure to childhood abuse. These results contribute to a better understanding of the extent of influence of differences in trauma exposure on pathophysiological processes in PD and may have implications for personalized medicine.

Analyzing microglial phenotypes across neuropathologies: a practical guide

As extremely sensitive immune cells, microglia act as versatile watchdogs of the central nervous system (CNS) that tightly control tissue homeostasis. Therefore, microglial activation is an early and easily detectable hallmark of virtually all neuropsychiatric, neuro-oncological, neurodevelopmental, neurodegenerative and neuroinflammatory diseases. The recent introduction of novel high-throughput technologies and several single-cell methodologies as well as advances in epigenetic analyses helped to identify new microglia expression profiles, enhancer-landscapes and local signaling cues that defined diverse previously unappreciated microglia states in the healthy and diseased CNS. Here, we give an overview on the recent developments in the field of microglia biology and provide a practical guide to analyze disease-associated microglia phenotypes in both the murine and human CNS, on several morphological and molecular levels. Finally, technical limitations, potential pitfalls and data misinterpretations are discussed as well.

Psychological Stress as a Risk Factor for Accelerated Cellular Aging and Cognitive Decline: The Involvement of Microglia-Neuron Crosstalk

The relationship between the central nervous system (CNS) and microglia is lifelong. Microglia originate in the embryonic yolk sac during development and populate the CNS before the blood-brain barrier forms. In the CNS, they constitute a self-renewing population. Although they represent up to 10% of all brain cells, we are only beginning to understand how much brain homeostasis relies on their physiological functions. Often compared to a double-edged sword, microglia hold the potential to exert neuroprotective roles that can also exacerbate neurodegeneration once compromised. Microglia can promote synaptic growth in addition to eliminating synapses that are less active. Synaptic loss, which is considered one of the best pathological correlates of cognitive decline, is a distinctive feature of major depressive disorder (MDD) and cognitive aging. Long-term psychological stress accelerates cellular aging and predisposes to various diseases, including MDD, and cognitive decline. Among the underlying mechanisms, stress-induced neuroinflammation alters microglial interactions with the surrounding parenchymal cells and exacerbates oxidative burden and cellular damage, hence inducing changes in microglia and neurons typical of cognitive aging. Focusing on microglial interactions with neurons and their synapses, this review discusses the disrupted communication between these cells, notably involving fractalkine signaling and the triggering receptor expressed on myeloid cells (TREM). Overall, chronic stress emerges as a key player in cellular aging by altering the microglial sensome, notably via fractalkine signaling deficiency. To study cellular aging, novel positron emission tomography radiotracers for TREM and the purinergic family of receptors show interest for human study.

Altered inflammatory response in FMRP-deficient microglia

Fragile X syndrome (FXS) is an inherited intellectual disability with a high risk for comorbid autism spectrum disorders. Since FXS is a genetic disease, patients are more susceptible to environmental factors aggravating symptomatology. However, this confounding interaction between FXS environmental and genetic risk factors is under-investigated. Here, Fmr1 knock-out (KO) mice and the immune stimulus lipopolysaccharide (LPS) were used to explore this interaction between FXS development and inflammation in microglia, the brain’s primary immune cell. Our results demonstrate that Fmr1 KO and wild-type (WT) microglia are not different in inflammatory outcomes without LPS. However, Fmr1 KO microglia produces an elevated pro-inflammatory and phagocytic response following LPS treatment when compared to WT microglia. Our experiments also revealed baseline differences in mitochondrial function and morphology between WT and Fmr1 KO microglia, which LPS treatment exaggerated. Our data suggest an altered inflammatory mechanism in Fmr1 KO microglia implicating a gene and environment interaction.

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Fmr1 KO microglia display elevated LPS-induced pro-inflammatory gene expressions

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Fmr1 KO microglia display elevated LPS-induced pro-inflammatory cytokine releases

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Fmr1 KO microglia demonstrate increased LPS-induced phagocytic responses

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Fmr1 KO microglial mitochondria have altered properties and LPS-stimulated responses

Altered synaptic connectivity and brain function in mice lacking microglial adapter protein Iba1

Growing evidence indicates that microglia impact brain function by regulating synaptic pruning and formation as well as synaptic transmission and plasticity. Iba1 (ionized Ca⁺²-binding adapter protein 1), encoded by the Allograft inflammatory factor 1 (Aif1) gene, is an actin-interacting protein in microglia. Although Iba1 has long been used as a cellular marker for microglia, its functional role remains unknown. Here, we used global, Iba1-deficient (Aif1 ^-/-) mice to characterize microglial activity, synaptic function, and behavior. Microglial imaging in acute hippocampal slices and fixed tissues from juvenile mice revealed that Aif1 ^-/- microglia display reductions in ATP-induced motility and ramification, respectively. Biochemical assays further demonstrated that Aif1 ^-/- brain tissues exhibit an altered expression of microglial-enriched proteins associated with synaptic pruning. Consistent with these changes, juvenile Aif1 ^-/- mice displayed deficits in the excitatory synapse number and synaptic drive assessed by neuronal labeling and whole-cell patch-clamp recording in acute hippocampal slices. Unexpectedly, microglial synaptic engulfment capacity was diminished in juvenile Aif1 ^-/- mice. During early postnatal development, when synapse formation is a predominant event in the hippocampus, the excitatory synapse number was still reduced in Aif1 ^-/- mice. Together, these findings support an overall role of Iba1 in excitatory synaptic growth in juvenile mice. Lastly, postnatal synaptic deficits persisted in adulthood and correlated with significant behavioral changes in adult Aif1 ^-/- mice, which exhibited impairments in object recognition memory and social interaction. These results suggest that Iba1 critically contributes to microglial activity underlying essential neuroglia developmental processes that may deeply influence behavior.

Characterization of exploratory patterns and hippocampal-prefrontal network oscillations during the emergence of free exploration

During free exploration, the emergence of patterned and sequential behavioral responses to an unknown environment reflects exploration traits and adaptation. However, the behavioral dynamics and neural substrates underlying the exploratory behavior remain poorly understood. We developed computational tools to quantify the exploratory behavior and performed in vivo electrophysiological recordings in a large arena in which mice made sequential excursions into unknown territory. Occupancy entropy was calculated to characterize the cumulative and moment-to-moment behavioral dynamics in explored and unexplored territories. Local field potential analysis revealed that the theta activity in the dorsal hippocampus (dHPC) was highly correlated with the occupancy entropy. Individual dHPC and prefrontal cortex (PFC) oscillatory activities could classify various aspects of free exploration. Initiation of exploration was accompanied by a coordinated decrease and increase in theta activity in PFC and dHPC, respectively. Our results indicate that dHPC and PFC work synergistically in shaping free exploration by modulating exploratory traits during emergence and visits to an unknown environment.

Brain Immunoinformatics: A Symmetrical Link between Informatics, Wet Lab and the Clinic

Breakthrough advances in informatics over the last decade have thoroughly influenced the field of immunology. The intermingling of machine learning with wet lab applications and clinical results has hatched the newly defined immunoinformatics society. Immunoinformatics of the central neural system, referred to as neuroimmunoinformatics (NII), investigates symmetrical and asymmetrical interactions of the brain-immune interface. This interdisciplinary overview on NII is addressed to bioscientists and computer scientists. We delineate the dominating trajectories and field-shaping achievements and elaborate on future directions using bridging language and terminology. Computation, varying from linear modeling to complex deep learning approaches, fuels neuroimmunology through three core directions. Firstly, by providing big-data analysis software for high-throughput methods such as next-generation sequencing and genome-wide association studies. Secondly, by designing models for the prediction of protein morphology, functions, and symmetrical and asymmetrical protein–protein interactions. Finally, NII boosts the output of quantitative pathology by enabling the automatization of tedious processes such as cell counting, tracing, and arbor analysis. The new classification of microglia, the brain’s innate immune cells, was an NII achievement. Deep sequencing classifies microglia in “sensotypes” to accurately describe the versatility of immune responses to physiological and pathological challenges, as well as to experimental conditions such as xenografting and organoids. NII approaches complex tasks in the brain-immune interface, recognizes patterns and allows for hypothesis-free predictions with ultimate targeted individualized treatment strategies, and personalizes disease prognosis and treatment response.

The semantics of microglia activation: neuroinflammation, homeostasis, and stress

Microglia are emerging as critical regulators of neuronal function and behavior in nearly every area of neuroscience. Initial reports focused on classical immune functions of microglia in pathological contexts, however, immunological concepts from these studies have been applied to describe neuro-immune interactions in the absence of disease, injury, or infection. Indeed, terms such as 'microglia activation' or 'neuroinflammation' are used ubiquitously to describe changes in neuro-immune function in disparate contexts; particularly in stress research, where these terms prompt undue comparisons to pathological conditions. This creates a barrier for investigators new to neuro-immunology and ultimately hinders our understanding of stress effects on microglia. As more studies seek to understand the role of microglia in neurobiology and behavior, it is increasingly important to develop standard methods to study and define microglial phenotype and function. In this review, we summarize primary research on the role of microglia in pathological and physiological contexts. Further, we propose a framework to better describe changes in microglia1 phenotype and function in chronic stress. This approach will enable more precise characterization of microglia in different contexts, which should facilitate development of microglia-directed therapeutics in psychiatric and neurological disease.

Social isolation-related depression accelerates ethanol intake via microglia-derived neuroinflammation

Social isolation is common in modern society and is a contributor to depressive disorders. People with depression are highly vulnerable to alcohol use, and abusive alcohol consumption is a well-known obstacle to treating depressive disorders. Using a mouse model involving isolation stress (IS) and/or ethanol intake, we investigated the mutual influence between IS-derived depressive and ethanol-seeking behaviors along with the underlying mechanisms. IS increased ethanol craving, which robustly exacerbated depressive-like behaviors. Ethanol intake activated the mesolimbic dopaminergic system, as evidenced by dopamine/tyrosine hydroxylase double-positive signals in the ventral tegmental area and c-Fos activity in the nucleus accumbens. IS-induced ethanol intake also reduced serotonergic activity, via microglial hyperactivation in raphe nuclei, that was notably attenuated by a microglial inhibitor (minocycline). Our study demonstrated that microglial activation is a key mediator in the vicious cycle between depression and alcohol consumption. We also propose that dopaminergic reward might be involved in this pathogenicity.

Complement and microglia dependent synapse elimination in brain development

Synapse elimination, also known as synaptic pruning, is a critical step in the maturation of neural circuits during brain development. Mounting evidence indicates that the complement cascade of the innate immune system plays an important role in synapse elimination. Studies indicate that excess synapses during development are opsonized by complement proteins and subsequently phagocytosed by microglia which expresses complement receptors. The process is regulated by diverse molecular signals, including complement inhibitors that affect the activation of complement, as well as signals that affect microglial recruitment and activation. These signals may promote or inhibit the removal of specific sets of synapses during development. The complement-microglia system has also been implicated in the pathogenesis of several developmental brain disorders, suggesting that the dysregulation of mechanisms of synapse pruning may underlie the specific circuitry defects in these diseases. Here, we review the latest evidence on the molecular and cellular mechanisms of complement-dependent and microglia-dependent synapse elimination during brain development, and highlight the potential of this system as a therapeutic target for developmental brain disorders. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology Neurological Diseases > Stem Cells and Development Immune System Diseases > Molecular and Cellular Physiology.

CX3C-chemokine receptor 1 modulates cognitive dysfunction induced by sleep deprivation

Background:

Microglia plays an indispensable role in the pathological process of sleep deprivation (SD). Here, the potential role of microglial CX3C-chemokine receptor 1 (CX3CR1) in modulating the cognition decline during SD was evaluated in terms of microglial neuroinflammation and synaptic pruning. In this study, we aimed to investigat whether the interference in the microglial function by the CX3CR1 knockout affects the CNS's response to SD.

Methods:

Middle-aged wild-type (WT) C57BL/6 and CX3CR1^−/− mice were either subjected to SD or allowed normal sleep (S) for 8 h to mimic the pathophysiological changes of middle-aged people after staying up all night. After which, behavioral and histological tests were used to explore their different changes.

Results:

CX3CR1 deficiency prevented SD-induced cognitive impairments, unlike WT groups. Compared with the CX3CR1^−/− S group, the CX3CR1^−/− SD mice reported a markedly decreased microglia and cellular oncogene fos density in the dentate gyrus (DG), decreased expression of pro-inflammatory cytokines, and decreased microglial phagocytosis-related factors, whereas increased levels of anti-inflammatory cytokines in the hippocampus and a significant increase in the density of spines of the DG were also noted.

Conclusions:

These findings suggest that CX3CR1 deficiency leads to different cerebral behaviors and responses to SD. The inflammation-attenuating activity and the related modification of synaptic pruning are possible mechanism candidates, which indicate CX3CR1 as a candidate therapeutic target for the prevention of the sleep loss-induced cognitive impairments.

Microbes, metabolites and (synaptic) malleability, oh my! The effect of the microbiome on synaptic plasticity

The microbiome influences the emotional and cognitive phenotype of its host, as well as the neurodevelopment and pathophysiology of various brain processes and disorders, via the well‐established microbiome–gut–brain axis. Rapidly accumulating data link the microbiome to severe neuropsychiatric disorders in humans, including schizophrenia, Alzheimer's and Parkinson's. Moreover, preclinical work has shown that perturbation of the microbiome is closely associated with social, cognitive and behavioural deficits. The potential of the microbiome as a diagnostic and therapeutic tool is currently undercut by a lack of clear mechanistic understanding of the microbiome–gut–brain axis. This review establishes the hypothesis that the mechanism by which this influence is carried out is synaptic plasticity – long‐term changes to the physical and functional neuronal structures that enable the brain to undertake learning, memory formation, emotional regulation and more. By examining the different constituents of the microbiome–gut–brain axis through the lens of synaptic plasticity, this review explores the diverse aspects by which the microbiome shapes the behaviour and mental wellbeing of the host. Key elements of this complex bi‐directional relationship include neurotransmitters, neuronal electrophysiology, immune mediators that engage with both the central and enteric nervous systems and signalling cascades that trigger long‐term potentiation of synapses. The importance of establishing mechanistic correlations along the microbiome–gut–brain axis cannot be overstated as they hold the potential for furthering current understanding regarding the vast fields of neuroscience and neuropsychiatry. This review strives to elucidate the promising theory of microbiome‐driven synaptic plasticity in the hope of enlightening current researchers and inspiring future ones.

Microglia Regulate Neuronal Circuits in Homeostatic and High-Fat Diet-Induced Inflammatory Conditions

Microglia are brain resident macrophages, which actively survey the surrounding microenvironment and promote tissue homeostasis under physiological conditions. During this process, microglia participate in synaptic remodeling, neurogenesis, elimination of unwanted neurons and cellular debris. The complex interplay between microglia and neurons drives the formation of functional neuronal connections and maintains an optimal neural network. However, activation of microglia induced by chronic inflammation increases synaptic phagocytosis and leads to neuronal impairment or death. Microglial dysfunction is implicated in almost all brain diseases and leads to long-lasting functional deficiency, such as hippocampus-related cognitive decline and hypothalamus-associated energy imbalance (i.e., obesity). High-fat diet (HFD) consumption triggers mediobasal hypothalamic microglial activation and inflammation. Moreover, HFD-induced inflammation results in cognitive deficits by triggering hippocampal microglial activation. Here, we have summarized the current knowledge of microglial characteristics and biological functions and also reviewed the molecular mechanism of microglia in shaping neural circuitries mainly related to cognition and energy balance in homeostatic and diet-induced inflammatory conditions.

Microglia as hackers of the matrix: sculpting synapses and the extracellular space

Microglia shape the synaptic environment in health and disease, but synapses do not exist in a vacuum. Instead, pre- and postsynaptic terminals are surrounded by extracellular matrix (ECM), which together with glia comprise the four elements of the contemporary tetrapartite synapse model. While research in this area is still just beginning, accumulating evidence points toward a novel role for microglia in regulating the ECM during normal brain homeostasis, and such processes may, in turn, become dysfunctional in disease. As it relates to synapses, microglia are reported to modify the perisynaptic matrix, which is the diffuse matrix that surrounds dendritic and axonal terminals, as well as perineuronal nets (PNNs), specialized reticular formations of compact ECM that enwrap neuronal subsets and stabilize proximal synapses. The interconnected relationship between synapses and the ECM in which they are embedded suggests that alterations in one structure necessarily affect the dynamics of the other, and microglia may need to sculpt the matrix to modify the synapses within. Here, we provide an overview of the microglial regulation of synapses, perisynaptic matrix, and PNNs, propose candidate mechanisms by which these structures may be modified, and present the implications of such modifications in normal brain homeostasis and in disease.

Mechanisms governing activity-dependent synaptic pruning in the developing mammalian CNS

Almost 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this period, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synapses, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell-death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programmes may go awry in neurodevelopmental disorders of the human CNS, focusing on autism spectrum disorder and schizophrenia. Together, our aim is to give an overview of how the field of activity-dependent pruning research has evolved, led to exciting new questions and guided the identification of new, therapeutically relevant mechanisms that result in aberrant circuit development in neurodevelopmental disorders.

Microglia-specific knock-down of Bmal1 improves memory and protects mice from high fat diet-induced obesity

Microglia play a critical role in maintaining neural function. While microglial activity follows a circadian rhythm, it is not clear how this intrinsic clock relates to their function, especially in stimulated conditions such as in the control of systemic energy homeostasis or memory formation. In this study, we found that microglia-specific knock-down of the core clock gene, Bmal1, resulted in increased microglial phagocytosis in mice subjected to high-fat diet (HFD)-induced metabolic stress and likewise among mice engaged in critical cognitive processes. Enhanced microglial phagocytosis was associated with significant retention of pro-opiomelanocortin (POMC)-immunoreactivity in the mediobasal hypothalamus in mice on a HFD as well as the formation of mature spines in the hippocampus during the learning process. This response ultimately protected mice from HFD-induced obesity and resulted in improved performance on memory tests. We conclude that loss of the rigorous control implemented by the intrinsic clock machinery increases the extent to which microglial phagocytosis can be triggered by neighboring neurons under metabolic stress or during memory formation. Taken together, microglial responses associated with loss of Bmal1 serve to ensure a healthier microenvironment for neighboring neurons in the setting of an adaptive response. Thus, microglial Bmal1 may be an important therapeutic target for metabolic and cognitive disorders with relevance to psychiatric disease.

Substantial acetylcholine reduction in multiple brain regions of Mecp2-deficient female rats and associated behavioral abnormalities

Rett syndrome (RTT) is a neurodevelopmental disorder with X-linked dominant inheritance caused mainly by mutations in the methyl-CpG-binding protein 2 (MECP2) gene. The effects of various Mecp2 mutations have been extensively assessed in mouse models, but none adequately mimic the symptoms and pathological changes of RTT. In this study, we assessed the effects of Mecp2 gene deletion on female rats (Mecp2^+/−) and found severe impairments in social behavior [at 8 weeks (w), 12 w, and 23 w of age], motor function [at 16 w and 26 w], and spatial cognition [at 29 w] as well as lower plasma insulin-like growth factor (but not brain-derived neurotrophic factor) and markedly reduced acetylcholine (30%–50%) in multiple brain regions compared to female Mecp2^+/+ rats [at 29 w]. Alternatively, changes in brain monoamine levels were relatively small, in contrast to reports on mouse Mecp2 mutants. Female Mecp2-deficient rats express phenotypes resembling RTT and so may provide a robust model for future research on RTT pathobiology and treatment.

Microglia control glutamatergic synapses in the adult mouse hippocampus

Microglia cells are active players in regulating synaptic development and plasticity in the brain. However, how they influence the normal functioning of synapses is largely unknown. In this study, we characterized the effects of pharmacological microglia depletion, achieved by administration of PLX5622, on hippocampal CA3‐CA1 synapses of adult wild type mice. Following microglial depletion, we observed a reduction of spontaneous and evoked glutamatergic activity associated with a decrease of dendritic spine density. We also observed the appearance of immature synaptic features and higher levels of plasticity. Microglia depleted mice showed a deficit in the acquisition of the Novel Object Recognition task. These events were accompanied by hippocampal astrogliosis, although in the absence ofneuroinflammatory condition. PLX‐induced synaptic changes were absent in Cx3cr1^−/− mice, highlighting the role of CX3CL1/CX3CR1 axis in microglia control of synaptic functioning. Remarkably, microglia repopulation after PLX5622 withdrawal was associated with the recovery of hippocampal synapses and learning functions. Altogether, these data demonstrate that microglia contribute to normal synaptic functioning in the adult brain and that their removal induces reversible changes in organization and activity of glutamatergic synapses.

mTOR-related synaptic pathology causes autism spectrum disorder-associated functional hyperconnectivity

Postmortem studies have revealed increased density of excitatory synapses in the brains of individuals with autism spectrum disorder (ASD), with a putative link to aberrant mTOR-dependent synaptic pruning. ASD is also characterized by atypical macroscale functional connectivity as measured with resting-state fMRI (rsfMRI). These observations raise the question of whether excess of synapses causes aberrant functional connectivity in ASD. Using rsfMRI, electrophysiology and in silico modelling in Tsc2 haploinsufficient mice, we show that mTOR-dependent increased spine density is associated with ASD -like stereotypies and cortico-striatal hyperconnectivity. These deficits are completely rescued by pharmacological inhibition of mTOR. Notably, we further demonstrate that children with idiopathic ASD exhibit analogous cortical-striatal hyperconnectivity, and document that this connectivity fingerprint is enriched for ASD-dysregulated genes interacting with mTOR or Tsc2. Finally, we show that the identified transcriptomic signature is predominantly expressed in a subset of children with autism, thereby defining a segregable autism subtype. Our findings causally link mTOR-related synaptic pathology to large-scale network aberrations, revealing a unifying multi-scale framework that mechanistically reconciles developmental synaptopathy and functional hyperconnectivity in autism.

Antibiotics Treatment Modulates Microglia-Synapses Interaction

‘Dysbiosis’ of the adult gut microbiota, in response to challenges such as infection, altered diet, stress, and antibiotics treatment has been recently linked to pathological alteration of brain function and behavior. Moreover, gut microbiota composition constantly controls microglia maturation, as revealed by morphological observations and gene expression analysis. However, it is unclear whether microglia functional properties and crosstalk with neurons, known to shape and modulate synaptic development and function, are influenced by the gut microbiota. Here, we investigated how antibiotic-mediated alteration of the gut microbiota influences microglial and neuronal functions in adult mice hippocampus. Hippocampal microglia from adult mice treated with oral antibiotics exhibited increased microglia density, altered basal patrolling activity, and impaired process rearrangement in response to damage. Patch clamp recordings at CA3-CA1 synapses revealed that antibiotics treatment alters neuronal functions, reducing spontaneous postsynaptic glutamatergic currents and decreasing synaptic connectivity, without reducing dendritic spines density. Antibiotics treatment was unable to modulate synaptic function in CX3CR1-deficient mice, pointing to an involvement of microglia–neuron crosstalk through the CX3CL1/CX3CR1 axis in the effect of dysbiosis on neuronal functions. Together, our findings show that antibiotic alteration of gut microbiota impairs synaptic efficacy, suggesting that CX3CL1/CX3CR1 signaling supporting microglia is a major player in in the gut–brain axis, and in particular in the gut microbiota-to-neuron communication pathway.

Primed for addiction: A critical review of the role of microglia in the neurodevelopmental consequences of adolescent alcohol drinking

Alcohol is one of the most widely used recreational substances worldwide, with drinking frequently initiated during adolescence. The developmental state of the adolescent brain makes it vulnerable to initiating alcohol use, often in high doses, and particularly susceptible to alcohol-induced brain changes. Microglia, the brain parenchymal macrophages, have been implicated in mediating some of these effects, though the role that these cells play in the progression from alcohol drinking to dependence remains unclear. Microglia are uniquely positioned to sense and respond to central nervous system insult, and are now understood to exhibit innate immune memory, or "priming," altering their future functional responses based on prior exposures. In alcohol use disorders (AUDs), the role of microglia is debated. Whereas microglial activation can be pathogenic, contributing to neuroinflammation, tissue damage, and behavioral changes, or protective, it can also engage protective functions, providing support and mediating the resolution of damage. Understanding the role of microglia in adolescent AUDs is complicated by the fact that microglia are thought to be involved in developmental processes such as synaptic refinement and myelination, which underlie the functional maturation of multiple brain systems in adolescence. Thus, the role microglia play in the impact of alcohol use in adolescence is likely multifaceted. Long-term sequelae may be due to a failure to recover from EtOH-induced tissue damage, altered neurodevelopmental trajectories, and/or persistent changes to microglial responsivity and function. Here, we review critically the literature surrounding the effects of alcohol on microglia in models of adolescent alcohol misuse. We attempt to disentangle what is known about microglia from other neuroimmune effectors, to which we apply recent discoveries on the role of microglia in development and plasticity. Considered altogether, these studies challenge assumptions that proinflammatory microglia drive addiction. Alcohol priming microglia and thereby perturbing their homeostatic roles in neurodevelopment, especially during critical periods of plasticity such as adolescence, may have more serious implications for the neuropathogenesis of AUDs in adolescents.