On place and time: microglia in embryonic and perinatal brain development

Post-natal developmental changes in the composition of the rat neocortical N-glycome

Asparagine-linked glycosylation (N-glycosylation) plays a key role in many neurodevelopmental processes, including neural cell adhesion, neurite outgrowth and axon targeting. However, little is known about the dynamics of N-glycosylation during brain development and, in particular, how the N-glycome of the developing neocortex differs from that of the adult. The aim of this study, therefore, was to perform a thorough characterization of N-glycosylation in both the adult and neonatal rat neocortex in order to gain insights into the types of changes occurring in the N-glycome during neurodevelopment. To this end, we used hydrophilic interaction ultraperformance liquid chromatography coupled to electrospray ionization quadrupole time-of-flight mass spectrometry to compare the adult neocortical N-glycome with that of 24- and 48-h neonates. We report that the abundance of complex N-glycans is significantly lower in adults compared with neonates. Furthermore, the proportion of charged complex N-glycans is also greatly reduced. This decrease in the abundance of complex N-glycans is offset by a corresponding increase in the proportion of truncated and, to a lesser extent, hybrid N-glycans. Lastly, we report that although the proportion of oligomannose N-glycans remains constant at around 24%, the distribution of high-mannose subtypes shifts from predominantly large subtypes in neonates to smaller subtypes in the adult. In summary, our findings indicate that N-glycan synthesis in the rat neocortex is fundamentally different in neonates compared with adults with a general shift occurring from large, sialylated N-glycans towards smaller, neutral structures as neonates develop into adults, coupled with a parallel shift towards smaller oligomannose structures.

Neurobehavioral outcomes in young adults with perinatally acquired HIV

BACKGROUND

With combination antiretroviral therapy (cART), infants with perinatally acquired HIV (pHIV) are living into adolescence and adulthood. Worldwide, many have not received cART in the first years of life, and challenges of adolescence complicate transition to adulthood. Neurobehavioral outcomes in pHIV young adults (pHIVAd) are infrequently reported.

OBJECTIVES

To examine neurobehavioral characteristics of pHIVAd ages 21-30 years, and to compare them with age-matched young adults infected in the second or third decade of life (HIVagematch), and older adults with similar duration HIV disease (HIVOA).

METHODS

A comprehensive neuropsychological test battery and questionnaires to determine cognitive function and mood, and reviews of neuromedical and behavioral records were undertaken in three groups of 13 individuals each. Descriptive analysis and bivariate techniques were used for comparisons.

RESULTS

Rates of cognitive impairment were highest in pHIVAd (85%) compared with HIVagematch (38%) and HIVOA (62%). pHIVAd had the worst scores in global cognition, speed of information processing, working memory, and verbal fluency (0.5--1.0 SD below other groups). There was a trend for higher rates of psychiatric dysfunction (predominantly mood disorders) in pHIVAd (85%) compared with HIV-agematch (46%) and HIVOA (54%). Only four pHIVAd reported employment or enrollment in school. Four had autoimmune disorders.

CONCLUSION

These pHIVAd displayed high rates of cognitive, psychiatric, and autoimmune dysfunction, greater than age-matched or HIV duration-matched comparators. Although this small study is largely descriptive in nature, it suggests that a lack of cART in early life may result in long-term neurobehavioral and immune abnormalities manifesting into adulthood.

The serotonin 2B receptor is required in neonatal microglia to limit neuroinflammation and sickness behavior in adulthood

Severe peripheral infections induce an adaptive sickness behavior and an innate immune reaction in various organs including the brain. On the long term, persistent alteration of microglia, the brain innate immune cells, is associated with an increased risk of psychiatric disorders. It is thus critical to identify genes and mechanisms controlling the intensity and duration of the neuroinflammation induced by peripheral immune challenges. We tested the hypothesis that the 5-HT_2B receptor, the main serotonin receptor expressed by microglia, might represent a valuable candidate. First, we observed that Htr2b^-/- mice, knock-out for the 5-HT_2B receptor gene, developed, when exposed to a peripheral lipopolysaccharide (LPS) challenge, a stronger weight loss compared to wild-type mice; in addition, comparison of inflammatory markers in brain, 4 and 24 hr after LPS injection, showed that Htr2b deficiency leads to a prolonged neuroinflammation. Second, to assess the specific contribution of the microglial 5-HT_2B receptor, we investigated the response to LPS of conditional knock-out mice invalidated for Htr2b in microglia only. We found that deletion of Htr2b in microglia since birth is sufficient to cause enhanced weight loss and increased neuroinflammatory response upon LPS injection at adult stage. In contrast, mice deleted for microglial Htr2b in adulthood responded normally to LPS, revealing a neonatal developmental effect. These results highlight the role of microglia in the response to a peripheral immune challenge and suggest the existence of a developmental, neonatal period, during which instruction of microglia through 5-HT_2B receptors is necessary to prevent microglia overreactivity in adulthood.

Long-term effects of pre-gestational stress and perinatal venlafaxine treatment on neurobehavioral development of female offspring

Preclinical studies suggest that stress-related disorders even prior gestation can cause long-term changes at the level of neurobehavioral adaptations. Therefore, it is critical to consider undergoing antidepressant therapy which could reverse the negative consequences in the offspring. Venlafaxine is widely used in clinical practice; however insufficient amount of well-controlled studies verified the safety of venlafaxine therapy during gestation and lactation. The aim of this work was to investigate the effects of perinatal venlafaxine therapy on selected neurobehavioral variables in mothers and their female offspring using a model of maternal adversity. Pre-gestational stressed and non-stressed Wistar rat dams were treated with either venlafaxine (10 mg/kg/day) or vehicle during pregnancy and lactation. We have shown that pre-gestational stress decreased the number of pups with a significant reduction in the number of males but not females. Furthermore, we found that offspring of stressed and treated mothers exhibited anxiogenic behavior in juvenile and adolescent age. However, during adulthood pre-gestational stress significantly increased anxiety-like behavior of female, with venlafaxine treatment normalizing the state to control levels. Additionally, we found that even maternal stress prior gestation can have long-term impact on adult number of hippocampal immature neurons of the female offspring. A number of questions related to the best treatment options for maternal depression still remains, however present data may provide greater insight into the possible outcomes associated with perinatal venlafaxine therapy.

Mitochondrial-related effects of pentabromophenol, tetrabromobisphenol A, and triphenyl phosphate on murine BV-2 microglia cells

The predominant reliance on bromated flame retardants (BFRs) is diminishing with expanded use of alternative organophosphate flame retardants. However, exposure related issues for susceptible populations, the developing, infirmed, or aged, remain given environmental persistence and home-environment detection. In this regard, reports of flame retardant (FR)-related effects on the innate immune system suggest process by which a spectrum of adverse health effects could manifest across the life-span. As representative of the nervous system innate immune system, the current study examined changes in microglia following exposure to representative FRs, pentabromophenol (PBP), tetrabromobisphenol A (2,2',6,6',-tetrabromo-4,4'-isopropylidine diphenol; TBBPA) and triphenyl phosphate (TPP). Following 18hr exposure of murine BV-2 cells, at dose levels resulting in ≥80% viability (10 and 40 μM), limited alterations in pro-inflammatory responses were observed however, changes were observed in mitochondrial respiration. Basal respiration was altered by PBP; ATP-linked respiration by PBP and TBBPA, and maximum respiration by all three FRs. Basal glycolytic rate was altered by PBP and TBBPA and compensatory glycolysis by all three. Phagocytosis was decreased for PBP and TBBPA. NLRP3 inflammasome activation was assessed using BV-2-ASC (apoptosis-associated speck-like protein containing a CARD) reporter cells to visualize aggregate formation. PBP, showed a direct stimulation of aggregate formation and properties as a NLRP3 inflammasome secondary trigger. TBBPA showed indications of possible secondary triggering activity while no changes were seen with TPP. Thus, the data suggests an effect of all three FRs on mitochondria metabolism yet, different functional outcomes including, phagocytic capability and NLRP3 inflammasome activation.

Biphasic Impact of Prenatal Inflammation and Macrophage Depletion on the Wiring of Neocortical Inhibitory Circuits

The etiology of neurodevelopmental disorders is linked to defects in parvalbumin (PV)-expressing cortical interneurons and to prenatal immune challenges. Mouse models of maternal immune activation (MIA) and microglia deficits increase the postnatal density of PV interneurons, raising the question of their functional integration. Here, we show that MIA and embryonic depletion of macrophages including microglia have a two-step impact on PV interneurons wiring onto their excitatory target neurons in the barrel cortex. In adults, both challenges reduced the inhibitory drive from PV interneurons, as reported in neurodevelopmental disorders. In juveniles, however, we found an increased density of PV neurons, an enhanced strength of unitary connections onto excitatory cells, and an aberrant horizontal inhibition with a reduced lateral propagation of sensory inputs in vivo. Our results provide a comprehensive framework for understanding the impact of prenatal immune challenges onto the developmental trajectory of inhibitory circuits that leads to pathological brain wiring.

Microglial ontogeny, diversity and neurodevelopmental functions

Microglia are instrumental to the development, function, homeostasis and pathologies of the central nervous system. These brain-resident macrophages arise early in embryogenesis and seed the developing brain, where they differentiate in response to cues provided by their neural niche. Throughout life, microglia regulate the neural tissue through a variety of cellular functions influenced by intrinsic and extrinsic factors. Despite their importance, we are only starting to uncover how microglia colonize the brain, adopt distinct functional states during development and the long-term impact of early alteration of their functions. This review highlights the latest knowledge on the ontogeny of microglia, their developmental trajectory and emerging roles. Characterizing these processes will be critical for our understanding of both brain physiology and pathologies.

Neuronal and glial regulation of CNS angiogenesis and barriergenesis

Neurovascular pathologies of the central nervous system (CNS), which are associated with barrier dysfunction, are leading causes of death and disability. The roles that neuronal and glial progenitors and mature cells play in CNS angiogenesis and neurovascular barrier maturation have been elucidated in recent years. Yet how neuronal activity influences these processes remains largely unexplored. Here, we discuss our current understanding of how neuronal and glial development affects CNS angiogenesis and barriergenesis, and outline future directions to elucidate how neuronal activity might influence these processes. An understanding of these mechanisms is crucial for developing new interventions to treat neurovascular pathologies.

Microglial regional heterogeneity and its role in the brain

Microglia have been recently shown to manifest a very interesting phenotypical heterogeneity across different regions in the mammalian central nervous system (CNS). However, the underlying mechanism and functional meaning of this phenomenon are currently unclear. Baseline diversities of adult microglia in their cell number, cellular and subcellular structures, molecular signature as well as relevant functions have been discovered. But recent transcriptomic studies using bulk RNAseq and single-cell RNAseq have produced conflicting results on region-specific signatures of microglia. It is highly speculative whether such spatial heterogeneity contributes to varying sensitivities of individual microglia to the same physiological and pathological signals in different CNS regions, and hence underlie their functional relevance for CNS disease development. This review aims to thoroughly summarize up-to-date knowledge on this specific topic and provide some insights on the potential underlying mechanisms, starting from microgliogenesis. Understanding regional heterogeneity of microglia in the context of their diverse neighboring neurons and other glia may provide an important clue for future development of innovative therapies for neuropsychiatric disorders.

Neural progenitor cells mediated by H2A.Z.2 regulate microglial development via Cxcl14 in the embryonic brain

Microglia, the resident immune cells of the central nervous system, play an important role in the brain. Microglia have a special spatiotemporal distribution during the development of the cerebral cortex. Neural progenitor cells (NPCs) are the main source of neural-specific cells in the early brain. It is unclear whether NPCs affect microglial development and what molecular mechanisms control early microglial localization. H2A.Z.2, a histone variant of H2A, has a key role in gene expression regulation, genomic stability, and chromatin remodeling, but its function in brain development is not fully understood. Here, we found that the specific deletion of H2A.Z.2 in neural progenitor cells led to an abnormal increase in microglia in the ventricular zone/subventricular zone (VZ/SVZ) of the embryonic cortex. Mechanistically, H2A.Z.2 regulated microglial development by incorporating G9a into the promoter region of Cxcl14 and promoted H3k9me2 modification to inhibit the transcription of Cxcl14 in neural progenitor cells. Meanwhile, we found that the deletion of H2A.Z.2 in microglia itself had no significant effect on microglial development in the early cerebral cortex. Our findings demonstrate a key role of H2A.Z.2 in neural progenitor cells in controlling microglial development and broaden our knowledge of 2 different types of cells that may affect each other through crosstalk in the central nervous system.

Beyond defense: regulation of neuronal morphogenesis and brain functions via Toll-like receptors

Toll-like receptors (TLRs) are well known as critical pattern recognition receptors that trigger innate immune responses. In addition, TLRs are expressed in neurons and may act as the gears in the neuronal detection/alarm system for making good connections. As neuronal differentiation and circuit formation take place along with programmed cell death, neurons face the challenge of connecting with appropriate targets while avoiding dying or dead neurons. Activation of neuronal TLR3, TLR7 and TLR8 with nucleic acids negatively modulates neurite outgrowth and alters synapse formation in a cell-autonomous manner. It consequently influences neural connectivity and brain function and leads to deficits related to neuropsychiatric disorders. Importantly, neuronal TLR activation does not simply duplicate the downstream signal pathways and effectors of classical innate immune responses. The differences in spatial and temporal expression of TLRs and their ligands likely account for the diverse signaling pathways of neuronal TLRs. In conclusion, the accumulated evidence strengthens the idea that the innate immune system of neurons serves as an alarm system that responds to exogenous pathogens as well as intrinsic danger signals and fine-tune developmental processes of neurons.

PTEN in Autism and Neurodevelopmental Disorders

Phosphatase and tensin homolog (PTEN) is a classical tumor suppressor that antagonizes phosphatidylinositol 3-phosphate kinase (PI3K)/AKT signaling. Although there is a strong association of PTEN germline mutations with cancer syndromes, they have also been described in a subset of patients with autism spectrum disorders with macrocephaly characterized by impairments in social interactions and communication, repetitive behavior and, occasionally, epilepsy. To investigate PTEN's role during neurodevelopment and its implication for autism, several conditional Pten knockout mouse models have been generated. These models are valuable tools to understand PTEN's spatiotemporal roles during neurodevelopment. In this review, we will highlight the anatomical and phenotypic results from animal studies and link them to cellular and molecular findings.

Microglia cytoarchitecture in the brain of adenosine A2A receptor knockout mice: Brain region and sex specificities

Microglia cells exert a critical role in brain development, mainly supported by their immune functions, which predicts an impact on the genesis of psychiatric disorders. In fact, microglia stress during gestation is, for instance, associated with chronic anxiety and cognitive deficits accompanied by long-lasting, region- and sex-specific changes in microglia morphology. We recently reported that the pattern of microglia morphologic plasticity, which is sex-determined, impacts on anxious-like behaviour and cognition. We also reported that the pharmacologic blockade of adenosine A_2A receptors (A_{2 A} R) is able to reshape microglia morphology, in a sex-specific manner and with behavioural sequelae. In order to better understand the role of A_{2 A} R in the sex differentiation of microglia, we now compared their morphology in wild-type and A_{2 A} R knockout male and female C57BL/6 mice in two cardinal brain regions implicated in anxiety-like behaviour and cognition, the prefrontal cortex (PFC) and the dorsal hippocampus (dHIP). We report interregional differences between PFC and dHIP in a sex-specific manner: while males presented more complex microglia in the dHIP, microglia from females had a more complex morphology in the PFC. Surprisingly, the genetic deletion of A_{2 A} R did not alter these sex differences, but promoted the exclusive remodelling (increase in complexity) in PFC microglia from females. These findings further support the existence of a heterogeneous microglial network, distinct between sexes and brain regions, and help characterizing the role of A_{2 A} R in the sex- and brain region-specific morphologic differentiation of microglia.

TREM2 is required for microglial instruction of astrocytic synaptic engulfment in neurodevelopment

Variants in the microglial receptor TREM2 confer risk for multiple neurodegenerative diseases. However, it remains unknown how this receptor functions on microglia to modulate these diverse neuropathologies. To understand the role of TREM2 on microglia more generally, we investigated changes in microglial function in Trem2-/- mice. We found that loss of TREM2 impairs normal neurodevelopment, resulting in reduced synapse number across the cortex and hippocampus in 1-month-old mice. This reduction in synapse number was not due directly to alterations in interactions between microglia and synapses. Rather, TREM2 was required for microglia to limit synaptic engulfment by astrocytes during development. While these changes were largely normalized later in adulthood, high fat diet administration was sufficient to reinitiate TREM2-dependent modulation of synapse loss. Together, this identifies a novel role for microglia in instructing synaptic pruning by astrocytes to broadly regulate appropriate synaptic refinement, and suggests novel candidate mechanisms for how TREM2 and microglia could influence synaptic loss in brain injury and disease.

Robust elimination of genome-damaged cells safeguards against brain somatic aneuploidy following Knl1 deletion

The brain is a genomic mosaic shaped by cellular responses to genome damage. Here, we manipulate somatic genome stability by conditional Knl1 deletion from embryonic mouse brain. KNL1 mutations cause microcephaly and KNL1 mediates the spindle assembly checkpoint, a safeguard against chromosome missegregation and aneuploidy. We find that following Knl1 deletion, segregation errors in mitotic neural progenitor cells give rise to DNA damage on the missegregated chromosomes. This triggers rapid p53 activation and robust apoptotic and microglial phagocytic responses that extensively eliminate cells with somatic genome damage, thus causing microcephaly. By leaving only karyotypically normal progenitors to continue dividing, these mechanisms provide a second safeguard against brain somatic aneuploidy. Without Knl1 or p53-dependent safeguards, genome-damaged cells are not cleared, alleviating microcephaly, but paradoxically leading to total pre-weaning lethality. Thus, mitotic genome damage activates robust responses to eliminate somatic mutant cells, which if left unpurged, can impact brain and organismal fitness.

Microglia: Brain cells on the move

In the last decade, tremendous progress has been made in understanding the biology of microglia - i.e. the fascinating immigrated resident immune cell population of the central nervous system (CNS). Recent literature reviews have largely dealt with the plentiful functions of microglia in CNS homeostasis, development and pathology, and the influences of sex and the microbiome. In this review, the intriguing aspect of their physical plasticity during CNS development will get specific attention. Microglia move around (mobility) and reshape their processes (motility). Microglial migration into and inside the CNS is most prominent throughout development and consequently most of the data described in this review concern mobility and motility in the changing environment of the developing brain. Here, we first define microglia based on their highly specialized age- and region-dependent gene expression signature and associated functional heterogeneity. Next, we describe their origin, the migration route of immature microglial cells towards the CNS, the mechanisms underlying their invasion of the CNS, and their spatiotemporal localization and surveying behaviour inside the developing CNS. These processes are dependent on microglial mobility and motility which are determined by the microenvironment of the CNS. Therefore, we further zoom in on the changing environment during CNS development. We elaborate on the extracellular matrix and the respective integrin receptors on microglia and we discuss the purinergic and molecular signalling in microglial mobility. In the last section, we discuss the physiological and pathological functions of microglia in which mobility and motility are involved to stress the importance of microglial 'movement'.

Reverse genetic screen reveals that Il34 facilitates yolk sac macrophage distribution and seeding of the brain

Microglia are brain-resident macrophages, which have specialized functions important in brain development and in disease. They colonize the brain in early embryonic stages, but few factors that drive the migration of yolk sac macrophages (YSMs) into the embryonic brain, or regulate their acquisition of specialized properties, are currently known. Here, we present a CRISPR/Cas9-based in vivo reverse genetic screening pipeline to identify new microglia regulators using zebrafish. Zebrafish larvae are particularly suitable due to their external development, transparency and conserved microglia features. We targeted putative microglia regulators, by Cas9/gRNA complex injections, followed by Neutral-Red-based visualization of microglia. Microglia were quantified automatically in 3-day-old larvae using a software tool we called SpotNGlia. We identified that loss of zebrafish colony-stimulating factor 1 receptor (Csf1r) ligand, Il34, caused reduced microglia numbers. Previous studies on the role of IL34 in microglia development in vivo were ambiguous. Our data, and a concurrent paper, show that, in zebrafish, il34 is required during the earliest seeding of the brain by microglia. Our data also indicate that Il34 is required for YSM distribution to other organs. Disruption of the other Csf1r ligand, Csf1, did not reduce microglia numbers in mutants, whereas overexpression increased the number of microglia. This shows that Csf1 can influence microglia numbers, but might not be essential for the early seeding of the brain. In all, we identified il34 as a modifier of microglia colonization, by affecting distribution of YSMs to target organs, validating our reverse genetic screening pipeline in zebrafish.This article has an associated First Person interview with the joint first authors of the paper.

Impact of microbiota on central nervous system and neurological diseases: the gut-brain axis

Development of central nervous system (CNS) is regulated by both intrinsic and peripheral signals. Previous studies have suggested that environmental factors affect neurological activities under both physiological and pathological conditions. Although there is anatomical separation, emerging evidence has indicated the existence of bidirectional interaction between gut microbiota, i.e., (diverse microorganisms colonizing human intestine), and brain. The cross-talk between gut microbiota and brain may have crucial impact during basic neurogenerative processes, in neurodegenerative disorders and tumors of CNS. In this review, we discuss the biological interplay between gut-brain axis, and further explore how this communication may be dysregulated in neurological diseases. Further, we highlight new insights in modification of gut microbiota composition, which may emerge as a promising therapeutic approach to treat CNS disorders.

New insights on synaptic dysfunction in neuropsychiatric disorders

Growing evidence implicates synaptic proteins in the pathogenesis of neuropsychiatric disorders such as autism spectrum disorder (ASD), intellectual disability (ID) and schizophrenia. In fact, mutations in genes encoding synaptic proteins are enriched and overlap among different conditions highlighting the complex and pleiotropic nature of these disorders. In this review, we discuss recently described candidate genes that affect excitatory synapse function and result in changes in spine number and morphology. Spine pathology has been observed in several animal models of disease and in human brain post-mortem samples from ID, ASD, and schizophrenia patients. Recent data point to convergent mechanisms, such as dysregulation of the actin cytoskeleton and dysfunction of microglia synaptic remodeling, underlying dendritic spine dysgenesis. Interestingly, the reversion of important pathologic features, including spine abnormalities, has been observed in adult animal models of neuropsychiatric disorders, suggesting that therapies may not be restricted to a specific developmental window. Shedding light on the specific mechanisms impacted in neuropsychiatric disorders will undeniably contribute to the development of more directed and personalized therapies.

Basic Concept of Microglia Biology and Neuroinflammation in Relation to Psychiatry

The hypothesis that the neuroimmune system plays a role in the pathogenesis of different psychiatric disorders, including schizophrenia, depression, and bipolar disease, has attained increasing interest over the past years. Previously thought to have the sole purpose of protecting the central nervous system (CNS) from harmful stimuli, it is now known that the central immune system is critically involved in regulating physiological processes including neurodevelopment, synaptic plasticity, and circuit maintenance. Hence, alterations in microglia - the main immune cell of the CNS - and/or inflammatory factors do not unequivocally connote ongoing neuroinflammation or neuroinflammatory processes per se but rather might signify changes in brain homoeostasis. Despite this, psychiatric research tends to equate functional changes in microglia or alterations in other immune mediators with neuroinflammation. It is the main impetus of this chapter to overcome some of the current misconceptions and possible oversimplifications with respect to neuroinflammation and microglia activity in psychiatry. In order to do so, we will first provide an overview of the basic concepts of neuroinflammation and neuroinflammatory processes. We will then focus on microglia with respect to their ontogeny and immunological and non-immunological functions presenting novel insights on how microglia communicate with other cell types of the central nervous system to ensure proper brain functioning. And lastly, we will delineate the non-immunological functions of inflammatory cytokines in order to address the possible misconception of equating alterations in central cytokine levels with ongoing central inflammation. We hereby hope to help unravel the functional relevance of neuroimmune dysfunctions in psychiatric illnesses and provide future research directions in the field of psychoneuroimmunology.

Developmental roles of microglia: A window into mechanisms of disease

Microglia are engineers of the central nervous system (CNS) both in health and disease. In addition to the canonical immunological roles of clearing damaging entities and limiting the spread of toxicity and death, microglia remodel the CNS throughout life. While they have been extensively studied in disease and injury, due to their highly variable functions, their precise role in these contexts still remains uncertain. Over the past decade, we have greatly expanded our understanding of microglial function, including their essential homeostatic roles during development. Here, we review these developmental roles, identify parallels in disease, and speculate whether developmental mechanisms re-emerge in disease and injury. Developmental Dynamics 248:98-117, 2019. © 2018 Wiley Periodicals, Inc.

Neuroinflammation in preterm babies and autism spectrum disorders

Genetic anomalies have a role in autism spectrum disorders (ASD). Each genetic factor is responsible for a small fraction of cases. Environment factors, like preterm delivery, have an important role in ASD. Preterm infants have a 10-fold higher risk of developing ASD. Preterm birth is often associated with maternal/fetal inflammation, leading to a fetal/neonatal inflammatory syndrome. There are demonstrated experimental links between fetal inflammation and the later development of behavioral symptoms consistent with ASD. Preterm infants have deficits in connectivity. Most ASD genes encode synaptic proteins, suggesting that ASD are connectivity pathologies. Microglia are essential for normal synaptogenesis. Microglia are diverted from homeostatic functions towards inflammatory phenotypes during perinatal inflammation, impairing synaptogenesis. Preterm infants with ASD have a different phenotype from term born peers. Our original hypothesis is that exposure to inflammation in preterm infants, combined with at risk genetic background, deregulates brain development leading to ASD.

Embryonic Neocortical Microglia Express Toll-Like Receptor 9 and Respond to Plasmid DNA Injected into the Ventricle: Technical Considerations Regarding Microglial Distribution in Electroporated Brain Walls

Microglia, the resident immune cells in the CNS, play multiple roles during development. In the embryonic cerebral wall, microglia modulate the functions of neural stem/progenitor cells through their distribution in regions undergoing cell proliferation and/or differentiation. Previous studies using CX3CR1-GFP transgenic mice demonstrated that microglia extensively survey these regions. To simultaneously visualize microglia and neural-lineage cells that interact with each other, we applied the in utero electroporation (IUE) technique, which has been widely used for gene-transfer in neurodevelopmental studies, to CX3CR1-GFP mice (males and females). However, we unexpectedly faced a technical problem: although microglia are normally distributed homogeneously throughout the mid-embryonic cortical wall with only limited luminal entry, the intraventricular presence of exogenously derived plasmid DNAs induced microglia to accumulate along the apical surface of the cortex and aggregate in the choroid plexus. This effect was independent of capillary needle puncture of the brain wall or application of electrical pulses. The microglial response occurred at plasmid DNA concentrations lower than those routinely used for IUE, and was mediated by activation of Toll-like receptor 9 (TLR9), an innate immune sensor that recognizes unmethylated cytosine-phosphate guanosine motifs abundant in microbial DNA. Administration of plasmid DNA together with oligonucleotide 2088, the antagonist of TLR9, partially restored the dispersed intramural localization of microglia and significantly decreased luminal accumulation of these cells. Thus, via TLR9, intraventricular plasmid DNA administration causes aberrant distribution of embryonic microglia, suggesting that the behavior of microglia in brain primordia subjected to IUE should be carefully interpreted.

The Gut-Microglia Connection: Implications for Central Nervous System Diseases

The importance of the gut microbiome in central nervous system (CNS) diseases has long been recognized; however, research into this connection is limited, in part, owing to a lack of convincing mechanisms because the brain is a distant target of the gut. Previous studies on the brain revealed that most of the CNS diseases affected by the gut microbiome are closely associated with microglial dysfunction. Microglia, the major CNS-resident macrophages, are crucial for the immune response of the CNS against infection and injury, as well as for brain development and function. However, the current understanding of the mechanisms controlling the maturation and function of microglia is obscure, especially regarding the extrinsic factors affecting microglial function during the developmental process. The gut microflora has been shown to significantly influence microglia from before birth until adulthood, and the metabolites generated by the microbiota regulate the inflammation response mediated by microglia in the CNS; this inspired our hypothesis that microglia act as a critical mediator between the gut microbiome and CNS diseases. Herein, we highlight and discuss current findings that show the influence of host microbiome, as a crucial extrinsic factor, on microglia within the CNS. In addition, we summarize the CNS diseases associated with both the host microbiome and microglia and explore the potential pathways by which the gut bacteria influence the pathogenesis of CNS diseases. Our work is thus a comprehensive theoretical foundation for studies on the gut-microglia connection in the development of CNS diseases; and provides great potential for researchers to target pathways associated with the gut-microglia connection and overcome CNS diseases.

Region-Specific Phenotypes of Microglia: The Role of Local Regulatory Cues

Microglia are ubiquitous, macrophage like cells within the central nervous system (CNS) that play critical roles in supporting neuronal health and viability. They can also influence neuronal membrane properties and synaptic connectivity, positioning microglia as key cellular players in both physiological and pathological contexts. Microglia have generally been assumed to be equivalent throughout the CNS, but accumulating evidence indicates that their properties vary substantially across distinct CNS regions. In comparison to our understanding of neuronal diversity and its functional importance, our knowledge about causes and consequences of microglial regional heterogeneity is extremely limited. To fully understand how microglia influence the function of specific neuronal populations and shape heightened susceptibility of some neurons to damage and disease, greater focus on microglial heterogeneity is needed.

Microglia extensively survey the developing cortex via the CXCL12/CXCR4 system to help neural progenitors to acquire differentiated properties

Neocortical development proceeds through the formation of new zones in which neural-lineage cells are organized based on their differentiation status. Although microglia initially distribute homogeneously throughout the growing cerebral wall, they accumulate in the inner cytogenic zone, the ventricular zone (VZ) and the subventricular zone (SVZ) in the mid-embryonic stage. However, the roles of these cells remain to be elucidated. In this study, we found that microglia, despite being only a minor population of the cells that constitute the cerebral wall, promote the differentiation of neural progenitor cells by frequently moving throughout the cortex; their migration is mediated by the CXCL12/CXCR4 system. Pulse-chase experiments confirmed that microglia help Pax6⁺ stem-like cells to differentiate into Tbr2⁺ intermediate progenitors. Further, monitoring of microglia by live imaging showed that administration of AMD3100, an antagonist of CXCR4, dampened microglial movement and decreased microglial surveillance throughout the cortex. In particular, arrest of microglial motion led to a prominent decrease in the abundance of Tbr2⁺ cells in the SVZ. Based on our findings, we propose that extensive surveillance by microglia contributes to the efficient functioning of these cells, thereby regulating the differentiation of neural stem-like cells.

Embryonic hematopoiesis modulates the inflammatory response and larval hematopoiesis in Drosophila

Recent lineage tracing analyses have significantly improved our understanding of immune system development and highlighted the importance of the different hematopoietic waves. The current challenge is to understand whether these waves interact and whether this affects the function of the immune system. Here we report a molecular pathway regulating the immune response and involving the communication between embryonic and larval hematopoietic waves in Drosophila. Down-regulating the transcription factor Gcm specific to embryonic hematopoiesis enhances the larval phenotypes induced by over-expressing the pro-inflammatory Jak/Stat pathway or by wasp infestation. Gcm works by modulating the transduction of the Upd cytokines to the site of larval hematopoiesis and hence the response to chronic (Jak/Stat over-expression) and acute (wasp infestation) immune challenges. Thus, homeostatic interactions control the function of the immune system in physiology and pathology. Our data also indicate that a transiently expressed developmental pathway has a long-lasting effect on the immune response.

Connecting the nervous and the immune systems in evolution

Despite their great importance for biomedical research, the intricate network of relationships between macro- and microglia, in terms of development, function and evolution, remains poorly understood. Drawing inspiration from the recent meeting “Of Glia and Microglia”, held at the University of Strasbourg in December 2017, we here discuss the outstanding questions in the seemingly disparate fields of glial development, physiology and evolution, and also provide suggestions for how the field should move forward.

Volker Hartenstein and Angela Giangrande discuss recent advances and future directions in glial biology and evolution in the context of a recent scientific conference. Their Comment illustrates the importance of interdisciplinary approaches to answering outstanding questions in biology.

Immunological Processes in Schizophrenia Pathology: Potential Biomarkers?

Accumulating evidence suggests that the pathophysiology or schizophrenia involves alterations in immune functions, both peripherally and centrally. Immunopsychiatric research has provided a number of candidate biomarkers that could aid estimating the risk of developing schizophrenia and/or predicting its clinical course or outcomes. This chapter summarizes the findings of immune dysfunctions along the clinical course of schizophrenia and discusses their potential value as predictive, trait or state biomarkers. Given the convergence of findings deriving from immunology, epidemiology, and genetics, the possibility of identifying immune-based biomarkers of schizophrenia seems realistic. Despite these promises, however, the field has realized that immune dysfunctions in schizophrenia may be as heterogeneous as the disorder itself. While challenging for psychiatric nosology, this heterogeneity offers the opportunity to define patient subgroups based on the presence or absence of distinct immune dysfunctions. This stratification may be clinically relevant for schizophrenic patients as it may help establishing personalized add-on therapies or preventive interventions with immunomodulating drugs.

Microbiome Influences Prenatal and Adult Microglia in a Sex-Specific Manner

Microglia are embryonically seeded macrophages that contribute to brain development, homeostasis, and pathologies. It is thus essential to decipher how microglial properties are temporally regulated by intrinsic and extrinsic factors, such as sexual identity and the microbiome. Here, we found that microglia undergo differentiation phases, discernable by transcriptomic signatures and chromatin accessibility landscapes, which can diverge in adult males and females. Remarkably, the absence of microbiome in germ-free mice had a time and sexually dimorphic impact both prenatally and postnatally: microglia were more profoundly perturbed in male embryos and female adults. Antibiotic treatment of adult mice triggered sexually biased microglial responses revealing both acute and long-term effects of microbiota depletion. Finally, human fetal microglia exhibited significant overlap with the murine transcriptomic signature. Our study shows that microglia respond to environmental challenges in a sex- and time-dependent manner from prenatal stages, with major implications for our understanding of microglial contributions to health and disease.

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Microglia undergo sequential phases of differentiation during development

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The maternal microbiome influences microglial properties during prenatal stages

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The absence of the microbiome has a sex- and time-specific impact on microglia

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Microbiome depletions have acute and long-term effects on microglial properties

Environment matters: microglia function and dysfunction in a changing world

The immune system is our interface with the environment, and immune molecules such as cytokines and chemokines and the cells that produce them within the brain, notably microglia, are critical for normal brain development. This recognition has in recent years led to the working hypothesis that inflammatory events during pregnancy or the early postnatal period, for example, in response to infection, may disrupt the normal developmental trajectory of microglia and consequently their interactions with neurons, thereby contributing to the risk for neurological disorders. The current article outlines recent findings on the impact of diverse, pervasive environmental challenges, beyond infection, including air pollution and maternal stress; and their impact on microglial development and its broad implications for neural pathologies.