Morphological and Phagocytic Profile of Microglia in the Developing Rat Cerebellum

Impacts of age and environment on postnatal microglial activity: Consequences for cognitive function following early life adversity

Early life adversity (ELA) increases the likelihood of later-life neuropsychiatric disorders and cognitive dysfunction. Importantly, ELA, neuropsychiatric disorders, and cognitive deficits all involve aberrant immune signaling. Microglia are the primary neuroimmune cells and regulate brain development. Microglia are particularly sensitive to early life insults, which can program their responses to future challenges. ELA in the form of maternal separation (MS) in rats alters later-life microglial morphology and the inflammatory profile of the prefrontal cortex, a region important for cognition. However, the role of microglial responses during MS in the development of later cognition is not known. Therefore, here we aimed to determine whether the presence of microglia during MS mediates long-term impacts on adult working memory. Clodronate liposomes were used to transiently deplete microglia from the brain, while empty liposomes were used as a control. We hypothesized that if microglia mediate the long-term impacts of ELA on working memory in adulthood, then depleting microglia during MS would prevent these deficits. Importantly, microglial function shifts throughout the neonatal period, so an exploratory investigation assessed whether depletion during the early versus late neonatal period had different effects on adult working memory. Surprisingly, empty liposome treatment during the early, but not late, postnatal period induced microglial activity changes that compounded with MS to impair working memory in females. In contrast, microglial depletion later in infancy impaired later life working memory in females, suggesting that microglial function during late infancy plays an important role in the development of cognitive function. Together, these findings suggest that microglia shift their sensitivity to early life insults across development. Our findings also highlight the potential for MS to impact some developmental processes only when compounded with additional neuroimmune challenges in a sex-dependent manner.

Brain region-specific alterations in gene expression trajectories in the offspring born from influenza A virus infected mice

Influenza A virus (IAV) infection during pregnancy can increase the risk for neurodevelopmental disorders in the offspring, however, the underlying neurobiological mechanisms are largely unknown. To recapitulate viral infection, preclinical studies have traditionally focused on using synthetic viral mimetics, rather than live IAV, to examine consequences of maternal immune activation (MIA)-dependent processes on offspring. In contrast, few studies have used live IAV to assess effects on global gene expression, and none to date have addressed whether moderate IAV, mimicking seasonal influenza disease, alters normal gene expression trajectories in different brain regions across different stages of development. Herein, we show that moderate IAV infection during pregnancy, which causes mild maternal disease and no overt foetal complications in utero, induces lasting effects on the offspring into adulthood. We observed behavioural changes in adult offspring, including disrupted prepulse inhibition, dopaminergic hyper-responsiveness, and spatial recognition memory deficits. Gene profiling in the offspring brain from neonate to adolescence revealed persistent alterations to normal gene expression trajectories in the prefronal cortex, hippocampus, hypothalamus and cerebellum. Alterations were found in genes involved in inflammation and neurogenesis, which were predominately dysregulated in neonatal and early adolescent offspring. Notably, late adolescent offspring born from IAV infected mice displayed altered microglial morphology in the hippocampus. In conclusion, we show that moderate IAV during pregnancy perturbs neurodevelopmental trajectories in the offspring, including alterations in the neuroinflammatory gene expression profile and microglial number and morphology in the hippocampus, resulting in behavioural changes in adult offspring. Such early perturbations may underlie the vulnerability in human offspring for the later development of neurodevelopmental disorders, including schizophrenia. Our work highlights the importance of using live IAV in developing novel preclinical models that better recapitulate the real-world impact of inflammatory insults during pregnancy on offspring neurodevelopmental trajectories and disease susceptibility later in life.

Microglial Refinement of A-Fiber Projections in the Postnatal Spinal Cord Dorsal Horn Is Required for Normal Maturation of Dynamic Touch

Sensory systems are shaped in postnatal life by the refinement of synaptic connectivity. In the dorsal horn of the spinal cord, somatosensory circuits undergo postnatal activity-dependent reorganization, including the refinement of primary afferent A-fiber terminals from superficial to deeper spinal dorsal horn laminae which is accompanied by decreases in cutaneous sensitivity. Here, we show in the mouse that microglia, the resident immune cells in the CNS, phagocytose A-fiber terminals in superficial laminae in the first weeks of life. Genetic perturbation of microglial engulfment during the initial postnatal period in either sex prevents the normal process of A-fiber refinement and elimination, resulting in an altered sensitivity of dorsal horn cells to dynamic tactile cutaneous stimulation, and behavioral hypersensitivity to dynamic touch. Thus, functional microglia are necessary for the normal postnatal development of dorsal horn sensory circuits. In the absence of microglial engulfment, superfluous A-fiber projections remain in the dorsal horn, and the balance of sensory connectivity is disrupted, leading to lifelong hypersensitivity to dynamic touch.

Bridging Retinal and Cerebral Neurodegeneration: A Focus on Crosslinks between Alzheimer-Perusini's Disease and Retinal Dystrophies

In the early stages of Alzheimer-Perusini's disease (AD), individuals often experience vision-related issues such as color vision impairment, reduced contrast sensitivity, and visual acuity problems. As the disease progresses, there is a connection with glaucoma and age-related macular degeneration (AMD) leading to retinal cell death. The retina's involvement suggests a link with the hippocampus, where most AD forms start. A thinning of the retinal nerve fiber layer (RNFL) due to the loss of retinal ganglion cells (RGCs) is seen as a potential AD diagnostic marker using electroretinography (ERG) and optical coherence tomography (OCT). Amyloid beta fragments (Aβ), found in the eye's vitreous and aqueous humor, are also present in the cerebrospinal fluid (CSF) and accumulate in the retina. Aβ is known to cause tau hyperphosphorylation, leading to its buildup in various retinal layers. However, diseases like AD are now seen as mixed proteinopathies, with deposits of the prion protein (PrP) and α-synuclein found in affected brains and retinas. Glial cells, especially microglial cells, play a crucial role in these diseases, maintaining immunoproteostasis. Studies have shown similarities between retinal and brain microglia in terms of transcription factor expression and morphotypes. All these findings constitute a good start to achieving better comprehension of neurodegeneration in both the eye and the brain. New insights will be able to bring the scientific community closer to specific disease-modifying therapies.

Abnormal Microglial Density and Morphology in the Brain of Cyclooxygenase 2 Knockin Mice

Prostaglandin E2 (PGE2) is a signaling molecule produced by cyclooxygenase-2 (COX-2) that is important in healthy brain development. Anomalies in the COX-2/PGE2 pathway due to genetic or environmental factors have been linked to Autism Spectrum Disorders (ASD). Our previous studies showed that COX-2 deficient (COX-2-KI) mice exhibit sex-dependent molecular changes in the brain and associated autism-related behaviors. Here, we aim to determine the effect of COX-2-KI on microglial density and morphology in the developing brain. Microglia normally transition between an amoeboid or ramified morphology depending on their surroundings and are important for the development of the healthy brain, assisting with synaptogenesis, synaptic pruning, and phagocytosis. We use COX-2-KI male and female mice to evaluate microglia density, morphology, and branch length and number in five brain regions (cerebellum, hippocampus, olfactory bulb, prefrontal cortex, and thalamus) at the gestational day 19 (G19) and postnatal day 25 (PN25). We discovered that COX2-KI females were affected at G19 with increased microglial density, altered percentage of amoeboid and ramified microglia, affected branch length, and decreased branching networks in a region-specific manner; these effects persisted to PN25 in select regions. Interestingly, while limited changes were found in G19 COX-2-KI males, at PN25 we found increased microglial density, higher percentages of ramified microglia, and increased branch counts, and length observed in nearly all brain regions tested. Overall, we show for the first time that the COX-2 deficiency in our ASD mouse model influences microglia morphology in a sex- and region- and stage-dependent manner.

IL-4 shapes microglia-dependent pruning of the cerebellum during postnatal development

Interleukin-4 (IL-4) is a type 2 cytokine with pleiotropic functions in adaptive immunity, allergies, and cognitive processes. Here, we show that low levels of IL-4 in the early postnatal stage delineate a critical period in which microglia extensively prune cerebellar neurons. Elevating the levels of this cytokine via peripheral injection, or using a mouse model of allergic asthma, leads to defective pruning, permanent increase in cerebellar granule cells, and circuit alterations. These animals also show a hyperkinetic and impulsive-like phenotype, reminiscent of attention-deficit hyperactivity disorder (ADHD). These alterations are blocked in Il4rαfl/fl::Cx3cr1-CreER mice, which are deficient in IL-4 receptor signaling in microglia. These findings demonstrate a previously unknown role for IL-4 during a neuroimmune critical period of cerebellar maturation and provide a first putative mechanism for the comorbidity between allergic disease and ADHD observed in humans.

Microglial tissue surveillance: The never-resting gardener in the developing and adult CNS

Immunosurveillance by microglia is a dynamic process in the central nervous system (CNS) with versatile functions to maintain tissue homeostasis and provide immune defense. A tightly controlled microglia network throughout the CNS parenchyma facilitates efficient immunosurveillance, where each cell guards a certain tissue territory. Each cell is constantly surveilling its environment and the surrounding cells, screening for pathogens but also removing cell debris and metabolites, grooming neighboring cells and facilitating cellular crosstalk. In the absence of inflammation, this "tissue surveillance" by microglia presents an essential process for CNS homeostasis and development. In this review, we provide a summary on different tissue surveillance functions mediated by microglia, the underlying molecular machineries, and how defects, such as genetic mutations, can alter these surveillance mechanisms and cause disease onset.

Movers and shakers: Microglial dynamics and modulation of neural networks

Microglia are multifaceted cells that act as immune sentinels, with important roles in pathological events, but also as integral contributors to the normal development and function of neural circuits. In the last decade, our understanding of the contributions these cells make to synaptic health and dysfunction has expanded at a dizzying pace. Here we review the known mechanisms that govern the dynamics of microglia allowing these motile cells to interact with synapses, and recruit microglia to specific sites on neurons. We then review the molecular signals that may underlie the function of microglia in synaptic remodeling. The emerging picture from the literature suggests that microglia are highly sensitive cells, reacting to neuronal signals with dynamic and specific actions tuned to the need of specific synapses and networks.

Dendrimer-Conjugated Glutamate Carboxypeptidase II Inhibitor Restores Microglial Changes in a Rabbit Model of Cerebral Palsy

We have previously shown that maternal endotoxin exposure leads to a phenotype of cerebral palsy and pro-inflammatory microglia in the brain in neonatal rabbits. "Activated" microglia overexpress the enzyme glutamate carboxypeptidase II (GCPII) that hydrolyzes N-acetylaspartylglutamate to N-acetylaspartate and glutamate, and we have shown previously that inhibiting microglial GCPII is neuroprotective. Glutamate-induced injury and associated immune signaling can alter microglial responses including microglial process movements for surveillance and phagocytosis. We hypothesize that inhibition of GCPII activity could alter microglial phenotype and normalize microglial process movement/dynamics. Newborn rabbit kits exposed to endotoxin in utero, when treated with dendrimer-conjugated 2-(phosphonomethyl)-pentanedioic acid (D-2PMPA), a potent and selective inhibitor of microglial GCPII, showed profound changes in microglial phenotype within 48 h of treatment. Live imaging of hippocampal microglia in ex vivo brain slice preparations revealed larger cell body and phagocytic cup sizes with less stable microglia processes in CP kits compared to healthy controls. D-2PMPA treatment led to significant reversal of microglial process stability to healthy control levels. Our results emphasize the importance of microglial process dynamics in determining the state of microglial function in the developing brain and demonstrate how GCPII inhibition specifically in microglia can effectively change the microglial process motility to healthy control levels, potentially impacting migration, phagocytosis, and inflammatory functions.

The appearance of phagocytic microglia in the postnatal brain of Niemann Pick type C mice is developmentally regulated and underscores shortfalls in fine odor discrimination

The loss of NPC1 or NPC2 function results in cholesterol and sphingolipid dyshomeostasis that impairs developmental trajectories, predisposing the postnatal brain to the appearance of pathological signs, including progressive and stereotyped Purkinje cell loss and microgliosis. Despite increasing evidence reporting the activation of pro-inflammatory microglia as a cardinal event of NPC1 disease progression at symptomatic stages both in patients and preclinical models, how microglia cells respond to altered neurodevelopmental dynamics remains not completely understood. To gain an insight on this issue, we have characterized patterns of microglia activation in the early postnatal cerebellum and young adult olfactory bulb of the hypomorphic Npc1^nmf164 mouse model. Previous evidence has shown that both these areas display a number of anomalies affecting neuron and glial cell proliferation and differentiation, which largely anticipate cellular changes and clinical signs, raising our interest on how microglia interplay to these changes. Even so, to separate the contribution of cues provided by the dysfunctional microenvironment we have also studied microglia isolated from mice of increasing ages and cultured in vitro for 1 week. Our findings show that microglia of both cerebellum and olfactory bulb of Npc1^nmf164 mice adopt an activated phenotype, characterized by increased cell proliferation, enlarged soma size and de-ramified processes, as well as a robust phagocytic activity, in a time- and space-specific manner. Enhanced phagocytosis associates with a profound remodeling of gene expression signatures towards gene products involved in chemotaxis, cell recognition and engulfment, including Cd68 and Trem2. These early changes in microglia morphology and activities are induced by region-specific developmental anomalies that likely anticipate alterations in neuronal connectivity. As a proof of concept, we show that microglia activation within the granule cell layer and glomerular layer of the olfactory bulb of Npc1^nmf164 mice is associated with shortfalls in fine odor discrimination.

The immune cell profile of the developing rat brain

Little is known about the peripheral immune cell (PIC) profile of the developing brain despite growing appreciation for these cells in the mature nervous system. To address this gap, the PIC profile, defined as which cells are present, where they are located, and for how long, was examined in the developing rat using spectral flow cytometry. Select regions of the rat brain (cerebellum, hippocampus, and hypothalamus) were examined at embryonic day 20, and postnatal days 0, 7 and 16. At their peak (E20), PICs were most abundant in the cerebellum, then the hippocampus and hypothalamus. Within the PIC pool, monocytes were most prevalent in all regions and time points, and shifted from being majority classical at E20 to non-classical by PN7. T cells increased over time, and shifted from majority cytotoxic to T-helper cells by PN7. This suggests the PIC profile transitions from reactive to adaptive and surveilling in the second postnatal week. NK cells and mast cells increased temporarily, and mast cells were restricted to the hippocampus and hypothalamus, suggesting they may play a specific role in the development of those regions. Mimicking a viral infection by administration of Poly I:C increased the influx of PICs into the neonatal brain, particularly of NK cells and in the case of males only, non-classical monocytes. This work provides a map for researchers as they study immune cell contributions to healthy and pathological brain development.

Microglia morphophysiological diversity and its implications for the CNS

Microglia are mononuclear phagocytes of mesodermal origin that migrate to the central nervous system (CNS) during the early stages of embryonic development. After colonizing the CNS, they proliferate and remain able to self-renew throughout life, maintaining the number of microglia around 5-12% of the cells in the CNS parenchyma. They are considered to play key roles in development, homeostasis and innate immunity of the CNS. Microglia are exceptionally diverse in their morphological characteristics, actively modifying the shape of their processes and soma in response to different stimuli. This broad morphological spectrum of microglia responses is considered to be closely correlated to their diverse range of functions in health and disease. However, the morphophysiological attributes of microglia, and the structural and functional features of microglia-neuron interactions, remain largely unknown. Here, we assess the current knowledge of the diverse microglial morphologies, with a focus on the correlation between microglial shape and function. We also outline some of the current challenges, opportunities, and future directions that will help us to tackle unanswered questions about microglia, and to continue unravelling the mysteries of microglia, in all its shapes.

Microglia-dependent remodeling of neuronal circuits

Microglia are tissue-resident macrophages responsible for the surveillance, neuronal support, and immune defense of the brain parenchyma. Recently, the role played by microglia in the formation and function of neuronal circuits has garnered substantial attention. During development, microglia have been shown to engulf neuronal precursors and participate in pruning mechanisms while, in the mature brain, they influence synaptic signaling, provide trophic support and shape synaptic plasticity. Recently, studies have unveiled different microglial characteristics associated with specific brain regions. This emerging view suggests that the maturation and function of distinct neuronal circuits may be potentially associated with the molecular identity microglia adopts across the brain. Here, we review and summarize the known role of these cells in the thalamus, hippocampus, cortex, and cerebellum. We focus on in vivo studies to highlight the characteristics of microglia that may be important in the remodeling of these neuronal circuits and in relation to neurodevelopmental and neuropsychiatric disorders.

A tool for mapping microglial morphology, morphOMICs, reveals brain-region and sex-dependent phenotypes

Environmental cues influence the highly dynamic morphology of microglia. Strategies to characterize these changes usually involve user-selected morphometric features, which preclude the identification of a spectrum of context-dependent morphological phenotypes. Here we develop MorphOMICs, a topological data analysis approach, which enables semiautomatic mapping of microglial morphology into an atlas of cue-dependent phenotypes and overcomes feature-selection biases and biological variability. We extract spatially heterogeneous and sexually dimorphic morphological phenotypes for seven adult mouse brain regions. This sex-specific phenotype declines with maturation but increases over the disease trajectories in two neurodegeneration mouse models, with females showing a faster morphological shift in affected brain regions. Remarkably, microglia morphologies reflect an adaptation upon repeated exposure to ketamine anesthesia and do not recover to control morphologies. Finally, we demonstrate that both long primary processes and short terminal processes provide distinct insights to morphological phenotypes. MorphOMICs opens a new perspective to characterize microglial morphology.

Colombo et al. build a morphological spectrum of over 40,000 microglia across development and disease with a topological data analysis approach that allows mapping of new conditions along these sex-region-specific and brain-region-specific atlases.

Astroglial and microglial pathology in Down syndrome: Focus on Alzheimer's disease

Down syndrome (DS) arises from the triplication of human chromosome 21 and is considered the most common genetic cause of intellectual disability. Glial cells, specifically astroglia and microglia, display pathological alterations that might contribute to DS neuropathological alterations. Further, in middle adulthood, people with DS develop clinical symptoms associated with premature aging and Alzheimer's disease (AD). Overexpression of the amyloid precursor protein (APP) gene, encoded on chromosome 21, leads to increased amyloid-β (Aβ) levels and subsequent formation of Aβ plaques in the brains of individuals with DS. Amyloid-β deposition might contribute to astroglial and microglial reactivity, leading to neurotoxic effects and elevated secretion of inflammatory mediators. This review discusses evidence of astroglial and microglial alterations that might be associated with the AD continuum in DS.

Prenatal effects of alprazolam treatment on the immature cerebellum of rats

The use of benzodiazepines (BZDs) during pregnancy, especially alprazolam, is common and its impact on the fetal neural tissue is not known. In this sense, the present study aimed to investigate the effects of prenatal treatment with alprazolam on the cerebellum of Wistar rat pups. Thirty animals (24 females and six males, CEUA protocol 014/17) were separated into pairs for copulation. Females were divided into three groups: Control (CT), treatment 1 (T1, 1.25 mg per animal), and treatment 2, which is an overdose (T2, 30 mg per animal). Alprazolam was administered 10 days before copulation and throughout pregnancy. We evaluated the number and weight of pups and the macroscopic changes in the brain. Eight neonates (n = 8) from each group were used in the following analyses: Cellular and chromatin density, gliosis, synaptic density, inflammation, and oxidative stress. The results showed no significant differences regarding the number of pups, body weight, and macroscopic changes. The morphological study focused on the external granular layer (EGL) that is presented only in the immature cerebellum. Here, we detected more cells after alprazolam treatment; the T2 group showed large nuclei and some pyknotic nuclei; also, both treated groups presented an increase in the euchromatin density compared with the control. The molecular and biochemical analyses used the total protein extract of the entire cerebellum and showed an increased expression of Iba-1 and NF-κBp65 but without indication of inflammation or degeneration in the T1 group. Overdose of alprazolam presented an increased level of oxidative degradation of lipids. The treatment with alprazolam during pregnancy involved cellular and molecular changes in the immature cerebellum.

Effects of developmental exposure to FireMaster® 550 (FM 550) on microglia density, reactivity and morphology in a prosocial animal model

Microglia are known to shape brain sex differences critical for social and reproductive behaviors. Chemical exposures can disrupt brain sexual differentiation but there is limited data regarding how they may impact microglia distribution and function. We focused on the prevalent flame retardant mixture Firemaster 550 (FM 550) which is used in foam-based furniture and infant products including strollers and nursing pillows because it disrupts sexually dimorphic behaviors. We hypothesized early life FM 550 exposure would disrupt microglial distribution and reactivity in brain regions known to be highly sexually dimorphic or associated with social disorders in humans. We used prairie voles (Microtus ochrogaster) because they display spontaneous prosocial behaviors not seen in rats or mice and are thus a powerful model for studying chemical exposure-related impacts on social behaviors and their underlying neural systems. We have previously demonstrated that perinatal FM 550 exposure sex-specifically impacts socioemotional behaviors in prairie voles. We first established that, unlike in rats, the postnatal colonization of the prairie vole brain is not sexually dimorphic. Vole dams were then exposed to FM 550 (0, 500, 1000, 2000 µg/day) via subcutaneous injections through gestation, and pups were directly exposed beginning the day after birth until weaning. Adult offspring's brains were assessed for number and type (ramified, intermediate, ameboid) of microglia in the medial prefrontal cortex (mPFC), cerebellum (lobules VI-VII) and amygdala. Effects were sex- and dose-specific in the regions of interests. Overall, FM 550 exposure resulted in reduced numbers of microglia in most regions examined, with the 1000 µg FM 550 exposed males particularly affected. To further quantify differences in microglia morphology in the 1000 µg FM 550 group, Sholl and skeleton analysis were carried out on individual microglia. Microglia from control females had a more ramified phenotype compared to control males while 1000 µg FM 550-exposed males had decreased branching and ramification compared to same-sex controls. Future studies will examine the impact on the exposure to FM 550 on microglia during development given the critical role of these cells in shaping neural circuits.

Cell death in development, maintenance, and diseases of the nervous system

Cell death, be it of neurons or glial cells, marks the development of the nervous system. Albeit relatively less so than in tissues such as the gut, cell death is also a feature of nervous system homeostasis-especially in context of adult neurogenesis. Finally, cell death is commonplace in acute brain injuries, chronic neurodegenerative diseases, and in some central nervous system tumors such as glioblastoma. Recent studies are enumerating the various molecular modalities involved in the execution of cells. Intimately linked with cell death are mechanisms of disposal that remove the dead cell and bring about a tissue-level response. Heretofore, the association between these methods of dying and physiological or pathological responses has remained nebulous. It is envisioned that careful cartography of death and disposal may reveal novel understandings of disease states and chart new therapeutic strategies in the near future.

Comparison of Microglial Morphology and Function in Primary Cerebellar Cell Cultures on Collagen and Collagen-Mimetic Hydrogels

Neuronal-glial cell cultures are usually grown attached to or encapsulated in an adhesive environment as evenly distributed networks lacking tissue-like cell density, organization and morphology. In such cultures, microglia have activated amoeboid morphology and do not display extended and intensively branched processes characteristic of the ramified tissue microglia. We have recently described self-assembling functional cerebellar organoids promoted by hydrogels containing collagen-like peptides (CLPs) conjugated to a polyethylene glycol (PEG) core. Spontaneous neuronal activity was accompanied by changes in the microglial morphology and behavior, suggesting the cells might play an essential role in forming the functional neuronal networks in response to the peptide signalling. The present study examines microglial cell morphology and function in cerebellar cell organoid cultures on CLP-PEG hydrogels and compares them to the cultures on crosslinked collagen hydrogels of similar elastomechanical properties. Material characterization suggested more expressed fibril orientation and denser packaging in crosslinked collagen than CLP-PEG. However, CLP-PEG promoted a significantly higher microglial motility (determined by time-lapse imaging) accompanied by highly diverse morphology including the ramified (brightfield and confocal microscopy), more active Ca2+ signalling (intracellular Ca2+ fluorescence recordings), and moderate inflammatory cytokine level (ELISA). On the contrary, on the collagen hydrogels, microglial cells were significantly less active and mostly round-shaped. In addition, the latter hydrogels did not support the neuron synaptic activity. Our findings indicate that the synthetic CLP-PEG hydrogels ensure more tissue-like microglial morphology, motility, and function than the crosslinked collagen substrates.

Gestational Bisphenol A Exposure Impacts Embryonic Hypothalamic Microglia Numbers, Ramification, and Phagocytic Cups

Microglia are a resident population of phagocytic immune cells that reside within the central nervous system (CNS). During gestation, they are highly sensitive to their surrounding environment and can alter their physiology to respond to perceived neural insults, potentially leading to adverse influences on nearby neural progenitors. Given that bisphenol A (BPA) itself can impact developing brains, and that microglia express estrogen receptors to which BPA can bind, here we asked whether fetal microglia are responsive to gestational BPA exposure. Accordingly, we exposed pregnant dams to control or 50 mg of BPA per kg diet during gestation to investigate the impact of maternal BPA on embryonic hypothalamic microglia. Gestational BPA exposure from embryonic day 0.5 (E0.5) to E15.5 resulted in a significant increase in the number of microglia present in the hypothalamus of both male and female embryos. Staining for microglial activation using CD68 showed no change between control and prenatal BPA-exposed microglia, regardless of sex. Similarly, analysis of cultured embryonic brains demonstrated that gestational BPA exposure failed to change the secretion of cytokines or chemokines, regardless of embryo sex or the dose (50 μg of BPA per kg or 50 mg of BPA per kg maternal diet) of BPA treatment. In contrast, live-cell imaging of microglia dynamics in E15.5 control and gestationally-exposed BPA hypothalamic slices showed increased ramification of microglia exposed to BPA. Moreover, live-cell imaging also revealed a significant increase in the number of microglial phagocytic cups visible following exposure to gestational BPA. Together, these results suggest that gestational BPA exposure impacts embryonic hypothalamic microglia, perhaps leading them to alter their interactions with developing neural programs.

Guidelines for a Morphometric Analysis of Prokaryotic and Eukaryotic Cells by Scanning Electron Microscopy

The invention of a scanning electron microscopy (SEM) pushed the imaging methods and allowed for the observation of cell details with a high resolution. Currently, SEM appears as an extremely useful tool to analyse the morphology of biological samples. The aim of this paper is to provide a set of guidelines for using SEM to analyse morphology of prokaryotic and eukaryotic cells, taking as model cases Escherichia coli bacteria and B-35 rat neuroblastoma cells. Herein, we discuss the necessity of a careful sample preparation and provide an optimised protocol that allows to observe the details of cell ultrastructure (≥ 50 nm) with a minimum processing effort. Highlighting the versatility of morphometric descriptors, we present the most informative parameters and couple them with molecular processes. In this way, we indicate the wide range of information that can be collected through SEM imaging of biological materials that makes SEM a convenient screening method to detect cell pathology.

Phagocytic microglia in development: Are they what they eat?

Microglia, the innate immune cells of the brain, are indispensable for proper brain development. As professional phagocytes, microglia engulf other cells within distinct developmental niches to sculpt the architecture of the brain. Here, I highlight the age-, brain region-, and substrate-dependent diversity of developmental phagocytosis, and pose the idea that phagocytosis may, in turn, drive changes in microglia phenotype. Ultimately, phagocytosis might be just as important for shaping microglia function as it is for shaping the brain.

Comparative Review of Microglia and Monocytes in CNS Phagocytosis

Macrophages maintain tissue homeostasis by phagocytosing and removing unwanted materials such as dead cells and cell debris. Microglia, the resident macrophages of the central nervous system (CNS), are no exception. In addition, a series of recent studies have shown that microglia phagocytose the neuronal synapses that form the basis of neural circuit function. This discovery has spurred many neuroscientists to study microglia. Importantly, in the CNS parenchyma, not only microglia but also blood-derived monocytes, which essentially differentiate into macrophages after infiltration, exert phagocytic ability, making the study of phagocytosis in the CNS even more interesting and complex. In particular, in the diseased brain, the phagocytosis of tissue-damaging substances, such as myelin debris in multiple sclerosis (MS), has been shown to be carried out by both microglia and blood-derived monocytes. However, it remains largely unclear why blood-derived monocytes need to invade the parenchyma, where microglia are already abundant, to assist in phagocytosis. We will also discuss whether this phagocytosis can affect the fate of the phagocytosing cell itself as well as the substance being phagocytosed and the surrounding environment in addition to future research directions. In this review, we will introduce recent studies to answer a question that often arises when studying microglial phagocytosis: under what circumstances and to what extent blood-derived monocytes infiltrate the CNS and contribute to phagocytosis. In addition, the readers will learn how recent studies have experimentally distinguished between microglia and infiltrating monocytes. Finally, we aim to contribute to the progress of phagocytosis research by discussing the effects of phagocytosis on phagocytic cells.

Little cells of the little brain: microglia in cerebellar development and function

Microglia are long-lived resident macrophages of the brain with diverse roles that span development, adulthood, and aging. Once thought to be a relatively homogeneous population, there is a growing recognition that microglia are highly specialized to suit their specific brain region. Cerebellar microglia represent an example of such specialization, exhibiting a dynamical, transcriptional, and immunological profile that differs from that of other microglial populations. Here we review the evidence that cerebellar microglia shape the cerebellar environment and are in turn shaped by it. We examine the roles microglia play in cerebellar function, development, and aging. The emerging findings on cerebellar microglia may also provide insights into disease processes involving cerebellar dysfunction.

Sex differences in dopamine innervation and microglia are altered by synthetic progestin in neonatal medial prefrontal cortex

The synthetic progestin 17-α-hydroxyprogesterone caproate (17-OHPC) is commonly prescribed to pregnant women with a history of preterm delivery, despite little evidence of efficacy. The timing of 17-OHPC administration coincides with fetal mesocortical dopamine pathway development, yet the potential effects on cortical development and cognition are almost unknown. In rodent models, exposure to 17-OHPC significantly increased dopaminergic innervation of the medial prefrontal cortex (mPFC), an aberrant pattern of connectivity that may underlie deficits in cognitive flexibility observed in adulthood. In the present study, tyrosine hydroxylase (TH) immunoreactivity was used to determine whether 17-OHPC altered dopaminergic innervation of the mPFC during a neonatal period of synaptogenesis in males and females. Although there were no differences in the amount of TH-immunoreactive (-IR) fibres, there was a sex difference in TH-IR fibre distribution in deep layers of the prelimbic area (PL) mPFC; males had a narrower pattern of dopaminergic innervation than females. 17-OHPC exposure abolished these sex-specific patterns, such that 17-OHPC females had a narrower pattern in the PL than control females. In the infralimbic mPFC (IL), 17-OHPC males had a broader pattern of distribution of TH-immunoreactivity than control males with no differences in the amount of TH-IR fibres. 17-OHPC also created a sex difference in which males had a lower TH-IR fibre density than females. We also examined microglia, brain macrophages that play a key role in sculpting dopaminergic axon outgrowth in development, using phenotype as an indirect measure of microglial activity. Females had a greater number of reactive stout microglia compared to males in the PL, and males had more active round microglia than females in the IL. 17-OHPC treatment abolished the sex differences in both regions. These findings demonstrate that developmental exposure to 17-OHPC can exert differential effects in males and females and may diminish sex differences in cortical maturation.

Analysis of homozygous and heterozygous Csf1r knockout in the rat as a model for understanding microglial function in brain development and the impacts of human CSF1R mutations

Mutations in the human CSF1R gene have been associated with dominant and recessive forms of neurodegenerative disease. Here we describe the impacts of Csf1r mutation in the rat on development of the brain. Diffusion imaging indicated small reductions in major fiber tracts that may be associated in part with ventricular enlargement. RNA-seq profiling revealed a set of 105 microglial markers depleted in all brain regions of the Csf1rko rats. There was no evidence of region or sex-specific expression of microglia-associated transcripts. Other than the microglial signature, Csf1rko had no effect on any neuronal or region-specific transcript cluster. Expression of markers of oligodendrocytes, astrocytes, dopaminergic neurons and Purkinje cells was minimally affected. However, there were defects in dendritic arborization of doublecortin-positive neurogenic precursors and expression of poly-sialylated neural cell adhesion molecule (PS-NCAM) in the dentate gyrus of the hippocampus. Heterozygous Csf1rko rats had no detectable brain phenotype. We conclude that most brain developmental processes occur normally in the absence of microglia and that CSF1R haploinsufficiency is unlikely to cause leukoencephalopathy.

MMP13 Expression Is Increased Following Mutant α-Synuclein Exposure and Promotes Inflammatory Responses in Microglia

α-Synuclein is a 140-amino acid protein that readily misfolds and is associated with the Lewy body pathology found in sporadic and genetic forms of Parkinson's disease. We and others have shown that wild-type α-synuclein is a damage-associated molecular pattern that directly elicits a proinflammatory response in microglia through toll-like receptor activation. Here we investigated the direct effect of oligomeric mutant α-synuclein (A53T) on microglia morphology and activation. We found that misfolded A53T increased quantitative measures of amoeboid cell morphology, NFκB nuclear translocation and the expression of prototypical proinflammatory molecules. We also demonstrated that A53T increased expression of MMP13, a matrix metalloproteinase that remodels the extracellular matrix. To better understand the role of MMP13 in synucleinopathies, we further characterized the role of MMP13 in microglial signaling. We showed exposure of microglia to MMP13 induced a change in morphology and promoted the release of TNFα and MMP9. Notably, IL1β was not released indicating that the pathway involved in MMP13 activation of microglia may be different than the A53T pathway. Lastly, MMP13 increased the expression of CD68 suggesting that the lysosomal pathway might be altered by this MMP. Taken together this study shows that mutant α-synuclein directly induces a proinflammatory phenotype in microglia, which includes the expression of MMP13. In turn, MMP13 directly alters microglia supporting the need for multi-target therapies to treat Parkinson's disease patients.

Developmental Maturation of the Cerebellar White Matter-an Instructive Environment for Cerebellar Inhibitory Interneurons

In the developing cerebellum, the nascent white matter (WM) serves as an instructive niche for cerebellar cortical inhibitory interneurons. As their Pax2 expressing precursors transit the emerging WM, their laminar fate is programmed. The source(s) and nature of the signals involved remain unknown. Here, we used immunocytochemistry to follow the cellular maturation of the murine cerebellar WM during this critical period. During the first few days of postnatal development, when most Pax2 expressing cells are formed and many of them reach the cerebellar gray matter, only microglial cells can be identified in the territories through which Pax2 cells migrate. From p4 onward, cells expressing the oligodendrocytic or astrocyte markers, CNP-1, MBP or GFAP, started to appear in the nascent WM. Expression of macroglial markers increased with cerebellar differentiation, yet deep nuclei remained GFAP-negative at all ages. The progressive spread of maturing glia did not correlate with the exit of Pax2 cells from the WM, as indicated by the extensive mingling of these cells up to p15. Whereas sonic hedgehog-associated p75^NTR expression could be verified in granule cell precursors, postmitotic Pax2 cells are p75^NTR negative at all ages analyzed. Thus, if Pax2 cells, like their precursors, are sensitive to sonic hedgehog, this does not affect their expression of p75^NTR. Our findings document that subsequently generated sets of Pax2 expressing precursors of inhibitory cerebellar interneurons are confronted with a dynamically changing complement of cerebellar glia. The eventual identification of fate-defining pathways should profit from the covariation with glial maturation predicted by the present findings.

NPC1 deficiency impairs cerebellar postnatal development of microglia and climbing fiber refinement in a mouse model of Niemann-Pick disease type C

Little is known about the effects of NPC1 deficiency in brain development and whether these effects contribute to neurodegeneration in Niemann–Pick disease type C (NPC). Degeneration of cerebellar Purkinje cells occurs at an earlier stage and to a greater extent in NPC; therefore, we analyzed the effect of NPC1 deficiency on microglia and on climbing fiber synaptic refinement during cerebellar postnatal development using the Npc1^nmf164 mouse. Our analysis revealed that NPC1 deficiency leads to early phenotypic changes in microglia that are not associated with an innate immune response. However, the lack of NPC1 in Npc1^nmf164 mice significantly affected the early development of microglia by delaying the radial migration, increasing the proliferation and impairing the differentiation of microglia precursor cells during postnatal development. Additionally, increased phagocytic activity of differentiating microglia was observed at the end of the second postnatal week in Npc1^nmf164 mice. Moreover, significant climbing fiber synaptic refinement deficits along with an increased engulfment of climbing fiber synaptic elements by microglia were found in Npc1^nmf164 mice, suggesting that profound developmental defects in microglia and synaptic connectivity might precede and predispose Purkinje cells to early neurodegeneration in NPC.

Summary: Genetic deficiency of Npc1 impairs postnatal development of microglia and climbing fiber synaptic pruning in the mouse cerebellum.

Estradiol Increases Microglial Response to Lipopolysaccharide in the Ventromedial Hypothalamus during the Peripubertal Sensitive Period in Female Mice

Sensitive periods are times of development during which the effects of experience are unusually strong and long lasting. The peripubertal period has emerged as one such sensitive period, and a single administration of lipopolysaccharide (LPS) during this time reduces hormone-induced sexual behavior in adult female mice. During periods of high synaptic turnover, maturation, and elimination, as occurs during this sensitive period, microglia are particularly active. Estradiol also regulates microglial numbers, morphology, and activation. In addition, a good deal of evidence suggests that estradiol may confer this vulnerability to the effects of a stressor during the peripubertal period. Therefore, we investigated the effects of estradiol on microglial morphology, cytokine levels, and the sickness response to LPS. Estradiol levels were manipulated by implanting an estradiol-filled SILASTIC capsule (or oil-filled control) in ovariectomized mice or by administering the aromatase inhibitor, formestane (or oil control), to ovary-intact mice. We found that (1) estradiol elevates basal microglial Iba1 immunoreactivity in the ventromedial nucleus of the hypothalamus (VMH), (2) LPS induces higher levels of proinflammatory cytokines in the presence of estradiol, and (3) LPS causes hypothermia in the presence of estradiol. Taken together, these data suggest that estradiol enhances the effect of LPS during the pubertal sensitive period.

Inflammation Induced by Natural Neuronal Death and LPS Regulates Neural Progenitor Cell Proliferation in the Healthy Adult Brain

Inflammation is typically considered a negative response to injury or insult; however, recent advances demonstrate that inflammatory cells regulate development, plasticity, and homeostasis through anticytotoxic, progenerative responses. Here, we extend analyses of neuroinflammation to natural neurodegenerative and homeostatic states by exploiting seasonal plasticity in cytoarchitecture of the avian telencephalic song control nucleus, high vocal center [HVC (proper name)], in the songbird Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). We report that local injection of the endotoxin lipopolysaccharide into HVC of birds in both breeding (high circulating testosterone level) and nonbreeding (low circulating testosterone level) conditions increased neural progenitor cell proliferation in the nearby but distinct ventricular zone. Additionally, we found that oral administration of the anti-inflammatory drug minocycline during seasonal regression of HVC reduced microglia activation in HVC and prevented the normal proliferative response in the ventricular zone to apoptosis in HVC. Our results suggest that local neuroinflammation positively regulates neural progenitor cell proliferation and, in turn, contributes to the previously described repatterning of HVC cytoarchitecture following seasonally induced neuronal loss.

Undisturbed climbing fiber pruning in the cerebellar cortex of CX3 CR1-deficient mice

Pruning, the elimination of excess synapses is a phenomenon of fundamental importance for correct wiring of the central nervous system. The establishment of the cerebellar climbing fiber (CF)-to-Purkinje cell (PC) synapse provides a suitable model to study pruning and pruning-relevant processes during early postnatal development. Until now, the role of microglia in pruning remains under intense investigation. Here, we analyzed migration of microglia into the cerebellar cortex during early postnatal development and their possible contribution to the elimination of CF-to-PC synapses. Microglia enrich in the PC layer at pruning-relevant time points giving rise to the possibility that microglia are actively involved in synaptic pruning. We investigated the contribution of microglial fractalkine (CX₃ CR1) signaling during postnatal development using genetic ablation of the CX₃ CR1 receptor and an in-depth histological analysis of the cerebellar cortex. We found an aberrant migration of microglia into the granule and the molecular layer. By electrophysiological analysis, we show that defective fractalkine signaling and the associated migration deficits neither affect the pruning of excess CFs nor the development of functional parallel fiber and inhibitory synapses with PCs. These findings indicate that CX₃ CR1 signaling is not mandatory for correct cerebellar circuit formation. MAIN POINTS: Ablation of CX₃ CR1 results in a transient migration defect in cerebellar microglia. CX₃ CR1 is not required for functional pruning of cerebellar climbing fibers. Functional inhibitory and parallel fiber synapse development with Purkinje cells is undisturbed in CX₃ CR1-deficient mice.

Cerebellar Cells Self-Assemble into Functional Organoids on Synthetic, Chemically Crosslinked ECM-Mimicking Peptide Hydrogels

Hydrogel-supported neural cell cultures are more in vivo-relevant compared to monolayers formed on glass or plastic substrates. However, there is a lack of synthetic microenvironment available for obtaining standardized and easily reproducible cultures characterized by tissue-mimicking cell composition, cell-cell interactions, and functional networks. Synthetic peptides representing the biological properties of the extracellular matrix (ECM) proteins have been reported to promote the adhesion-driven differentiation and functional maturation of neural cells. Thus, such peptides can serve as building blocks for engineering a standardized, all-synthetic environment. In this study, we have compared the effect of two chemically crosslinked hydrogel compositions on primary cerebellar cells: collagen-like peptide (CLP), and CLP with an integrin-binding motif arginine-glycine-aspartate (CLP-RGD), both conjugated to polyethylene glycol molecular templates (PEG-CLP and PEG-CLP-RGD, respectively) and fabricated as self-supporting membranes. Both compositions promoted a spontaneous organization of primary cerebellar cells into tissue-like clusters with fast-rising Ca2+ signals in soma, reflecting action potential generation. Notably, neurons on PEG-CLP-RGD had more neurites and better synaptic efficiency compared to PEG-CLP. For comparison, poly-L-lysine-coated glass and plastic surfaces did not induce formation of such spontaneously active networks. Additionally, contrary to the hydrogel membranes, glass substrates functionalized with PEG-CLP and PEG-CLP-RGD did not sufficiently support cell attachment and, subsequently, did not promote functional cluster formation. These results indicate that not only chemical composition but also the hydrogel structure and viscoelasticity are essential for bioactive signaling. The synthetic strategy based on ECM-mimicking, multifunctional blocks in registry with chemical crosslinking for obtaining tissue-like mechanical properties is promising for the development of fast and well standardized functional in vitro neural models and new regenerative therapies.

Microglial Corpse Clearance: Lessons From Macrophages

From development to aging and disease, the brain parenchyma is under the constant threat of debris accumulation, in the form of dead cells and protein aggregates. To prevent garbage buildup, the brain is equipped with efficient phagocytes: the microglia. Microglia are similar, but not identical to other tissue macrophages, and in this review, we will first summarize the differences in the origin, lineage and population maintenance of microglia and macrophages. Then, we will discuss several principles that govern macrophage phagocytosis of apoptotic cells (efferocytosis), including the existence of redundant recognition mechanisms ("find-me" and "eat-me") that lead to a tight coupling between apoptosis and phagocytosis. We will then describe that resulting from engulfment and degradation of apoptotic cargo, phagocytes undergo an epigenetic, transcriptional and metabolic rewiring that leads to trained immunity, and discuss its relevance for microglia and brain function. In summary, we will show that neuroimmunologists can learn many lessons from the well-developed field of macrophage phagocytosis biology.

Regulatory Control of Microglial Phagocytosis by Estradiol and Prostaglandin E2 in the Developing Rat Cerebellum

Microglia are essential to sculpting the developing brain, and they achieve this in part through the process of phagocytosis which is regulated by microenvironmental signals associated with cell death and synaptic connectivity. In the rat cerebellum, microglial phagocytosis reaches its highest activity during the third postnatal week of development but the factors regulating this activity are unknown. A signaling pathway, involving prostaglandin E₂ (PGE₂) stimulation of the estrogen synthetic enzyme aromatase, peaks during the 2nd postnatal week and is a critical regulator of Purkinje cell maturation. We explored the relationship between the PGE₂-estradiol pathway and microglia in the maturing cerebellum. Toward that end, we treated developing rat pups with pharmacological inhibitors of estradiol and PGE₂ synthesis and then stained microglia with the universal marker Iba1 and quantified microglia engaged in phagocytosis as well as phagocytic cups in the vermis and cerebellar hemispheres. Inhibition of aromatase reduced the number of phagocytic cups in the vermis, but not in the cerebellar hemisphere at postnatal day 17. Similar results were found after treatment with nimesulide and indomethacin, inhibitors of the PGE₂-producing enzymes cyclooxygenase 1 and 2. In contrast, treatment with estradiol or PGE₂ had little effect on microglial phagocytosis in the developing cerebellum. Thus, endogenous estrogens and prostaglandins upregulate the phagocytic activity of microglia during a select window of postnatal cerebellar development, but exogenous treatment with these same signaling molecules does not further increase the already high levels of phagocytosis. This may be due to an upper threshold or evidence of resistance to exogenous perturbation.

Location of neonatal microglia drives small extracellular vesicles content and biological functions in vitro

Combining proteomics and systems biology approaches, we demonstrate that neonatal microglial cells derived from two different CNS locations, cortex and spinal cord, and cultured in vitro displayed different phenotypes upon different physiological or pathological conditions. These cells also exhibited greater variability in terms of cellular and small extracellular vesicles (sEVs) protein content and levels. Bioinformatic data analysis showed that cortical microglia exerted anti-inflammatory and neurogenesis/tumorigenesis properties, while the spinal cord microglia were more inflammatory. Interestingly, while both sEVs microglia sources enhanced growth of DRGs processes, only the spinal cord-derived sEVs microglia under LPS stimulation significantly attenuated glioma proliferation. These results were confirmed using the neurite outgrowth assay on DRGs cells and glioma proliferation analysis in 3D spheroid cultures. Results from these in vitro assays suggest that the microglia localized at different CNS regions can ensure different biological functions. Together, this study indicates that neonatal microglia locations regulate their physiological and pathological functional fates and could affect the high prevalence of brain vs spinal cord gliomas in adults.

Microglia in the developing retina

Microglia are increasingly shown to be key players in neuron development and synapse connectivity. However, the underlying mechanisms by which microglia regulate neuron function remain poorly understood in part because such analysis is challenging in the brain where neurons and synapses are intermingled and connectivity is only beginning to be mapped. Here, we discuss the features and function of microglia in the ordered mammalian retina where the laminar organization of neurons and synapses facilitates such molecular studies. We discuss microglia origins and consider the evidence for molecularly distinct microglia subpopulations and their potential for differential roles with a particular focus on the early stages of retina development. We then review the models and methods used for the study of these cells and discuss emerging data that link retina microglia to the genesis and survival of particular retina cell subtypes. We also highlight potential roles for microglia in shaping the development and organization of the vasculature and discuss cellular and molecular mechanisms involved in this process. Such insights may help resolve the mechanisms by which retinal microglia impact visual function and help guide studies of related features in brain development and disease.

Sex-specific effects of prenatal valproic acid exposure on sociability and neuroinflammation: Relevance for susceptibility and resilience in autism

Autism spectrum disorder (ASD) is a group of neurodevelopmental disorders with an incidence four times higher in boys than in girls. By analyzing the effect of sex in a mouse model of ASD, we were able to identify immune alterations that could underlie this sex bias. Pregnant mice were injected subcutaneously with 600 mg/kg of valproic acid (VPA) or saline at gestational day 12.5. Their male and female offspring were evaluated in a social interaction test at adulthood, and only male VPA mice showed reduced sociability levels and a lack of preference for the social stimulus over a novel object. We then analyzed the corticosterone (CORT) response to an inflammatory stimulus, as a measure of the hypothalamus-pituitary-adrenal (HPA) function, and the neuroinflammatory state in adult and young animals. Adult VPA males exhibited increased basal CORT levels, while VPA females showed levels comparable to controls. As male mice showed a blunted CORT response at PD21 when compared to female mice, we propose that this early dimorphism could explain the different effects of VPA on HPA function. In addition, prenatal VPA exposure resulted in altered astroglial and microglial cell density levels in the cerebellum and dentate gyrus of adult mice. These neuroinflammatory effects were more pronounced in females than males, and appeared at early developmental stages. Hence, these postnatal glial density differences could underlie the behavioral alterations observed in adulthood, when only males show a social deficit. Our work contributes to the understanding of biological mechanisms affected by VPA on male and female rodents and shed light on the study of possible resilience mechanisms in the female population and/or susceptibility to ASD in boys.

Transcriptional and Translational Differences of Microglia from Male and Female Brains

Sex differences in brain structure and function are of substantial scientific interest because of sex-related susceptibility to psychiatric and neurological disorders. Neuroinflammation is a common denominator of many of these diseases, and thus microglia, as the brain's immunocompetent cells, have come into focus in sex-specific studies. Here, we show differences in the structure, function, and transcriptomic and proteomic profiles in microglia freshly isolated from male and female mouse brains. We show that male microglia are more frequent in specific brain areas, have a higher antigen-presenting capacity, and appear to have a higher potential to respond to stimuli such as ATP, reflected in higher baseline outward and inward currents and higher protein expression of purinergic receptors. Altogether, we provide a comprehensive resource to generate and validate hypotheses regarding brain sex differences.

Generation of complex human organoid models including vascular networks by incorporation of mesodermal progenitor cells

Organoids derived from human pluripotent stem cells are interesting models to study mechanisms of morphogenesis and promising platforms for disease modeling and drug screening. However, they mostly remain incomplete as they lack stroma, tissue resident immune cells and in particular vasculature, which create important niches during development and disease. We propose, that the directed incorporation of mesodermal progenitor cells (MPCs) into organoids will overcome the aforementioned limitations. In order to demonstrate the feasibility of the method, we generated complex human tumor as well as neural organoids. We show that the formed blood vessels display a hierarchic organization and mural cells are assembled into the vessel wall. Moreover, we demonstrate a typical blood vessel ultrastructure including endothelial cell-cell junctions, a basement membrane as well as luminal caveolae and microvesicles. We observe a high plasticity in the endothelial network, which expands, while the organoids grow and is responsive to anti-angiogenic compounds and pro-angiogenic conditions such as hypoxia. We show that vessels within tumor organoids connect to host vessels following transplantation. Remarkably, MPCs also deliver Iba1⁺ cells that infiltrate the neural tissue in a microglia-like manner.

Sex-Dependent Effects of Perinatal Inflammation on the Brain: Implication for Neuro-Psychiatric Disorders

Individuals born preterm have higher rates of neurodevelopmental disorders such as schizophrenia, autistic spectrum, and attention deficit/hyperactivity disorders. These conditions are often sexually dimorphic and with different developmental trajectories. The etiology is likely multifactorial, however, infections both during pregnancy and in childhood have emerged as important risk factors. The association between sex- and age-dependent vulnerability to neuropsychiatric disorders has been suggested to relate to immune activation in the brain, including complex interactions between sex hormones, brain transcriptome, activation of glia cells, and cytokine production. Here, we will review sex-dependent effects on brain development, including glia cells, both under normal physiological conditions and following perinatal inflammation. Emphasis will be given to sex-dependent effects on brain regions which play a role in neuropsychiatric disorders and inflammatory reactions that may underlie early-life programming of neurobehavioral disturbances later in life.

Sex differences in neuroimmunity as an inherent risk factor

Identifying and understanding the sources of inherent risk to neurodevelopmental disorders is a fundamental goal of neuroscience. Being male or being exposed to inflammation early in life are two known risk factors, but they are only infrequently associated with each other. Cellular and molecular mechanisms mediating the masculinization of the brain in animal models reveal a consistent role for inflammatory signaling molecules and immune cells in the healthy male brain. Why this is so remains in the realm of speculation but may have its origins in the maternal immune system. Masculinization of the brain occurs during a restricted critical period that begins in utero and overlaps with the sensitive period during which maternal immune activation negatively impacts the developing brain. The convergence of maleness and early life inflammation as risk factors for neuropsychiatric disorders compels us to consider whether sexual differentiation of the brain in males creates an inherent and greater risk than that experienced by females.

Morphology of Microglia Across Contexts of Health and Disease

Microglia, the brain's resident macrophages, are incredibly plastic and dynamic cells. In this chapter, we aim to describe and classify the many morphological changes they can display in normal development, aging, and disease. Although microglia in healthy adult brain tissue are often ramified with small somas, they can undergo massive and rapid morphological shifts in response to stimuli, becoming amoeboid or hypertrophic. Older animals occasionally contain dystrophic, senescent, and gitter cell-like microglia, and brain injury can be accompanied by an increase in rod cells. By a careful study of microglial morphology, coupled with ultrastructural insights gleaned using electron microscopy, insights can be provided into the functions performed by these various morphological phenotypes.

Caspase-3 Mediated Cell Death in the Normal Development of the Mammalian Cerebellum

Caspase-3, onto which there is a convergence of the intrinsic and extrinsic apoptotic pathways, is the main executioner of apoptosis. We here review the current literature on the intervention of the protease in the execution of naturally occurring neuronal death (NOND) during cerebellar development. We will consider data on the most common altricial species (rat, mouse and rabbit), as well as humans. Among the different types of neurons and glia in cerebellum, there is ample evidence for an intervention of caspase-3 in the regulation of NOND of the post-mitotic cerebellar granule cells (CGCs) and Purkinje neurons, as a consequence of failure to establish proper synaptic contacts with target (secondary cell death). It seems possible that the GABAergic interneurons also undergo a similar type of secondary cell death, but the intervention of caspase-3 in this case still remains to be clarified in full. Remarkably, CGCs also undergo primary cell death at the precursor/pre-migratory stage of differentiation, in this instance without the intervention of caspase-3. Glial cells, as well, undergo a process of regulated cell death, but it seems possible that expression of caspase-3, at least in the Bergmann glia, is related to differentiation rather than death.

Immune cells and CNS physiology: Microglia and beyond

In recent years, there has been great progress in the field of neuroimmunology, implicating both parenchymal macrophages (microglia) and meningeal immunity in homeostatic brain function. In this review, Norris and Kipnis discuss these two immune compartments and how they are critically segregated to preserve homoeostasis.

Recent advances have directed our knowledge of the immune system from a narrative of “self” versus “nonself” to one in which immune function is critical for homeostasis of organs throughout the body. This is also the case with respect to the central nervous system (CNS). CNS immunity exists in a segregated state, with a marked partition occurring between the brain parenchyma and meningeal spaces. While the brain parenchyma is patrolled by perivascular macrophages and microglia, the meningeal spaces are supplied with a diverse immune repertoire. In this review, we posit that such partition allows for neuro–immune crosstalk to be properly tuned. Convention may imply that meningeal immunity is an ominous threat to brain function; however, recent studies have shown that its presence may instead be a steady hand directing the CNS to optimal performance.

Sex- and region-specific differences in microglia phenotype and characterization of the peripheral immune response following early-life infection in neonatal male and female rats

Early-life infection has been shown to have profound effects on the brain and behavior across the lifespan, a phenomenon termed "early-life programming". Indeed, many neuropsychiatric disorders begin or have their origins early in life and have been linked to early-life immune activation (e.g. autism, ADHD, and schizophrenia). Furthermore, many of these disorders show a robust sex bias, with males having a higher risk of developing early-onset neurodevelopmental disorders. The concept of early-life programming is now well established, however, it is still unclear how such effects are initiated and then maintained across time to produce such a phenomenon. To begin to address this question, we examined changes in microglia, the immune cells of the brain, and peripheral immune cells in the hours immediately following early-life infection in male and female rats. We found that males showed a significant decrease in BDNF expression and females showed a significant increase in IL-6 expression in the cerebellum following E.coli infection on postnatal day 4; however, for most cytokines examined in the brain and in the periphery we were unable to identify any sex differences in the immune response, at least at the time points examined. Instead, neonatal infection with E.coli increased the expression of a number of cytokines in the brain of both males and females similarly including TNF-α, IL-1β, and CD11b (a marker of microglia activation) in the hippocampus and, in the spleen, TNF-α and IL-1β. We also found that protein levels of GRO-KC, MIP-1a, MCP1, IP-10, TNF-α, and IL-10 were elevated 8-hours postinfection, but this response was resolved by 24-hours. Lastly, we found that males have more thin microglia than females on P5, however, neonatal infection had no effect on any of the microglia morphologies we examined. These data show that sex differences in the acute immune response to neonatal infection are likely gene, region, and even time dependent. Future research should consider these factors in order to develop a comprehensive understanding of the immune response in males and females as these changes are likely the initiating agents that lead to the long-term, and often sex-specific, effects of early-life infection.

Sex Differences in the Neuroimmune System

While sex differences in the peripheral immune response have been studied extensively, sex differences in the neuroimmune response, including glial activation and associated cytokine production in the brain, is a recently emerging field. Advances in our understanding of sex differences in the neuroimmune response have important implications for understanding how neural circuits are shaped during early brain development, how activation of the immune system may impact cognitive function and behavior, and how inflammation may be associated with the risk of mental health disorders that have strong sex-biases. The goal of this mini review is to highlight recent work in the field of sex differences in neuroimmune function, with a particular focus on how microglia function is influenced by age and sex hormone exposure.

Fluoride activates microglia, secretes inflammatory factors and influences synaptic neuron plasticity in the hippocampus of rats

Epidemiological studies have reported that highly fluoridated drinking water may significantly decrease the Intelligence Quotient (IQ) of exposed children. It is thought that synaptic plasticity is the basis of learning and memory skills in developing children. However, the effect on synaptic plasticity by activated microglia induced via fluoride treatment is less clear. Our previous research showed that fluoride ions activated microglia which then released pro-inflammatory cytokines. In this study, hippocampal-dependent memory status was evaluated in rat models sub-chronically exposed to fluoride in their drinking water. Microglial activation in the hippocampus was examined using immunofluorescence staining and the expression of synaptophysin (SYP) and postsynaptic density protein 95 (PSD-95), Long-term potentiation (LTP) and the expression of Amino-3-hydroxy-5-methy-4-isoxazole propionate (AMPA) receptor subunit GluR2 as well as N-methyl-d-aspartate (NMDA) receptor subunit NMDAR2β of exposed rats. We found that fluoride exposure activated microglia and increased the expression of DAP12 and TREM2, as well as promoted pro-inflammatory cytokines secretion via ERK/MAPK and P38/MAPK signal pathways. Furthermore fluoride depressed LTP and decreased PSD-95 protein levels as well as expression of ionotropic glutamate receptors GluR2 and NMDAR2β. We concluded that the role of fluoride on synaptic plasticity may be associated with neuroinflammation induced by microglia.

Cerebellar injury and impaired function in a rabbit model of maternal inflammation induced neonatal brain injury

Cerebellum is involved in higher cognitive functions and plays important roles in neurological disorders. Cerebellar injury has been detected frequently in patients with preterm birth resulting in cognitive dysfunction later in life. Maternal infection and inflammation is associated with preterm birth and in neonatal brain injury. We have previously shown that intrauterine lipopolysaccharide (LPS) exposure induces white matter injury and microglial activation in the cerebral white matter tracts of neonatal rabbits, resulting in motor deficits consistent with the clinical findings of cerebral palsy (CP). Here we investigated whether intrauterine LPS exposure induced cerebellar inflammation and functional impairment. Timed-pregnant New Zealand white rabbits underwent a laparotomy on gestational day 28 (G28) and LPS (3200 EU, endotoxin group) was injected along the wall of the uterus as previously described. Controls did not receive surgical intervention. Kits born to control and endotoxin treated dams were euthanized on postnatal day (PND)1 (3 days post-injury) or PND5 (7 days post-injury) and cerebellum evaluated for presence of inflammation. The microglial morphology in cerebellar white matter areas was analyzed using Neurolucida and Neurolucida Explorer. mRNA expression of inflammatory cytokines was quantified by real-time-PCR. We found that intrauterine exposure to LPS induced intensive microglial activation in cerebellar white matter areas, as evidenced by increased numbers of activated microglia and morphological changes (amoeboid soma and retracted processes) that was accompanied by significant increases in pro-inflammatory cytokines. The Purkinje cell layer was less developed in endotoxin exposed kits than healthy controls. In kits that survived to PND 60, soma size and cell density of Purkinje cells were significantly decreased in endotoxin exposed kits compared to controls. The findings of altered Purkinje cell morphology were consistent with impaired cerebellar function as tested by eye-blink conditioning at 1 month of age. The results indicate that the cerebellum is vulnerable to perinatal insults and that therapies targeting cerebellar inflammation and injury may help in improving outcomes and function.