Generation of a microglial developmental index in mice and in humans reveals a sex difference in maturation and immune reactivity

Key roles of glial cells in the encephalopathy of prematurity

Across the globe, approximately one in 10 babies are born preterm, that is, before 37 weeks of a typical 40 weeks of gestation. Up to 50% of preterm born infants develop brain injury, encephalopathy of prematurity (EoP), that substantially increases their risk for developing lifelong defects in motor skills and domains of learning, memory, emotional regulation, and cognition. We are still severely limited in our abilities to prevent or predict preterm birth. No longer just the "support cells," we now clearly understand that during development glia are key for building a healthy brain. Glial dysfunction is a hallmark of EoP, notably, microgliosis, astrogliosis, and oligodendrocyte injury. Our knowledge of glial biology during development is exponentially expanding but hasn't developed sufficiently for development of effective neuroregenerative therapies. This review summarizes the current state of knowledge for the roles of glia in infants with EoP and its animal models, and a description of known glial-cell interactions in the context of EoP, such as the roles for border-associated macrophages. The field of perinatal medicine is relatively small but has worked passionately to improve our understanding of the etiology of EoP coupled with detailed mechanistic studies of pre-clinical and human cohorts. A primary finding from this review is that expanding our collaborations with computational biologists, working together to understand the complexity of glial subtypes, glial maturation, and the impacts of EoP in the short and long term will be key to the design of therapies that improve outcomes.

Molecular profiling of the hippocampus of children with autism spectrum disorder

Several lines of evidence point to a key role of the hippocampus in Autism Spectrum Disorders (ASD). Altered hippocampal volume and deficits in memory for person and emotion related stimuli have been reported, along with enhanced ability for declarative memories. Mouse models have demonstrated a critical role of the hippocampus in social memory dysfunction, associated with ASD, together with decreased synaptic plasticity. Chondroitin sulfate proteoglycans (CSPGs), a family of extracellular matrix molecules, represent a potential key link between neurodevelopment, synaptic plasticity, and immune system signaling. There is a lack of information regarding the molecular pathology of the hippocampus in ASD. We conducted RNAseq profiling on postmortem human brain samples containing the hippocampus from male children with ASD (n = 7) and normal male children (3-14 yrs old), (n = 6) from the NIH NeuroBioBank. Gene expression profiling analysis implicated molecular pathways involved in extracellular matrix organization, neurodevelopment, synaptic regulation, and immune system signaling. qRT-PCR and Western blotting were used to confirm several of the top markers identified. The CSPG protein BCAN was examined with multiplex immunofluorescence to analyze cell-type specific expression of BCAN and astrocyte morphology. We observed decreased expression of synaptic proteins PSD95 (p < 0.02) and SYN1 (p < 0.02), increased expression of the extracellular matrix (ECM) protease MMP9 (p < 0.03), and decreased expression of MEF2C (p < 0.03). We also observed increased BCAN expression with astrocytes in children with ASD, together with altered astrocyte morphology. Our results point to alterations in immune system signaling, glia cell differentiation, and synaptic signaling in the hippocampus of children with ASD, together with alterations in extracellular matrix molecules. Furthermore, our results demonstrate altered expression of genes implicated in genetic studies of ASD including SYN1 and MEF2C.

Current understanding of the Alzheimer's disease-associated microbiome and therapeutic strategies

Alzheimer's disease (AD) is a fatal progressive neurodegenerative disease. Despite tremendous research efforts to understand this complex disease, the exact pathophysiology of the disease is not completely clear. Recently, anti-Aβ antibodies have been shown to remove amyloid from the brain and slow the clinical progression of mild dementia by ~30%. However, exploring alternative strategies is crucial to understanding and developing more effective therapeutic interventions. In recent years, the microbiota-gut-brain axis has received significant attention in the AD field. Numerous studies have suggested that alterations in the gut microbiota composition are associated with the progression of AD, and several underlying mechanisms have been proposed. However, studies in this area are still in their infancy, and many aspects of this field are just beginning to be explored and understood. Gaining a deeper understanding of the intricate interactions and signaling pathways involved in the microbiota-AD interaction is crucial for optimizing therapeutic strategies targeting gut microbiota to positively impact AD. In this review, we aim to summarize the current understanding of the microbiota-gut-brain axis in AD. We will discuss the existing evidence regarding the role of gut microbiota in AD pathogenesis, suggested underlying mechanisms, biological factors influencing the microbiome-gut-brain axis in AD, and remaining questions in the field. Last, we will discuss potential therapeutic approaches to recondition the community of gut microbiota to alleviate disease progression. An ongoing exploration of the gut-brain axis and the development of microbiota-based therapies hold the potential for advancing AD management in the future.

Female and male microglia are not different in the dentate gyrus of postnatal day 10 mice

Microglia, the resident immune cells of the brain, support normal brain function and the brain's response to disease and injury. The hippocampal dentate gyrus (DG) is important for microglial study due to its central role in many behavioral and cognitive functions. Interestingly, microglia and related cells are distinct in female vs. male rodents, even in early life. Indeed, postnatal day (P)-dependent sex differences in number, density, and morphology of microglia have been reported in certain hippocampal subregions at specific ages. However, sex differences in the DG have not yet been assessed at P10, a translationally relevant time point as the rodent neuroanatomical eqivalent of human term gestation. To address this knowledge gap, Iba1+ cells in the DG (which are enriched in the Hilus and Molecular Layer) in female and male C57BL/6J mice were analyzed for their number (via stereology) and density (via stereology and via sampling). Next, Iba1+ cells were classified into morphology categories previously established in the literature. Finally, the percent of Iba1+ cells in each morphology category was multiplied by total cell number to generate a total number of Iba1+ cells in each category. Results show no sex difference in Iba1+ cell number, density, or morphology in the P10 Hilus or Molecular Layer. The lack of sex difference in Iba1+ cells in P10 DG using commonly-employed methodologies (sampling, stereology, morphology classification) provides a baseline from which to interpret microglia changes seen after injury.

Potential biological contributers to the sex difference in multiple sclerosis progression

Multiple sclerosis (MS) is an immune-mediated disease that targets the myelin sheath of central nervous system (CNS) neurons leading to axon injury, neuronal death, and neurological progression. Though women are more highly susceptible to developing MS, men that develop this disease exhibit greater cognitive impairment and accumulate disability more rapidly than women. Magnetic resonance imaging and pathology studies have revealed that the greater neurological progression seen in males correlates with chronic immune activation and increased iron accumulation at the rims of chronic white matter lesions as well as more intensive whole brain and grey matter atrophy and axon loss. Studies in humans and in animal models of MS suggest that male aged microglia do not have a higher propensity for inflammation, but may become more re-active at the rim of white matter lesions as a result of the presence of pro-inflammatory T cells, greater astrocyte activation or iron release from oligodendrocytes in the males. There is also evidence that remyelination is more efficient in aged female than aged male rodents and that male neurons are more susceptible to oxidative and nitrosative stress. Both sex chromosome complement and sex hormones contribute to these sex differences in biology.

Adolescent microglia stimulation produces long-lasting protection against chronic stress-induced behavioral abnormalities in adult male mice

Our previous studies had reported that microglia activation one day before stress exposure prevented the behavioral abnormalities induced by chronic stress in adult mice, and a 10-day interval between microglia stimulation and stress exposure can abolish the prophylactic effect of LPS preinjection on the behavioral abnormalities induced by chronic stress, which, however, could be rescued by repeated LPS injection. This suggests that increased stimulation of microglia results in animals developing a strong ability to prevent deleterious stress stimuli. Because microglia in the adolescent brain exhibit flexible immunological plasticity, we hypothesize that a single low-dose LPS injection during adolescence may provide long-lasting protection against behavioral abnormalities induced by chronic stress in adult mice. As expected, our results showed that a single injection of LPS (100 μg/kg) at post-natal day 28 (PND 28) prevented the development of abnormal behaviors and shifted neuroinflammatory responses toward an anti-inflammatory phenotype in adult mice treated with CSDS at their different stages of the age (PND 56, 140, and 252). Moreover, pretreatment with minocycline or PLX3397 to inhibit microglial activation abolished the prophylactic effect of LPS preinjection after PND 28 on behavioral abnormalities and neuroinflammatory responses induced by CSDS in adult mice at their different stages of the age, PND 56, 140, and 252. These results indicate that stimulation of microglia in adolescence may confer long-lasting protection against neuroinflammatory responses and behavioral abnormalities induced by chronic stress in adult mice. This may offer the potential for the development of a "vaccine-like strategy" to prevent mental disorders.

Immune signaling as a node of interaction between systems that sex-specifically develop during puberty and adolescence

Adolescence is pivotal for neural and behavioral development across species. During this period, maturation occurs in several biological systems, the most well-recognized being activation of the hypothalamic-pituitary-gonadal axis marking pubertal onset. Increasing comparative studies of sex differences have enriched our understanding of systems integration during neurodevelopment. In recent years, immune signaling has emerged as a key node of interaction between a variety of biological signaling processes. Herein, we review the age- and sex-specific changes that occur in neural, hypothalamic-pituitary, and microbiome systems during adolescence. We then describe how immune signaling interacts with these systems, and review recent preclinical evidence indicating that immune signaling may play a central role in integrating changes in their typical and atypical development during adolescence. Finally, we discuss the translational relevance of these preclinical studies to human health and wellness.

A Microglial Function for the Nerve Growth Factor: Predictions of the Unpredictable

Microglia are the only immune cell population present in the brain parenchyma. Their vantage position in the central nervous system (CNS) enables these myeloid cells to perform the most disparate of tasks: from the classical immune functions of fighting infections and surveilling the extracellular space for pathogens and damage, to sculpting the neuronal circuitry by pruning unnecessary synapses and assisting neurons in spine formation, aiding in the maintenance of brain homeostasis. The neurotrophin field has always been dominated by the neurocentric view that the primary target of these molecules must be neurons: this holds true even for the Nerve Growth Factor (NGF), which owes its popularity in the neuroscience community to its trophic and tropic activity towards sensory and sympathetic neurons in the peripheral nervous system, and cholinergic neurons in the CNS. The increasing evidence that microglia are an integral part of neuronal computation calls for a closer look as to whether these glial cells are capable of responding directly to NGF. In this review, we will first outline evidence in support of a role for NGF as a molecule mediating neuroimmune communication. Then, we will illustrate some of those non-immune features that have made microglial cells one of the hottest topics of this last decade. In conclusion, we will discuss evidence in support of a microglial function for NGF.

The E3 ubiquitin ligase RNF216/TRIAD3 is a key coordinator of the hypothalamic-pituitary-gonadal axis

Recessive mutations in RNF216/TRIAD3 cause Gordon Holmes syndrome (GHS), in which dysfunction of the hypothalamic-pituitary-gonadal (HPG) axis and neurodegeneration are thought to be core phenotypes. We knocked out Rnf216/Triad3 in a gonadotropin-releasing hormone (GnRH) hypothalamic cell line. Rnf216/Triad3 knockout (KO) cells had decreased steady-state GnRH and calcium transients. Rnf216/Triad3 KO adult mice had reductions in GnRH neuron soma size and GnRH production without changes in neuron densities. In addition, KO male mice had smaller testicular volumes that were accompanied by an abnormal release of inhibin B and follicle-stimulating hormone, whereas KO females exhibited irregular estrous cycling. KO males, but not females, had reactive microglia in the hypothalamus. Conditional deletion of Rnf216/Triad3 in neural stem cells caused abnormal microglia expression in males, but reproductive function remained unaffected. Our findings show that dysfunction of RNF216/TRIAD3 affects the HPG axis and microglia in a region- and sex-dependent manner, implicating sex-specific therapeutic interventions for GHS.

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Rnf216/Triad3 controls GnRH production and intrinsic hypothalamic cell activity

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Rnf216/Triad3 knockout male mice have greater reproductive impairments than females

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Rnf216/Triad3 controls the HPG axis differently in males and females

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Rnf216/Triad3 knockout male mice have reactive microglia in the hypothalamus

Immune Cell Contributors to the Female Sex Bias in Multiple Sclerosis and Experimental Autoimmune Encephalomyelitis

Multiple sclerosis (MS) is a chronic, autoimmune, demyelinating disease of the central nervous system (CNS) that leads to axonal damage and accumulation of disability. Relapsing-remitting MS (RR-MS) is the most frequent presentation of MS and this form of MS is three times more prevalent in females than in males. This female bias in MS is apparent only after puberty, suggesting a role for sex hormones in this regulation; however, very little is known of the biological mechanisms that underpin the sex difference in MS onset. Experimental autoimmune encephalomyelitis (EAE) is an animal model of RR-MS that presents more severely in females in certain mouse strains and thus has been useful to study sex differences in CNS autoimmunity. Here, we overview the immunopathogenesis of MS and EAE and how immune mechanisms in these diseases differ between a male and female. We further describe how females exhibit more robust myelin-specific T helper (Th) 1 immunity in MS and EAE and how this sex bias in Th cells is conveyed by sex hormone effects on the T cells, antigen presenting cells, regulatory T cells, and innate lymphoid cell populations.

Exploring Sex-Related Differences in Microglia May Be a Game-Changer in Precision Medicine

One area of microglial biology that has been relatively neglected until recently is sex differences and this is in spite of the fact that sex is a risk factor in several diseases that are characterized by neuroinflammation and, by extension, microglial activation. Why these sex differences exist is not known but the panoply of differences extend to microglial number, genotype and phenotype. Significantly, several of these sex-related differences are also evident in health and change during life emphasizing the dynamic and plastic nature of microglia. This review will consider how age impacts on sex-related differences in microglia and ask whether the advancement of personalized medicine demands that a greater focus is placed on studying sex-related differences in microglia in Alzheimer's disease, Parkinson's disease and models of inflammatory stress and trauma in order to make true progress in dealing with these conditions.

Microglial metabolic flexibility: emerging roles for lactate

Microglia, the resident macrophages of the central nervous system (CNS), play important functions in the healthy and diseased brain. In the emerging field of immunometabolism, progress has been made in understanding how cellular metabolism can orchestrate the key responses of tissue macrophages, such as phagocytosis and inflammation. However, very little is known about the metabolic control of microglia. Lactate, now recognized as a crucial metabolite and a central substrate in metabolic flexibility, is emerging not only as a novel bioenergetic fuel for microglial metabolism but also as a potential modulator of cellular function. Parallels with macrophages will help in understanding how microglial lactate metabolism is implicated in brain physiology and pathology, and how it could be targeted for therapeutic purposes.

Sex Differences in Alzheimer's Disease: Insights From the Multiomics Landscape

Alzheimer's disease (AD) has complex etiologies, and the impact of sex on AD varies over the course of disease development. The literature provides some evidence of sex-specific contributions to AD. However, molecular mechanisms of sex-biased differences in AD remain elusive. Multiomics data in tandem with systems biology approaches offer a new avenue to dissect sex-stratified molecular mechanisms of AD and to develop sex-specific diagnostic and therapeutic strategies for AD. Single-cell transcriptomic datasets and cell deconvolution of bulk tissue transcriptomic data provide additional insights into brain cell type-specific impact on sex-biased differences in AD. In this review, we summarize the impact of sex chromosomes and sex hormones on AD, the impact of sex-biased differences during AD development, and the interplay between sex and a major AD genetic risk factor, the APOE ε4 genotype, through the multiomics landscape. Several sex-biased molecular pathways such as neuroinflammation and bioenergetic metabolism have been identified. The importance of sex chromosome and sex hormones, as well as the associated pathways in AD pathogenesis, is further strengthened by findings from omics studies. Future research efforts should integrate the multiomics data from different brain regions and different cell types using systems biology approaches, and leverage the knowledge into a holistic examination of sex differences in AD. Advances in systems biology technologies and increasingly available large-scale multiomics datasets will facilitate future studies dissecting such complex signaling mechanisms to better understand AD pathogenesis in both sexes, with the ultimate goals of developing efficacious sex- and APOE-stratified preventive and therapeutic interventions for AD.

Microglia regulation of synaptic plasticity and learning and memory

Microglia are the resident macrophages of the central nervous system. Microglia possess varied morphologies and functions. Under normal physiological conditions, microglia mainly exist in a resting state and constantly monitor their microenvironment and survey neuronal and synaptic activity. Through the C1q, C3 and CR3 “Eat Me” and CD47 and SIRPα “Don’t Eat Me” complement pathways, as well as other pathways such as CX3CR1 signaling, resting microglia regulate synaptic pruning, a process crucial for the promotion of synapse formation and the regulation of neuronal activity and synaptic plasticity. By mediating synaptic pruning, resting microglia play an important role in the regulation of experience-dependent plasticity in the barrel cortex and visual cortex after whisker removal or monocular deprivation, and also in the regulation of learning and memory, including the modulation of memory strength, forgetfulness, and memory quality. As a response to brain injury, infection or neuroinflammation, microglia become activated and increase in number. Activated microglia change to an amoeboid shape, migrate to sites of inflammation and secrete proteins such as cytokines, chemokines and reactive oxygen species. These molecules released by microglia can lead to synaptic plasticity and learning and memory deficits associated with aging, Alzheimer’s disease, traumatic brain injury, HIV-associated neurocognitive disorder, and other neurological or mental disorders such as autism, depression and post-traumatic stress disorder. With a focus mainly on recently published literature, here we reviewed the studies investigating the role of resting microglia in synaptic plasticity and learning and memory, as well as how activated microglia modulate disease-related plasticity and learning and memory deficits. By summarizing the function of microglia in these processes, we aim to provide an overview of microglia regulation of synaptic plasticity and learning and memory, and to discuss the possibility of microglia manipulation as a therapeutic to ameliorate cognitive deficits associated with aging, Alzheimer’s disease, traumatic brain injury, HIV-associated neurocognitive disorder, and mental disorders.

Microglial heterogeneity in aging and Alzheimer's disease: Is sex relevant?

Neurodegenerative diseases and their associated cognitive decline are known to be more prevalent during aging. Recent evidence has uncovered the role of microglia, the immunocompetent cells of the brain, in dysfunctions linked to neurodegenerative diseases such as is Alzheimer's disease (AD). Similar to other pathologies, AD is shown to be sex-biased, with females being more at risk compared to males. While the mechanisms driving this prevalence are still unclear, emerging data suggest the sex differences present in microglia throughout life might lead to different responses of these cells in both health and disease. Furthermore, microglial cells have recently been recognized as a deeply heterogeneous population, with multiple subsets and/or phenotypes stemming from diverse parameters such as age, sex or state of health. Therefore, this review discusses microglial heterogeneity during aging in both basal conditions and AD with a focus on existing sex differences in this process.

A Comparative Biology of Microglia Across Species

Microglia are unique brain-resident, myeloid cells. They have received growing interest for their implication in an increasing number of neurodevelopmental, acute injury, and neurodegenerative disorders of the central nervous system (CNS). Fate-mapping studies establish microglial ontogeny from the periphery during development, while recent transcriptomic studies highlight microglial identity as distinct from other CNS cells and peripheral myeloid cells. This evidence for a unique microglial ontogeny and identity raises questions regarding their identity and functions across species. This review will examine the available evidence for microglia in invertebrate and vertebrate species to clarify similarities and differences in microglial identity, ontogeny, and physiology across species. This discussion highlights conserved and divergent microglial properties through evolution. Finally, we suggest several interesting research directions from an evolutionary perspective to adequately understand the significance of microglia emergence. A proper appreciation of microglia from this perspective could inform the development of specific therapies geared at targeting microglia in various pathologies.

Microglia-Mediated Neurodegeneration in Perinatal Brain Injuries

Perinatal brain injuries, including encephalopathy related to fetal growth restriction, encephalopathy of prematurity, neonatal encephalopathy of the term neonate, and neonatal stroke, are a major cause of neurodevelopmental disorders. They trigger cellular and molecular cascades that lead in many cases to permanent motor, cognitive, and/or behavioral deficits. Damage includes neuronal degeneration, selective loss of subclasses of interneurons, blocked maturation of oligodendrocyte progenitor cells leading to dysmyelination, axonopathy and very likely synaptopathy, leading to impaired connectivity. The nature and severity of changes vary according to the type and severity of insult and maturation stage of the brain. Microglial activation has been demonstrated almost ubiquitously in perinatal brain injuries and these responses are key cell orchestrators of brain pathology but also attempts at repair. These divergent roles are facilitated by a diverse suite of transcriptional profiles and through a complex dialogue with other brain cell types. Adding to the complexity of understanding microglia and how to modulate them to protect the brain is that these cells have their own developmental stages, enabling them to be key participants in brain building. Of note, not only do microglia help build the brain and respond to brain injury, but they are a key cell in the transduction of systemic inflammation into neuroinflammation. Systemic inflammatory exposure is a key risk factor for poor neurodevelopmental outcomes in preterm born infants. Based on these observations, microglia appear as a key cell target for neuroprotection in perinatal brain injuries. Numerous strategies have been developed experimentally to modulate microglia and attenuate brain injury based on these strong supporting data and we will summarize these.

Deletion of muscarinic acetylcholine receptor 3 in microglia impacts brain ischemic injury

Microglia are the immune cells of the brain and become activated during any type of brain injury. In the middle cerebral artery occlusion (MCAo) model, a mouse model for ischemic stroke, we have previously shown that microglia and invaded monocytes upregulate the expression of the muscarinic acetylcholine receptor 3 (M3R) in the ischemic lesion. Here we tested whether this upregulation has an impact on the pathogenesis of MCAo. We depleted the m3R receptor in microglia, but not in circulating monocytes by giving tamoxifen to CX3CR1-CreERT^+/+M3R^flox/flox (M3RKO^mi) animals 3 weeks prior to MCAo. We found that M3RKO^mi male mice had bigger lesions, more pronounced motor deficits after one week and cognitive deficits after about one month compared to control males. The density of Iba1⁺ cells was lower in the lesions of M3RKO male mice in the early, but not in the late disease phase. In females, these differences were not significant. By giving tamoxifen 1 week prior to MCAo, we depleted m3R in microglia and in circulating monocytes (M3RKO^mi/mo). Male M3RKO^mi/mo did not differ in lesion size, but had a lower survival rate, showed motor deficits and a reduced accumulation of Iba1⁺ positive cells into the lesion site. In conclusion, our data suggest that the upregulation of m3R in microglia and monocytes in stroke has a beneficial effect on the clinical outcome in male mice.

Sex- and region-biased depletion of microglia/macrophages attenuates CLN1 disease in mice

BACKGROUND

The neuronal ceroid lipofuscinoses (CLN diseases) are fatal lysosomal storage diseases causing neurodegeneration in the CNS. We have previously shown that neuroinflammation comprising innate and adaptive immune reactions drives axonal damage and neuron loss in the CNS of palmitoyl protein thioesterase 1-deficient (Ppt1-/-) mice, a model of the infantile form of the diseases (CLN1). Therefore, we here explore whether pharmacological targeting of innate immune cells modifies disease outcome in CLN1 mice.

METHODS

We applied treatment with PLX3397 (150 ppm in the chow), a potent inhibitor of the colony stimulating factor-1 receptor (CSF-1R) to target innate immune cells in CLN1 mice. Experimental long-term treatment was non-invasively monitored by longitudinal optical coherence tomography and rotarod analysis, as well as analysis of visual acuity, myoclonic jerks, and survival. Treatment effects regarding neuroinflammation, neural damage, and neurodegeneration were subsequently analyzed by histology and immunohistochemistry.

RESULTS

We show that PLX3397 treatment attenuates neuroinflammation in CLN1 mice by depleting pro-inflammatory microglia/macrophages. This leads to a reduction of T lymphocyte recruitment, an amelioration of axon damage and neuron loss in the retinotectal system, as well as reduced thinning of the inner retina and total brain atrophy. Accordingly, long-term treatment with the inhibitor also ameliorates clinical outcomes in CLN1 mice, such as impaired motor coordination, visual acuity, and myoclonic jerks. However, we detected a sex- and region-biased efficacy of CSF-1R inhibition, with male microglia/macrophages showing higher responsiveness toward depletion, especially in the gray matter of the CNS. This results in a better treatment outcome in male Ppt1-/- mice regarding some histopathological and clinical readouts and reflects heterogeneity of innate immune reactions in the diseased CNS.

CONCLUSIONS

Our results demonstrate a detrimental impact of innate immune reactions in the CNS of CLN1 mice. These findings provide insights into CLN pathogenesis and may guide in the design of immunomodulatory treatment strategies.

Potential microglia-based interventions for stroke

A large number of families worldwide suffer from the physical and mental burden posed by stroke. An increasing number of studies aimed at the prevention and treatment of stroke have been conducted. Specifically, manipulating the immune response to stroke is under intense investigation. Microglia are the principal immune cells in the brain and are the first line of defense against the pathophysiology induced by stroke. Increasing evidence has suggested that microglia play diverse roles that depend on dynamic interactions with neurons, astrocytes, and other neighboring cells both in the normal brain and under pathological conditions, including stroke. Moreover, there are dynamic alterations in microglial functions with respect to aging and sex differences in the human brain, which offer a deep understanding of the conditions of stroke patients of different ages and sex. Hence, we review the dynamic microglial reactions caused by aging, sex, and crosstalk with neighboring cells both in normal conditions and after stroke and relevant potential interventions.

Development and sensory experience dependent regulation of microglia in barrel cortex

The barrel cortex is within the primary somatosensory cortex of the rodent, and processes signals from the vibrissae. Much focus has been devoted to the function of neurons, more recently, the role of glial cells in the processing of sensory input has gained increasing interest. Microglia are the principal immune cells of the nervous system that survey and regulate the cellular constituents of the dynamic nervous system. We investigated the normal and disrupted development of microglia in barrel cortex by chronically depriving sensory signals via whisker trimming for the animals' first postnatal month. Using immunohistochemistry to label microglia, we performed morphological reconstructions as well as densitometry analyses as a function of developmental age and sensory experience. Findings suggest that both developmental age and sensory experience has profound impact on microglia morphology. Following chronic sensory deprivation, microglia undergo a morphological transition from a monitoring or resting state to an altered morphological state, by exhibiting expanded cell body size and retracted processes. Sensory restoration via whisker regrowth returns these morphological alterations back to age-matched control values. Our results indicate that microglia may be recruited to participate in the modulation of neuronal structural remodeling during developmental critical periods and in response to alteration in sensory input.

ImmGen report: sexual dimorphism in the immune system transcriptome

Sexual dimorphism in the mammalian immune system is manifested as more frequent and severe infectious diseases in males and, on the other hand, higher rates of autoimmune disease in females, yet insights underlying those differences are still lacking. Here we characterize sex differences in the immune system by RNA and ATAC sequence profiling of untreated and interferon-induced immune cell types in male and female mice. We detect very few differentially expressed genes between male and female immune cells except in macrophages from three different tissues. Accordingly, very few genomic regions display differences in accessibility between sexes. Transcriptional sexual dimorphism in macrophages is mediated by genes of innate immune pathways, and increases after interferon stimulation. Thus, the stronger immune response of females may be due to more activated innate immune pathways prior to pathogen invasion.

Sexual dimorphism is observed frequently in immune disorders, but the underlying insights are still unclear. Here the authors analyze transcriptome and epigenome changes induced by interferon in various mouse immune cell types, and find only a restricted set of sexual dimorphism genes in innate immunity and macrophages.

Emerging roles for hypothalamic microglia as regulators of physiological homeostasis

The hypothalamus is a crucial brain region that responds to external stressors and functions to maintain physiological homeostatic processes, such as core body temperature and energy balance. The hypothalamus regulates homeostasis by producing hormones that thereby influence the production of other hormones that then control the internal milieu of the body. Microglia are resident macrophages and phagocytic immune cells of the central nervous system (CNS), classically known for surveying the brain's environment, responding to neural insults, and disposing of cellular debris. Recent evidence has shown that microglia are also responsive to external stressors and can influence both the development and function of the hypothalamus in a sex-dependent manner. This emerging microglia-hypothalamic interaction raises the intriguing notion that microglia might play an unappreciated role in hypothalamic control of physiological homeostasis. In this review, we briefly outline how the hypothalamus regulates physiological homeostasis and then describe how this literature overlaps with our understanding of microglia's role in the CNS. We also outline the current literature demonstrating how microglia loss or activation affects the hypothalamus, and ultimately homeostasis. We conclude by proposing how microglia could be key regulators of homeostatic processes by sensing cues external to the CNS and transmitting them through the hypothalamus.

Neuroinflammation as a risk factor for attention deficit hyperactivity disorder

Attention Deficit Hyperactivity Disorder (ADHD) is a persistent, and impairing pediatric-onset neurodevelopmental condition. Its high prevalence, and recurrent controversy over its widespread identification and treatment, drive strong interest in its etiology and mechanisms. Emerging evidence for a role for neuroinflammation in ADHD pathophysiology is of great interest. This evidence includes 1) the above-chance comorbidity of ADHD with inflammatory and autoimmune disorders, 2) initial studies indicating an association with ADHD and increased serum cytokines, 3) preliminary evidence from genetic studies demonstrating associations between polymorphisms in genes associated with inflammatory pathways and ADHD, 4) emerging evidence that early life exposure to environmental factors may increase risk for ADHD via an inflammatory mechanism, and 5) mechanistic evidence from animal models of maternal immune activation documenting behavioral and neural outcomes consistent with ADHD. Prenatal exposure to inflammation is associated with changes in offspring brain development including reductions in cortical gray matter volume and the volume of certain cortical areas -parallel to observations associated with ADHD. Alterations in neurotransmitter systems, including the dopaminergic, serotonergic and glutamatergic systems, are observed in ADHD populations. Animal models provide strong evidence that development and function of these neurotransmitters systems are sensitive to exposure to in utero inflammation. In summary, accumulating evidence from human studies and animal models, while still incomplete, support a potential role for neuroinflammation in the pathophysiology of ADHD. Confirmation of this association and the underlying mechanisms have become valuable targets for research. If confirmed, such a picture may be important in opening new intervention routes.

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.

Depletion of embryonic microglia using the CSF1R inhibitor PLX5622 has adverse sex-specific effects on mice, including accelerated weight gain, hyperactivity and anxiolytic-like behaviour

Microglia are the resident immune cells in the central nervous system (CNS). Originally thought to be primarily responsible for disposing of cellular debris and responding to neural insults, emerging research now shows that microglia are highly dynamic cells involved in a variety of neurodevelopmental processes. The hypothalamus is a brain region critical for maintaining homeostatic processes such as energy balance, thirst, food intake, reproduction, and circadian rhythms. Given that microglia colonize the embryonic brain alongside key steps of hypothalamic development, here we tested whether microglia are required for the proper establishment of this brain region. The Colony-stimulating factor-1 receptor (Csf1r) is expressed by microglia, macrophages and osteoclasts, and is required for their proliferation, differentiation, and survival. Therefore, to eliminate microglia from the fetal brain, we treated pregnant dams with the CSF1R inhibitor PLX5622. We showed that approximately 99% of microglia were eliminated by embryonic day 15.5 (E15.5) after pregnant dams were placed on a PLX5622 diet starting at E3.5. Following microglia depletion, we observed elevated numbers of apoptotic cells accumulating throughout the developing hypothalamus. Once the PLX5622 diet was removed, microglia repopulated the postnatal brain within 7 days and did not appear to repopulate from Nestin+ precursors. Embryonic microglia depletion also resulted in a decreased litter size, as well as an increase in the number of pups that died within the first two postnatal days of life. In pups that survived, the elimination of microglia in the fetal brain resulted in a decrease in the number of Pro-opiomelanocortin (POMC) neurons and a concomitant accelerated weight gain starting at postnatal day 5 (P5), suggesting that microglia could be important for the development of cell types key to hypothalamic satiety centers. Moreover, surviving PLX5622 exposed animals displayed craniofacial and dental abnormalities, perhaps due to non-CNS effects of PLX5622 on macrophages and/or osteoclasts. Finally, depletion of microglia during embryogenesis had long-term sex-specific effects on behaviour, including the development of hyperactivity and anxiolytic-like behaviour in juvenile and adult female mice, respectively. Together, these data demonstrate an important role for microglia during the development of the embryonic hypothalamus, and perhaps the CNS more broadly.

Sex, Gut, and Microglia

Microglia are brain-resident macrophages whose function affects a myriad of physiological processes and can in turn be affected by peripheral factors. In a recent issue of Cell, Garel, Ginhoux and colleagues describe how gender, developmental stage, and microbiome contribute to the transcriptome of microglia (Thion et al., 2018).