Microglia and memory: modulation by early-life infection

Reduced parenteral glucose supply during preterm neonatal infection attenuates neurological and renal pathology associated with modulation of innate and Th1 immunity

BACKGROUND

Premature infants are highly susceptible to infections that can lead to sepsis with life-threatening organ dysfunctions. The clinical practice of high parenteral glucose supply in preterm infants can exacerbate infection outcomes through excessive glycolysis-induced inflammatory response. This in turn can affect the health of vital preterm organs, including the brain and kidneys. We hypothesized that reduced parenteral glucose supply to infected preterm newborns may help protect against pathology in these two key organs.

METHODS

Cesarean-delivered preterm pigs were nourished with high or low parenteral glucose levels (21 % vs. 5 %), infused with Staphylococcus epidermidis or saline, and cared in heated, oxygenated incubators for until 22 h. Blood, brain, and kidney samples were collected for histological, immunohistological, q-PCR, ELISA, and biochemical analyses.

RESULTS

Infection led to multiple pathological changes (e.g. edema), increased inflammation and tissue injury (indicated by gene expression data) in both brain and kidneys of preterm piglets. Reduced glucose supply in infected animals alleviated histopathological manifestations in the brain, and reduced brain inflammation with enhanced M2 microglial phenotype. Reduced glucose supply also decreased plasma creatinine, and the severity of renal edema, tubular vacuolization and dilatation. Multiple genes related to innate and Th1 immunity in both organs were dampened by reduced glucose supply. Correlation analysis showed that renal inflammation was more closely connected to systemic inflammation compared to neuroinflammation.

CONCLUSION

Reduced glucose supply can reduce neural and renal inflammation after infection, thereby protecting brain and kidney health in infected preterm neonates.

Brain interleukins and Alzheimer's disease

The central nervous system (CNS) is immune-privileged by several immuno-modulators as interleukins (ILs). ILs are cytokines secreted by immune cells for cell-cell signaling communications and affect the functions of the CNS. ILs were reported to orchestrate different molecular and cellular mechanisms of both physiological and pathological events, through overproduction or over-expression of their receptors. They interact with numerous receptors mediating pro-inflammatory and/or anti-inflammatory actions. Interleukins have been implicated to participate in neurodegenerative diseases. They play a critical role in Alzheimer's disease (AD) pathology which is characterized by the over-production of pro-inflammatory ILs. These may aggravate neurodegeneration, in addition to their contribution to detrimental mechanisms as oxidative stress, and excitotoxicity. However, recent research on the relation between ILs and AD revealed major discrepancies. Most of the major ILs were shown to play both pro- and anti-inflammatory roles in different experimental settings and models. The interactions between different ILs through shared pathways also add to the difficulty of drawing solid conclusions. In addition, targeting the different ILs has not yielded consistent results. The repeated failures of therapeutic drugs in treating AD necessitate the search for novel agents targeting multiple mechanisms of the disease pathology. In this context, the understanding of interleukins and their roles throughout the disease progression and interaction with other systems in the brain may provide promising therapeutic targets for the prevention or treatment of AD.

Role of Maternal Immune Factors in Neuroimmunology of Brain Development

Inflammation during pregnancy may occur due to various factors. This condition, in which maternal immune system activation occurs, can affect fetal brain development and be related to neurodevelopmental diseases. MIA interacts with the fetus's brain development through maternal antibodies, cytokines, chemokines, and microglial cells. Antibodies are associated with the development of the nervous system by two mechanisms: direct binding to brain inflammatory factors and binding to brain antigens. Cytokines and chemokines have an active presence in inflammatory processes. Additionally, glial cells, defenders of the nervous system, play an essential role in synaptic modulation and neurogenesis. Maternal infections during pregnancy are the most critical factors related to MIA; however, several studies show the relation between these infections and neurodevelopmental diseases. Infection with specific viruses, such as Zika, cytomegalovirus, influenza A, and SARS-CoV-2, has revealed effects on neurodevelopment and the onset of diseases such as schizophrenia and autism. We review the relationship between maternal infections during pregnancy and their impact on neurodevelopmental processes.

A Brief Historic Review of Research on Early Life Stress and Inflammation across the Lifespan

BACKGROUND

Extensive evidence from animal and human studies indicates that exposure to stress during sensitive developmental periods significantly increases the risk for psychiatric and physical disorders, resulting in reduced longevity. Chronic immune activation has been suggested as one pathway through which early adverse experiences may become biologically embedded. This paper highlights selected key findings and questions that first emerged in the literature and founded the field and then examines how research methods and questions have evolved over time.

SUMMARY

During the past decades, evidence from preclinical, clinical, and epidemiological studies has accumulated suggesting consequences of early life stress (ELS) exposure for immune function, particularly increased chronic inflammation or inflammatory responses. Scientific approaches to study the effects of ELS on the immune system have changed since the first studies on this topic were published.

KEY MESSAGES

Across different study designs, species, and methods, a consistent association between childhood adversity and a pro-inflammatory phenotype has been reported. We critically discuss which topics warrant further consideration and how current findings could be used to develop targeted interventions to prevent or reverse the biological embedding of ELS and resultant disease manifestations.

Developmental functions of microglia: Impact of psychosocial and physiological early life stress

Microglia play numerous important roles in brain development. From early embryonic stages through adolescence, these immune cells influence neuronal genesis and maturation, guide connectivity, and shape brain circuits. They also interact with other glial cells and structures, influencing the brain's supportive microenvironment. While this central role makes microglia essential, it means that early life perturbations to microglia can have widespread effects on brain development, potentially resulting in long-lasting behavioral impairments. Here, we will focus on the effects of early life psychosocial versus physiological stressors in rodent models. Psychosocial stress refers to perceived threats that lead to stress axes activation, including prenatal stress, or chronic postnatal stress, including maternal separation and resource scarcity. Physiological stress refers to physical threats, including maternal immune activation, postnatal infection, and traumatic brain injury. Differing sources of early life stress have varied impacts on microglia, and these effects are moderated by factors such as developmental age, brain region, and sex. Overall, these stressors appear to either 1) upregulate basal microglia numbers and activity throughout the lifespan, while possibly blunting their responsivity to subsequent stressors, or 2) shift the developmental curve of microglia, resulting in differential timing and function, impacting the critical periods they govern. Either could contribute to behavioral dysfunctions that occur after the resolution of early life stress. Exploring how different stressors impact microglia, as well as how multiple stressors interact to alter microglia's developmental functions, could deepen our understanding of how early life stress changes the brain's developmental trajectory. This article is part of the Special Issue on "Microglia".

Enhancing aggression in Henan gamecocks via augmentation of serotonergic-dopaminergic signaling and attenuation of neuroimmune response

Animal aggression is one of the most conserved behaviors. Excessive and inappropriate aggression was a serious social concern across species. After long-term selection under strict stress conditions, Henan gamecock serves as a good model for studying aggressive behavior. In this research, we constructed a Henan game chicken backcross population containing 25% Rhode Island Red (RIR), and conducted brain transcriptomics and serum metabolomics analyses on Henan gamecock (HGR) through its comparison with its female encounters (HGH) and the male backcross birds (BGR). The study revealed that seven differential metabolites in serum and 172 differentially expressed genes in the brain were commonly shared in both HGR vs. HGH and HGR vs. BGR comparisons. They exhibited the same patterns of modulation in Henan gamecocks, following either HGH < HGR > BGR or HGH > HGR < BGR style. Therein, some neurological genes involving in serotonergic and dopaminergic signaling were upregulated, while the levels of many genes related with neuro-immune function were decreased in Henan gamecock. In addition, many unknown genes specifically or highly expressed in the brain of the Henan gamecock were identified. These genes are potentially key candidates for enhancing the bird's aggression. Multi-omics joint analysis revealed that tyrosine metabolism and neuroactive ligand-receptor interaction were commonly affected. Overall, our results propose that the aggressiveness of Henan gamecocks can be heightened by the activation of the serotonergic-dopaminergic metabolic process in the brain, which concurrently impairs the neuroimmune system. Further research is needed to identify the function of these unknown genes on the bird's aggressive behavior.

Using Drosophila amyloid toxicity models to study Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia and is characterised by a progressive loss of neurons, which manifests as gradual memory decline, followed by cognitive loss. Despite the significant progress in identifying novel biomarkers and understanding the prodromal pathology and symptomatology, AD remains a significant unmet clinical need. Lecanemab and aducanumab, the only Food and Drug Administration approved drugs to exhibit some disease-modifying clinical efficacy, target Aβ amyloid, underscoring the importance of this protein in disease aetiology. Nevertheless, in the absence of a definitive cure, the utilisation of preclinical models remains imperative for the identification of novel therapeutic targets and the evaluation of potential therapeutic agents. Drosophila melanogaster is a model system that can be used as a research tool to investigate neurodegeneration and therapeutic interventions. The short lifespan, low price and ease of husbandry/rearing make Drosophila an advantageous model organism from a practical perspective. However, it is the highly conserved genome and similarity of Drosophila and human neurobiology which make flies a powerful tool to investigate neurodegenerative mechanisms. In addition, the ease of transgenic modifications allows for early proof of principle studies for future therapeutic approaches in neurodegenerative research. This mini review will specifically focus on utilising Drosophila as an in vivo model of amyloid toxicity in AD.

Contribution of microglia to the epileptiform activity that results from neonatal hypoxia

Microglia are described as the immune cells of the brain, their immune properties have been extensively studied since first described, however, their neural functions have only been explored over the last decade. Microglia have an important role in maintaining homeostasis in the central nervous system by surveying their surroundings to detect pathogens or damage cells. While these are the classical functions described for microglia, more recently their neural functions have been defined; they are critical to the maturation of neurons during embryonic and postnatal development, phagocytic microglia remove excess synapses during development, a process called synaptic pruning, which is important to overall neural maturation. Furthermore, microglia can respond to neuronal activity and, together with astrocytes, can regulate neural activity, contributing to the equilibrium between excitation and inhibition through a feedback loop. Hypoxia at birth is a serious neurological condition that disrupts normal brain function resulting in seizures and epilepsy later in life. Evidence has shown that microglia may contribute to this hyperexcitability after neonatal hypoxia. This review will summarize the existing data on the role of microglia in the pathogenesis of neonatal hypoxia and the plausible mechanisms that contribute to the development of hyperexcitability after hypoxia in neonates. This article is part of the Special Issue on "Microglia".

Destabilization of fear memory by Rac1-driven engram-microglia communication in hippocampus

Rac1 is a key regulator of the cytoskeleton and neuronal plasticity, and is known to play a critical role in psychological and cognitive brain disorders. To elucidate the engram specific Rac1 signaling in fear memory, a doxycycline (Dox)-dependent robust activity marking (RAM) system was used to label dorsal dentate gyrus (DG) engram cells in mice during contextual fear conditioning. Rac1 mRNA and protein levels in DG engram cells were peaked at 24 h (day 1) after fear conditioning and were more abundant in the fear engram cells than in the non-engram cells. Optogenetic activation of Rac1 in a temporal manner in DG engram cells before memory retrieval decreased the freezing level in the fear context. Optogenetic activation of Rac1 increased autophagy protein 7 (ATG7) expression in the DG engram cells and activated DG microglia. Microglia-specific transcriptomics and fluorescence in situ hybridization revealed that overexpression of ATG7 in the fear engram cells upregulated the mRNA of Toll-like receptor TLR2/4 in DG microglia. Knockdown of microglial TLR2/4 rescued fear memory destabilization induced by ATG7 overexpression or Rac1 activation in DG engram cells. These results indicate that Rac1-driven communications between engram cells and microglia contributes to contextual fear memory destabilization, and is mediated by ATG7 and TLR2/4, and suggest a novel mechanistic framework for the cytoskeletal regulator in fear memory interference.

Hofbauer cells and fetal brain microglia share transcriptional profiles and responses to maternal diet-induced obesity

Maternal immune activation is associated with adverse offspring neurodevelopmental outcomes, many mediated by in utero microglial programming. As microglia remain inaccessible throughout development, identification of noninvasive biomarkers reflecting fetal brain microglial programming could permit screening and intervention. We used lineage tracing to demonstrate the shared ontogeny between fetal brain macrophages (microglia) and fetal placental macrophages (Hofbauer cells) in a mouse model of maternal diet-induced obesity, and single-cell RNA-seq to demonstrate shared transcriptional programs. Comparison with human datasets demonstrated conservation of placental resident macrophage signatures between mice and humans. Single-cell RNA-seq identified common alterations in fetal microglial and Hofbauer cell gene expression induced by maternal obesity, as well as sex differences in these alterations. We propose that Hofbauer cells, which are easily accessible at birth, provide insights into fetal brain microglial programs and may facilitate the early identification of offspring vulnerable to neurodevelopmental disorders.

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.

The brain, gut, and bladder health nexus: A conceptual model linking stress and mental health disorders to overactive bladder in women

OBJECTIVE

A small, but growing literature links stressors and mental health disorders (MHDs) across the life course to overactive bladder (OAB) and urinary incontinence symptoms. Mechanisms by which stressors and MHDs may impact bladder health are not fully understood, limiting novel prevention and treatment efforts. Moreover, potential biopsychosocial mechanisms involving the brain and gut have not been considered in an integrated, comprehensive fashion.

METHODS

Members of the prevention of lower urinary tract symptoms Research Consortium developed conceptual models to inform research on biopsychosocial mechanisms through which stress and MDHs may impact bladder health among girls and women, focusing on brain and gut physiology.

RESULTS

Two conceptual models were developed-one to explain central (brain-based) and peripheral (gut-based) mechanisms linking stressors and MHDs to OAB and bladder health, and one to highlight bidirectional communication between the brain, gut, and bladder. Traumatic events, chronic stressors, and MHDs may lead to a maladaptive stress response, including dysregulated communication and signaling between the brain, gut, and bladder. Gut bacteria produce molecules and metabolites that alter production of neurotransmitters, amino acids, short-chain fatty acids, and inflammatory immune response molecules that mediate communication between the gut and brain. Microbiota signal neurogenesis, microglia maturation, and synaptic pruning; they also calibrate brain-gut-bladder axis communication through neurotransmission and synaptogenesis, potentially influencing bladder symptom development. Life course trajectories of risk may be prevented or interrupted by central and peripheral resources for neuropsychological resilience.

CONCLUSIONS

Depicted pathways, including brain-gut-bladder communication, have implications for research and development of novel prevention and treatment approaches.

The inflammatory response to birth requires MyD88 and is driven by both mother and offspring

Birth is an inflammatory event for the newborn, characterized by elevations in interleukin (IL)-6, IL-10, and tumor necrosis factor (TNF)-α peripherally and/or centrally, as well as changes in brain microglia. However, the mechanism(s) underlying these responses is unknown. Toll-like receptors (TLRs) play crucial roles in innate immunity and initiate inflammatory cascades upon recognition of endogenous or exogenous antigens. Most TLR signaling depends on the adaptor molecule myeloid differentiation primary response 88 (MyD88). We independently varied MyD88 gene status in mouse dams and their offspring to determine whether the inflammatory response to birth depends on MyD88 signaling and, if so, whether that signaling occurs in the offspring, the mother, or both. We find that the perinatal surges in plasma IL-6 and brain expression of TNF-α depend solely on MyD88 gene status of the offspring, whereas postnatal increases in plasma IL-10 and TNF-α depend on MyD88 in both the pup and dam. Interestingly, MyD88 genotype of the dam primarily drives differences in offspring brain microglial density and has robust effects on developmental neuronal cell death. Milk cytokines were evaluated as a possible source of postnatal maternal influence; although we found high levels of CXCL1/GROα and several other cytokines in ingested post-partum milk, their presence did not require MyD88. Thus, the inflammatory response previously described in the late-term fetus and newborn depends on MyD88 (and, by extension, TLRs), with signaling in both the dam and offspring contributing. Unexpectedly, naturally-occuring neuronal cell death in the newborn is modulated primarily by maternal MyD88 gene status.

Hofbauer cells and fetal brain microglia share transcriptional profiles and responses to maternal diet-induced obesity

Maternal immune activation is associated with adverse offspring neurodevelopmental outcomes, many mediated by in utero microglial programming. As microglia remain inaccessible throughout development, identification of noninvasive biomarkers reflecting fetal brain microglial programming could permit screening and intervention. We used lineage tracing to demonstrate the shared ontogeny between fetal brain macrophages (microglia) and fetal placental macrophages (Hofbauer cells) in a mouse model of maternal diet-induced obesity, and single-cell RNA-seq to demonstrate shared transcriptional programs. Comparison with human datasets demonstrated conservation of placental resident macrophage signatures between mice and humans. Single-cell RNA-seq identified common alterations in fetal microglial and Hofbauer cell gene expression induced by maternal obesity, as well as sex differences in these alterations. We propose that Hofbauer cells, which are easily accessible at birth, provide novel insights into fetal brain microglial programs, and may facilitate the early identification of offspring vulnerable to neurodevelopmental disorders in the setting of maternal exposures.

Abnormal expression of miR-3653-3p, caspase 1, IL-1β in peripheral blood of schizophrenia

Schizophrenia (SCZ) is a chronic, highly relapsing, severe mental disorder with an unclear etiology. Cytokine-mediated neuroimmune abnormalities have been repeatedly revealed. IL-1β was reported to play a vital role in expanding the inflammatory response. However, the underlying molecular mechanism is poorly understood. In this study, we found that miR-3653-3p with the NLRP3 binding site in Targetscan was differentially expressed in miRNA high-throughput sequencing in schizophrenia (SCZ), and indeed, its downregulation in SCZ peripheral blood was also verified by RT-qPCR (P-value = 0.015). Furthermore, we found that the mRNAs of caspase 1 and IL-1β are elevated in people who suffer from SCZ (P = 0.044 and P = 0.001, respectively). Moreover, the interaction of NLRP3, Caspase1, and IL-1β was found in the peripheral blood of patients with SCZ. The expression level of miR-3653-3p was negatively correlated with NLRP3 and IL-1β mRNA contents (r = 0.487, P = 0.04 and r = 0.508, P = 0.037, respectively). NLRP3 mRNA was positively correlated with caspase1 mRNA. Meanwhile, the expression of miR-3653-3p was also negatively correlated with negative symptom subscores of PANSS (r = 0.450, P = 0.046). IL-1β mRNA is positively correlated with the total scores of PANSS (r = 0.690, P = 0.002) and the sub-scores of general psychopathology of PANSS (r = 0.583, P = 0.014). Additionally, a significant positive relationship exists between IL-1β and the total duration (r = 0.638, P = 0.006). We found that the combination of miR-3653-3p, caspase 1, and IL-1β have better diagnostic values. The results indicate that miR-3653-3p, caspase 1, and IL-1β can potentially be biomarkers of SCZ, identifying negative symptoms or a chronic course. A further understanding of the involvement of IL-1β in SCZ may be a crucial molecular effector for the chronic course to intervene.

Galectin-1 as a marker for microglia activation in the aging brain

Microglia cells, the immune cells residing in the brain, express immune regulatory molecules that have a central role in the manifestation of age-related brain characteristics. Our hypothesis suggests that galectin-1, an anti-inflammatory member of the beta-galactoside-binding lectin family, regulates microglia and neuroinflammation in the aging brain. Through our in-silico analysis, we discovered a subcluster of microglia in the aged mouse brain that exhibited increased expression of galectin-1 mRNA. In our Western blotting experiments, we observed a decrease in galectin-1 protein content in our rat primary cortical cultures over time. Additionally, we found that the presence of lipopolysaccharide, an immune activator, significantly increased the expression of galectin-1 protein in microglial cells. Utilizing flow cytometry, we determined that a portion of the galectin-1 protein was localized on the surface of the microglial cells. As cultivation time increased, we observed a decrease in the expression of activation-coupled molecules in microglial cells, indicating cellular exhaustion. In our mixed rat primary cortical cell cultures, we noted a transition of amoeboid microglial cells labeled with OX42(CD11b/c) to a ramified, branched phenotype during extended cultivation, accompanied by a complete disappearance of galectin-1 expression. By analyzing the transcriptome of a distinct microglial subpopulation in an animal model of aging, we established a correlation between chronological aging and galectin-1 expression. Furthermore, our in vitro study demonstrated that galectin-1 expression is associated with the functional activation state of microglial cells exhibiting specific amoeboid morphological characteristics. Based on our findings, we identify galectin-1 as a marker for microglia activation in the context of aging.

Involvement of IL-1β-Mediated Necroptosis in Neurodevelopment Impairment after Neonatal Sepsis in Rats

The mechanism of long-term cognitive impairment after neonatal sepsis remains poorly understood, although long-lasting neuroinflammation has been considered the primary contributor. Necroptosis is actively involved in the inflammatory process, and in this study, we aimed to determine whether neonatal sepsis-induced long-term cognitive impairment was associated with activation of necroptosis. Rat pups on postnatal day 3 (P3) received intraperitoneal injections of lipopolysaccharide (LPS, 1 mg/kg) to induce neonatal sepsis. Intracerebroventricular injection of IL-1β-siRNA and necrostatin-1 (NEC1) were performed to block the production of IL-1β and activation of necroptosis in the brain, respectively. The Morris water maze task and fear conditioning test were performed on P28-P32 and P34-P35, respectively. Enzyme-linked immunosorbent assay (ELISA), quantitative real-time PCR (RT-PCR), and Western blotting were used to examine the expression levels of proinflammatory cytokines and necroptosis-associated proteins, such as receptor-interacting protein 1 (RIP1) and receptor-interacting protein 3 (RIP3). Sustained elevation of IL-1β level was observed in the brain after initial neonatal sepsis, which would last for at least 32 days. Sustained necroptosis activation was also observed in the brain. Knockdown of IL-1β expression in the brain alleviated necroptosis and improved long-term cognitive function. Direct inhibition of necroptosis also improved neurodevelopment and cognitive performance. This research indicated that sustained activation of necroptosis via IL-1β contributed to long-term cognitive dysfunction after neonatal sepsis.

Crosstalk of Highly Purified Microglia and Astrocytes in the Frame of Toll-like Receptor (TLR)2/1 Activation

The major immune cells of the central nervous systems (CNS) are microglia and astrocytes, subsets of the glial cell population. The crosstalk between glia via soluble signaling molecules plays an indispensable role for neuropathologies, brain development as well as homeostasis. However, the investigation of the microglia-astrocyte crosstalk has been hampered due to the lack of suitable glial isolation methods. In this study, we investigated for the first time the crosstalk between highly purified Toll-like receptor (TLR)2-knock out (TLR2-KO) and wild-type (WT) microglia and astrocytes. We examined the crosstalk of TLR2-KO microglia and astrocytes in the presence of WT supernatants of the respective other glial cell type. Interestingly, we observed a significant TNF release by TLR2-KO astrocytes, which were activated with Pam3CSK4-stimulated WT microglial supernatants, strongly indicating a crosstalk between microglia and astrocytes after TLR2/1 activation. Furthermore, transcriptome analysis using RNA-seq revealed a wide range of significant up- and down-regulated genes such as Cd300, Tnfrsf9 or Lcn2, which might be involved in the molecular conversation between microglia and astrocytes. Finally, co-culturing microglia and astrocytes confirmed the prior results by demonstrating a significant TNF release by WT microglia co-cultured with TLR2-KO astrocytes. Our findings suggest a molecular TLR2/1-dependent conversation between highly pure activated microglia and astrocytes via signaling molecules. Furthermore, we demonstrate the first crosstalk experiments using ∼100% pure microglia and astrocyte mono-/co-cultures derived from mice with different genotypes highlighting the urgent need of efficient glial isolation protocols, which particularly holds true for astrocytes.

Neural response to stress differs by sex in young adulthood

Increase in stress-related disorders in women begins post-puberty and persists throughout the lifespan. To characterize sex differences in stress response in early adulthood, we used functional magnetic resonance imaging while participants underwent a stress task in conjunction with serum cortisol levels and questionnaires assessing anxiety and mood. Forty-two healthy subjects aged 18-25 years participated (21M, 21F). Interaction of stress and sex in brain activation and connectivity were examined. Results demonstrated significant sex differences in brain activity with women exhibiting increased activation in regions that inhibit arousal compared to men during the stress paradigm. Women had increased connectivity among stress circuitry regions and default mode network, whereas men had increased connectivity between stress and cognitive control regions. In a subset of subjects (13F, 17M), we obtained gamma-aminobutyric acid (GABA) magnetic resonance spectroscopy in rostral anterior cingulate cortex (rostral ACC) and dorsolateral prefrotal cortex (dlPFC) and conducted exploratory analyses to relate GABA measurements with sex differences in brain activation and connectivity. Prefrontal GABA levels were negatively associated with inferior temporal gyrus activation in men and women and with ventromedial prefrontal cortex activation in men. Despite sex differences in neural response, we found similar subjective ratings of anxiety and mood, cortisol levels, and GABA levels between sexes, suggesting sex differences in brain activity result in similar behavioral responses among the sexes. These results help establish sex differences in healthy brain activity from which we can better understand sex differences underlying stress-associated illnesses.

Transcriptional and epigenetic regulation of microglia in substance use disorders

Microglia are widely known for their role in immune surveillance and for their ability to refine neurocircuitry during development, but a growing body of evidence suggests that microglia may also play a complementary role to neurons in regulating the behavioral aspects of substance use disorders. While many of these efforts have focused on changes in microglial gene expression associated with drug-taking, epigenetic regulation of these changes has yet to be fully understood. This review provides recent evidence supporting the role of microglia in various aspects of substance use disorder, with particular focus on changes to the microglial transcriptome and the potential epigenetic mechanisms driving these changes. Further, this review discusses the latest technical advances in low-input chromatin profiling and highlights the current challenges for studying these novel molecular mechanisms in microglia.

Modelling Microglial Innate Immune Memory In Vitro: Understanding the Role of Aerobic Glycolysis in Innate Immune Memory

Microglia, the resident macrophages of the central nervous system, play important roles in maintaining brain homeostasis and facilitating the brain's innate immune responses. Following immune challenges microglia also retain immune memories, which can alter responses to secondary inflammatory challenges. Microglia have two main memory states, training and tolerance, which are associated with increased and attenuated expression of inflammatory cytokines, respectively. However, the mechanisms differentiating these two distinct states are not well understood. We investigated mechanisms underlying training versus tolerance memory paradigms in vitro in BV2 cells using B-cell-activating factor (BAFF) or bacterial lipopolysaccharide (LPS) as a priming stimulus followed by LPS as a second stimulus. BAFF followed by LPS showed enhanced responses indicative of priming, whereas LPS followed by LPS as the second stimulus caused reduced responses suggestive of tolerance. The main difference between the BAFF versus the LPS stimulus was the induction of aerobic glycolysis by LPS. Inhibiting aerobic glycolysis during the priming stimulus using sodium oxamate prevented the establishment of the tolerized memory state. In addition, tolerized microglia were unable to induce aerobic glycolysis upon LPS restimulus. Therefore, we conclude that aerobic glycolysis triggered by the first LPS stimulus was a critical step in the induction of innate immune tolerance.

Lasting consequences on physiology and social behavior following cesarean delivery in prairie voles

Cesarean delivery is associated with diminished plasma levels of several 'birth-signaling' hormones, such as oxytocin and vasopressin. These same hormones have been previously shown to exert organizational effects when acting in early life. For example, our previous work found a broadly gregarious phenotype in prairie voles exposed to oxytocin at birth. Meanwhile, cesarean delivery has been previously associated with changes in social behavior and metabolic processes related to oxytocin and vasopressin. In the present study, we investigated the long-term neurodevelopmental consequences of cesarean delivery in prairie voles. After cross-fostering, vole pups delivered either via cesarean or vaginal delivery were studied throughout development. Cesarean-delivered pups responded to isolation differently in terms of their vocalizations (albeit in opposite directions in the two experiments), huddled in less cohesive groups under warmed conditions, and shed less heat. As young adults, we observed no differences in anxiety-like or alloparental behavior. However, in adulthood, cesarean-delivered voles of both sexes failed to form partner preferences with opposite sex conspecifics. In a follow-up study, we replicated this deficit in partner-preference formation among cesarean-delivered voles and were able to normalize pair-bonding behavior by treating cesarean-delivered vole pups with oxytocin (0.25 mg/kg) at delivery. Finally, we detected minor differences in regional oxytocin receptor expression within the brains of cesarean-delivered voles, as well as microbial composition of the gut. Gene expression changes in the gut epithelium indicated that cesarean-delivered male voles have altered gut development. These results speak to the possibility of unintended developmental consequences of cesarean delivery, which currently accounts for 32.9 % of deliveries in the U.S. and suggest that further research should be directed at whether hormone replacement at delivery influences behavioral outcomes in later life.

Effects of sex and pro-inflammatory cytokines on context discrimination memory

In learning and memory tasks, immune overactivation is associated with impaired performance, while normal immune activation is associated with optimal performance. In one specific domain of memory, context discrimination memory, peripheral immune stimulation has been shown to impair performance on the context-object discrimination memory task in male rats. In order to evaluate potential sex differences in this task, as well as potential mechanisms for the memory impairment, we evaluated the ability of peripheral immune stimulation to impair task performance in both males and females. Next, we examined whether treatment with interleukin-1 receptor antagonist (IL-1ra), a receptor antagonist for the pro-inflammatory cytokine interleukin (IL)-1β, was able to rescue the memory deficit. We examined microglial morphology in the hippocampus and cytokine mRNA and protein expression in the hippocampus and the periphery. Male rats displayed memory impairment in response to LPS, and this impairment was not rescued by IL-1ra. Female rats did not have significant memory impairments and IL-1ra administration improved memory following inflammation. A subset of cytokines and chemokines were increased only in LPS-treated males. Inflammation alone did not alter microglia morphology, but IL-1ra did in certain sub-regions of the hippocampus. Together, these results indicate that sex differences exist in the ability of a peripheral immune stimulus to influence context discrimination memory and specific cytokine signals may be altered in impaired males. This study highlights the importance of sex differences in response to inflammatory challenges, especially related to memory impairments in context discrimination memory.

Maternal immune activation alters social affective behavior and sensitivity to corticotropin releasing factor in male but not female rats

Prenatal infection increases risk for neurodevelopmental disorders such as autism in offspring. In rodents, prenatal administration of the viral mimic Polyinosinic: polycytidylic acid (Poly I: C) allows for investigation of developmental consequences of gestational sickness on offspring social behavior and neural circuit function. Because maternal immune activation (MIA) disrupts cortical development and sociability, we examined approach and avoidance in a rat social affective preference (SAP) task. Following maternal Poly I:C (0.5 mg/kg) injection on gestational day 12.5, male adult offspring (PN 60-64) exhibited atypical social interactions with stressed conspecifics whereas female SAP behavior was unaffected by maternal Poly I:C. Social responses to stressed conspecifics depend upon the insular cortex where corticotropin releasing factor (CRF) modulates synaptic transmission and SAP behavior. We characterized insular field excitatory postsynaptic potentials (fEPSP) in adult offspring of Poly I:C or control treated dams. Male MIA offspring showed decreased sensitivity to CRF (300 nM) while female MIA offspring showed greater sensitivity to CRF compared to sham offspring. These sex specific effects appear to be behaviorally relevant as CRF injected into the insula of male and female rats prior to social exploration testing had no effect in MIA male offspring but increased social interaction in female MIA offspring. We examined the cellular distribution of CRF receptor mRNA but found no effect of maternal Poly I:C in the insula. Together, these experiments reveal sex specific effects of prenatal infection on offspring responses to social affective stimuli and identify insular CRF signaling as a novel neurobiological substrate for autism risk.

Astrocytes regulate neuronal network activity by mediating synapse remodeling

Based on experience during our life, neuronal connectivity continuously changes through structural remodeling of synapses. Recent studies have shown that the complex interaction between astrocytes and synapses regulates structural synapse remodeling by inducing the formation and elimination of synapses, as well as their functional maturation. Defects in this astrocyte-mediated synapse remodeling cause problems in not only neuronal network activities but also animal behaviors. Moreover, in various neurological disorders, astrocytes have been shown to play central roles in the initiation and progression of synaptic pathophysiology through impaired interactions with synapses. In this review, we will discuss recent studies identifying the novel roles of astrocytes in neuronal circuit remodeling, focusing on synapse formation and elimination. We will also discuss the potential implication of defective astrocytic function in evoking various brain disorders.

Neuroinflammation in Alzheimer's Disease: Current Progress in Molecular Signaling and Therapeutics

Alzheimer's disease, a neurodegenerative disease with amyloid beta accumulation as a major hallmark, has become a dire global health concern as there is a lack of clear understanding of the causative agent. It is a major cause of dementia which is increasing exponentially with age. Alzheimer's disease is marked by tau hyperphosphorylation and amyloid beta accumulation that robs people of their memories. Amyloid beta deposition initiated a spectrum of microglia-activated neuroinflammation, and microglia and astrocyte activation elicited expressions of various inflammatory and anti-inflammatory cytokines. Neuroinflammation is one of the cardinal features of Alzheimer's disease. Pro-inflammatory cytokine signaling plays multifarious roles in neurodegeneration and neuroprotection. Induction of proinflammatory signaling leads to discharge of immune mediators which affect functions of neurons and cause cell death. Sluggish anti-inflammatory system also contributes to neuroinflammation. Numerous pathways like NFκB, p38 MAPK, Akt/mTOR, caspase, nitric oxide, and COX are involved in triggering brain immune cells like astrocytes and microglia to secrete inflammatory cytokines such as tumor necrosis factor, interleukins, and chemokines and participate in Alzheimer's disease pathology. PPAR-γ agonists tend to boost the phagocytosis of amyloid beta and decrease the inflammatory cytokine IL-1β. Recent findings suggest the cross-link between gut microbiota and neuroinflammation contributing in AD which has been explained in this study. The role of cellular, molecular pathways and involvement of inflammatory mediators in neuroinflammation has also been described; targeting them could be a potential therapeutic strategy for treatment of AD.

Neurodevelopmental outcome of infants who develop necrotizing enterocolitis: The gut-brain axis

Necrotizing enterocolitis (NEC) poses a significant risk for neurodevelopmental impairment in extremely preterm infants. The gut microbiota shapes the development of the gut, immune system, and the brain; and dysbiosis drive neonatal morbidities including NEC. In this chapter, we delineate a gut-brain axis linking gut microbiota to the adverse neurological outcomes in NEC patients. We propose that in NEC, immaturity of the microbiome along with aberrant gut microbiota-driven immaturity of the gut barrier and immune system can lead to effects including systemic inflammation and circulating microbial mediators. This nexus of gut microbiota-driven systemic effects further interacts with a likewise underdeveloped blood-brain barrier to regulate neuroinflammation and neurodevelopment. Targeting deviant gut-brain axis signaling presents an opportunity to improve the neurodevelopmental outcomes of NEC patients.

Role of gut-brain axis in neurodevelopmental impairment of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a common gastrointestinal disease of preterm infants with high morbidity and mortality. In survivors of NEC, one of the leading causes of long-term morbidity is the development of severe neurocognitive injury. The exact pathogenesis of neurodevelopmental delay in NEC remains unknown, but microbiota is considered to have dramatic effects on the development and function of the host brain via the gut-brain axis. In this review, we discuss the characteristics of microbiota of NEC, the impaired neurological outcomes, and the role of the complex interplay between the intestinal microbiota and brain to influence neurodevelopment in NEC. The increasing knowledge of microbial-host interactions has the potential to generate novel therapies for manipulating brain development in the future.

Sex Differences in Neurodevelopmental Disorders: A Key Role for the Immune System

Sex differences are prominent defining features of neurodevelopmental disorders. Understanding the sex biases in these disorders can shed light on mechanisms leading to relative risk and resilience for the disorders, as well as more broadly advance our understanding of how sex differences may relate to brain development. The prevalence of neurodevelopmental disorders is increasing, and the two most common neurodevelopmental disorders, Autism Spectrum Disorder (ASD) and Attention-Deficit/Hyperactivity Disorder (ADHD) exhibit male-biases in prevalence rates and sex differences in symptomology. While the causes of neurodevelopmental disorders and their sex differences remain to be fully understood, increasing evidence suggests that the immune system plays a critical role in shaping development. In this chapter we discuss sex differences in prevalence and symptomology of ASD and ADHD, review sexual differentiation and immune regulation of neurodevelopment, and discuss findings from human and rodent studies of immune dysregulation and perinatal immune perturbation as they relate to potential mechanisms underlying neurodevelopmental disorders. This chapter will give an overview of how understanding sex differences in neuroimmune function in the context of neurodevelopmental disorders could lend insight into their etiologies and better treatment strategies.

Prophylactic and therapeutic insights into trained immunity: A renewed concept of innate immune memory

Trained immunity is a renewed concept of innate immune memory that facilitates the innate immune system to have the capacity to remember and train cells via metabolic and transcriptional events to enable them to provide nonspecific defense against the subsequent encounters with a range of pathogens and acquire a quicker and more robust immune response, but different from the adaptive immune memory. Reversing the epigenetic changes or targeting the immunological pathways may be considered potential therapeutic approaches to counteract the hyper-responsive or hypo-responsive state of trained immunity. The efficient regulation of immune homeostasis and promotion or inhibition of immune responses is required for a balanced response. Trained immunity-based vaccines can serve as potent immune stimuli and help in the clearance of pathogens in the body through multiple or heterologous effects and confer protection against nonspecific and specific pathogens. This review highlights various features of trained immunity and its applications in developing novel therapeutics and vaccines, along with certain detrimental effects, challenges as well as future perspectives.

A Comparison of the Relationship Between C-Reactive Protein Levels and Cognitive Functions in Patients with Schizophrenia, First-Episode Psychosis, and Healthy Controls

Background

There are several hypotheses on what causes schizophrenia, some of which include inflammatory responses. Additionally, it might be challenging to control and treat cognitive abnormalities, which represent the primary symptoms, and may be related to inflammation. This study aims to determine whether there is a relationship between C-reactive protein levels and cognitive abilities by assessing neuropsychological tests of drug-free patients with schizophrenia and first-episode psychosis.

Methods

The patient group consisted of 36 patients with schizophrenia or "first-episode psychosis," while the control group comprised 31 healthy people. The control group consisted of healthy participants without any medical or psychiatric diseases. Structured Clinical Interview for DSM-5 axis I disorders was applied for diagnosis, while Wisconsin card sorting test, Stroop color and word test, trail making tests, Rey auditory verbal learning test, and digit span test were applied for cognitive assessment of both groups. Clinical characteristics of patients were evaluated by using the Scale for the Assessment of Positive Symptoms, the Scale for the Assessment of Negative Symptoms, and the Calgary Depression Scale for Schizophrenia. The patient group and healthy control group were evaluated in terms of inflammation levels. The C-reactive protein levels were measured, and their relationship with cognitive status was examined. The serum samples were analyzed by the immunoturbidimetric method in C-reactive protein C8000 Architect (Abbott, Ill, USA) to measure the C-reactive protein levels.

Results

C-reactive protein levels were found to be higher in the patient group (P = .003), while the Scale for the Assessment of Negative Symptoms and Scale for the Assessment of Positive Symptoms scores were found to be positively correlated with C-reactive protein levels. Cognitive functions in the patient group were significantly lower compared to the healthy group. There was a statistically weak correlation between C-reactive protein and the number of word color reading errors in the Stroop test, which was associated with complex and frontal attention; however, no correlation was found with digit span test, Rey auditory verbal learning test, or Wisconsin card sorting test points.

Conclusion

Elevated peripheral levels of C-reactive protein are associated with poorer cognitive function in patients with first-episode psychosis and schizophrenia, particularly, complex attention associated with the Stroop test. Inflammation may have an impact on cognitive impairment in psychosis.

Persistent muscle hyperalgesia after adolescent stress is exacerbated by a mild-nociceptive input in adulthood and is associated with microglia activation

Non-specific low back pain (LBP) is a major global disease burden and childhood adversity predisposes to its development. The mechanisms are largely unknown. Here, we investigated if adversity in young rats augments mechanical hyperalgesia and how spinal cord microglia contribute to this. Adolescent rats underwent restraint stress, control animals were handled. In adulthood, all rats received two intramuscular injections of NGF/saline or both into the lumbar multifidus muscle. Stress induced in rats at adolescence lowered low back pressure pain threshold (PPT; p = 0.0001) and paw withdrawal threshold (PWT; p = 0.0007). The lowered muscle PPT persisted throughout adulthood (p = 0.012). A subsequent NGF in adulthood lowered only PPT (d = 0.87). Immunohistochemistry revealed changes in microglia morphology: stress followed by NGF induced a significant increase in ameboid state (p < 0.05). Repeated NGF injections without stress showed significantly increased cell size in surveilling and bushy states (p < 0.05). Thus, stress in adolescence induced persistent muscle hyperalgesia that can be enhanced by a mild-nociceptive input. The accompanying morphological changes in microglia differ between priming by adolescent stress and by nociceptive inputs. This novel rodent model shows that adolescent stress is a risk factor for the development of LBP in adulthood and that morphological changes in microglia are signs of spinal mechanisms involved.

Infection, Learning, and Memory: Focus on Immune Activation and Aversive Conditioning

Here we review the effects of immune activation primarily via lipopolysaccharide (LPS), a cell wall component of Gram-negative bacteria, on hippocampal and non-hippocampal-dependent learning and memory. Rodent studies have found that LPS alters both the acquisition and consolidation of aversive learning and memory, such as those evoking evolutionarily adaptive responses like fear and disgust. The inhibitory effects of LPS on the acquisition and consolidation of contextual fear memory are discussed. LPS-induced alterations in the acquisition of taste and place-related conditioned disgust memory within bottle preference tasks and taste reactivity tests (taste-related), in addition to conditioned context avoidance tasks and the anticipatory nausea paradigm (place-related), are highlighted. Further, conditioned disgust memory consolidation may also be influenced by LPS-induced effects. Growing evidence suggests a central role of immune activation, especially pro-inflammatory cytokine activity, in eliciting the effects described here. Understanding how infection-induced immune activation alters learning and memory is increasingly important as bacterial and viral infections are found to present a risk of learning and memory impairment.

Birth triggers an inflammatory response in the neonatal periphery and brain

Birth is preceded by inflammation at the fetal/maternal interface. Additionally, the newborn experiences stimuli that under any other circumstance could elicit an immune response. It is unknown, however, whether birth elicits an inflammatory response in the newborn that extends to the brain. Moreover, it is unknown whether birth mode may alter such a response. To study these questions, we first measured corticosterone and pro- and anti-inflammatory cytokines in plasma of mouse offspring at several timepoints spaced closely before and after a vaginal or Cesarean birth. We found highest levels of IL-6 one day before birth and surges in corticosterone and IL-10 just after birth, regardless of birth mode. We next examined the neuroimmune response by measuring cytokine mRNA expression and microglial number and morphology in the paraventricular nucleus of the hypothalamus and hippocampus around the time of birth. We found a marked increase in TNF-α expression in both brain regions a day after birth, and rapid increases in microglial cell number in the first three days postnatal, with subtle differences by birth mode. To test whether the association between birth and cytokine production or expansion of microglia is causal, we manipulated birth timing. Remarkably, advancing birth by a day advanced the increases in all of the markers tested. Thus, birth triggers an immune response in the body and brain of offspring. Our results may provide a mechanism for effects of birth (e.g., acute changes in cell death and neural activation) previously reported in the newborn brain.

Inflammatory Signatures of Maternal Obesity as Risk Factors for Neurodevelopmental Disorders: Role of Maternal Microbiota and Nutritional Intervention Strategies

Obesity is a main risk factor for the onset and the precipitation of many non-communicable diseases. This condition, which is associated with low-grade chronic systemic inflammation, is of main concern during pregnancy leading to very serious consequences for the new generations. In addition to the prominent role played by the adipose tissue, dysbiosis of the maternal gut may also sustain the obesity-related inflammatory milieu contributing to create an overall suboptimal intrauterine environment. Such a condition here generically defined as “inflamed womb” may hold long-term detrimental effects on fetal brain development, increasing the vulnerability to mental disorders. In this review, we will examine the hypothesis that maternal obesity-related gut dysbiosis and the associated inflammation might specifically target fetal brain microglia, the resident brain immune macrophages, altering neurodevelopmental trajectories in a sex-dependent fashion. We will also review some of the most promising nutritional strategies capable to prevent or counteract the effects of maternal obesity through the modulation of inflammation and oxidative stress or by targeting the maternal microbiota.

Infectious disease and cognition in wild populations

Infectious disease is linked to impaired cognition across a breadth of host taxa and cognitive abilities, potentially contributing to variation in cognitive performance within and among populations. Impaired cognitive performance can stem from direct damage by the parasite, the host immune response, or lost opportunities for learning. Moreover, cognitive impairment could be compounded by factors that simultaneously increase infection risk and impair cognition directly, such as stress and malnutrition. As highlighted in this review, however, answers to fundamental questions remain unresolved, including the frequency, duration, and fitness consequences of infection-linked cognitive impairment in wild animal populations, the cognitive abilities most likely to be affected, and the potential for adaptive evolution of cognition in response to accelerating emergence of infectious disease.

Functional status of brainstem auditory pathway in babies born below 30 week gestation with necrotizing enterocolitis

OBJECTIVE

Recent studies showed that neonatal necrotizing enterocolitis (NEC) adversely affects the brainstem auditory pathway in babies born at 30-40 week gestation. We compared the functional status of the pathway between babies born below 30 week gestation with NEC and those without NEC for any differences to understand whether NEC also affects the pathway in babies born at a smaller gestation.

METHOD

Brainstem auditory evoked response was studied at term in NEC babies born below 30 week gestation. The data obtained were compared with age-matched non-NEC babies for any abnormalities, and then compared with previously reported NEC babies born at 30-34 week gestation for any differences.

RESULTS

Although the latencies of waves I and III did not differ significantly between NEC and non-NEC babies, wave V latency in NEC babies was longer than in non-NEC babies at all click rates used. In particular, I-V interpeak interval, reflecting brainstem conduction time, in NEC babies was significant longer than in non-NEC babies. Wave V amplitude and the V/I amplitude ratios in NEC babies was smaller than in non-NEC babies at some click rates. The I-V interval in our NEC babies born below 30 week gestation was longer than in previously reported NEC babies born at 30-34 week gestation at all click rates.

CONCLUSION

NEC babies born below 30 week gestation are associated with delayed brainstem conduction time. Functional status of the brainstem auditory pathway in NEC babies born below 30 week gestation is less favorable than that in those with greater gestation.

Human milk: From complex tailored nutrition to bioactive impact on child cognition and behavior

Human milk is a highly complex liquid food tailor-made to match an infant's needs. Beyond documented positive effects of breastfeeding on infant and maternal health, there is increasing evidence that milk constituents also impact child neurodevelopment. Non-nutrient milk bioactives would contribute to the (long-term) development of child cognition and behavior, a process termed 'Lactocrine Programming'. In this review we discuss the current state of the field on human milk composition and its links with child cognitive and behavioral development. To promote state-of-the-art methodologies and designs that facilitate data pooling and meta-analytic endeavors, we present detailed recommendations and best practices for future studies. Finally, we determine important scientific gaps that need to be filled to advance the field, and discuss innovative directions for future research. Unveiling the mechanisms underlying the links between human milk and child cognition and behavior will deepen our understanding of the broad functions of this complex liquid food, as well as provide necessary information for designing future interventions.

A method for the selective depletion of microglia in the dorsal hippocampus in the juvenile rat brain

BACKGROUND

To understand the role of microglia in brain function and development, methods have emerged to deplete microglia throughout the brain. Liposome-encapsulated clodronate (LEC) can be infused into the brain to deplete microglia in a brain-region and time-specific manner.

NEW METHOD

This study validates methodology to deplete microglia in the rat dorsal hippocampus (dHP) during a specific period of juvenile development. Stereotaxic surgery was performed to infuse LEC at postnatal day (P) 16 or 19 into dHP. Rat brains were harvested at various ages to determine specificity of infusion and duration of depletion.

RESULTS

P19 infusion of LEC into dHP with a 27G syringe depleted microglia in dHP subregions CA1, dentate gyrus (DG), and CA3 from P24-P30. There was also evidence of depletion in parietal cortex above the infusion site. P16 infusion of LEC with a 32 G syringe depleted microglia only in dHP subregions CA1 and DG from P21-P40.

COMPARISON WITH EXISTING METHOD(S)

Previous methods have infused LEC intra-hippocampally in adult rats or intra-cerebroventricularly in neonatal rats. This study is the first to publish methodology to deplete microglia in a brain-region specific manner during juvenile rat development.

CONCLUSIONS

The timing of LEC infusion during the juvenile period can be adjusted to achieve maximal microglia depletion by a specific postnatal day. A 27G needle results in LEC backflow during the infusion, but also allows LEC to reach all subregions of dHP. Infusion with a 32 G needle prevents backflow during infusion, but results in a more local spread of LEC within dHP.

Sustained peripheral immune hyper-reactivity (SPIHR): an enduring biomarker of altered inflammatory responses in adult rats after perinatal brain injury

Background

Chorioamnionitis (CHORIO) is a principal risk factor for preterm birth and is the most common pathological abnormality found in the placentae of preterm infants. CHORIO has a multitude of effects on the maternal–placental–fetal axis including profound inflammation. Cumulatively, these changes trigger injury in the developing immune and central nervous systems, thereby increasing susceptibility to chronic sequelae later in life. Despite this and reports of neural–immune changes in children with cerebral palsy, the extent and chronicity of the peripheral immune and neuroinflammatory changes secondary to CHORIO has not been fully characterized.

Methods

We examined the persistence and time course of peripheral immune hyper-reactivity in an established and translational model of perinatal brain injury (PBI) secondary to CHORIO. Pregnant Sprague–Dawley rats underwent laparotomy on embryonic day 18 (E18, preterm equivalent). Uterine arteries were occluded for 60 min, followed by intra-amniotic injection of lipopolysaccharide (LPS). Serum and peripheral blood mononuclear cells (PBMCs) were collected at young adult (postnatal day P60) and middle-aged equivalents (P120). Serum and PBMCs secretome chemokines and cytokines were assayed using multiplex electrochemiluminescent immunoassay. Multiparameter flow cytometry was performed to interrogate immune cell populations.

Results

Serum levels of interleukin-1β (IL-1β), IL-5, IL-6, C–X–C Motif Chemokine Ligand 1 (CXCL1), tumor necrosis factor-α (TNF-α), and C–C motif chemokine ligand 2/monocyte chemoattractant protein-1 (CCL2/MCP-1) were significantly higher in CHORIO animals compared to sham controls at P60. Notably, CHORIO PBMCs were primed. Specifically, they were hyper-reactive and secreted more inflammatory mediators both at baseline and when stimulated in vitro. While serum levels of cytokines normalized by P120, PBMCs remained primed, and hyper-reactive with a robust pro-inflammatory secretome concomitant with a persistent change in multiple T cell populations in CHORIO animals.

Conclusions

The data indicate that an in utero inflammatory insult leads to neural–immune changes that persist through adulthood, thereby conferring vulnerability to brain and immune system injury throughout the lifespan. This unique molecular and cellular immune signature including sustained peripheral immune hyper-reactivity (SPIHR) and immune cell priming may be a viable biomarker of altered inflammatory responses following in utero insults and advances our understanding of the neuroinflammatory cascade that leads to perinatal brain injury and later neurodevelopmental disorders, including cerebral palsy.

Sex-specific responses of the pubertal neuroimmune axis in CD-1 mice

The mechanistic relationship between the sexually dimorphic neuroimmune system and the sex-specific outcomes of a pubertal immune challenge is unclear. Therefore, we examined sex differences in the progression of cytotoxic microglial responses and blood-brain barrier (BBB) disruption to a peripubertal lipopolysaccharide (LPS) treatment in brain regions relevant to stress responses and cognitive function. Six-week-old (i.e., stress-sensitive pubertal period) male and female CD-1 mice were treated with LPS (1.5 ​mg/kg body weight, ip) or 0.9% saline (LPS-matched volume, ip). Sex and treatment differences in microglial (Iba1⁺) and apoptotic neuronal (caspase-3⁺/NeuN⁺) and non-neuronal (caspase-3⁺/NeuN⁻) expression were examined in the hippocampus, medial prefrontal cortex (mPFC), and paraventricular nucleus 24 ​h (sickness), one week (symptomatic recovery) and four weeks (early adulthood) post-treatment (n ​= ​8/group). Microglial morphology was quantified with fractal analyses. Group differences in BBB permeability to ¹⁴C-sucrose were examined 24 ​h (whole-brain, hippocampus, prefrontal cortex, hypothalamus, and cerebellum) and one week (whole-brain) post-treatment. The acute effects of pubertal LPS were specific to females (i.e., global BBB disruption, altered microglial expression and morphology in the mPFC and hippocampus, increased hippocampal apoptosis). The residual effects of pubertal LPS-induced sickness observed in microglia persisted into adulthood in a sex- and region-specific manner. In addition to highlighting these sex-specific responses of the pubertal neuroimmune system, we report baseline region-specific sex differences in microglia spanning puberty through adulthood. We propose that these sex differences in neuroimmune-neurovascular interactions during the stress-sensitive pubertal period create sex biases in stress-related disorders of brain and behaviour.

•

Pubertal LPS alters baseline sex differences in microglial numbers and morphology.

•

Pubertal CD-1 mice mount sexually dimorphic neuroimmune responses to systemic LPS.

•

Treatment effects on microglial expression and morphology differ by sex and region.

•

The acute LPS-induced effects were specific to females.

The emerging tale of microglia in psychiatric disorders

As the professional phagocytes of the brain, microglia orchestrate the immunological response and play an increasingly important role in maintaining homeostatic brain functions. Microglia are activated by pathological events or slight alterations in brain homeostasis. This activation is dependent on the context and type of stressor or pathology. Through secretion of cytokines, chemokines and growth factors, microglia can strongly influence the response to a stressor and can, therefore, determine the pathological outcome. Psychopathologies have repeatedly been associated with long-lasting priming and sensitization of cerebral microglia. This review focuses on the diversity of microglial phenotype and function in health and psychiatric disease. We first discuss the diverse homeostatic functions performed by microglia and then elaborate on context-specific spatial and temporal microglial heterogeneity. Subsequently, we summarize microglia involvement in psychopathologies, namely major depressive disorder, schizophrenia and bipolar disorder, with a particular focus on post-mortem studies. Finally, we postulate microglia as a promising novel therapeutic target in psychiatry through antidepressant and antipsychotic treatment.

Probiotics: Potential novel therapeutics for microbiota-gut-brain axis dysfunction across gender and lifespan

Probiotics are live microorganisms, which when administered in adequate amounts, present a health benefit for the host. While the beneficial effects of probiotics on gastrointestinal function are generally well recognized, new animal research and clinical studies have found that alterations in gut microbial communities can have a broad range of effects throughout the body. Non-intestinal sites impacted include the immune, endocrine, cardiovascular and the central nervous system (CNS). In particular, there has been a growing interest and appreciation about the role that gut microbiota may play in affecting CNS-related function through the 'microbiota-gut-brain axis'. Emerging evidence suggests potential therapeutic benefits of probiotics in several CNS conditions, such as anxiety, depression, autism spectrum disorders and Parkinson's disease. There may also be some gender-specific variances in terms of probiotic mediated effects, with the gut microbiota shaping and being concurrently molded by the hormonal environment governing differences between the sexes. Probiotics may influence the ability of the gut microbiome to affect a variety of biological processes in the host, including neurotransmitter activity, vagal neurotransmission, generation of neuroactive metabolites and inflammatory response mediators. Some of these may engage in cross talk with host sex hormones, such as estrogens, which could be of relevance in relation to their effects on stress response and cognitive health. This raises the possibility of gender-specific variation with regards to the biological action of probiotics, including that on the endocrine and central nervous systems. In this review we aim to describe the current understanding in relation to the role and use of probiotics in microbiota-gut-brain axis-related dysfunction. Furthermore, we will address the conceptualization and classification of probiotics in the context of gender and lifespan as well as how restoring gut microbiota composition by clinical or dietary intervention can help in supporting health outcomes other than those related to the gastrointestinal tract. We also evaluate how these new learnings may impact industrial effort in probiotic research and the discovery and development of novel and more personalized, condition-specific, beneficial probiotic therapeutic agents.

Microglia and Sensitive Periods in Brain Development

From embryonic neuronal migration to adolescent circuit refinement, the immune system plays an essential role throughout central nervous system (CNS) development. Immune signaling molecules serve as a common language between the immune system and CNS, allowing them to work together to modulate brain function both in health and disease. As the resident CNS macrophage, microglia comprise the majority of immune cells in the brain. Much like their peripheral counterparts, microglia survey their environment for pathology, clean up debris, and propagate inflammatory responses when necessary. Beyond this, recent studies have highlighted that microglia perform a number of complex tasks during neural development, from directing neuronal and axonal positioning to pruning synapses, receptors, and even whole cells. In this chapter, we discuss this literature within the framework that immune activation during discrete windows of neural development can profoundly impact brain function long-term, and thus the risk of neurodevelopmental and neuropsychiatric disorders. In this chapter, we review three sensitive developmental periods - embryonic wiring, early postnatal synaptic pruning, and adolescent circuit refinement - in order to highlight the diversity of functions that microglia perform in building a brain. In reviewing this literature, it becomes obvious that timing matters, perhaps more so than the nature of the immune activation itself; largely conserved patterns of microglial response to diverse insults result in different functional impacts depending on the stage of brain maturation at the time of the challenge.

Primetime for microglia: When stress and infection collide

Inflammation during critical windows of development contributes to behavioral affect later in life. In this of Neuron, Cao et al. (2021) demonstrate a novel mechanism through which early life Tlr4-dependent inflammation in microglia permanently alters neuronal function and leaves male mice susceptible to stress-induced depressive-like behaviors.

Traumatic Injury to the Developing Brain: Emerging Relationship to Early Life Stress

Despite the high incidence of brain injuries in children, we have yet to fully understand the unique vulnerability of a young brain to an injury and key determinants of long-term recovery. Here we consider how early life stress may influence recovery after an early age brain injury. Studies of early life stress alone reveal persistent structural and functional impairments at adulthood. We consider the interacting pathologies imposed by early life stress and subsequent brain injuries during early brain development as well as at adulthood. This review outlines how early life stress primes the immune cells of the brain and periphery to elicit a heightened response to injury. While the focus of this review is on early age traumatic brain injuries, there is also a consideration of preclinical models of neonatal hypoxia and stroke, as each further speaks to the vulnerability of the brain and reinforces those characteristics that are common across each of these injuries. Lastly, we identify a common mechanistic trend; namely, early life stress worsens outcomes independent of its temporal proximity to a brain injury.

The effects of early-life immune activation on microglia-mediated neuronal remodeling and the associated ontogeny of hippocampal-dependent learning in juvenile rats

Many neurodevelopmental disorders and associated learning deficits have been linked to early-life immune activation or ongoing immune dysregulation (Laskaris et al., 2016; O'Connor et al., 2014; Frick et al., 2013). Neuroscientists have begun to understand how the maturation of neural circuits allows for the emergence of cognitive and learning behaviors; yet we know very little about how these developing neural circuits are perturbed by certain events, including risk-factors such as early-life immune activation and immune dysregulation. To answer these questions, we examined the impact of early-life immune activation on the emergence of hippocampal-dependent learning in juvenile male and female rats using a well-characterized hippocampal-dependent learning task and we investigated the corresponding, dynamic multicellular interactions in the hippocampus that may contribute to these learning deficits. We found that even low levels of immune activation can result in hippocampal-depedent learning deficits days later, but only when this activation occurs during a sensitive period of development. The initial immune response and associated cytokine production in the hippocampus resolved within 24 h, several days prior to the observed learning deficit, but notably the initial immune response was followed by altered microglial-neuronal communication and synapse remodeling that changed the structure of hippocampal neurons during this period of juvenile brain development. We conclude that immune activation or dysregulation during a sensitive period of hippocampal development can precipitate the emergence of learning deficits via a multi-cellular process that may be initiated by, but not the direct result of the initial cytokine response. SIGNIFICANCE STATEMENT: Many neurodevelopmental disorders have been linked to early-life immune activation or immune dysregulation; however, very little is known about how dynamic changes in neuroimmune cells mediate the transition from normal brain function to the early stages of cognitive disorders, or how changes in immune signaling are subsequently integrated into developing neuronal networks. The current experiments examined the consequences of immune activation on the cellular and molecular changes that accompany the emergence of learning deficits during a sensitive period of hippocampal development. These findings have the potential to significantly advance our understanding of how early-life immune activation or dysregulation can result in the emergence of cognitive and learning deficits that are the largest source of years lived with disability in humans.

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.