Microglial activation in a neuroinflammational animal model of schizophrenia--a pilot study

Maternal immune activation-induced PPARγ-dependent dysfunction of microglia associated with neurogenic impairment and aberrant postnatal behaviors in offspring

Maternal infection during pregnancy is an important factor involved in the pathogenesis of brain disorders in the offspring. Mounting evidence from maternal immune activation (MIA) animals indicates that microglial priming may contribute to neurodevelopmental abnormalities in the offspring. Because peroxisome proliferator-activated receptor gamma (PPARγ) activation exerts neuroprotective effects by regulating neuroinflammatory response, it is a pharmacological target for treating neurogenic disorders. We investigated the effect of PPARγ-dependent microglial activation on neurogenesis and consequent behavioral outcomes in male MIA-offspring. Pregnant dams on gestation day 18 received Poly(I:C) (1, 5, or 10 mg/kg; i.p.) or the vehicle. The MIA model that received 10 mg/kg Poly(I:C) showed significantly increased inflammatory responses in the maternal serum and fetal hippocampus, followed by cognitive deficits, which were highly correlated with hippocampal neurogenesis impairment in prepubertal male offspring. The microglial population in hippocampus increased, displayed decreased processes and larger soma, and had a higher expression of the CD11b, which is indicative of the M1 phenotype (classical activation). Activation of the PPARγ pathway by pioglitazone in the MIA offspring rescued the imbalance of the microglial activation and ameliorated the MIA-induced suppressed neurogenesis and cognitive impairments and anxiety behaviors. In an in vitro experiment, PPARγ-induced M2 microglia (alternative activation) promoted the proliferation and differentiation of neural precursor cells. These results indicated that the MIA-induced long-term changes in microglia phenotypes were associated with hippocampal neurogenesis and neurobehavioral abnormalities in offspring. Modulation of the microglial phenotypes was associated with a PPARγ-mediated neuroprotective mechanism in the MIA offspring and may serve as a potential therapeutic approach for prenatal immune activation-induced neuropsychiatric disorders.

Maternal Inflammation and Neurodevelopmental Programming: A Review of Preclinical Outcomes and Implications for Translational Psychiatry

Early disruptions to neurodevelopment are highly relevant to understanding both psychiatric risk and underlying pathophysiology that can be targeted by new treatments. Much convergent evidence from the human literature associates inflammation during pregnancy with later neuropsychiatric disorders in offspring. Preclinical models of prenatal inflammation have been developed to examine the causal maternal physiological and offspring neural mechanisms underlying these findings. Here we review the strengths and limitations of preclinical models used for these purposes and describe selected studies that have shown maternal immune impacts on the brain and behavior of offspring. Maternal immune activation in mice, rats, nonhuman primates, and other mammalian model species have demonstrated convergent outcomes across methodologies. These outcomes include shifts and/or disruptions in the normal developmental trajectory of molecular and cellular processes in the offspring brain. Prenatal developmental origins are critical to a mechanistic understanding of maternal immune activation-induced alterations to microglia and immune molecules, brain growth and development, synaptic morphology and physiology, and anxiety- and depression-like, sensorimotor, and social behaviors. These phenotypes are relevant to brain functioning across domains and to anxiety and mood disorders, schizophrenia, and autism spectrum disorder, in which they have been identified. By turning a neurodevelopmental lens on this body of work, we emphasize the importance of acute changes to the prenatal offspring brain in fostering a better understanding of potential mechanisms for intervention. Collectively, overlapping results across maternal immune activation studies also highlight the need to examine preclinical offspring neurodevelopment alterations in terms of a multifactorial immune milieu, or immunome, to determine potential mechanisms of psychiatric risk.

Programming Bugs: Microbiota and the Developmental Origins of Brain Health and Disease

It has been nearly 30 years since Dr. David Barker first highlighted the importance of prenatal factors in contributing to the developmental origins of adult disease. This concept was later broadened to include postnatal events. It is clear that the interaction between genetic predisposition and early life environmental exposures is key in this regard. However, recent research has also identified another important factor in the microbiota-the trillions of microorganisms that inhabit key body niches, including the vagina and gastrointestinal tract. Because the composition of these maternal microbiome sites has been linked to maternal metabolism and is also vertically transmitted to offspring, changes in the maternal microbiota are poised to significantly affect the newborn. In fact, several lines of evidence show that the gut microbiota interacts with diet, drugs, and stress both prenatally and postnatally and that these exogenous factors could also affect the dynamic changes in the microbiota composition occurring during pregnancy. Animal models have shown great utility in illuminating how these disruptions result in behavioral and brain morphological phenotypes reminiscent of psychiatric disorders (anxiety, depression, schizophrenia, and autism spectrum disorders). Increasing evidence points to critical interactions among the microbiota, host genetics, and both the prenatal and postnatal environments to temporally program susceptibility to psychiatric disorders later in life. Sex-specific phenotypes may be programmed through the influence of the microbiota on the hypothalamic-pituitary-adrenal axis and neuroimmune system.

Microglial Activation and Psychotic Disorders: Evidence from Pre-clinical and Clinical Studies

Clinical and pre-clinical studies have demonstrated an important role of neuroinflammation in the etiology of schizophrenia. While the underlying mechanisms remain poorly understood, there are some studies demonstrating an association between maternal immune activation and behavioral changes in adult offspring and identifying early life infection as a trigger for schizophrenia; in addition, inflammatory markers were found to be increased in the schizophrenic post-mortem brain. During maternal immune activation, pro-inflammatory mediators such as cytokines, chemokines, antibodies, and acute-phase proteins are released in the maternal bloodstream, thus increasing the permeability of the placental barrier and the fetal blood-brain barrier, allowing the inflammatory mediators to enter the fetal brain. In the central nervous system (CNS), these pro-inflammatory mediators are able to activate microglial cells that can release pro-inflammatory cytokines such as tumor necrosis factor alpha (TNF-α), interleukin (IL)-1β, and IL-6. As a consequence, circulating immune cells may infiltrate the brain, increasing cytokine levels and releasing antibodies that aggravate the neuroinflammation. Neuroinflammation may affect processes that are pivotal for normal brain maturation such as myelination, synaptic pruning, and neuronal remodeling. Microglial cell activation and pro-inflammatory mediators have been extensively studied in schizophrenic post-mortem brain samples. Some results of these investigations demonstrated an increase in microglial activation markers, cytokines, and chemokines in post-mortem brain samples from individuals with schizophrenia. In contrast, there are studies that have demonstrated low levels of microglial activation makers in the schizophrenic post-mortem brain. Thus, based on the important role of neuroinflammation as a trigger in the development of schizophrenia, this chapter aims (1) to enumerate evidence of neuroinflammation and microglial activation from pre-clinical schizophrenia models, (2) to show links between schizophrenia and neuroinflammation in clinical studies, and (3) to identify mechanisms by which microglial activation may influence in the development of schizophrenia.

Brain changes in a maternal immune activation model of neurodevelopmental brain disorders

The developing brain is sensitive to a variety of insults. Epidemiological studies have identified prenatal exposure to infection as a risk factor for a range of neurological disorders, including autism spectrum disorder and schizophrenia. Animal models corroborate this association and have been used to probe the contribution of gene-environment interactions to the etiology of neurodevelopmental disorders. Here we review the behavior and brain phenotypes that have been characterized in MIA offspring, including the studies that have looked at the interaction between maternal immune activation and genetic risk factors for autism spectrum disorder or schizophrenia. These phenotypes include behaviors relevant to autism, schizophrenia, and other neurological disorders, alterations in brain anatomy, and structural and functional neuronal impairments. The link between maternal infection and these phenotypic changes is not fully understood, but there is increasing evidence that maternal immune activation induces prolonged immune alterations in the offspring's brain which could underlie epigenetic alterations which in turn may mediate the behavior and brain changes. These concepts will be discussed followed by a summary of the pharmacological interventions that have been tested in the maternal immune activation model.

Controversies and prospects about microglia in maternal immune activation models for neurodevelopmental disorders

Activation of the maternal immune system during pregnancy is a well-established risk factor for neuropsychiatric disease in the offspring, yet, the underlying mechanisms leading to altered brain function remain largely undefined. Microglia, the resident immune cells of the brain, are key to adequate development of the central nervous system (CNS), and are prime candidates to mediate maternal immune activation (MIA)-induced brain abnormalities. As such, the effects of MIA on the immunological phenotype of microglia has been widely investigated. However, contradicting results due to differences in read-out and methodological approaches impede final conclusions on MIA-induced microglial alterations. The aim of this review is to critically discuss the evidence for an activated microglial phenotype upon MIA.

Maternal immune activation altered microglial immunoreactivity in the brain of postnatal day 2 rat offspring

Microglia, the resident immune cells of the central nervous system, play critical roles in neurodevelopment, synaptic pruning, and neuronal wiring. Early in development, microglia migrate via the tangential and radial migration pathways to their final destinations and mature gradually, a process that includes morphological changes. Recent research has implicated microglial abnormality in the etiology of schizophrenia. Since prenatal exposure to viral or bacterial infections due to maternal immune activation (MIA) leads to increased risk of schizophrenia in the offspring during adulthood, the present study systematically investigated how MIA induced by polyinosinic:polycytidylic acid (a mimic of viral double-stranded RNA) affected microglial immunoreactivity along the migration and maturation trajectories in the brains of male and female rat offspring on postnatal day (PND) 2. The immunohistochemistry revealed significant changes in the density of IBA-1 immunoreactive cells in the corpus callosum, somatosensory cortex, striatum, and the subregions of the hippocampus of the MIA offspring. The male and female MIA offspring displayed markedly altered microglial immunoreactivity in both the tangential and radial migration, as well as maturation, pathways when compared to their sex- and age-matched controls as evidenced by morphology-based cell counting. Given the important roles of microglia in synaptic pruning and neuronal wiring and survival, these changes may lead to structural and functional neurodevelopmental abnormalities, and so contribute to the functional deficits observed in juvenile and adult MIA offspring. Future research is required to systematically determine how MIA affects microglial migration and maturation in rat offspring.

Environmentally enriched pigs have transcriptional profiles consistent with neuroprotective effects and reduced microglial activity

Environmental enrichment (EE) is widely used to study the effects of external factors on brain development, function and health in rodent models, but very little is known of the effects of EE on the brain in a large animal model such as the pig. Twenty-four young pigs (aged 5 weeks at start of study, 1:1 male: female ratio) were housed in environmentally enriched (EE) pens and provided with additional enrichment stimulation (a bag filled with straw) once daily. Litter, weight and sex matched controls n= (24) were housed in barren (B) conditions. Behaviour was recorded on alternate days from study day 10. After 21 days, RNA-sequencing of the frontal cortex of male piglets culled one hour after the enrichment stimulation, but not those at 4 h after stimulation, showed upregulation of genes involved in neuronal activity and synaptic plasticity in the EE compared to the B condition. This result is mirrored in the behavioural response to the stimulation which showed a peak in activity around the 1 h time-point. By contrast, EE piglets displayed a signature consistent with a relative decrease in microglial activity compared to those in the B condition. These results confirm those from rodents, suggesting that EE may also confer neuronal health benefits in large mammal models, through a potential relative reduction in neuroinflammatory process and increase in neuroprotection driven by an enrichment-induced increase in behavioural activity.

Microglia activation in the offspring of prenatal Poly I: C exposed rats: a PET imaging and immunohistochemistry study

Background

The well-known ‘pyrotherapy’ of Julius Wagner-Jauregg might be the beginning of the study on the immunological concepts of schizophrenia. As the primary immune effector cells in the brain, microglia play a pivotal role in neuroinflammatory processes. Maternal viral infection during pregnancy is associated with an increased risk for psychiatric disorders with presumed neurodevelopmental origin, including autism spectrum disorders and schizophrenia. The present study was to quantify microglia activation in vivo in the mature offspring of rats exposed to polyriboinosinic–polyribocytidilicacid (Poly I:C) during pregnancy using ¹¹C-PK11195 positron emission tomography (PET) and immunohistochemistry.

Objective

The study aimed to quantify microglia activation in vivo in the prefrontal cortex and hippocampus in mature offspring of prenatal Poly I:C exposed rats.

Methods

Offspring of Poly I:C-treated dams were the model group, offspring of saline-treated dams were the control group. Behavioural test for two groups was taken by spontaneous activity, prepulse inhibition (PPI) and latent inhibition (LI) test (including active avoidance conditioning task and passive avoidance conditioning task). Randomly selected successful model rats were assessed by behavioural test in the model group and control group rats. ¹¹C-PK11195 micro-PET/CT and immunohistochemistry were performed on the selected rats to measure microglia activation.

Results

The treatment group showed hyperlocomotion and deficits in PPI and LI compared with the control group. The treatment group also showed an increased ¹¹C-PK11195 uptake ratio in the prefrontal cortex (t=−3.990, p=0.003) and hippocampus (t=−4.462, p=0.001). The number of activated microglia cells was significantly higher in the treatment group than in the control group (hippocampus: t=8.204, p<0.001; prefrontal: t=6.995, p<0.001). Within the treatment group, there were significant correlations between the behavioural parameters and the activation of microglia as measured by PET and immunohistochemistry.

Conclusions

The present study demonstrated microglia activation in vivo in the prefrontal cortex and hippocampus in mature offspring of prenatal Poly I:C exposed rats. This study suggests that microglia activation may play a possible or potential role in the pathogenesis of schizophrenia.

Late gestation immune activation increases IBA1-positive immunoreactivity levels in the corpus callosum of adult rat offspring

Animal models of maternal immune activation study the effects of infection, an environmental risk factor for schizophrenia, on brain development. Microglia activation and cytokine upregulation may have key roles in schizophrenia neuropathology. We hypothesised that maternal immune activation induces changes in microglia and cytokines in the brains of the adult offspring. Maternal immune activation was induced by injecting polyriboinosinic:polyribocytidylic acid into pregnant rats on gestational day (GD) 10 or GD19, with brain tissue collected from the offspring at adulthood. We observed no change in Iba1, Gfap, IL1-β and TNF-α mRNA levels in the cingulate cortex (CC) in adult offspring exposed to maternal immune activation. Prenatal exposure to immune activation had a significant main effect on microglial IBA1-positive immunoreactive material (IBA1+IRM) in the corpus callosum; post-hoc analyses identified a significant increase in GD19 offspring, but not GD10. No change in was observed in the CC. In contrast, maternal immune activation had a significant main effect on GFAP+IRM in the CC at GD19 (not GD10); post-hoc analyses only identified a strong trend towards increased GFAP+IRM in the GD19 offspring, with no white matter changes. This suggests late gestation maternal immune activation causes subtle alterations to microglia and astrocytes in the adult offspring.

Inflammatory and cytotoxic effects of bifenthrin in primary microglia and organotypic hippocampal slice cultures

BACKGROUND

Pyrethroids, such as bifenthrin (BF), are among the most widely used class of insecticides that pose serious risks to human and wildlife health. Pyrethroids are proposed to affect astrocytic functions and to cause neuron injury in the central nervous system (CNS). Microglia are key cells involved in innate immune responses in the CNS, and microglia activation has been linked to inflammation and neurotoxicity. However, little information is known about the effects of BF-induced toxicity in primary microglial cells as well as in organotypic hippocampal slice cultures (OHSCs).

METHODS

Oxidative stress and inflammatory responses induced by BF were evaluated in primary microglial cells and OHSCs incubated with different concentrations of BF (1-20 μM) for 4 and 24 h. mRNA and protein synthesis of cyclooxygenase-2 (COX-2), tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), nuclear erythroid-2 like factor-2 (Nrf-2), and microsomal prostaglandin synthase-1 (mPGES-1) was also studied by qPCR and Western blot. Cell viability was analyzed by MTT-tetrazolio (MTT) and lactate dehydrogenase (LDH) assays. Neurotoxicity in OHSCs was analyzed by propidium iodide (PI) staining and confocal microscopy.

RESULTS

Exposure of microglial cells to BF for 24 h resulted in a dose-dependent reduction in the number of viable cells. At sub-cytotoxic concentrations, BF increased reactive oxygen species (ROS), TNF-alpha synthesis, and prostaglandin E2 (PGE2) production, at both 4- and 24-h time points, respectively. Furthermore, BF incubation decreased superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) activities and increased lipid peroxidation, protein oxidation, and H2O2 formation. In addition, BF significantly induced protein synthesis and mRNA expression of oxidative and inflammatory mediators after 4 and 24 h, including Nrf-2, COX-2, mPGES-1, and nuclear factor kappaB (NF-kappaB). A 24-h exposure of OHSCs to BF also increased neuronal death compared to untreated controls. Furthermore, depletion of microglia from OHSCs potently enhanced neuronal death induced by BF.

CONCLUSIONS

Overall, BF exhibited cytotoxic effects in primary microglial cells, accompanied by the induction of various inflammatory and oxidative stress markers including the Nrf-2/COX-2/mPGES-1/NF-kappaB pathways. Moreover, the study provided evidence that BF induced neuronal death in OHSCs and suggests that microglia exert a protective function against BF toxicity.

Prenatal Immune Challenge in Mice Leads to Partly Sex-Dependent Behavioral, Microglial, and Molecular Abnormalities Associated with Schizophrenia

Epidemiological studies revealed that environmental factors comprising prenatal infection are strongly linked to risk for later development of neuropsychiatric disorders such as schizophrenia. Considering strong sex differences in schizophrenia and its increased prevalence in males, we designed a methodological approach to investigate possible sex differences in pathophysiological mechanisms. Prenatal immune challenge was modeled by systemic administration of the viral mimic polyinosinic-polycytidylic acid (Poly I:C) to C57BL/6 mice at embryonic day 9.5. The consequences on behavior, gene expression, and microglia—brain immune cells that are critical for normal development—were characterized in male vs. female offspring at adulthood. The cerebral cortex, hippocampus, and cerebellum, regions where structural and functional alterations were mainly described in schizophrenia patients, were selected for cellular and molecular analyses. Confocal and electron microscopy revealed most pronounced differences in microglial distribution, arborization, cellular stress, and synaptic interactions in the hippocampus of male vs. female offspring exposed to Poly I:C. Sex differences in microglia were also measured under both steady-state and Poly I:C conditions. These microglial alterations were accompanied by behavioral impairment, affecting for instance sensorimotor gating, in males. Consistent with these results, increased expression of genes related to inflammation was measured in cerebral cortex and hippocampus of males challenged with Poly I:C. Overall, these findings suggest that schizophrenia's higher incidence in males might be associated, among other mechanisms, with an increased microglial reactivity to prenatal immune challenges, hence determining disease outcomes into adulthood.

Maternal immune activation in neurodevelopmental disorders

Converging lines of evidence from basic science and clinical studies suggest a relationship between maternal immune activation (MIA) and neurodevelopmental disorders such as autism spectrum disorder (ASD) and schizophrenia. The mechanisms through which MIA increases the risk of neurodevelopmental disorders have become a subject of intensive research. This review aims to describe how dysregulation of microglial function and immune mechanisms may link MIA and neurodevelopmental pathologies. We also summarize the current evidence in animal models of MIA. Developmental Dynamics 247:588-619, 2018. © 2017 Wiley Periodicals, Inc.

Tumour necrosis factor-mediated homeostatic synaptic plasticity in behavioural models: testing a role in maternal immune activation

The proinflammatory cytokine tumour necrosis factor-alpha (TNFα) has long been characterized for its role in the innate immune system, but more recently has been found to have a distinct role in the nervous system that does not overlap with other proinflammatory cytokines. Through regulation of neuronal glutamate and GABA receptor trafficking, TNF mediates a homeostatic form of synaptic plasticity, but plays no direct role in Hebbian forms of plasticity. As yet, there is no evidence to suggest that this adaptive plasticity plays a significant role in normal development, but it does maintain neuronal circuit function in the face of several types of disruption. This includes developmental plasticity in primary sensory cortices, as well as modulating the response to antidepressants, chronic antipsychotics and drugs of abuse. TNF is also a prominent component of the neuroinflammation occurring in most neuropathologies, but the role of TNF-mediated synaptic plasticity in this context remains to be determined. We tested this in a maternal immune activation (MIA) model of neurodevelopmental disorders. Using TNF^-/- mice, we observed that TNF is not required for the expression of abnormal social or anxious behaviour in this model. This indicates that TNF does not uniquely contribute to the development of neuronal dysfunction in this model, and suggests that during neuroinflammatory events, compensation between the various proinflammatory cytokines is the norm.This article is part of the themed issue 'Integrating Hebbian and homeostatic plasticity'.

Bridging Autism Spectrum Disorders and Schizophrenia through inflammation and biomarkers - pre-clinical and clinical investigations

In recent years, evidence supporting a link between inflammation and neuropsychiatric disorders has been mounting. Autism spectrum disorders (ASD) and schizophrenia share some clinical similarities which we hypothesize might reflect the same biological basis, namely, in terms of inflammation. However, the diagnosis of ASD and schizophrenia relies solely on clinical symptoms, and to date, there is no clinically useful biomarker to diagnose or monitor the course of such illnesses.

The focus of this review is the central role that inflammation plays in ASD and schizophrenia. It spans from pre-clinical animal models to clinical research and excludes in vitro studies. Four major areas are covered: (1) microglia, the inflammatory brain resident myeloid cells, (2) biomarkers, including circulating cytokines, oxidative stress markers, and microRNA players, known to influence cellular processes at brain and immune levels, (3) effect of anti-psychotics on biomarkers and other predictors of response, and (4) impact of gender on response to immune activation, biomarkers, and response to anti-psychotic treatments.

Towards an Integrated View of Early Molecular Changes Underlying Vulnerability to Social Stress in Psychosis

Psychotic disorders are heterogeneous and complex, involving many putative causal factors interacting along the course of disease development. Many of the factors implicated in the pathogenesis of psychosis also appear to be involved in disease onset and subsequent neuroprogression. Herein, we highlight the pertinent literature implicating inflammation and oxidative stress in the pathogenesis of psychosis, and the potential contribution of N-methyl-D-aspartate receptors (NMDARs). We also emphasize the role of peripubertal social stress in psychosis, and the ways in which hippocampal dysfunction can mediate dysregulation of the hypothalamic-pituitary-adrenal axis and cortisol release. Finally, we propose a model wherein inflammation and oxidative stress act as a first hit, producing altered parvalbumin interneuron development, NMDAR hypofunction, microglial priming, and sensitivity to a second hit of peripubertal social stress. With a greater understanding of how these factors interact, it may be possible to detect, prevent, and treat psychosis more effectively.

Deep brain stimulation during early adolescence prevents microglial alterations in a model of maternal immune activation

In recent years schizophrenia has been recognized as a neurodevelopmental disorder likely involving a perinatal insult progressively affecting brain development. The poly I:C maternal immune activation (MIA) rodent model is considered as a neurodevelopmental model of schizophrenia. Using this model we and others demonstrated the association between neuroinflammation in the form of altered microglia and a schizophrenia-like endophenotype. Therapeutic intervention using the anti-inflammatory drug minocycline affected altered microglia activation and was successful in the adult offspring. However, less is known about the effect of preventive therapeutic strategies on microglia properties. Previously we found that deep brain stimulation of the medial prefrontal cortex applied pre-symptomatically to adolescence MIA rats prevented the manifestation of behavioral and structural deficits in adult rats. We here studied the effects of deep brain stimulation during adolescence on microglia properties in adulthood. We found that in the hippocampus and nucleus accumbens, but not in the medial prefrontal cortex, microglial density and soma size were increased in MIA rats. Pro-inflammatory cytokine mRNA was unchanged in all brain areas before and after implantation and stimulation. Stimulation of either the medial prefrontal cortex or the nucleus accumbens normalized microglia density and soma size in main projection areas including the hippocampus and in the area around the electrode implantation. We conclude that in parallel to an alleviation of the symptoms in the rat MIA model, deep brain stimulation has the potential to prevent the neuroinflammatory component in this disease.

Maternal immune activation results in complex microglial transcriptome signature in the adult offspring that is reversed by minocycline treatment

Maternal immune activation (MIA) during pregnancy has been linked to an increased risk of developing psychiatric pathologies in later life. This link may be bridged by a defective microglial phenotype in the offspring induced by MIA, as microglia have key roles in the development and maintenance of neuronal signaling in the central nervous system. The beneficial effects of the immunomodulatory treatment with minocycline on schizophrenic patients are consistent with this hypothesis. Using the MIA mouse model, we found an altered microglial transcriptome and phagocytic function in the adult offspring accompanied by behavioral abnormalities. The changes in microglial phagocytosis on a functional and transcriptional level were similar to those observed in a mouse model of Alzheimer's disease hinting to a related microglial phenotype in neurodegenerative and psychiatric disorders. Minocycline treatment of adult MIA offspring reverted completely the transcriptional, functional and behavioral deficits, highlighting the potential benefits of therapeutic targeting of microglia in psychiatric disorders.

Neuroinflammation and Oxidative Stress in Psychosis and Psychosis Risk

Although our understanding of psychotic disorders has advanced substantially in the past few decades, very little has changed in the standard of care for these illnesses since the development of atypical anti-psychotics in the 1990s. Here, we integrate new insights into the pathophysiology with the increasing interest in early detection and prevention. First, we explore the role of N-methyl-d-aspartate receptors in a subpopulation of cortical parvalbumin-containing interneurons (PVIs). Postmortem and preclinical data has implicated these neurons in the positive and negative symptoms, as well as the cognitive dysfunction present in schizophrenia. These neurons also appear to be sensitive to inflammation and oxidative stress during the perinatal and peripubertal periods, which may be mediated in large part by aberrant synaptic pruning. After exploring some of the molecular mechanisms through which neuroinflammation and oxidative stress are thought to exert their effects, we highlight the progress that has been made in identifying psychosis prior to onset through the identification of individuals at clinical high risk for psychosis (CHR). By combining our understanding of psychosis pathogenesis with the increasing characterization of endophenotypes that precede frank psychosis, it may be possible to identify patients before they present with psychosis and intervene to reduce the burden of the disease to both patients and families.

Maternal Immune Activation and Autism Spectrum Disorder: From Rodents to Nonhuman and Human Primates

A subset of women who are exposed to infection during pregnancy have an increased risk of giving birth to a child who will later be diagnosed with a neurodevelopmental or neuropsychiatric disorder. Although epidemiology studies have primarily focused on the association between maternal infection and an increased risk of offspring schizophrenia, mounting evidence indicates that maternal infection may also increase the risk of autism spectrum disorder. A number of factors, including genetic susceptibility, the intensity and timing of the infection, and exposure to additional aversive postnatal events, may influence the extent to which maternal infection alters fetal brain development and which disease phenotype (autism spectrum disorder, schizophrenia, other neurodevelopmental disorders) is expressed. Preclinical animal models provide a test bed to systematically evaluate the effects of maternal infection on fetal brain development, determine the relevance to human central nervous system disorders, and to evaluate novel preventive and therapeutic strategies. Maternal immune activation models in mice, rats, and nonhuman primates suggest that the maternal immune response is the critical link between exposure to infection during pregnancy and subsequent changes in brain and behavioral development of offspring. However, differences in the type, severity, and timing of prenatal immune challenge paired with inconsistencies in behavioral phenotyping approaches have hindered the translation of preclinical results to human studies. Here we highlight the promises and limitations of the maternal immune activation model as a preclinical tool to study prenatal risk factors for autism spectrum disorder, and suggest specific changes to improve reproducibility and maximize translational potential.

Effects of early or late prenatal immune activation in mice on behavioral and neuroanatomical abnormalities relevant to schizophrenia in the adulthood

Maternal immune activation (MIA) during pregnancy in rodents increases the risk of the offspring to develop schizophrenia-related behaviors, suggesting a relationship between the immune system and the brain development. Here we tested the hypothesis that MIA induced by the viral mimetic polyinosinic-polycytidylic acid (poly I:C) in early or late gestation of mice leads to behavioral and neuroanatomical disorders in the adulthood. On gestational days (GDs) 9 or 17 pregnant dams were treated with poly I:C or saline via intravenous route and the offspring behaviors were measured during adulthood. Considering the progressive structural neuroanatomical alterations in the brain of individuals with schizophrenia, we used magnetic resonance imaging (MRI) to perform brain morphometric analysis of the offspring aged one year. MIA on GD9 or GD17 led to increased basal locomotor activity, enhanced motor responses to ketamine, a psychotomimetic drug, and reduced time spent in the center of the arena, suggesting an increased anxiety-like behavior. In addition, MIA on GD17 reduced glucose preference in the offspring. None of the treatments altered the relative volume of the lateral ventricles. However, a decrease in brain volume, especially for posterior structures, was observed for one-year-old animals treated with poly I:C compared with control groups. Thus, activation of the maternal immune system at different GDs lead to neuroanatomical and behavioral alterations possibly related to the positive and negative symptoms of schizophrenia. These results provide insights on neuroimmunonological and neurodevelopmental aspects of certain psychopathologies, such as schizophrenia.

The immune system as a novel regulator of sex differences in brain and behavioral development

Sexual differentiation of the brain occurs early in life as a result of sex-typical hormone action and sex chromosome effects. Immunocompetent cells are being recognized as underappreciated regulators of sex differences in brain and behavioral development, including microglia, astrocytes, and possibly other less well studied cell types, including T cells and mast cells. Immunocompetent cells in the brain are responsive to steroid hormones, but their role in sex-specific brain development is an emerging field of interest. This Review presents a summary of what is currently known about sex differences in the number, morphology, and signaling profile of immune cells in the developing brain and their role in the early-life programming of sex differences in brain and behavior. We review what is currently known about sex differences in the response to early-life perturbations, including stress, inflammation, diet, and environmental pollutants. We also discuss how and why understanding sex differences in the developing neuroimmune environment may provide insight into understanding the etiology of several neurodevelopmental disorders. This Review also highlights what remains to be discovered in this emerging field of developmental neuroimmunology and underscores the importance of filling in these knowledge gaps. © 2016 Wiley Periodicals, Inc.

Microglia and Brain Plasticity in Acute Psychosis and Schizophrenia Illness Course: A Meta-Review

OBJECTIVE

Schizophrenia poses a tremendous health, social, and economic burden upon patients and society, indicating current treatment options remain inadequate. Recent findings from several lines of evidence have pointed to the importance of immune system involvement in not only premorbid neurodevelopmental but also subsequent symptom generation and aging processes of brain change in schizophrenia. In this meta-review, we use the summarized evidence from recent quantitative systematic reviews (SRs) and meta-analyses of several subspecialties to critically evaluate the hypothesis that immune-related processes shape the symptomatic presentation and illness course of schizophrenia, both directly and indirectly through altered neuroplasticity.

METHODS

We performed a data search in PubMed for English language SRs and meta-analyses from 2010 to 2017. The methodological quality of the SRs was assessed with the AMSTAR instrument. In addition, we review in this paper 11 original publications on translocator protein (TSPO) positron emission tomography (PET) imaging in schizophrenia.

RESULTS

We reviewed 26 SRs and meta-analyses. Evidence from clinical observational studies of inflammatory or immunological markers and randomized controlled drug trials of immunomodulatory compounds as add-on in the treatment of schizophrenia suggests psychotic exacerbations are accompanied by immunological changes different from those seen in non-acute states, and that the symptoms of schizophrenia can be modified by compounds such as non-steroidal anti-inflammatory drug and minocycline. Information derived from post-mortem brain tissue analysis and PET neuroimaging studies to evaluate microglial activation have added new perspectives to the available evidence, yet these results are very heterogeneous. Each research domain comes with unique opportunities as well as inherent limitations. A better understanding of the (patho-)physiology of microglial cells and their role in neuroplasticity is key to interpreting the immune-related findings in the context of schizophrenia illness exacerbations and progression.

CONCLUSION

Evidence from clinical studies analyzing patients' blood and cerebrospinal fluid samples, neuroimaging and post-mortem brain tissue suggests that aberrant immune responses may define schizophrenia illness' course through altered neuroplasticity representing abnormal aging processes. Most findings are however prone to bias and confounding, and often non-specific to schizophrenia, and a multidisciplinary translational approach is needed to consolidate these findings and link them to other schizophrenia hypotheses.

Schizophrenia associated sensory gating deficits develop after adolescent microglia activation

Maternal infection during pregnancy is a well-established risk factor for schizophrenia in the adult offspring. Consistently, prenatal Poly(I:C) treatment in mice has been validated to model behavioral and neurodevelopmental abnormalities associated with schizophrenia. By using the Poly(I:C) BALB/c mouse model, we investigated the functional profile of microglia by flow cytometry in relation to progressive behavioral changes from adolescence to adulthood. Prenatal Poly(I:C) treatment induced the expected sensory gating deficits (pre-pulse inhibition (PPI) of the acoustic startle response) in 100day-old adult offspring, but only in female not in male descendants. No PPI-deficits were present in 30day-old adolescent mice. Sensory gating deficits in adult females were preceded by a strong M1-type microglia polarization pattern during puberty as determined by flow cytometric analysis of multiple pro- and anti-inflammatory surface markers. Microglia activation in females did not persist until adulthood and was absent in behaviorally unaffected male descendants. Further, the specific activation pattern of microglia was not mirrored by a similar activation of peripheral immune cells. We conclude that prenatal Poly(I:C) treatment induces post pubertal deficits in sensory gating which are specifically preceded by a pro-inflammatory activation pattern of microglia during puberty.

Microglia depletion in early life programs persistent changes in social, mood-related, and locomotor behavior in male and female rats

Microglia, the innate immune cells of the central nervous system, regulate brain development by promoting cell genesis, pruning synapses, and removing dying, newly-born or progenitor cells. However, the role of microglia in the early life programming of behavior under normal conditions is not well characterized. We used central infusion of liposomal clodronate to selectively deplete microglia from the neonatal rat brain and subsequently assessed the impact of microglial depletion on programming of juvenile and adult motivated behaviors. Liposomal clodronate treatment on postnatal days one and four led to greater than 70% loss of forebrain microglia by postnatal day 6 that lasted for approximately ten days. Neonatal microglia depletion led to reduced juvenile and adult anxiety behavior on the elevated plus maze and open field test, and increased locomotor activity. On a test of juvenile social play, microglial depletion led to decreased chase behaviors relative to control animals. There was no change in active social behavior in adults on a reciprocal social interaction test, but there was decreased passive interaction time and an increased number of social avoidance behaviors in clodronate treated rats relative to controls. There was an overall decrease in behavioral despair on the forced swim test in adult rats treated neonatally with clodronate. Females, but not males, treated neonatally with clodronate showed a blunted corticosterone response after acute stress in adulthood. These results show that microglia are important for the early life programming of juvenile and adult motivated behavior.

Prenatal immune activation causes hippocampal synaptic deficits in the absence of overt microglia anomalies

Prenatal exposure to infectious or inflammatory insults can increase the risk of developing neuropsychiatric disorder in later life, including schizophrenia, bipolar disorder, and autism. These brain disorders are also characterized by pre- and postsynaptic deficits. Using a well-established mouse model of maternal exposure to the viral mimetic polyriboinosinic-polyribocytidilic acid [poly(I:C)], we examined whether prenatal immune activation might cause synaptic deficits in the hippocampal formation of pubescent and adult offspring. Based on the widely appreciated role of microglia in synaptic pruning, we further explored possible associations between synaptic deficits and microglia anomalies in offspring of poly(I:C)-exposed and control mothers. We found that prenatal immune activation induced an adult onset of presynaptic hippocampal deficits (as evaluated by synaptophysin and bassoon density). The early-life insult further caused postsynaptic hippocampal deficits in pubescence (as evaluated by PSD95 and SynGAP density), some of which persisted into adulthood. In contrast, prenatal immune activation did not change microglia (or astrocyte) density, nor did it alter their activation phenotypes. The prenatal manipulation did also not cause signs of persistent systemic inflammation. Despite the absence of overt glial anomalies or systemic inflammation, adult offspring exposed to prenatal immune activation displayed increased hippocampal IL-1β levels. Taken together, our findings demonstrate that age-dependent synaptic deficits and abnormal pro-inflammatory cytokine expression can occur during postnatal brain maturation in the absence of microglial anomalies or systemic inflammation.

Emotional Contagion is not Altered in Mice Prenatally Exposed to Poly (I:C) on Gestational Day 9

Prenatal immune activation has been associated with increased risk of developing schizophrenia. The polyinosinic-polycytidylic acid (Poly(I:C)) mouse model replicates some of the endophenotype characteristic of this disorder but the social deficits observed in schizophrenia patients have not been well studied in this model. Therefore we aimed to investigate social behavior, in particular emotional contagion for pain, in this mouse model. We injected pregnant mouse dams with Poly(I:C) or saline (control) on gestation day 9 (GD9) and we evaluated their offspring in the pre-pulse inhibition (PPI) test at age 50–55 days old to confirm the reliability of our model. Mice were then evaluated in an emotional contagion test immediately followed by the light/dark test to explore post-test anxiety-like behavior at 10 weeks of age. In the emotional contagion test, an observer (prenatally exposed to Poly(I:C) or to saline) witnessed a familiar wild-type (WT) mouse (demonstrator) receiving electric foot shocks. Our results replicate the sensory gating impairments in the Poly(I:C) offspring but we only observed minor group differences in the social tasks. One of the differences we found was that demonstrators deposited fewer feces in the presence of control observers than of observers prenatally exposed to Poly(I:C), which we suggest could be due to the observers’ behavior. We discuss the findings in the context of age, sex and day of prenatal injection, suggesting that Poly(I:C) on GD9 may be a valuable tool to assess other symptoms or symptom clusters of schizophrenia but perhaps not comprising the social domain.

Prenatal immune activation in mice blocks the effects of environmental enrichment on exploratory behavior and microglia density

Adverse environmental factors including prenatal maternal infection are capable of inducing long-lasting behavioral and neural alterations which can enhance the risk to develop schizophrenia. It is so far not clear whether supportive postnatal environments are able to modify such prenatally-induced alterations. In rodent models, environmental enrichment influences behavior and cognition, for instance by affecting endocrinologic, immunologic, and neuroplastic parameters. The current study was designed to elucidate the influence of postnatal environmental enrichment on schizophrenia-like behavioral alterations induced by prenatal polyI:C immune stimulation at gestational day 9 in mice. Adult offspring were tested for amphetamine-induced locomotion, social interaction, and problem-solving behavior as well as expression of dopamine D1 and D2 receptors and associated molecules, microglia density and adult neurogenesis. Prenatal polyI:C treatment resulted in increased dopamine sensitivity and dopamine D2 receptor expression in adult offspring which was not reversed by environmental enrichment. Prenatal immune activation prevented the effects of environmental enrichment which increased exploratory behavior and microglia density in NaCl treated mice. Problem-solving behavior as well as the number of immature neurons was affected by neither prenatal immune stimulation nor postnatal environmental enrichment. The behavioral and neural alterations that persist into adulthood could not generally be modified by environmental enrichment. This might be due to early neurodevelopmental disturbances which could not be rescued or compensated for at a later developmental stage.

Flow cytometric characterization of microglia in the offspring of PolyI:C treated mice

The neuropathology of schizophrenia has been reported to be closely associated with microglial activation. In a previous study, using the prenatal PolyI:C schizophrenia animal model, we showed an increase in cell numbers and a reduction in microglial branching in 30-day-old PolyI:C descendants, which suggests that there is microglial activation during adolescence. To provide more information about the activation state, we aimed to examine the expression levels of Iba1, which was reported to be up-regulated in activated microglia. We used a flow cytometric approach and investigated CD11b and CD45, two additional markers for the identification of microglial cells. We demonstrated that intracellular staining against Iba1 can be used as a reliable flow cytometric method for identification of microglial cells. Prenatal PolyI:C treatment had long-term effects on CD11b and CD45 expression. It also resulted in a trend towards increased Iba1 expression. Imbalance in CD11b, CD45, and Iba1 expression might contribute to impaired synaptic surveillance and enhanced activation/inflammatory activity of microglia in adult offspring.

Stress and neuroinflammation: a systematic review of the effects of stress on microglia and the implications for mental illness

RATIONALE

Psychosocial stressors are a well-documented risk factor for mental illness. Neuroinflammation, in particular elevated microglial activity, has been proposed to mediate this association. A number of preclinical studies have investigated the effect of stress on microglial activity. However, these have not been systematically reviewed before.

OBJECTIVES

This study aims to systematically review the effects of stress on microglia, as indexed by the histological microglial marker ionised calcium binding adaptor molecule 1 (Iba-1), and consider the implications of these for the role of stress in the development of mental disorders.

METHODS

A systematic review was undertaken using pre-defined search criteria on PubMed and EMBASE. Inclusion and data extraction was agreed by two independent researchers after review of abstracts and full text.

RESULTS

Eighteen studies met the inclusion criteria. These used seven different psychosocial stressors, including chronic restraint, social isolation and repeated social defeat in gerbils, mice and/or rats. The hippocampus (11/18 studies) and prefrontal cortex (13/18 studies) were the most frequently studied areas. Within the hippocampus, increased Iba-1 levels of between 20 and 200 % were reported by all 11 studies; however, one study found this to be a duration-dependent effect. Of those examining the prefrontal cortex, ∼75 % found psychosocial stress resulted in elevated Iba-1 activity. Elevations were also consistently seen in the nucleus accumbens, and under some stress conditions in the amygdala and paraventricular nucleus.

CONCLUSIONS

There is consistent evidence that a range of psychosocial stressors lead to elevated microglial activity in the hippocampus and good evidence that this is also the case in other brain regions. These effects were seen with early-life/prenatal stress, as well as stressors in adulthood. We consider these findings in terms of the two-hit hypothesis, which proposes that early-life stress primes microglia, leading to a potentiated response to subsequent stress. The implications for understanding the pathoaetiology of mental disorders and the development of new treatments are also considered.

A New Outlook on Mental Illnesses: Glial Involvement Beyond the Glue

Mental illnesses have long been perceived as the exclusive consequence of abnormalities in neuronal functioning. Until recently, the role of glial cells in the pathophysiology of mental diseases has largely been overlooked. However recently, multiple lines of evidence suggest more diverse and significant functions of glia with behavior-altering effects. The newly ascribed roles of astrocytes, oligodendrocytes and microglia have led to their examination in brain pathology and mental illnesses. Indeed, abnormalities in glial function, structure and density have been observed in postmortem brain studies of subjects diagnosed with mental illnesses. In this review, we discuss the newly identified functions of glia and highlight the findings of glial abnormalities in psychiatric disorders. We discuss these preclinical and clinical findings implicating the involvement of glial cells in mental illnesses with the perspective that these cells may represent a new target for treatment.

Late prenatal immune activation causes hippocampal deficits in the absence of persistent inflammation across aging

BACKGROUND

Prenatal exposure to infection and/or inflammation is increasingly recognized to play an important role in neurodevelopmental brain disorders. It has recently been postulated that prenatal immune activation, especially when occurring during late gestational stages, may also induce pathological brain aging via sustained effects on systemic and central inflammation. Here, we tested this hypothesis using an established mouse model of exposure to viral-like immune activation in late pregnancy.

METHODS

Pregnant C57BL6/J mice on gestation day 17 were treated with the viral mimetic polyriboinosinic-polyribocytidilic acid (poly(I:C)) or control vehicle solution. The resulting offspring were first tested using cognitive and behavioral paradigms known to be sensitive to hippocampal damage, after which they were assigned to quantitative analyses of inflammatory cytokines, microglia density and morphology, astrocyte density, presynaptic markers, and neurotrophin expression in the hippocampus throughout aging (1, 5, and 22 months of age).

RESULTS

Maternal poly(I:C) treatment led to a robust increase in inflammatory cytokine levels in late gestation but did not cause persistent systemic or hippocampal inflammation in the offspring. The late prenatal manipulation also failed to cause long-term changes in microglia density, morphology, or activation, and did not induce signs of astrogliosis in pubescent, adult, or aged offspring. Despite the lack of persistent inflammatory or glial anomalies, offspring of poly(I:C)-exposed mothers showed marked and partly age-dependent deficits in hippocampus-regulated cognitive functions as well as impaired hippocampal synaptophysin and brain-derived neurotrophic factor (BDNF) expression.

CONCLUSIONS

Late prenatal exposure to viral-like immune activation in mice causes hippocampus-related cognitive and synaptic deficits in the absence of chronic inflammation across aging. These findings do not support the hypothesis that this form of prenatal immune activation may induce pathological brain aging via sustained effects on systemic and central inflammation. We further conclude that poly(I:C)-based prenatal immune activation models are reliable in their effectiveness to induce (hippocampal) neuropathology across aging, but they appear unsuited for studying the role of chronic systemic or central inflammation in brain aging.

Assessing brain immune activation in psychiatric disorders: clinical and preclinical PET imaging studies of the 18-kDa translocator protein

Accumulating evidence from different lines of research suggests an involvement of the immune system in the pathophysiology of several psychiatric disorders. During recent years, a series of positron emission tomography (PET) studies have been published using radioligands for the translocator protein (TSPO) to study microglia activation in schizophrenia, bipolar I disorder, major depression, autism spectrum disorder, and drug abuse. The results have been somewhat conflicting, which could be due to differences both in patient sample characteristics and in PET methods. In particular, further work is needed to address both methodological and biological sources of variability in TSPO levels, a process in which the use of animal models and small animal PET systems can be a valuable tool. Given this development, PET studies of immune activation have the potential to further increase our understanding of disease mechanisms in psychiatric disorders, which is a requisite in the search for new treatment approaches. Furthermore, molecular imaging could become an important clinical tool for identifying specific subgroups of patients or disease stages that would benefit from treatment targeting the immune system.

Maternal immune activation evoked by polyinosinic:polycytidylic acid does not evoke microglial cell activation in the embryo

Several studies have indicated that inflammation during pregnancy increases the risk for the development of neuropsychiatric disorders in the offspring. Morphological brain abnormalities combined with deviations in the inflammatory status of the brain can be observed in patients of both autism and schizophrenia. It was shown that acute infection can induce changes in maternal cytokine levels which in turn are suggested to affect fetal brain development and increase the risk on the development of neuropsychiatric disorders in the offspring. Animal models of maternal immune activation reproduce the etiology of neurodevelopmental disorders such as schizophrenia and autism. In this study the poly (I:C) model was used to mimic viral immune activation in pregnant mice in order to assess the activation status of fetal microglia in these developmental disorders. Because microglia are the resident immune cells of the brain they were expected to be activated due to the inflammatory stimulus. Microglial cell density and activation level in the fetal cortex and hippocampus were determined. Despite the presence of a systemic inflammation in the pregnant mice, there was no significant difference in fetal microglial cell density or immunohistochemically determined activation level between the control and inflammation group. These data indicate that activation of the fetal microglial cells is not likely to be responsible for the inflammation induced deficits in the offspring in this model.

Neurodevelopmental Animal Models Reveal the Convergent Role of Neurotransmitter Systems, Inflammation, and Oxidative Stress as Biomarkers of Schizophrenia: Implications for Novel Drug Development

Schizophrenia is a life altering disease with a complex etiology and pathophysiology, and although antipsychotics are valuable in treating the disorder, certain symptoms and/or sufferers remain resistant to treatment. Our poor understanding of the underlying neuropathological mechanisms of schizophrenia hinders the discovery and development of improved pharmacological treatment, so that filling these gaps is of utmost importance for an improved outcome. A vast amount of clinical data has strongly implicated the role of inflammation and oxidative insults in the pathophysiology of schizophrenia. Preclinical studies using animal models are fundamental in our understanding of disease development and pathology as well as the discovery and development of novel treatment options. In particular, social isolation rearing (SIR) and pre- or postnatal inflammation (PPNI) have shown great promise in mimicking the biobehavioral manifestations of schizophrenia. Furthermore, the "dual-hit" hypothesis of schizophrenia states that a first adverse event such as genetic predisposition or a prenatal insult renders an individual susceptible to develop the disease, while a second insult (e.g., postnatal inflammation, environmental adversity, or drug abuse) may be necessary to precipitate the full-blown syndrome. Animal models that emphasize the "dual-hit" hypothesis therefore provide valuable insight into understanding disease progression. In this Review, we will discuss SIR, PPNI, as well as possible "dual-hit" animal models within the context of the redox-immune-inflammatory hypothesis of schizophrenia, correlating such changes with the recognized monoamine and behavioral alterations of schizophrenia. Finally, based on these models, we will review new therapeutic options, especially those targeting immune-inflammatory and redox pathways.

Adaptive Immunity in Schizophrenia: Functional Implications of T Cells in the Etiology, Course and Treatment

Schizophrenia is a severe and highly complex neurodevelopmental disorder with an unknown etiopathology. Recently, immunopathogenesis has emerged as one of the most compelling etiological models of schizophrenia. Over the past few years considerable research has been devoted to the role of innate immune responses in schizophrenia. The findings of such studies have helped to conceptualize schizophrenia as a chronic low-grade inflammatory disorder. Although the contribution of adaptive immune responses has also been emphasized, however, the precise role of T cells in the underlying neurobiological pathways of schizophrenia is yet to be ascertained comprehensively. T cells have the ability to infiltrate brain and mediate neuro-immune cross-talk. Conversely, the central nervous system and the neurotransmitters are capable of regulating the immune system. Neurotransmitter like dopamine, implicated widely in schizophrenia risk and progression can modulate the proliferation, trafficking and functions of T cells. Within brain, T cells activate microglia, induce production of pro-inflammatory cytokines as well as reactive oxygen species and subsequently lead to neuroinflammation. Importantly, such processes contribute to neuronal injury/death and are gradually being implicated as mediators of neuroprogressive changes in schizophrenia. Antipsychotic drugs, commonly used to treat schizophrenia are also known to affect adaptive immune system; interfere with the differentiation and functions of T cells. This understanding suggests a pivotal role of T cells in the etiology, course and treatment of schizophrenia and forms the basis of this review.

Inflammational Animal Models for Schizophrenia

Abstract. Inflammational-immunological processes within the pathophysiology of schizophrenia seem to play an important role. Early signals of neurobiological changes in the embryonal phase of brain in later patients with schizophrenia might lead to activation of the immunological system, for example, of cytokines and microglial cells. Microglia then induces – via the neurotoxic activities of these cells as an overreaction – a rarification of synaptic connections in frontal and temporal brain regions, that is, reduction of the neuropil. Promising inflammational animal models for schizophrenia with high validity can be used today to mimic behavioral as well as neurobiological findings in patients, for example, the well-known neurochemical alterations of dopaminergic, glutamatergic, serotonergic, and other neurotransmitter systems. Also the microglial activation can be modeled well within one of this models, that is, the inflammational PolyI:C animal model of schizophrenia, showing a time peak in late adolescence/early adulthood. The exact mechanism, by which activated microglia cells then triggers further neurodegeneration, must now be investigated in broader detail. Thus, these animal models can be used to understand the pathophysiology of schizophrenia better especially concerning the interaction of immune activation, inflammation, and neurodegeneration. This could also lead to the development of anti-inflammational treatment options and of preventive interventions.

Antineuronal antibodies against neurotransmitter receptors and synaptic proteins in schizophrenia: current knowledge and clinical implications

When Eugen Bleuler coined the term 'schizophrenia' he believed that various causes of illness may underlie this disease. Currently, neurodevelopmental abnormalities and consecutive impairments in dopaminergic and glutamatergic neurotransmission are considered as major causes of schizophrenia. However, there are various indications for involvement of immune processes, at least in subgroups of patients. Circulating antineuronal antibodies provide a promising link between the well-described disturbances in neurotransmission and the immune hypothesis of schizophrenia. This review summarizes important studies that have examined the role of glutamate, dopamine, acetylcholine and serotonin receptor autoantibodies, and other antineuronal antibodies against synaptic proteins in the serum of patients diagnosed with schizophrenia. Currently, it is not known whether the presence of antineuronal antibodies in blood should be considered as a causal or disease-modulating factor in schizophrenia. Due to emerging evidence regarding the important role of the blood-brain barrier, combined testing of serum and cerebrospinal fluid is likely to be more appropriate to answer this question than pure serum analyses. We suggest implementation of such testing in first-onset and treatment-resistant patients as part of the diagnostic process. In addition, future clinical trials should evaluate if immunotherapy (e.g. cortisone pulse therapy, intravenous immunoglobulins, plasmapheresis, rituximab, or cyclophosphamide) is helpful in cases with a neuroinflammatory component.

Kynurenine 3-Monooxygenase: An Influential Mediator of Neuropathology

Mounting evidence demonstrates that kynurenine metabolism may play an important pathogenic role in the development of multiple neurological and neuropsychiatric disorders. The kynurenine pathway consists of two functionally distinct branches that generate both neuroactive and oxidatively reactive metabolites. In the brain, the rate-limiting enzyme for one of these branches, kynurenine 3-monooxygenase (KMO), is predominantly expressed in microglia and has emerged as a pivotal point of metabolic regulation. KMO substrate and expression levels are upregulated by pro-inflammatory cytokines and altered by functional genetic mutations. Increased KMO metabolism results in the formation of metabolites that activate glutamate receptors and elevate oxidative stress, while recent evidence has revealed neurodevelopmental consequences of reduced KMO activity. Together, the evidence suggests that KMO is positioned at a critical metabolic junction to influence the development or trajectory of a myriad of neurological diseases. Understanding the mechanism(s) by which alterations in KMO activity are able to impair neuronal function, and viability will enhance our knowledge of related disease pathology and provide insight into novel therapeutic opportunities. This review will discuss the influence of KMO on brain kynurenine metabolism and the current understanding of molecular mechanisms by which altered KMO activity may contribute to neurodevelopment, neurodegenerative, and neuropsychiatric diseases.

Brain in flames - animal models of psychosis: utility and limitations

The neurodevelopmental hypothesis of schizophrenia posits that schizophrenia is a psychopathological condition resulting from aberrations in neurodevelopmental processes caused by a combination of environmental and genetic factors which proceed long before the onset of clinical symptoms. Many studies discuss an immunological component in the onset and progression of schizophrenia. We here review studies utilizing animal models of schizophrenia with manipulations of genetic, pharmacologic, and immunological origin. We focus on the immunological component to bridge the studies in terms of evaluation and treatment options of negative, positive, and cognitive symptoms. Throughout the review we link certain aspects of each model to the situation in human schizophrenic patients. In conclusion we suggest a combination of existing models to better represent the human situation. Moreover, we emphasize that animal models represent defined single or multiple symptoms or hallmarks of a given disease.

Minocycline alleviates behavioral deficits and inhibits microglial activation in the offspring of pregnant mice after administration of polyriboinosinic-polyribocytidilic acid

Epidemiological studies have indicated that maternal infection during pregnancy may lead to a higher incidence of schizophrenia in the offspring. Activation of microglia is a key event in the reaction of the cerebral immune system to pathological changes. It can be hypothesized that microglia contribute to the neuropathology of schizophrenia. In this study, at embryonic day (ED) 9 pregnant mice were treated with intraperitoneal injection of polyriboinosinic-polyribocytidilic acid (Poly I:C) at a single dose of 20 mg/kg. At postnatal day 42, descendants were treated with minocycline (40 mg/kg) or saline for consecutive 14 days. Behavioral changes (locomotor activity, social interaction, and prepulse inhibition) were examined and the number of microglia was assessed after the treatment. The adult offspring exposed to Poly I:C at ED 9 showed behavioral changes (hyperlocomotion, deficits in social interaction and prepulse inhibition) and significant microglial activation in these brain areas (hippocampus, thalamus, and cerebral cortex) compared to those in saline-injected group. Moreover, minocycline attenuated the behavioral deficits and inhibited the activated microglia. These findings suggest that maternal infection may contribute to microglial activation in the offspring. In addition, the effect of minocycline in this immune model may be related to the inhibition of microglial activation.

The risk for behavioural deficits is determined by the maternal immune response to prenatal immune challenge in a neurodevelopmental model

BACKGROUND

Schizophrenia is a highly disabling psychiatric disorder with a proposed neurodevelopmental basis. One mechanism through which genetic and environmental risk factors might act is by triggering persistent brain inflammation, as evidenced by long-lasting neuro-immunological disturbances in patients. Our goal was to investigate whether microglia activation is a neurobiological correlate to the altered behaviour in the maternal immune activation (MIA) model, a well-validated animal model with relevance to schizophrenia. A recent observation in the MIA model is the differential maternal body weight response to the immune stimulus, correlated with a different behavioural outcome in the offspring. Although it is generally assumed that the differences in maternal weight response reflect differences in cytokine response, this has not been investigated so far. Our aim was to investigate whether (i) the maternal weight response to MIA reflects differences in the maternal cytokine response, (ii) the differential behavioural phenotype of the offspring extends to depressive symptoms such as anhedonia and (iii) there are changes in chronic microglia activation dependent on the behavioural phenotype.

METHODS

Based on a dose-response study, MIA was induced in pregnant rats by injecting 4mg/kg Poly I:C at gestational day 15. Serum samples were collected to assess the amount of TNF-α in the maternal blood following MIA. MIA offspring were divided into weight loss (WL; n=14) and weight gain (WG; n=10) groups, depending on the maternal body weight response to Poly I:C. Adult offspring were behaviourally phenotyped for prepulse inhibition, locomotor activity with and without amphetamine and MK-801 challenge, and sucrose preference. Finally, microglia activation was scored on CD11b- and Iba1-immunohistochemically stained sections.

RESULTS

Pregnant dams that lost weight following MIA showed increased levels of TNF-α compared to controls, unlike dams that gained weight following MIA. Poly I:C WL offspring showed the most severe behavioural outcome. Poly I:C WG offspring, on the other hand, did not show clear behavioural deficits. Most interestingly a reduced sucrose preference indicative of anhedonia was found in Poly I:C WL but not Poly I:C WG offspring compared to controls. Finally, there were no significant differences in microglia activation scores between any of the investigated groups.

CONCLUSIONS

The individual maternal immune response to MIA is an important determinant of the behavioural outcome in offspring, including negative symptoms such as anhedonia. We failed to find any significant difference in the level of microglia activation between Poly I:C WL, Poly I:C WG and control offspring.

Maternal immune activation and abnormal brain development across CNS disorders

Epidemiological studies have shown a clear association between maternal infection and schizophrenia or autism in the progeny. Animal models have revealed maternal immune activation (mIA) to be a profound risk factor for neurochemical and behavioural abnormalities in the offspring. Microglial priming has been proposed as a major consequence of mIA, and represents a critical link in a causal chain that leads to the wide spectrum of neuronal dysfunctions and behavioural phenotypes observed in the juvenile, adult or aged offspring. Such diversity of phenotypic outcomes in the mIA model are mirrored by recent clinical evidence suggesting that infectious exposure during pregnancy is also associated with epilepsy and, to a lesser extent, cerebral palsy in children. Preclinical research also suggests that mIA might precipitate the development of Alzheimer and Parkinson diseases. Here, we summarize and critically review the emerging evidence that mIA is a shared environmental risk factor across CNS disorders that varies as a function of interactions between genetic and additional environmental factors. We also review ongoing clinical trials targeting immune pathways affected by mIA that may play a part in disease manifestation. In addition, future directions and outstanding questions are discussed, including potential symptomatic, disease-modifying and preventive treatment strategies.

Effect of maternal immune activation on the kynurenine pathway in preadolescent rat offspring and on MK801-induced hyperlocomotion in adulthood: amelioration by COX-2 inhibition

Infections during pregnancy and subsequent maternal immune activation (MIA) increase risk for schizophrenia in offspring. The progeny of rodents injected with the viral infection mimic polyI:C during gestation display brain and behavioural abnormalities but the underlying mechanisms are unknown. Since the blood kynurenine pathway (KP) of tryptophan degradation impacts brain function and is strongly regulated by the immune system, we tested if KP changes occur in polyI:C offspring at preadolescence. We also tested whether MK801-induced hyperlocomotion, a behaviour characteristic of adult polyI:C offspring, is prevented by adolescent treatment with celecoxib, a COX-2 inhibitor that impacts the KP. Pregnant rats were treated with polyI:C (4mg/kg, i.v.) or vehicle on gestational day 19. Serum levels of KP metabolites were measured in offspring of polyI:C or vehicle treated dams at postnatal day (PND) 31-33 using HPLC/GCMS. Additional polyI:C or vehicle exposed offspring were given celecoxib or vehicle between PND 35 and 46 and tested with MK801 (0.3mg/kg) in adulthood (PND>90). Prenatal polyI:C resulted in increases in the serum KP neurotoxic metabolite quinolinic acid at PND 31-33 (105%, p=0.014). In contrast, the neuroprotective kynurenic acid and its precursor kynurenine were significantly decreased (28% p=0.027, and 31% p=0.033, respectively). Picolinic acid, another neuroprotective KP metabolite, was increased (31%, p=0.014). Adolescent treatment with celecoxib (2.5 and 5mg/kg/day, i.p.) prevented the development of MK801-induced hyperlocomotion in adult polyI:C offspring. Our study reveals the blood KP as a potential mechanism by which MIA interferes with postnatal brain maturation and associated behavioural disturbances and emphasises the preventative potential of inflammation targeting drugs.

Minocycline rescues decrease in neurogenesis, increase in microglia cytokines and deficits in sensorimotor gating in an animal model of schizophrenia

Adult neurogenesis in the hippocampus is impaired in schizophrenic patients and in an animal model of schizophrenia. Amongst a plethora of regulators, the immune system has been shown repeatedly to strongly modulate neurogenesis under physiological and pathological conditions. It is well accepted, that schizophrenic patients have an aberrant peripheral immune status, which is also reflected in the animal model. The microglia as the intrinsic immune competent cells of the brain have recently come into focus as possible therapeutic targets in schizophrenia. We here used a maternal immune stimulation rodent model of schizophrenia in which polyinosinic-polycytidilic acid (Poly I:C) was injected into pregnant rats to mimic an anti-viral immune response. We identified microglia IL-1β and TNF-α increase constituting the factors correlating best with decreases in net-neurogenesis and impairment in pre-pulse inhibition of a startle response in the Poly I:C model. Treatment with the antibiotic minocycline (3mg/kg/day) normalized microglial cytokine production in the hippocampus and rescued neurogenesis and behavior. We could also show that enhanced microglial TNF-α and IL-1β production in the hippocampus was accompanied by a decrease in the pro-proliferative TNFR2 receptor expression on neuronal progenitor cells, which could be attenuated by minocycline. These findings strongly support the idea to use anti-inflammatory drugs to target microglia activation as an adjunctive therapy in schizophrenic patients.

Perineuronal nets and schizophrenia: the importance of neuronal coatings

Schizophrenia is a complex brain disorder associated with deficits in synaptic connectivity. The insidious onset of this illness during late adolescence and early adulthood has been reported to be dependent on several key processes of brain development including synaptic refinement, myelination and the physiological maturation of inhibitory neural networks. Interestingly, these events coincide with the appearance of perineuronal nets (PNNs), reticular structures composed of components of the extracellular matrix that coat a variety of cells in the mammalian brain. Until recently, the functions of the PNN had remained enigmatic, but are now considered to be important in development of the central nervous system, neuronal protection and synaptic plasticity, all elements which have been associated with schizophrenia. Here, we review the emerging evidence linking PNNs to schizophrenia. Future studies aimed at further elucidating the functions of PNNs will provide new insights into the pathophysiology of schizophrenia leading to the identification of novel therapeutic targets with the potential to restore normal synaptic integrity in the brain of patients afflicted by this illness.