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

Beyond the looking glass: recent advances in understanding the impact of environmental exposures on neuropsychiatric disease

The etiologic pathways leading to neuropsychiatric diseases remain poorly defined. As genomic technologies have advanced over the past several decades, considerable progress has been made linking neuropsychiatric disorders to genetic underpinnings. Interest and consideration of nongenetic risk factors (e.g., lead exposure and schizophrenia) have, in contrast, lagged behind heritable frameworks of explanation. Thus, the association of neuropsychiatric illness to environmental chemical exposure, and their potential interactions with genetic susceptibility, are largely unexplored. In this review, we describe emerging approaches for considering the impact of chemical risk factors acting alone and in concert with genetic risk, and point to the potential role of epigenetics in mediating exposure effects on transcription of genes implicated in mental disorders. We highlight recent examples of research in nongenetic risk factors in psychiatric disorders that point to potential shared biological mechanisms-synaptic dysfunction, immune alterations, and gut-brain interactions. We outline new tools and resources that can be harnessed for the study of environmental factors in psychiatric disorders. These tools, combined with emerging experimental evidence, suggest that there is a need to broadly incorporate environmental exposures in psychiatric research, with the ultimate goal of identifying modifiable risk factors and informing new treatment strategies for neuropsychiatric disease.

Maternal Immune Activation by Poly I:C as a preclinical Model for Neurodevelopmental Disorders: A focus on Autism and Schizophrenia

Maternal immune activation (MIA) in response to a viral infection during early and mid-gestation has been linked through various epidemiological studies to a higher risk for the child to develop autism or schizophrenia-related symptoms.. This has led to the establishment of the pathogen-free poly I:C-induced MIA animal model for neurodevelopmental disorders, which shows relatively high construct and face validity. Depending on the experimental variables, particularly the timing of poly I:C administration, different behavioural and molecular phenotypes have been described that relate to specific symptoms of neurodevelopmental disorders such as autism spectrum disorder and/or schizophrenia. We here review and summarize epidemiological evidence for the effects of maternal infection and immune activation, as well as major findings in different poly I:C MIA models with a focus on poly I:C exposure timing, behavioural and molecular changes in the offspring, and characteristics of the model that relate it to autism spectrum disorder and schizophrenia.

Maternal stressors and the developmental origins of neuropsychiatric risk

The maternal environment during pregnancy is critical for fetal development and perinatal perturbations can prime offspring disease risk. Here, we briefly review evidence linking two well-characterized maternal stressors - psychosocial stress and infection - to increased neuropsychiatric risk in offspring. In the current climate of increasing obesity and globalization of the Western-style diet, maternal overnutrition emerges as a pressing public health concern. We focus our attention on recent epidemiological and animal model evidence showing that, like psychosocial stress and infection, maternal overnutrition can also increase offspring neuropsychiatric risk. Using lessons learned from the psychosocial stress and infection literature, we discuss how altered maternal and placental physiology in the setting of overnutrition may contribute to abnormal fetal development and resulting neuropsychiatric outcomes. A better understanding of converging pathophysiological pathways shared between stressors may enable development of interventions against neuropsychiatric illnesses that may be beneficial across stressors.

Schizophrenia and Influenza at the Centenary of the 1918-1919 Spanish Influenza Pandemic: Mechanisms of Psychosis Risk

Associations between influenza infection and psychosis have been reported since the eighteenth century, with acute "psychoses of influenza" documented during multiple pandemics. In the late 20th century, reports of a season-of-birth effect in schizophrenia were supported by large-scale ecological and sero-epidemiological studies suggesting that maternal influenza infection increases the risk of psychosis in offspring. We examine the evidence for the association between influenza infection and schizophrenia risk, before reviewing possible mechanisms via which this risk may be conferred. Maternal immune activation models implicate placental dysfunction, disruption of cytokine networks, and subsequent microglial activation as potentially important pathogenic processes. More recent neuroimmunological advances focusing on neuronal autoimmunity following infection provide the basis for a model of infection-induced psychosis, potentially implicating autoimmunity to schizophrenia-relevant protein targets including the N-methyl-D-aspartate receptor. Finally, we outline areas for future research and relevant experimental approaches and consider whether the current evidence provides a basis for the rational development of strategies to prevent schizophrenia.

Microglial Function in the Effects of Early-Life Stress on Brain and Behavioral Development

The putative effects of early-life stress (ELS) on later behavior and neurobiology have been widely investigated. Recently, microglia have been implicated in mediating some of the effects of ELS on behavior. In this review, findings from preclinical and clinical literature with a specific focus on microglial alterations induced by the exposure to ELS (i.e., exposure to behavioral stressors or environmental agents and infection) are summarized. These studies were utilized to interpret changes in developmental trajectories based on the time at which the stress occurred, as well as the paradigm used. ELS and microglial alterations were found to be associated with a wide array of deficits including cognitive performance, memory, reward processing, and processing of social stimuli. Four general conclusions emerged: (1) ELS interferes with microglial developmental programs, including their proliferation and death and their phagocytic activity; (2) this can affect neuronal and non-neuronal developmental processes, which are dynamic during development and for which microglial activity is instrumental; (3) the effects are extremely dependent on the time point at which the investigation is carried out; and (4) both pre- and postnatal ELS can prime microglial reactivity, indicating a long-lasting alteration, which has been implicated in behavioral abnormalities later in life.

Increased extracellular free-water in adult male rats following in utero exposure to maternal immune activation

BACKGROUND

In previous work, we applied novel in vivo imaging methods to reveal that white matter pathology in patients with first-episode psychosis (FEP) is mainly characterized by excessive extracellular free-water, and to a lesser extent by cellular processes, such as demyelination. Here, we apply a back-translational approach to evaluate whether or not a rodent model of maternal immune activation (MIA) induces patterns of white matter pathology that we observed in patients with FEP. To this end, we examined free-water and tissue-specific white matter alterations in rats born to mothers exposed to the viral mimic polyriboinosinic-polyribocytidylic acid (Poly-I:C) in pregnancy, which is widely used to produce alterations relevant to schizophrenia and is characterized by a robust neuroinflammatory response.

METHOD

Pregnant dams were injected on gestational day 15 with the viral mimic Poly-I:C (4 mg/kg) or saline. Diffusion-weighted magnetic resonance images were acquired from 17 male offspring (9 Poly-I:C and 8 saline) on postnatal day 90, after the emergence of brain structural and behavioral abnormalities. The free-water fraction (FW) and tissue-specific fractional anisotropy (FA_T), as well as conventional fractional anisotropy (FA) were computed across voxels traversing a white matter skeleton. Voxel-wise and whole-brain averaged white matter were tested for significant microstructural alterations in immune-challenged, relative to saline-exposed offspring.

RESULTS

Compared to saline-exposed offspring, those exposed to maternal Poly-I:C displayed increased extracellular FW averaged across voxels comprising a white matter skeleton (t(15) = 2.74; p = 0.01). Voxel-wise analysis ascribed these changes to white matter within the corpus callosum, external capsule and the striatum. In contrast, no significant between-group differences emerged for FA_T or for conventional FA, measured across average and voxel-wise white matter.

CONCLUSION

We identified excess FW across frontal white matter fibers of rats exposed to prenatal immune activation, analogous to our "bedside" observation in FEP patients. Findings from this initial experiment promote use of the MIA model to examine pathological pathways underlying FW alterations observed in patients with schizophrenia. Establishing these mechanisms has important implications for clinical studies, as free-water imaging reflects a feasible biomarker that has so far yielded consistent findings in the early stages of schizophrenia.

Maternal viral infection causes global alterations in porcine fetal microglia

Maternal infections during pregnancy are associated with increased risk of neurodevelopmental disorders, although the precise mechanisms remain to be elucidated. Previously, we established a maternal immune activation (MIA) model using swine, which results in altered social behaviors of piglet offspring. These behavioral abnormalities occurred in the absence of microglia priming. Thus, we examined fetal microglial activity during prenatal development in response to maternal infection with live porcine reproductive and respiratory syndrome virus. Fetuses were obtained by cesarean sections performed 7 and 21 d postinoculation (dpi). MIA fetuses had reduced brain weights at 21 dpi compared to controls. Furthermore, MIA microglia increased expression of major histocompatibility complex class II that was coupled with reduced phagocytic and chemotactic activity compared to controls. High-throughput gene-expression analysis of microglial-enriched genes involved in neurodevelopment, the microglia sensome, and inflammation revealed differential regulation in primary microglia and in whole amygdala tissue. Microglia density was increased in the fetal amygdala at 7 dpi. Our data also reveal widespread sexual dimorphisms in microglial gene expression and demonstrate that the consequences of MIA are sex dependent. Overall, these results indicate that fetal microglia are significantly altered by maternal viral infection, presenting a potential mechanism through which MIA impacts prenatal brain development and function.

Elevated Gestational IL-13 During Fetal Development Is Associated With Hyperactivity and Inattention in Eight-Year-Old Children

Maternal immune activation (MIA) during fetal development leads to behavioral and psychological disorders in the offspring. Concomitantly, insufficient supply of polyunsaturated fatty acids (PUFAs) is suspected to contribute to early neuronal maldevelopment due to the immune modulatory capabilities of PUFAs. However, human data are missing considering both of these aspects and their impact on children's behavioral outcomes. In line, this study aimed to elucidate the influence of gestational cytokines and PUFA-containing lipids during late pregnancy on behavioral sequelae in childhood, particularly focusing on an immune activation shaped by a history of maternal atopic diseases instead of a pathogen-mediated immune response. Based on the prospective mother-child cohort LINA we assessed the unstimulated blood cytokine profiles and concentrations of PUFA-containing lipids of 293 mothers at the 34th week of pregnancy. Maternal history of atopic diseases was obtained from questionnaires and behavior in eight-year-old children was assessed by the standardized Strength and Difficulties Questionnaires (SDQ) generating scores for hyperactivity/inattention, emotional symptoms, conduct problems, and peer relationship problems. Elevated IL-13 increased the risk for the child to show behavioral difficulties, in particular, hyperactive/inattentive behavior [adj. OR (95% CI): 2.47 (1.51-4.02), n = 255 vs. 38] at the age of eight years. Although the presence of maternal atopic dermatitis (AD) was associated with increased gestational IL-13 concentrations [adj. MR (95% CI): 1.17 (1.04-1.32)], no effect on children's behavioral difficulties was observed. However, a decrease in the PUFA containing lipid species PC aa C38:6 was not only associated with an increased gestational IL-13 concentration but also mediated the indirect effect of low PC aa C38:6 concentrations on children's abnormal behavior independent of maternal AD. We additionally assessed whether maternal IL-13 and PC aa C38:6 concentrations translate their effect by altering children's cord blood PC aa C38:6 and IL-13. While also the children's cord blood IL-13 was related to children's behavior, no effect of children's PC aa C38:6 was observed. This is the first study demonstrating that elevated gestational IL-13 increases the risk for children to develop behavioral difficulties. Analyses suggest that a reduced supply of gestational PC aa C38:6 contributes to elevated gestational IL-13 leading to behavioral sequelae in the offspring.

Recent Advances in the Early Intervention in Schizophrenia: Future Direction from Preclinical Findings

PURPOSE OF REVIEW

In the past decade, there has been increasing interest in the potential benefit of early intervention in schizophrenia. Patients with schizophrenia show cognitive impairment for several years preceding the onset of psychosis. The author discusses the recent topics on prevention of schizophrenia.

RECENT FINDINGS

Preclinical findings suggest that maternal immune activation (MIA) produces cognitive deficits as a prodromal symptom in juvenile offspring in rodents. Treatment with anti-inflammatory compounds, such as D-serine, 7,8-dihydroxyflavone (a TrkB agonist), sulforaphane (or its precursor glucoraphanin), and TPPU (1-trifluoromethoxyphenyl-3-(1-propionylpiperidin-4-yl) urea: a soluble epoxide hydrolase inhibitor), during adolescence might prevent the onset of behavioral abnormalities and parvalbumin immunoreactivity in the medial prefrontal cortex of adult offspring after MIA. Based on the role of inflammation and cognitive impairment in the prodromal state, early intervention using anti-inflammatory compounds (i.e., D-serine, sodium benzoate, TrkB agonist, Nrf2 agonist, soluble epoxide hydrolase inhibitor) may reduce the risk of subsequent transition to schizophrenia.

Maternal Immune Activation during Pregnancy Alters the Behavior Profile of Female Offspring of Sprague Dawley Rats

Sex differences are documented in psychiatric and neurological disorders, yet most preclinical animal research has been conducted in males only. There is a need to better understand of the nature of sex differences in brain disease in order to meet the needs of psychiatric patients. We present the behavior profile of adult female offspring produced using a maternal immune activation (MIA) model where pregnant rats receive an immune stimulant and the offspring typically show various abnormalities consistent with psychiatric illnesses such as schizophrenia and autism. The results in female offspring were compared to a previously published cohort of their male siblings (Lins et al., 2018). We examined prepulse inhibition (PPI), sociability, MK-801-induced locomotor activity, crossmodal object recognition (CMOR), and oddity discrimination; behaviors relevant to the positive, negative, and cognitive symptoms of schizophrenia. No between-treatment differences in PPI or locomotor activity were noted. Tactile memory was observed in the control and treated female offspring, visual recognition memory was deficient in the polyinosinic:polycytidylic acid (polyI:C) offspring only, and both groups lacked crossmodal recognition. PolyI:C offspring were impaired in oddity preference and had reduced preference for a stranger conspecific in a sociability assay. Systemic maternal CXCL1, IL-6, and TNF-a levels 3 h after polyI:C treatment were determined, but no relationship was found between these cytokines and the behavior seen in the adult female offspring. Overall, female offspring of polyI:C-treated dams display an array of behavior abnormalities relevant to psychiatric illnesses such as schizophrenia similar to those previously reported in male rats.

Microglia: Brain cells on the move

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

Mapping the impact of exposure to maternal immune activation on juvenile Wistar rat brain macro- and microstructure during early post-natal development

Maternal immune activation is consistently associated with elevated risk for multiple psychiatric disorders in the affected offspring. Related to this, an important goal of our work is to explore the impact of maternal immune activation effects across the lifespan. In this context, we recently reported the effects of polyriboinosinic-polyribocytidylic acid-induced maternal immune activation at gestational day 15, immediately prior to birth, at gestational day 21 and again at post-natal day 21, providing a systematic assessment of plasma interleukin 6, body temperature and weight alterations in pregnant rats and preliminary evidence for gross morphological changes and microglial neuropathology in both male and female offsprings at these time points. Here, we sought to complement and extend these data by characterising in more detail the mesoscale impact of gestational polyriboinosinic-polyribocytidylic acid exposure at gestational day 15 on the neuroanatomy of the juvenile (post-natal day 21) rat brain using high-resolution, ex vivo anatomical magnetic resonance imaging in combination with atlas-based segmentation. Our preliminary data suggest subtle neuroanatomical effects of gestational polyriboinosinic-polyribocytidylic acid exposure (n = 10) relative to saline controls (n = 10) at this time-point. Specifically, we found an increase in the relative volume of the diagonal domain in polyriboinosinic-polyribocytidylic acid offspring (p < 0.01 uncorrected), which just failed to pass stringent multiple comparisons correction (actual q = 0.07). No statistically significant microstructural alterations were detectable using diffusion tensor imaging. Further studies are required to map the proximal effects of maternal immune activation on the developing rodent brain from foetal to early post-natal life and confirm our findings herein.

Maternal immune activation, central nervous system development and behavioral phenotypes

Maternal immune activation (MIA) refers to a maternal immune system triggered by infectious or infectious-like stimuli. A cascade of cytokines and immunologic alterations are transmitted to the fetus, resulting in adverse phenotypes most notably in the central nervous system. Epidemiologic studies implicate maternal infections in a variety of neuropsychiatric disorders, most commonly autism spectrum disorders and schizophrenia. In animal models, MIA causes neurochemical and anatomic changes in the brain that correspond to those found in humans with the disorders. As our understanding of the interactions between environment, genetics, and immune system grows, the role of alternative, noninfectious risk factors, such as prenatal stress, obesity, and the gut microbiome also becomes clearer. This review considers how infectious and noninfectious etiologies activate the maternal immune system. Their impact on fetal programming and neuropsychiatric disorders in offspring is examined in the context of human and animal studies.

Placental Macrophages: A Window Into Fetal Microglial Function in Maternal Obesity

Fetal placental macrophages and microglia (resident brain macrophages) have a common origin in the fetal yolk sac. Yolk-sac-derived macrophages comprise the permanent pool of brain microglia throughout an individual's lifetime. Inappropriate fetal microglial priming may therefore have lifelong neurodevelopmental consequences, but direct evaluation of microglial function in a living fetus or neonate is impossible. We sought to test the hypothesis that maternal obesity would prime both placental macrophages and fetal brain microglia to overrespond to an immune challenge, thus providing a window into microglial function using placental cells. Obesity was induced in C57BL/6 J mice using a 60% high-fat diet. On embryonic day 17.5, fetal brain microglia and corresponding CD11b + placental cells were isolated from fresh tissue. Cells were treated with media or lipopolysaccharide (LPS). Tumor necrosis factor-alpha (TNF-α) production by stimulated and unstimulated cells was quantified via ELISA. We demonstrate for the first time that the proinflammatory cytokine production of CD11b + placental cells is strongly correlated with that of brain microglia (Spearman's ρ = 0.73, p = 0.002) in the setting of maternal obesity. Maternal obesity-exposed CD11b + cells had an exaggerated response to LPS compared to controls, with a 5.1-fold increase in TNF-α production in placentas (p = 0.003) and 3.8-fold increase in TNF-α production in brains (p = 0.002). In sex-stratified analyses, only male obesity-exposed brains and placentas had significant increase in TNF-α production in response to LPS. Taken together, these data suggest that maternal obesity primes both placental macrophages and fetal brain microglia to overproduce a proinflammatory cytokine in response to immune challenge. Male brain and placental immune response is more marked than female in this setting. Given that fetal microglial priming may impact neuroimmune function throughout the lifespan, these data could provide insight into the male predominance of certain neurodevelopmental morbidities linked to maternal obesity, including cognitive dysfunction, autism spectrum disorder, and ADHD. Placental CD11b+ macrophages may have the potential to serve as an accessible biomarker of aberrant fetal brain immune activation in maternal obesity. This finding may have broader implications for assaying the impact of other maternal exposures on fetal brain development.