Maternal immune activation in rodent models: A systematic review of neurodevelopmental changes in gene expression and epigenetic modulation in the offspring brain

Maternal immune activation affects female offspring whisker movements during object exploration in a rat model of neurodevelopmental disorders

Poly I:C rat offspring are used to investigate the effects of in utero exposure to maternal immune activation (MIA) and have been suggested as a model of neurodevelopmental disorders (NDD). The behavioural symptoms of this model are diverse and can vary with external factors, including the choice of background strain and husbandry practices. Measuring whisker movements provides quantitative, robust measurements of sensory, motor and cognitive behaviours in rodents. In this study, whisker movements were investigated in 50-day-old male and female offspring of MIA-exposed rat dams and compared to age-matched offspring of control (vehicle) dams. Rat offspring were filmed using high-speed videography in a sequential object exploration task with smooth and textured objects. Poly I:C treatment effects were found in female offspring that did not increase whisker mean angular position during object exploration, especially for the smooth object, indicating an attentional deficit. Whisker tracking during object exploration is demonstrated here, for the first time, as a useful, quick and non-invasive tool to identify both treatment effects and sex differences in a model of MIA-induced NDDs.

Synaptic proteome perturbations after maternal immune activation: Identification of embryonic and adult hippocampal changes

BACKGROUND

Maternal immune activation (MIA) triggers neurobiological changes in offspring, potentially reshaping the molecular synaptic landscape, with the hippocampus being particularly vulnerable. However, critical details regarding developmental timing of these changes and whether they differ between males and females remain unclear.

METHODS

We induced MIA in C57BL/6J mice on gestational day nine using the viral mimetic poly(I:C) and performed mass spectrometry-based proteomic analyses on hippocampal synaptoneurosomes of embryonic (E18) and adult (20 ± 1 weeks) MIA offspring.

RESULTS

In the embryonic synaptoneurosomes, MIA led to lipid, polysaccharide, and glycoprotein metabolism pathway disruptions. In the adult synaptic proteome, we observed a dynamic shift toward transmembrane trafficking, intracellular signalling cascades, including cell death and growth, and cytoskeletal organisation. In adults, many associated pathways overlapped between males and females. However, we found distinct sex-specific enrichment of dopaminergic and glutamatergic pathways. We identified 50 proteins altered by MIA in both embryonic and adult samples (28 with the same directionality), mainly involved in presynaptic structure and synaptic vesicle function. We probed human phenome-wide association study data in the cognitive and psychiatric domains, and 49 of the 50 genes encoding these proteins were significantly associated with the investigated phenotypes.

CONCLUSIONS

Our data emphasise the dynamic effects of viral-like MIA on developing and mature hippocampi and provide novel targets for study following prenatal immune challenges. The 22 proteins that changed directionality from the embryonic to adult hippocampus, suggestive of compensatory over-adaptions, are particularly attractive for future investigations.

Prenatal exposure to common infections and newborn DNA methylation: A prospective, population-based study

BACKGROUND

Infections during pregnancy have been robustly associated with adverse mental and physical health outcomes in offspring, yet the underlying molecular pathways remain largely unknown. Here, we examined whether exposure to common infections in utero associates with DNA methylation (DNAm) patterns at birth and whether this in turn relates to offspring health outcomes in the general population.

METHODS

Using data from 2,367 children from the Dutch population-based Generation R Study, we first performed an epigenome-wide association study to identify differentially methylated sites and regions at birth associated with prenatal infection exposure. We also examined the influence of infection timing by using self-reported cumulative infection scores for each trimester. Second, we sought to develop an aggregate methylation profile score (MPS) based on cord blood DNAm as an epigenetic proxy of prenatal infection exposure and tested whether this MPS prospectively associates with offspring health outcomes, including psychiatric symptoms, BMI, and asthma at ages 13-16 years. Third, we investigated whether prenatal infection exposure associates with offspring epigenetic age acceleration - a marker of biological aging. Across all analysis steps, we tested whether our findings replicate in 864 participants from an independent population-based cohort (ALSPAC, UK).

RESULTS

We observed no differentially methylated sites or regions in cord blood in relation to prenatal infection exposure, after multiple testing correction. 33 DNAm sites showed suggestive associations (p < 5e10 - 5; of which one was also nominally associated in ALSPAC), indicating potential links to genes associated with immune, neurodevelopmental, and cardiovascular pathways. While the MPS of prenatal infections associated with maternal reports of infections in the internal hold out sample in the Generation R Study (R2incremental = 0.049), it did not replicate in ALSPAC (R2incremental = 0.001), and it did not prospectively associate with offspring health outcomes in either cohort. Moreover, we observed no association between prenatal exposure to infections and epigenetic age acceleration across cohorts and clocks.

CONCLUSION

In contrast to prior studies, which reported DNAm differences in offspring exposed to severe infections in utero, we do not find evidence for associations between self-reported clinically evident common infections during pregnancy and DNAm or epigenetic aging in cord blood within the general pediatric population. Future studies are needed to establish whether associations exist but are too subtle to be statistically meaningful with present sample sizes, whether they replicate in a cohort with a more similar infection score as our discovery cohort, whether they occur in different tissues than cord blood, and whether other biological pathways may be more relevant for mediating the effect of prenatal common infection exposure on downstream offspring health outcomes.

Examining longitudinal associations between prenatal exposure to infections and child brain morphology

BACKGROUND

Maternal infection during pregnancy has been identified as a prenatal risk factor for the later development of psychopathology in exposed offspring. Neuroimaging data collected during childhood has suggested a link between prenatal exposure to maternal infection and child brain structure and function, potentially offering a neurobiological explanation for the emergence of psychopathology. Additionally, preclinical studies utilizing repeated measures of neuroimaging data suggest that effects of prenatal maternal infection on the offspring's brain may normalize over time (i.e., catch-up growth). However, it remains unclear whether exposure to prenatal maternal infection in humans is related to long-term differential neurodevelopmental trajectories. Hence, this study aimed to investigate the association between prenatal exposure to infections on child brain development over time using repeated measures MRI data.

METHODS

We leveraged data from a population-based cohort, Generation R, in which we examined prospectively assessed self-reported infections at each trimester of pregnancy (N = 2,155). We further used three neuroimaging assessments (at mean ages 8, 10 and 14) to obtain cortical and subcortical measures of the offspring's brain morphology with MRI. Hereafter, we applied linear mixed-effects models, adjusting for several confounding factors, to estimate the association of prenatal maternal infection with child brain development over time.

RESULTS

We found that prenatal exposure to infection in the third trimester was associated with a slower decrease in volumes of the pars orbitalis, rostral anterior cingulate and superior frontal gyrus, and a faster increase in the middle temporal gyrus. In the temporal pole we observed a divergent pattern, specifically showing an increase in volume in offspring exposed to more infections compared to a decrease in volume in offspring exposed to fewer infections. We further observed associations in other frontal and temporal lobe structures after exposure to infections in any trimester, though these did not survive multiple testing correction.

CONCLUSIONS

Our results suggest that prenatal exposure to infections in the third trimester may be associated with slower age-related growth in the regions: pars orbitalis, rostral anterior cingulate and superior frontal gyrus, and faster age-related growth in the middle temporal gyrus across childhood, suggesting a potential sensitive period. Our results might be interpreted as an extension of longitudinal findings from preclinical studies, indicating that children exposed to prenatal infections could exhibit catch-up growth. However, given the lack of differences in brain volume between various infection groups at baseline, there may instead be either a longitudinal deviation or a subtle temporal deviation. Subsequent well-powered studies that extend into the period of full brain development (∼25 years) are needed to confirm whether the observed phenomenon is indeed catch-up growth, a longitudinal deviation, or a subtle temporal deviation.

Maternal Immune Activation imprints translational dysregulation and differential MAP2 phosphorylation in descendant neural stem cells

Alterations induced by maternal immune activation (MIA) during gestation impact the subsequent neurodevelopment of progeny, a process that in humans, has been linked to the development of several neuropsychiatric conditions. To undertake a comprehensive examination of the molecular mechanisms governing MIA, we have devised an in vitro model based on neural stem cells (NSCs) sourced from fetuses carried by animals subjected to Poly I:C treatment. These neural progenitors demonstrate proliferative capacity and can be effectively differentiated into both neurons and glial cells. Transcriptomic, proteomic, and phosphoproteomic analyses conducted on these cellular models, in conjunction with counterparts from control treatments, revealed discernible shifts in the expression levels of a specific subset of proteins implicated in neuronal function. Noteworthy, we found an absence of congruence between these alterations at the transcriptomic level, suggesting that differences in protein translation contribute to the observed dysregulation. Furthermore, the phosphoproteomic data highlighted a discernible discrepancy in the basal phosphorylation of proteins between differentiated cells from both experimental groups, particularly within proteins associated with cytoskeletal architecture and synaptic functionality, notably those belonging to the MAP family. Observed alterations in MAP phosphorylation were found to potentially have functional consequences as they correlate with changes in neuronal plasticity and the establishment of neuronal synapses. Our data agrees with previous published observations and further underscore the importance of MAP2 phosphorylation state on its function and the impact that this protein has in neuronal structure and function.

Epigenetic disruptions in the offspring hypothalamus in response to maternal infection

DNA methylation is an epigenetic modification that can alter gene expression, and the incidence can vary across developmental stages, inflammatory conditions, and sexes. The effects of viral maternal viral infection and sex on the DNA methylation patterns were studied in the hypothalamus of a pig model of immune activation during development. DNA methylation at single-base resolution in regions of high CpG density was measured on 24 individual hypothalamus samples using reduced representation bisulfite sequencing. Differential over- and under-methylated sites were identified and annotated to proximal genes and corresponding biological processes. A total of 120 sites were differentially methylated (FDR-adjusted p-value < 0.05) between maternal infection or sex groups. Among the 66 sites differentially methylated between groups exposed to inflammatory signals and control, most sites were over-methylated in the challenged group and included sites in the promoter regions of genes SIRT3 and NRBP1. Among the 54 differentially methylated sites between females and males, most sites were over-methylated in females and included sites in the promoter region of genes TNC and EIF4G1. The analysis of the genes proximal to the differentially methylated sites suggested that biological processes potentially impacted include immune response, neuron migration and ensheathment, peptide signaling, adaptive thermogenesis, and tissue development. These results suggest that translational studies should consider that the prolonged effect of maternal infection during gestation may be enacted through epigenetic regulatory mechanisms that may differ between sexes.

Pulmonary maternal immune activation does not cross the placenta but leads to fetal metabolic adaptation

The fetal development of organs and functions is vulnerable to perturbation by maternal inflammation which may increase susceptibility to disorders after birth. Because it is not well understood how the placenta and fetus respond to acute lung- inflammation, we characterize the response to maternal pulmonary lipopolysaccharide exposure across 24 h in maternal and fetal organs using multi-omics, imaging and integrative analyses. Unlike maternal organs, which mount strong inflammatory immune responses, the placenta upregulates immuno-modulatory genes, in particular the IL-6 signaling suppressor Socs3. Similarly, we observe no immune response in the fetal liver, which instead displays metabolic changes, including increases in lipids containing docosahexaenoic acid, crucial for fetal brain development. The maternal liver and plasma display similar metabolic alterations, potentially increasing bioavailability of docosahexaenoic acid for the mother and fetus. Thus, our integrated temporal analysis shows that systemic inflammation in the mother leads to a metabolic perturbation in the fetus.

GABA system as the cause and effect in early development

GABA is the primary inhibitory neurotransmitter in the adult brain and through its actions on GABAARs, it protects against excitotoxicity and seizure activity, ensures temporal fidelity of neurotransmission, and regulates concerted rhythmic activity of neuronal populations. In the developing brain, the development of GABAergic neurons precedes that of glutamatergic neurons and the GABA system serves as a guide and framework for the development of other brain systems. Despite this early start, the maturation of the GABA system also continues well into the early postnatal period. In this review, we organize evidence around two scenarios based on the essential and protracted nature of GABA system development: 1) disruptions in the development of the GABA system can lead to large scale disruptions in other developmental processes (i.e., GABA as the cause), 2) protracted maturation of this system makes it vulnerable to the effects of developmental insults (i.e., GABA as the effect). While ample evidence supports the importance of GABA/GABAAR system in both scenarios, large gaps in existing knowledge prevent strong mechanistic conclusions.

PolyI:C Maternal Immune Activation on E9.5 Causes the Deregulation of Microglia and the Complement System in Mice, Leading to Decreased Synaptic Spine Density

Maternal immune activation (MIA) is a risk factor for multiple neurodevelopmental disorders; however, animal models developed to explore MIA mechanisms are sensitive to experimental factors, which has led to complexity in previous reports of the MIA phenotype. We sought to characterize an MIA protocol throughout development to understand how prenatal immune insult alters the trajectory of important neurodevelopmental processes, including the microglial regulation of synaptic spines and complement signaling. We used polyinosinic:polycytidylic acid (polyI:C) to induce MIA on gestational day 9.5 in CD-1 mice, and measured their synaptic spine density, microglial synaptic pruning, and complement protein expression. We found reduced dendritic spine density in the somatosensory cortex starting at 3-weeks-of-age with requisite increases in microglial synaptic pruning and phagocytosis, suggesting spine density loss was caused by increased microglial synaptic pruning. Additionally, we showed dysregulation in complement protein expression persisting into adulthood. Our findings highlight disruptions in the prenatal environment leading to alterations in multiple dynamic processes through to postnatal development. This could potentially suggest developmental time points during which synaptic processes could be measured as risk factors or targeted with therapeutics for neurodevelopmental disorders.

Maternal inflammation regulates fetal emergency myelopoiesis

Neonates are highly susceptible to inflammation and infection. Here, we investigate how late fetal liver (FL) mouse hematopoietic stem and progenitor cells (HSPCs) respond to inflammation, testing the hypothesis that deficits in the engagement of emergency myelopoiesis (EM) pathways limit neutrophil output and contribute to perinatal neutropenia. We show that fetal HSPCs have limited production of myeloid cells at steady state and fail to activate a classical adult-like EM transcriptional program. Moreover, we find that fetal HSPCs can respond to EM-inducing inflammatory stimuli in vitro but are restricted by maternal anti-inflammatory factors, primarily interleukin-10 (IL-10), from activating EM pathways in utero. Accordingly, we demonstrate that the loss of maternal IL-10 restores EM activation in fetal HSPCs but at the cost of fetal demise. These results reveal the evolutionary trade-off inherent in maternal anti-inflammatory responses that maintain pregnancy but render the fetus unresponsive to EM activation signals and susceptible to infection.

Neuroimmune mechanisms in autism etiology - untangling a complex problem using human cellular models

Autism spectrum disorder (ASD) affects 1 in 36 people and is more often diagnosed in males than in females. Core features of ASD are impaired social interactions, repetitive behaviors and deficits in verbal communication. ASD is a highly heterogeneous and heritable disorder, yet its underlying genetic causes account only for up to 80% of the cases. Hence, a subset of ASD cases could be influenced by environmental risk factors. Maternal immune activation (MIA) is a response to inflammation during pregnancy, which can lead to increased inflammatory signals to the fetus. Inflammatory signals can cross the placenta and blood brain barriers affecting fetal brain development. Epidemiological and animal studies suggest that MIA could contribute to ASD etiology. However, human mechanistic studies have been hindered by a lack of experimental systems that could replicate the impact of MIA during fetal development. Therefore, mechanisms altered by inflammation during human pre-natal brain development, and that could underlie ASD pathogenesis have been largely understudied. The advent of human cellular models with induced pluripotent stem cell (iPSC) and organoid technology is closing this gap in knowledge by providing both access to molecular manipulations and culturing capability of tissue that would be otherwise inaccessible. We present an overview of multiple levels of evidence from clinical, epidemiological, and cellular studies that provide a potential link between higher ASD risk and inflammation. More importantly, we discuss how stem cell-derived models may constitute an ideal experimental system to mechanistically interrogate the effect of inflammation during the early stages of brain development.

Molecular mapping of a core transcriptional signature of microglia-specific genes in schizophrenia

Besides playing a central role in neuroinflammation, microglia regulate synaptic development and is involved in plasticity. Converging lines of evidence suggest that these different processes play a critical role in schizophrenia. Furthermore, previous studies reported altered transcription of microglia genes in schizophrenia, while microglia itself seems to be involved in the etiopathology of the disease. However, the regional specificity of these brain transcriptional abnormalities remains unclear. Moreover, it is unknown whether brain and peripheral expression of microglia genes are related. Thus, we investigated the expression of a pre-registered list of 10 genes from a core signature of human microglia both at brain and peripheral levels. We included 9 independent Gene Expression Omnibus datasets (764 samples obtained from 266 individuals with schizophrenia and 237 healthy controls) from 8 different brain regions and 3 peripheral tissues. We report evidence of a widespread transcriptional alteration of microglia genes both in brain tissues (we observed a decreased expression in the cerebellum, associative striatum, hippocampus, and parietal cortex of individuals with schizophrenia compared with healthy controls) and whole blood (characterized by a mixed altered expression pattern). Our results suggest that brain underexpression of microglia genes may represent a candidate transcriptional signature for schizophrenia. Moreover, the dual brain-whole blood transcriptional alterations of microglia/macrophage genes identified support the model of schizophrenia as a whole-body disorder and lend weight to the use of blood samples as a potential source of biological peripheral biomarkers.

Immunogenetics of autism spectrum disorder: A systematic literature review

The aetiology of autism spectrum disorder (ASD) is complex and, partly, accounted by genetic factors. Nonetheless, the genetic underpinnings of ASD are poorly defined. The presence of immune dysregulations in autistic individuals, and their families, supports a role of the immune system and its genetic regulators. Albeit immune responses belong either to the innate or adaptive arms, the overall immune system genetics is broad, and encompasses a multitude of functionally heterogenous pathways which may have different influences on ASD. Hence, to gain insights on the immunogenetic underpinnings of ASD, we conducted a systematic literature review of previous immune genetic and transcription studies in ASD. We defined a list of immune genes relevant to ASD and explored their neuro-immune function. Our review confirms the presence of immunogenetic variability in ASD, accounted by inherited variations of innate and adaptive immune system genes and genetic expression changes in the blood and post-mortem brain of autistic individuals. Besides their immune function, the identified genes control neurodevelopment processes (neuronal and synaptic plasticity) and are highly expressed in pre/peri-natal periods. Hence, our synthesis bolsters the hypothesis that perturbation in immune genes may contribute to ASD by derailing the typical trajectory of neurodevelopment. Our review also helped identifying some of the limitations of prior immunogenetic research in ASD. Thus, alongside clarifying the neurodevelopment role of immune genes, we outline key considerations for future work into the aetiology of ASD and possible novel intervention targets.

Prenatal risk factors and postnatal cannabis exposure: Assessing dual models of schizophrenia-like rodents

Schizophrenia (SCZ) is a multifactorial neurodevelopmental disorder caused by genetic and environmental alterations, especially during prenatal stages. On the other hand, cannabis consumption in adolescence has been also linked to an increased risk of developing SCZ. The combination of both hits has been proposed as the dual hit hypothesis of SCZ. We systematically reviewed prenatal environmental alterations and cannabis consumption during adolescence that are associated with an increased risk of SCZ, following the PRISMA model. The analysis focused on dual animal models where the first hit is prenatal environmental exposure and the second hit consists of postnatal cannabis exposure. The articles were evaluated by three independent reviewers based on inclusion criteria. We extracted the first author´s name, year, model species, sex and analysis. The articles reported on dual murine models and their effects on weight, behavior, genetics, electrophysiology and brain structure and function. We conclude that the defects caused by the dual hits depend on the sex of the model, as well as type of hits.

Arketamine for cognitive impairment in psychiatric disorders

Cognitive impairment has been observed in patients with various psychiatric disorders, including schizophrenia, major depressive disorder (MDD), and bipolar disorder (BD). Although modern therapeutic drugs can improve certain symptoms (i.e., psychosis, depression) in these patients, these drugs have not been found to improve cognitive impairment. The N-methyl-D-aspartate receptor antagonist (R,S)-ketamine has attracted attention as a rapidly acting antidepressant. In addition to its robust antidepressant effects, (R,S)-ketamine has been suggested to improve cognitive impairment in patients with MDD and BD, despite causing cognitive impairment in healthy control subjects. (R,S)-ketamine is a racemic mixture of equal amounts of (R)-ketamine (or arketamine) and (S)-ketamine (or esketamine). Arketamine has been found to have more potent antidepressant-like actions than esketamine in rodents. Interestingly, arketamine, but not esketamine, has been suggested to improve phencyclidine-induced cognitive deficits in mice. Furthermore, arketamine has been suggested to ameliorate cognitive deficits in rodent offspring after maternal immune activation. In the current article, it is proposed that arketamine has therapeutic potential for treating cognitive impairment in patients with psychiatric disorders. Additionally, the potential role of the gut-microbiome-brain axis in cognitive impairment in psychiatric disorders is discussed.

Maternal IL-10 restricts fetal emergency myelopoiesis

Neonates, in contrast to adults, are highly susceptible to inflammation and infection. Here we investigate how late fetal liver (FL) mouse hematopoietic stem and progenitor cells (HSPC) respond to inflammation, testing the hypothesis that deficits in engagement of emergency myelopoiesis (EM) pathways limit neutrophil output and contribute to perinatal neutropenia. We show that despite similar molecular wiring as adults, fetal HSPCs have limited production of myeloid cells at steady state and fail to activate a classical EM transcriptional program. Moreover, we find that fetal HSPCs are capable of responding to EM-inducing inflammatory stimuli in vitro , but are restricted by maternal anti-inflammatory factors, primarily interleukin-10 (IL-10), from activating EM pathways in utero . Accordingly, we demonstrate that loss of maternal IL-10 restores EM activation in fetal HSPCs but at the cost of premature parturition. These results reveal the evolutionary trade-off inherent in maternal anti-inflammatory responses that maintain pregnancy but render the fetus unresponsive to EM activation signals and susceptible to infection.

HIGHLIGHTS

The structure of the HSPC compartment is conserved from late fetal to adult life.Fetal HSPCs have diminished steady-state myeloid cell production compared to adult.Fetal HSPCs are restricted from engaging in emergency myelopoiesis by maternal IL-10.Restriction of emergency myelopoiesis may explain neutropenia in septic neonates.

eTOC BLURB

Fetal hematopoietic stem and progenitor cells are restricted from activating emergency myelopoiesis pathways by maternal IL-10, resulting in inadequate myeloid cell production in response to inflammatory challenges and contributing to neonatal neutropenia.

The effect of rifampicin on expression of the toll-like receptor system genes in the forebrain cortex of rats prenatally exposed to alcohol

Ethanol causes long-term changes in the toll-like receptor (TLR) system, promoting activation of neuroinflammation pathways. Alcohol use during pregnancy causes neuroinflammatory processes in the fetus; this can lead to the development of symptoms of fetal alcohol spectrum disorder (FASD). Our study has shown that prenatal alcohol exposure (PAE) induced long-term changes in the TLR system genes (Tlr3, Tlr4, Ticam, Hmgb1, cytokine genes) in the forebrain cortex of rat pups. Administration of rifampicin (Rif), which can reduce the level of pro-inflammatory mediators in various pathological conditions of the nervous system, normalized the altered expression level of the studied TLR system genes. This suggests that Rif can prevent the development of persistent neuroinflammatory events in the forebrain cortex of rat pups caused by dysregulation in the TLR system.

Maternal immune activation and role of placenta in the prenatal programming of neurodevelopmental disorders

Maternal infection during pregnancy, leading to maternal immune activation (mIA) and cytokine release, increases the offspring risk of developing a variety of neurodevelopmental disorders (NDDs), including schizophrenia. Animal models have provided evidence to support these mechanistic links, with placental inflammatory responses and dysregulation of placental function implicated. This leads to changes in fetal brain cytokine balance and altered epigenetic regulation of key neurodevelopmental pathways. The prenatal timing of such mIA-evoked changes, and the accompanying fetal developmental responses to an altered in utero environment, will determine the scope of the impacts on neurodevelopmental processes. Such dysregulation can impart enduring neuropathological changes, which manifest subsequently in the postnatal period as altered neurodevelopmental behaviours in the offspring. Hence, elucidation of the functional changes that occur at the molecular level in the placenta is vital in improving our understanding of the mechanisms that underlie the pathogenesis of NDDs. This has notable relevance to the recent COVID-19 pandemic, where inflammatory responses in the placenta to SARS-CoV-2 infection during pregnancy and NDDs in early childhood have been reported. This review presents an integrated overview of these collective topics and describes the possible contribution of prenatal programming through placental effects as an underlying mechanism that links to NDD risk, underpinned by altered epigenetic regulation of neurodevelopmental pathways.

Altered Purinergic Signaling in Neurodevelopmental Disorders: Focus on P2 Receptors

With the umbrella term 'neurodevelopmental disorders' (NDDs) we refer to a plethora of congenital pathological conditions generally connected with cognitive, social behavior, and sensory/motor alterations. Among the possible causes, gestational and perinatal insults have been demonstrated to interfere with the physiological processes necessary for the proper development of fetal brain cytoarchitecture and functionality. In recent years, several genetic disorders caused by mutations in key enzymes involved in purine metabolism have been associated with autism-like behavioral outcomes. Further analysis revealed dysregulated purine and pyrimidine levels in the biofluids of subjects with other NDDs. Moreover, the pharmacological blockade of specific purinergic pathways reversed the cognitive and behavioral defects caused by maternal immune activation, a validated and now extensively used rodent model for NDDs. Furthermore, Fragile X and Rett syndrome transgenic animal models as well as models of premature birth, have been successfully utilized to investigate purinergic signaling as a potential pharmacological target for these diseases. In this review, we examine results on the role of the P2 receptor signaling in the etiopathogenesis of NDDs. On this basis, we discuss how this evidence could be exploited to develop more receptor-specific ligands for future therapeutic interventions and novel prognostic markers for the early detection of these conditions.

Maternal Immune Activation Induces Adolescent Cognitive Deficits Preceded by Developmental Perturbations in Cortical Reelin Signalling

Exposure to maternal immune activation (MIA) in utero significantly elevates the risk of developing schizophrenia and other neurodevelopmental disorders. To understand the biological mechanisms underlying the link between MIA and increased risk, preclinical animal models have focussed on specific signalling pathways in the brain that mediate symptoms associated with neurodevelopmental disorders such as cognitive dysfunction. Reelin signalling in multiple brain regions is involved in neuronal migration, synaptic plasticity and long-term potentiation, and has been implicated in cognitive deficits. However, how regulation of Reelin expression is affected by MIA across cortical development and associated cognitive functions remains largely unclear. Using a MIA rat model, here we demonstrate cognitive deficits in adolescent object-location memory in MIA offspring and reductions in Reln expression prenatally and in the adult prefrontal cortex. Further, developmental disturbances in gene/protein expression and DNA methylation of downstream signalling components occurred subsequent to MIA-induced Reelin dysregulation and prior to cognitive deficits. We propose that MIA-induced dysregulation of Reelin signalling contributes to the emergence of prefrontal cortex-mediated cognitive deficits through altered NMDA receptor function, resulting in inefficient long-term potentiation. Our data suggest a developmental window during which attenuation of Reelin signalling may provide a possible therapeutic target.

Maternal Immune Activation Induced by Prenatal Lipopolysaccharide Exposure Leads to Long-Lasting Autistic-like Social, Cognitive and Immune Alterations in Male Wistar Rats

Several studies have supported the association between maternal immune activation (MIA) caused by exposure to pathogens or inflammation during critical periods of gestation and an increased susceptibility to the development of various psychiatric and neurological disorders, including autism and other neurodevelopmental disorders (NDDs), in the offspring. In the present work, we aimed to provide extensive characterization of the short- and long-term consequences of MIA in the offspring, both at the behavioral and immunological level. To this end, we exposed Wistar rat dams to Lipopolysaccharide and tested the infant, adolescent and adult offspring across several behavioral domains relevant to human psychopathological traits. Furthermore, we also measured plasmatic inflammatory markers both at adolescence and adulthood. Our results support the hypothesis of a deleterious impact of MIA on the neurobehavioral development of the offspring: we found deficits in the communicative, social and cognitive domains, together with stereotypic-like behaviors and an altered inflammatory profile at the systemic level. Although the precise mechanisms underlying the role of neuroinflammatory states in neurodevelopment need to be clarified, this study contributes to a better understanding of the impact of MIA on the risk of developing behavioral deficits and psychiatric illness in the offspring.

Maternal behaviours and adult offspring behavioural deficits are predicted by maternal TNFα concentration in a rat model of neurodevelopmental disorders

Exposure to inflammatory stressors during fetal development is a major risk factor for neurodevelopmental disorders (NDDs) in adult offspring. Maternal immune activation (MIA), induced by infection, causes an acute increase in pro-inflammatory cytokines which can increase the risk for NDDs directly by inducing placental and fetal brain inflammation, or indirectly through affecting maternal care behaviours thereby affecting postnatal brain development. Which of these two potential mechanisms dominates in increasing offspring risk for NDDs remains unclear. Here, we show that acute systemic maternal inflammation induced by the viral mimetic polyinosinic:polycytidylic acid (poly I:C) on gestational day 15 of rat pregnancy affects offspring and maternal behaviour, offspring cognition, and expression of NDD-relevant genes in the offspring brain. Dams exposed to poly I:C elicited an acute increase in the pro-inflammatory cytokine tumour necrosis factor (TNF; referred to here as TNFα), which predicted disruption of key maternal care behaviours. Offspring of poly I:C-treated dams showed early behavioural and adult cognitive deficits correlated to the maternal TNFα response, but, importantly, not with altered maternal care. We also found interacting effects of sex and treatment on GABAergic gene expression and DNA methylation in these offspring in a brain region-specific manner, including increased parvalbumin expression in the female adolescent frontal cortex. We conclude that the MIA-induced elevation of TNFα in the maternal compartment affects fetal neurodevelopment leading to altered offspring behaviour and cognition. Our results suggest that a focus on prenatal pathways affecting fetal neurodevelopment would provide greater insights into the mechanisms underpinning the TNFα-mediated genesis of altered offspring behaviour and cognition following maternal inflammation.

Maternal immune activation as an epidemiological risk factor for neurodevelopmental disorders: Considerations of timing, severity, individual differences, and sex in human and rodent studies

Epidemiological evidence suggests that one's risk of being diagnosed with a neurodevelopmental disorder (NDD)-such as autism, ADHD, or schizophrenia-increases significantly if their mother had a viral or bacterial infection during the first or second trimester of pregnancy. Despite this well-known data, little is known about how developing neural systems are perturbed by events such as early-life immune activation. One theory is that the maternal immune response disrupts neural processes important for typical fetal and postnatal development, which can subsequently result in specific and overlapping behavioral phenotypes in offspring, characteristic of NDDs. As such, rodent models of maternal immune activation (MIA) have been useful in elucidating neural mechanisms that may become dysregulated by MIA. This review will start with an up-to-date and in-depth, critical summary of epidemiological data in humans, examining the association between different types of MIA and NDD outcomes in offspring. Thereafter, we will summarize common rodent models of MIA and discuss their relevance to the human epidemiological data. Finally, we will highlight other factors that may interact with or impact MIA and its associated risk for NDDs, and emphasize the importance for researchers to consider these when designing future human and rodent studies. These points to consider include: the sex of the offspring, the developmental timing of the immune challenge, and other factors that may contribute to individual variability in neural and behavioral responses to MIA, such as genetics, parental age, the gut microbiome, prenatal stress, and placental buffering.

The impact of maternal immune activation on embryonic brain development

The adult brain is a complex structure with distinct functional sub-regions, which are generated from an initial pool of neural epithelial cells within the embryo. This transition requires a number of highly coordinated processes, including neurogenesis, i.e., the generation of neurons, and neuronal migration. These take place during a critical period of development, during which the brain is particularly susceptible to environmental insults. Neurogenesis defects have been associated with the pathogenesis of neurodevelopmental disorders (NDDs), such as autism spectrum disorder and schizophrenia. However, these disorders have highly complex multifactorial etiologies, and hence the underlying mechanisms leading to aberrant neurogenesis continue to be the focus of a significant research effort and have yet to be established. Evidence from epidemiological studies suggests that exposure to maternal infection in utero is a critical risk factor for NDDs. To establish the biological mechanisms linking maternal immune activation (MIA) and altered neurodevelopment, animal models have been developed that allow experimental manipulation and investigation of different developmental stages of brain development following exposure to MIA. Here, we review the changes to embryonic brain development focusing on neurogenesis, neuronal migration and cortical lamination, following MIA. Across published studies, we found evidence for an acute proliferation defect in the embryonic MIA brain, which, in most cases, is linked to an acceleration in neurogenesis, demonstrated by an increased proportion of neurogenic to proliferative divisions. This is accompanied by disrupted cortical lamination, particularly in the density of deep layer neurons, which may be a consequence of the premature neurogenic shift. Although many aspects of the underlying pathways remain unclear, an altered epigenome and mitochondrial dysfunction are likely mechanisms underpinning disrupted neurogenesis in the MIA model. Further research is necessary to delineate the causative pathways responsible for the variation in neurogenesis phenotype following MIA, which are likely due to differences in timing of MIA induction as well as sex-dependent variation. This will help to better understand the underlying pathogenesis of NDDs, and establish therapeutic targets.

Hippocampal Changes Elicited by Metabolic and Inflammatory Stressors following Prenatal Maternal Infection

The hippocampus participates in spatial navigation and behavioral processes, displays molecular plasticity in response to environmental challenges, and can play a role in neuropsychiatric diseases. The combined effects of inflammatory prenatal and postnatal challenges can disrupt the hippocampal gene networks and regulatory mechanisms. Using a proven pig model of viral maternal immune activation (MIA) matched to controls and an RNA-sequencing approach, the hippocampal transcriptome was profiled on two-month-old female and male offspring assigned to fasting, mimetic viral, or saline treatments. More than 2600 genes presented single or combined effects (FDR-adjusted p-value < 0.05) of MIA, postnatal stress, or sex. Biological processes and pathways encompassing messenger cyclic adenosine 3',5'-monophosphate (cAMP) signaling were enriched with genes including gastric inhibitory polypeptide receptor (GIPR) predominantly over-expressed in the MIA-exposed fasting males relative to groups that differed in sex, prenatal or postnatal challenge. While this pattern was amplified in fasting offspring, the postnatal inflammatory challenge appeared to cancel out the effects of the prenatal challenge. The transcription factors C-terminal binding protein 2 (CTBP2), RE1 silencing transcription factor (REST), signal transducer and activator of transcription 1 (STAT1), and SUZ12 polycomb repressive complex 2 subunit were over-represented among the genes impacted by the prenatal and postnatal factors studied. Our results indicate that one environmental challenge can influence the effect of another challenge on the hippocampal transcriptome. These findings can assist in the identification of molecular targets to ameliorate the effects of pre-and post-natal stressors on hippocampal-associated physiology and behavior.

SARS-CoV-2 (COVID-19) as a possible risk factor for neurodevelopmental disorders

Pregnant women constitute one of the most vulnerable populations to be affected by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, the cause of coronavirus disease 2019. SARS-CoV-2 infection during pregnancy could negatively impact fetal brain development via multiple mechanisms. Accumulating evidence indicates that mother to fetus transmission of SARS-CoV-2 does occur, albeit rarely. When it does occur, there is a potential for neuroinvasion via immune cells, retrograde axonal transport, and olfactory bulb and lymphatic pathways. In the absence of maternal to fetal transmission, there is still the potential for negative neurodevelopmental outcomes as a consequence of disrupted placental development and function leading to preeclampsia, preterm birth, and intrauterine growth restriction. In addition, maternal immune activation may lead to hypomyelination, microglial activation, white matter damage, and reduced neurogenesis in the developing fetus. Moreover, maternal immune activation can disrupt the maternal or fetal hypothalamic-pituitary-adrenal (HPA) axis leading to altered neurodevelopment. Finally, pro-inflammatory cytokines can potentially alter epigenetic processes within the developing brain. In this review, we address each of these potential mechanisms. We propose that SARS-CoV-2 could lead to neurodevelopmental disorders in a subset of pregnant women and that long-term studies are warranted.

Dynamic changes in spatiotemporal transcriptome reveal maternal immune dysregulation of autism spectrum disorder

Maternal immune activation (MIA) during pregnancy is known to be an environmental risk factor for neurodevelopment and autism spectrum disorder (ASD). However, it is unclear at which fetal brain developmental windows and regions MIA induces ASD-related neurodevelopmental transcriptional abnormalities. The non-chasm differentially expressed genes (DEGs) involved in MIA inducing ASD during fetal brain developmental windows were identified by performing the differential expression analysis and comparing the common DEGs among MIA at four different gestational development windows, ASD with multiple brain regions from human patients and mouse models, and human and mouse embryonic brain developmental trajectory. The gene set and functional enrichment analyses were performing to identify MIA dysregulated ASD-related the fetal neurodevelopmental windows and brain regions and function annotations. Additionally, the networks were constructed using Cytoscape for visualization. MIA at E12.5 and E14.5 increased the risk of distinct brain regions for ASD. MIA-driven transcriptional alterations of non-chasm DEGs, during the coincidence brain developmental windows between human and mice, involving ASD-relevant synaptic components, as well as immune- and metabolism-related functions and pathways. Furthermore, a great number of non-chasm brain development-, immune-, and metabolism-related DEGs were overlapped in at least two existing ASD-associated databases, suggesting that the others could be considered as the candidate targets to construct the model mice for explaining the pathological changes of ASD when environmental factors (MIA) and gene mutation effects co-occur. Overall, our search supported that transcriptome-based MIA dysregulated the brain development-, immune-, and metabolism-related non-chasm DEGs at specific embryonic brain developmental window and region, leading to abnormal embryonic neurodevelopment, to induce the increasing risk of ASD.

Clustering of cognitive phenotypes identifies susceptible and resilient offspring in a rat model of maternal immune activation and early-life stress

Schizophrenia and other neurodevelopmental disorders often have very heterogeneous symptoms, especially regarding cognition: while some individuals may exhibit deficient cognition, others are relatively unaffected. Studies using developmental animal models often ignore phenotypic heterogeneity in favour of traditional treatment/control comparisons. This may result in resilient or unaffected individuals masking the effects of susceptible individuals if grouped together. Here, we used maternal immune activation and limited bedding and nesting, respectively, as a two-hit neurodevelopmental model for schizophrenia. Both factors reduced cognitive function in a novel object recognition (NOR) task. While we found treatment group effects on cognitive phenotypes, behavioural clustering identified three subpopulations exposed to either insult: those exhibiting ‘typical’ cognitive performance on the NOR, an intermediate phenotype, or a marked deficit. These clusters included offspring from each treatment group, although both intermediate and marked deficit clusters were composed primarily of offspring from treated groups. Clustering allowed stratification within treatment groups into ‘susceptible’ and ‘resilient’ individuals, while also identifying conserved phenotypes across treatment groups. Using unbiased cluster analyses in preclinical models can better characterize phenotypes and enables a better understanding of both face and construct validity of phenotypic heterogeneity. The use of unbiased clustering techniques may help identify potential markers associated with individual susceptibility and resilience in neurodevelopmental disorder models.

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Maternal immune activation and early life stress affect recognition memory.

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New groups were identified using unbiased clustering based on task performance.

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These clusters reflected high-, low-, and intermediary recognition memory.

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Treatment offspring were distributed across all three unbiased clusters.

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These findings suggest some offspring will be resilient to developmental insults.

Impact of Maternal Immune Activation on Nonhuman Primate Prefrontal Cortex Development: Insights for Schizophrenia

Late adolescence is a period of dynamic change in the brain as humans learn to navigate increasingly complex environments. In particular, prefrontal cortical (PFC) regions undergo extensive remodeling as the brain is fine-tuned to orchestrate cognitive control over attention, reasoning, and emotions. Late adolescence also presents a uniquely vulnerable period as neurodevelopmental illnesses, such as schizophrenia, become evident and worsen into young adulthood. Challenges in early development, including prenatal exposure to infection, may set the stage for a cascade of maladaptive events that ultimately result in aberrant PFC connectivity and function before symptoms emerge. A growing body of research suggests that activation of the mother's immune system during pregnancy may act as a disease primer, in combination with other environmental and genetic factors, contributing to an increased risk of neurodevelopmental disorders, including schizophrenia. Animal models provide an invaluable opportunity to examine the course of brain and behavioral changes in offspring exposed to maternal immune activation (MIA). Although the vast majority of MIA research has been carried out in rodents, here we highlight the translational utility of the nonhuman primate (NHP) as a model species more closely related to humans in PFC structure and function. In this review, we consider the protracted period of brain and behavioral maturation in the NHP, describe emerging findings from MIA NHP offspring in the context of rodent preclinical models, and lastly explore the translational relevance of the NHP MIA model to expand understanding of the etiology and developmental course of PFC pathology in schizophrenia.

Inflammation and Autophagy: A Convergent Point between Autism Spectrum Disorder (ASD)-Related Genetic and Environmental Factors: Focus on Aluminum Adjuvants

Autism spectrum disorder (ASD), schizophrenia, and bipolar disorder are genetically complex and heterogeneous neurodevelopmental disorders (NDDs) resulting from genetic factors and gene-environment (GxE) interactions for which onset occurs in early brain development. Recent progress highlights the link between ASD and (i) immunogenetics, neurodevelopment, and inflammation, and (ii) impairments of autophagy, a crucial neurodevelopmental process involved in synaptic pruning. Among various environmental factors causing risk for ASD, aluminum (Al)-containing vaccines injected during critical periods have received special attention and triggered relevant scientific questions. The aim of this review is to discuss the current knowledge on the role of early inflammation, immune and autophagy dysfunction in ASD as well as preclinical studies which question Al adjuvant impacts on brain and immune maturation. We highlight the most recent breakthroughs and the lack of epidemiological, pharmacokinetic and pharmacodynamic data constituting a "scientific gap". We propose additional research, such as genetic studies that could contribute to identify populations at genetic risk, improving diagnosis, and potentially the development of new therapeutic tools.

Cell-type-specific epigenetic effects of early life stress on the brain

Early life stress (ELS) induces long-term phenotypic adaptations that contribute to increased vulnerability to a host of neuropsychiatric disorders. Epigenetic mechanisms, including DNA methylation, histone modifications and non-coding RNA, are a proposed link between environmental stressors, alterations in gene expression, and phenotypes. Epigenetic modifications play a primary role in shaping functional differences between cell types and can be modified by environmental perturbations, especially in early development. Together with contributions from genetic variation, epigenetic mechanisms orchestrate patterns of gene expression within specific cell types that contribute to phenotypic variation between individuals. To date, many studies have provided insights into epigenetic changes resulting from ELS. However, most of these studies have examined heterogenous brain tissue, despite evidence of cell-type-specific epigenetic modifications in phenotypes associated with ELS. In this review, we focus on rodent and human studies that have examined epigenetic modifications induced by ELS in select cell types isolated from the brain or associated with genes that have cell-type-restricted expression in neurons, microglia, astrocytes, and oligodendrocytes. Although significant challenges remain, future studies using these approaches can enable important mechanistic insight into the role of epigenetic variation in the effects of ELS on brain function.

Maternal immune activation in rats induces dysfunction of placental leucine transport and alters fetal brain growth

Maternal infection during pregnancy increases the offspring risk of developing a variety of neurodevelopmental disorders (NDDs), including schizophrenia. While the mechanisms remain unclear, dysregulation of placental function is implicated. We hypothesised that maternal infection, leading to maternal immune activation and stimulated cytokine production, alters placental and yolk sac amino acid transport, affecting fetal brain development and thus NDD risk. Using a rat model of maternal immune activation induced by the viral mimetic polyinosinic:polycytidylic acid (poly(I:C)), we investigated placental and yolk sac expression of system L amino acid transporter subtypes which transport several essential amino acids including branched-chain amino acids (BCAA), maternal and fetal BCAA concentration, placental 14C-leucine transport activity and associated impacts on fetal growth and development. Poly(I:C) treatment increased acutely maternal IL-6 and TNFα concentration, contrasting with IL-1β. Transcriptional responses for these pro-inflammatory cytokines were found in placenta and yolk sac following poly(I:C) treatment. Placental and yolk sac weights were reduced by poly(I:C) treatment, yet fetal body weight was unaffected, while fetal brain weight was increased. Maternal plasma BCAA concentration was reduced 24 h post-poly(I:C) treatment, yet placental, but not yolk sac, BCAA concentration was increased. Placental and yolk sac gene expression of Slc7a5, Slc7a8 and Slc43a2 encoding LAT1, LAT2 and LAT4 transporter subtypes respectively, was altered by poly(I:C) treatment. Placental 14C-leucine transport was significantly reduced 24 h post-treatment, contrasting with a significant increase six days following poly(I:C) treatment. Maternal immune activation induces dysregulated placental transport of amino acids affecting fetal brain development, and NDD risk potential in offspring.

Developmental programming of macrophages by early life adversity

Macrophages are central elements of all organs, where they have a multitude of physiological and pathological functions. The first macrophages are produced during fetal development, and most adult organs retain populations of fetal-derived macrophages that self-maintain without major input of hematopoietic stem cell-derived monocytes. Their developmental origins make macrophages highly susceptible to environmental perturbations experienced in early life, in particular the fetal period. It is now well recognized that such adverse developmental conditions contribute to a wide range of diseases later in life. This chapter explores the notion that macrophages are key targets of environmental adversities during development, and mediators of their long-term impact on health and disease. We first briefly summarize our current understanding of macrophage ontogeny and their biology in tissues and consider potential mechanisms by which environmental stressors may mediate fetal programming. We then review evidence for programming of macrophages by adversities ranging from maternal immune activation and diet to environmental pollutants and toxins, which have disease relevance for different organ systems. Throughout this chapter, we contemplate appropriate experimental strategies to study macrophage programming. We conclude by discussing how our current knowledge of macrophage programming could be conceptualized, and finally highlight open questions in the field and approaches to address them.

Genetic Variants Associated With Resilience in Human and Animal Studies

Resilience is broadly defined as the ability to maintain or regain functioning in the face of adversity and is influenced by both environmental and genetic factors. The identification of specific genetic factors and their biological pathways underpinning resilient functioning can help in the identification of common key factors, but heterogeneities in the operationalisation of resilience have hampered advances. We conducted a systematic review of genetic variants associated with resilience to enable the identification of general resilience mechanisms. We adopted broad inclusion criteria for the definition of resilience to capture both human and animal model studies, which use a wide range of resilience definitions and measure very different outcomes. Analyzing 158 studies, we found 71 candidate genes associated with resilience. OPRM1 (Opioid receptor mu 1), NPY (neuropeptide Y), CACNA1C (calcium voltage-gated channel subunit alpha1 C), DCC (deleted in colorectal carcinoma), and FKBP5 (FKBP prolyl isomerase 5) had both animal and human variants associated with resilience, supporting the idea of shared biological pathways. Further, for OPRM1, OXTR (oxytocin receptor), CRHR1 (corticotropin-releasing hormone receptor 1), COMT (catechol-O-methyltransferase), BDNF (brain-derived neurotrophic factor), APOE (apolipoprotein E), and SLC6A4 (solute carrier family 6 member 4), the same allele was associated with resilience across divergent resilience definitions, which suggests these genes may therefore provide a starting point for further research examining commonality in resilience pathways.