Early life stress inhibits expression of a novel innate immune pathway in the developing hippocampus

Early Life Adversity, Microbiome, and Inflammatory Responses

Early life adversity has a profound impact on physical and mental health. Because the central nervous and immune systems are not fully mature at birth and continue to mature during the postnatal period, a bidirectional interaction between the central nervous system and the immune system has been hypothesized, with traumatic stressors during childhood being pivotal in priming individuals for later adult psychopathology. Similarly, the microbiome, which regulates both neurodevelopment and immune function, also matures during childhood, rendering this interaction between the brain and the immune system even more complex. In this review, we provide evidence for the role of the immune response and the microbiome in the deleterious effects of early life adversity, both in humans and rodent models.

Modulation of gut microbiota composition due to early weaning stress induces depressive behavior during the juvenile period in mice

BACKGROUND

The gut microbiota plays an important role in the development of behavior and immunity in infants and juveniles. Early weaning (EW), a form of social stress in mice, leads to increased anxiety and an enhanced stress response in the hypothalamic-pituitary-adrenal axis during adulthood. Early life stress also modulates the immune system and increases vulnerability to infection. However, studies investigating the causal relationships among juvenile stress, microbiota changes, and immune and behavioral deficits are limited. Therefore, we hypothesized that EW alters gut microbiota composition and impairs the development of the nervous and immune systems.

RESULTS

EW mice moved longer distances in the marble-burying test and had longer immobility times in the tail suspension test than normal weaning (NW) mice. In parallel, the gut microbiome composition differed between NW and EW mice, and the abundance of Erysipelotrichacea in EW mice at 8 weeks of age was lower than that in NW mice. In an empirical study, germ-free mice colonized with the gut microbiota of EW mice (GF-EW mice) demonstrated higher depressive behavior than GF mice colonized with normal weaning microbiota (GF-NW mice). Immune cell profiles were also affected by the EW microbiota colonization; the number of CD4 + T cells in the spleen was reduced in GF-EW mice.

CONCLUSION

Our results suggest that EW-induced alterations in the gut microbiota cause depressive behaviors and modulate the immune system.

The Stress Phenotyping Framework: A multidisciplinary biobehavioral approach for assessing and therapeutically targeting maladaptive stress physiology

Although dysregulated stress biology is becoming increasingly recognized as a key driver of lifelong disparities in chronic disease, we presently have no validated biomarkers of toxic stress physiology; no biological, behavioral, or cognitive treatments specifically focused on normalizing toxic stress processes; and no agreed-upon guidelines for treating stress in the clinic or evaluating the efficacy of interventions that seek to reduce toxic stress and improve human functioning. We address these critical issues by (a) systematically describing key systems and mechanisms that are dysregulated by stress; (b) summarizing indicators, biomarkers, and instruments for assessing stress response systems; and (c) highlighting therapeutic approaches that can be used to normalize stress-related biopsychosocial functioning. We also present a novel multidisciplinary Stress Phenotyping Framework that can bring stress researchers and clinicians one step closer to realizing the goal of using precision medicine-based approaches to prevent and treat stress-associated health problems.

Father's care uniquely influences male neurodevelopment

Mammalian infants depend on parental care for survival, with numerous consequences for their behavioral development. We investigated the epigenetic and neurodevelopmental mechanisms mediating the impact of early biparental care on development of alloparenting behavior, or caring for offspring that are not one's own. We find that receiving high parental care early in life leads to slower epigenetic aging of both sexes and widespread male-specific differential expression of genes related to synaptic transmission and autism in the nucleus accumbens. Examination of parental care composition indicates that high-care fathers promote a male-specific increase in excitatory synapses and increases in pup retrieval behavior as juveniles. Interestingly, females raised by high-care fathers have the opposite behavioral response and display fewer pup retrievals. These results support the concept that neurodevelopmental trajectories are programmed by different features of early-life parental care and reveal that male neurodevelopmental processes are uniquely sensitive to care by fathers.

Variation along P2RX7 interacts with early traumas on severity of anxiety suggesting a role for neuroinflammation

Emotional stress is a leading risk factor in the development of neuropsychiatric disorders possibly via immune activation. P2X7 receptors promote neuroinflammation, and research suggests a relationship between chromosome region 12q2431, in which the P2X7R gene is located, and development of mood disorders, however, few studies concentrate on its association with anxiety. Our aim was to investigate the effects of P2RX7 variation in interaction with early childhood traumas and recent stressors on anxiety. 1752 participants completed questionnaires assessing childhood adversities and recent negative life events, provided data on anxiety using the Brief Symptom Inventory, and were genotyped for 681 SNPs in the P2RX7 gene, 335 of which passed quality control and were entered into linear regression models followed by a linkage disequilibrium-based clumping procedure to identify clumps of SNPs with a significant main or interaction effect. We identified a significant clump with top SNP rs67881993 and containing a set of 29SNPs that are in high LD, which significantly interacted with early childhood traumas but not with recent stress conveying a protective effect against increased anxiety in those exposed to early adversities. Our study demonstrated that P2RX7 variants interact with distal and more etiological stressors in influencing the severity of anxiety symptoms, supporting previous scarce results and demonstrating its role in moderating the effects of stress.

Early Life Stress, Neuroinflammation, and Psychiatric Illness of Adulthood

Stress exposure during early stages of life elevates the risk of developing psychopathologies and psychiatric illness in later life. The brain and immune system are not completely developed by birth and therefore continue develop after birth; this post birth development is influenced by several psychosocial factors; hence, early life stress (ELS) exposure can alter brain structural development and function. A growing number of experimental animal and observational human studies have investigated the link between ELS exposure and increased risk of psychopathology through alternations in the immune system, by evaluating inflammation biomarkers. Recent studies, including brain imaging, have also shed light on the mechanisms by which both the innate and adaptive immune systems interact with neural circuits and neurotransmitters, which affect psychopathology. Herein, we discuss the link between the experience of stress in early life and lifelong alterations in the immune system, which subsequently lead to the development of various psychiatric illnesses.

Early life stress impairs synaptic pruning in the developing hippocampus

Early life adversity impairs normal hippocampal function and connectivity in various mammalian species, including humans and rodents. According to the 'cumulative model' the number of early adversities can be summed up to determine the risk for developing psychopathology later in life. In contrast, the 'dimensional model' argues that 'Deprivation' and 'Threat' impact different developmental processes that should not be added in determining clinical outcomes. Here we examine these predictions in male and female mice exposed to a single adversity - limited bedding (LB) - versus mice exposed to multiple adversities - unpredictable postnatal stress (UPS) - focusing on microglia-mediated synaptic pruning in the developing hippocampus. Exposure to both LB and UPS reduced the ramification of microglia, impaired their ability to phagocytose synaptic material in vivo and ex vivo, and decreased expression of TREM2. Abnormal phagocytic activity was associated with increased spine density in CA1 pyramidal neurons that was seen in 17-day-old groups and persisted in peri-pubescent 29-day-old LB and UPS mice. Exposure to LB caused more severe impairment in microglial ramification and synaptic engulfment compared to UPS, outcomes that were accompanied by a UPS-specific increase in the expression of several genes implicated in synaptic pruning. We propose that despite being a single stressor, LB represents a more severe form of early deprivation, and that appropriate levels of hippocampal stimulation during the second and third weeks of life are necessary to support normal microglial ramification and synaptic pruning. Further, impaired synaptic pruning during this critical period of hippocampal development contributes to the abnormal hippocampal function and connectivity seen in UPS and LB later in life.

Oxytocin and microglia in the development of social behaviour

Oxytocin is a well-established regulator of social behaviour. Microglia, the resident immune cells of the central nervous system, regulate brain development and maintenance in health and disease. Oxytocin and microglia interact: microglia appear to regulate the oxytocin system and are, in turn, regulated by oxytocin, which appears to have anti-inflammatory effects. Both microglia and oxytocin are regulated in sex-specific ways. Oxytocin and microglia may work together to promote experience-dependent circuit refinement through multiple developmental-sensitive periods contributing to individual differences in social behaviour. This article is part of the theme issue 'Interplays between oxytocin and other neuromodulators in shaping complex social behaviours'.

Effects of early life adversities upon memory processes and cognition in rodent models

Exposure to stressors in early postnatal life induces long-lasting modifications in brainfunction.Thisplasticity,an essential characteristic of the brain that enables adaptation to the environment, may also induce impairments in some psychophysiological functions, including learning and memory. Early life stress (ELS) has long-term effects on thehypothalamic-pituitary-adrenal axisresponse to stressors, and has been reported to lead toneuroinflammation,altered levelsof neurotrophic factors, modifications inneurogenesis andsynaptic plasticity,with changes in neurotransmitter systems and network functioning. In this review, we focus on early postnatal stress in animal models and their effects on learning and memory.Many studies have reported ELS-induced impairments in different types of memories, including spatial memory, fear memory, recognition (both for objects and social) memory, working memory and reversal learning. Studies are not always in agreement, however, no effects, or sometimes facilitation, being reported, depending on the nature and intensity of the early intervention, as well as the age when the outcome was evaluated and the sex of the animals. When considering processes occurring after consolidation, related with memory maintenance or modification, there are a very reduced number of reports. Future studies addressing the mechanisms underlying memory changes for ELS should shed some light on the understanding of the different effects induced by stressors of different types and intensities on cognitive functions.

Glia-Driven Brain Circuit Refinement Is Altered by Early-Life Adversity: Behavioral Outcomes

Early-life adversity (ELA), often clinically referred to as "adverse childhood experiences (ACE)," is the exposure to stress-inducing events in childhood that can result in poor health outcomes. ELA negatively affects neurodevelopment in children and adolescents resulting in several behavioral deficits and increasing the risk of developing a myriad of neuropsychiatric disorders later in life. The neurobiological mechanisms by which ELA alters neurodevelopment in childhood have been the focus of numerous reviews. However, a comprehensive review of the mechanisms affecting adolescent neurodevelopment (i.e., synaptic pruning and myelination) is lacking. Synaptic pruning and myelination are glia-driven processes that are imperative for brain circuit refinement during the transition from adolescence to adulthood. Failure to optimize brain circuitry between key brain structures involved in learning and memory, such as the hippocampus and prefrontal cortex, leads to the emergence of maladaptive behaviors including increased anxiety or reduced executive function. As such, we review preclinical and clinical literature to explore the immediate and lasting effects of ELA on brain circuit development and refinement. Finally, we describe a number of therapeutic interventions best-suited to support adolescent neurodevelopment in children with a history of ELA.

The emerging tale of microglia in psychiatric disorders

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

Microglial synaptic pruning on axon initial segment spines of dentate granule cells: Sexually dimorphic effects of early-life stress and consequences for adult fear response

Axon initial segments (AIS) of dentate granule cells in the hippocampus exhibit prominent spines (AISS) during early development that are associated with microglial contacts. In the present study, we investigated whether developmental changes in AISS could be modified by early-life stress (ELS), specifically neonatal maternal separation (MS), through stress hormones and microglial activation and examined the potential behavioural consequences. We examined AISS at postnatal day (PND)5, 15 and 50, using Golgi-Cox staining and anatomical analysis. Neurone-microglial interaction was assessed using antibodies against ankyrin-G, PSD-95 and Iba1, for AIS, AISS and microglia visualisation, respectively, in normally reared and neonatal maternally separated male and female rats. We observed a higher density of AISS in ELS rats at both PND15 and PND50 compared to controls. Effects were more pronounced in females than males. AIS-associated microglia in ELS rats showed a hyper-ramified morphology and less co-localisation with PSD-95 compared to controls at PND15. ELS-associated alteration in microglial morphology and synaptic pruning was mimicked by treatment of acute hippocampal slices of normally reared rats with vasopressin. ELS rats exhibited increased freezing behaviour during auditory fear memory testing, which was more pronounced in female subjects and corresponded with increased Fos expression in dorsal and ventral dentate granule cells. Thus, microglial synaptic pruning in dentate AIS of hippocampus is influenced by ELS, with demonstrable sex bias regarding its anatomical characteristics and subsequent fear-induced defensive behaviours.

Challenges in researching the immune pathways between early life adversity and psychopathology

Exposure to childhood adversity is a critical risk factor for the development of psychopathology. A growing field of research examines how exposure to childhood adversity is translated into biological risk for psychopathology through alterations in immune system functioning, most notably heightened levels of inflammation biomarkers. Though our knowledge about how childhood adversity can instantiate biological risk for psychopathology is growing, there remain many challenges and gaps in the field to understand how inflammation from childhood adversity contributes to psychopathology. This paper reviews research on the inflammatory outcomes arising from childhood adversity and presents four major challenges that future research must address: (a) the measurement of childhood adversity, (b) the measurement of inflammation, (c) the identification of mediators between childhood adversity and inflammation, and (d) the identification of moderators of inflammatory outcomes following childhood adversity. We discuss synergies and inconsistencies in the literature to summarize the current understanding of the association between childhood adversity, a proinflammatory phenotype, and the biological risk for psychopathology. We discuss the clinical implications of the inflammatory links between childhood adversity and psychopathology, including possibilities for intervention. Finally, this review conclude by delineates future directions for research, including issues of how best to detect, prevent, and understand these “hidden wounds” of childhood adversity.

Early life stress causes sex-specific changes in adult fronto-limbic connectivity that differentially drive learning

It is currently unclear whether early life stress (ELS) affects males and females differently. However, a growing body of work has shown that sex moderates responses to stress and injury, with important insights into sex-specific mechanisms provided by work in rodents. Unfortunately, most of the ELS studies in rodents were conducted only in males, a bias that is particularly notable in translational work that has used human imaging. Here we examine the effects of unpredictable postnatal stress (UPS), a mouse model of complex ELS, using high resolution diffusion magnetic resonance imaging. We show that UPS induces several neuroanatomical alterations that were seen in both sexes and resemble those reported in humans. In contrast, exposure to UPS induced fronto-limbic hyper-connectivity in males, but either no change or hypoconnectivity in females. Moderated-mediation analysis found that these sex-specific changes are likely to alter contextual freezing behavior in males but not in females.

Early Life Stress Exacerbates Outcome after Traumatic Brain Injury

The neurocognitive impairments associated with mild traumatic brain injury (TBI) often resolve within 1-2 weeks; however, a subset of people exhibit persistent cognitive dysfunction for weeks to months after injury. The factors that contribute to these persistent deficits are unknown. One potential risk factor for worsened outcome after TBI is a history of stress experienced by a person early in life. Early life stress (ELS) includes maltreatment such as neglect, and interferes with the normal construction of cortical and hippocampal circuits. We hypothesized that a history of ELS contributes to persistent learning and memory dysfunction following a TBI. To explore this interaction, we modeled ELS by separating Sprague Dawley pups from their nursing mothers from post-natal days 2-14 for 3 h daily. At 2 months of age, male rats received sham surgery or mild to moderate parasagittal fluid-percussion brain injury. We found that the combination of ELS with TBI in adulthood impaired hippocampal-dependent learning, as assessed with contextual fear conditioning, the water maze task, and spatial working memory. Cortical atrophy was significantly exacerbated in TBI animals exposed to ELS compared with normal-reared TBI animals. Changes in corticosterone in response to restraint stress were prolonged in TBI animals that received ELS compared with TBI animals that were normally reared or sham animals that received ELS. Our findings indicate that ELS is a risk factor for worsened outcome after TBI, and results in persistent learning and memory deficits, worsened cortical pathology, and an exacerbation of the hormonal stress response.

Pioglitazone alleviates maternal sleep deprivation-induced cognitive deficits in male rat offspring by enhancing microglia-mediated neurogenesis

Maternal sleep disturbance in pregnancy causes cognitive impairments and emotional disorders in offspring. Microglia-mediated inflammatory processes contribute to prenatal stress-induced neurodevelopmental deficits. Peroxisome proliferator-activated receptor gamma (PPARγ) activation underlies the switching of microglial activation phenotypes, which has emerged as a pharmacological target for regulating neuroinflammatory responses in the treatment of neuropsychiatric disorders. Here we investigated the effects of PPARγ-dependent microglial activation on neurogenesis and cognitive behavioral outcomes in male rat offspring exposed to maternal sleep deprivation (MSD) for 72 h from days 18-21 of pregnancy. In the Morris water maze test, male MSD rat offspring needed more time than control offspring to escape to the hidden platform and spent less time in the target quadrant when the hidden platform was removed. In MSD rat offspring, microglial density as determined by immunofluorescence was higher, microglia showed fewer and shorter processes, and neurogenesis in the hippocampus was significantly reduced. Levels of mRNA encoding pro-inflammatory markers IL-6, TNFα, and IL-1β were higher in male MSD offspring, whereas levels of anti-inflammatory markers Arg1, IL-4, and IL-10 were lower, as was PPARγ expression in the hippocampus. PPARγ activation by pioglitazone (30 mg/kg/day, i.p., 7 d) mitigated these negative effects of MSD, rescuing hippocampal neurogenesis and improving cognitive function. The PPARγ inhibitor GW9662 (1 mg/kg/day, i.p., 7 d) eliminated the effects of pioglitazone. Conditioned medium from pioglitazone-treated microglia promoted proliferation and differentiation of neural progenitor cells. These results suggest that MSD-induced deficits in spatial learning and memory can be ameliorated through PPARγ-dependent modulation of microglial phenotypes.

From early adversities to immune activation in psychiatric disorders: the role of the sympathetic nervous system

Increased peripheral levels of cytokines and central microglial activation have been reported in patients with psychiatric disorders. The degree of both innate and adaptive immune activation is also associated with worse clinical outcomes and poor treatment response in these patients. Understanding the possible causes and mechanisms leading to this immune activation is therefore an important and necessary step for the development of novel and more effective treatment strategies for these patients. In this work, we review the evidence of literature pointing to childhood trauma as one of the main causes behind the increased immune activation in patients with psychiatric disorders. We then discuss the potential mechanisms linking the experience of early life adversity (ELA) to innate immune activation. Specifically, we focus on the innervation of the bone marrow from sympathetic nervous system (SNS) as a new and emerging mechanism that has the potential to bridge the observed increases in both central and peripheral inflammatory markers in patients exposed to ELA. Experimental studies in laboratory rodents suggest that SNS activation following early life stress exposure causes a shift in the profile of innate immune cells, with an increase in proinflammatory monocytes. In turn, these cells traffic to the brain and influence neural circuitry, which manifests as increased anxiety and other relevant behavioural phenotypes. To date, however, very few studies have been conducted to explore this candidate mechanism in humans. Future research is also needed to clarify whether these pathways could be partially reversible to improve prevention and treatment strategies in the future.

Synergistic effects of early life mild adversity and chronic social defeat on rat brain microglia and cytokines

Exposure to early life stress affects the development and function of the brain and when followed by adversities in adulthood, the negative effects of stress are enhanced. Microglia has been proposed as a potential mediator of this phenomenon. In the present study, we investigated the long-term effects of mild early life stress, the consequences of a stressor in adulthood as well as their interaction on microglial and cytokine (PPARγ, IL-1β and TNFα) levels in the brain of adult male rats. As an early life stress we used a model of maternal neglect, in which the dam is present but non-accessible to the pup for 15 min during postnatal days 10-13; as a stressor in adulthood we exposed animals to chronic social defeat (CSD) for 3 weeks. We determined in the hippocampus, prefrontal cortex and amygdala, the number of Iba-1+ microglial cells, the number of PPARγ+ cells as well as the relative expression of PPARγ, IL-1β and TNFα mRNA by qPCR. Following exposure to CSD, the number of Iba-1+ cells was increased in the hippocampus and the prefrontal cortex of adult animals exposed to mild early life stress, while in the absence of CSD no such difference was observed. Moreover, following CSD PPARγ levels were increased in the hippocampus of adult males exposed as neonates to "maternal neglect". Our findings support the notion that early life stress, even a mild one, primes microglia and enhances its reactivity to a second stressful event, later in life, in accord with the "two-hit" hypothesis.

Inflammation and post-traumatic stress disorder

While post-traumatic stress disorder (PTSD) is currently diagnosed based solely on classic psychological and behavioral symptoms, a growing body of evidence has highlighted a link between this disorder and alterations in the immune and inflammatory systems. Epidemiological studies have demonstrated that PTSD is associated with significantly increased rates of physical comorbidities in which immune dysregulation is involved, such as metabolic syndrome, atherosclerotic cardiovascular disease, and autoimmune diseases. In line with this, a number of blood biomarker studies have reported that compared to healthy controls, individuals with PTSD exhibit significantly elevated levels of proinflammatory markers, such as interleukin-1β, interleukin-6, tumor necrosis factor-α, and C-reactive protein. Moreover, various lines of animal and human research have suggested that inflammation is not only associated with PTSD but also can play an important role in its pathogenesis and pathophysiology. In this review, we first summarize evidence suggestive of increased inflammation in PTSD. We then examine findings that suggest possible mechanisms of inflammation in this disorder in terms of two different but interrelated perspectives: putative causes of increased proinflammatory activities and potential consequences that inflammation generates. Given that there is currently a dearth of treatment options for PTSD, possibilities of new therapeutic approaches using pharmacological and non-pharmacological treatments/interventions that have anti-inflammatory effects are also discussed. Despite the increasing attention given to the inflammatory pathology of PTSD, there remains much to be elucidated, including more detailed mechanisms of inflammation, potential usefulness of inflammatory biomarkers as diagnostic and prognostic markers, and efficacy of novel treatment strategies targeting inflammation.

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.

Developmental Trajectories of Early Life Stress and Trauma: A Narrative Review on Neurobiological Aspects Beyond Stress System Dysregulation

Early life stressors display a high universal prevalence and constitute a major public health problem. Prolonged psychoneurobiological alterations as sequelae of early life stress (ELS) could represent a developmental risk factor and mediate risk for disease, leading to higher physical and mental morbidity rates in later life. ELS could exert a programming effect on sensitive neuronal brain networks related to the stress response during critical periods of development and thus lead to enduring hyper- or hypo-activation of the stress system and altered glucocorticoid signaling. In addition, alterations in emotional and autonomic reactivity, circadian rhythm disruption, functional and structural changes in the brain, as well as immune and metabolic dysregulation have been lately identified as important risk factors for a chronically impaired homeostatic balance after ELS. Furthermore, human genetic background and epigenetic modifications through stress-related gene expression could interact with these alterations and explain inter-individual variation in vulnerability or resilience to stress. This narrative review presents relevant evidence from mainly human research on the ten most acknowledged neurobiological allostatic pathways exerting enduring adverse effects of ELS even decades later (hypothalamic-pituitary-adrenal axis, autonomic nervous system, immune system and inflammation, oxidative stress, cardiovascular system, gut microbiome, sleep and circadian system, genetics, epigenetics, structural, and functional brain correlates). Although most findings back a causal relation between ELS and psychobiological maladjustment in later life, the precise developmental trajectories and their temporal coincidence has not been elucidated as yet. Future studies should prospectively investigate putative mediators and their temporal sequence, while considering the potentially delayed time-frame for their phenotypical expression. Better screening strategies for ELS are needed for a better individual prevention and treatment.

Early-life adversity programs long-term cytokine and microglia expression within the HPA axis in female Japanese quail

Stress exposure during pre and post-natal development can have persistent and often dysfunctional effects on several physiological systems, including immune function, affecting the ability to combat infection. The neuro-immune response is inextricably linked to the action of the Hypothalamic Pituitary Adrenal (HPA) axis. Cytokines released from neuro-immune cells, including microglia, activate the HPA axis while glucocorticoids in turn regulate cytokine release from microglia. Because of the close links between these two physiological systems, coupled with potential for persistent changes to HPA axis activity following developmental stress, components of the neuro-immune system could be targets for developmental programming. However, little is known of any programming effects of developmental stress on neuro-immune function. We investigated whether developmental stress exposure via elevated pre-natal corticosterone (CORT) or post-natal unpredictable food availability, had long-term effects on pro (IL-1β) and anti-inflammatory (IL-10) cytokine and microglia-dependent gene (CSF1R) expression within HPA axis tissues in a precocial bird, the Japanese quail (Coturnix japonica). Following post-natal stress, we observed increased IL-1β expression in the pituitary gland, reduced IL-10 expression in the amygdala and hypothalamus and reduced CSF1R expression within the hypothalamus and pituitary gland. Post-natal stress disrupted the ratio of IL-1β:IL-10 expression within the hippocampus and hypothalamus. Pre-natal stress only increased IL-1β expression in the pituitary gland. We found no evidence for interactive or cumulative effects across life stages on basal cytokine and glia expression in adulthood. We show that post-natal stress may have a larger impact than elevated pre-natal CORT on basal immunity in HPA axis specific brain regions, with changes in cytokine homeostasis and microglia abundance. These results provide evidence for post-natal programming of a pro-inflammatory neuro-immune phenotype at the expense of reduced microglia, which could have implications for CNS health and subsequent neuro-immune responses.

Early life stress and trauma: developmental neuroendocrine aspects of prolonged stress system dysregulation

Experience of early life stress (ELS) and trauma is highly prevalent in the general population and has a high public health impact, as it can trigger a health-related risk cascade and lead to impaired homeostatic balance and elevated cacostatic load even decades later. The prolonged neuropsychobiological impact of ELS can, thus, be conceptualized as a common developmental risk factor for disease associated with increased physical and mental morbidity in later life. ELS during critical periods of brain development with elevated neuroplasticity could exert a programming effect on particular neuronal networks related to the stress response and lead to enduring neuroendocrine alterations, i.e., hyper- or hypoactivation of the stress system, associated with adult hypothalamic-pituitary-adrenal axis and glucocorticoid signaling dysregulation. This paper reviews the pathophysiology of the human stress response and provides evidence from human research on the most acknowledged stress axis-related neuroendocrine pathways exerting the enduring adverse effects of ELS and mediating the cumulative long-term risk of disease vulnerability in adulthood.

Does the Stress of Laboratory Life and Experimentation on Animals Adversely Affect Research Data? A Critical Review

Recurrent acute and/or chronic stress can affect all vertebrate species, and can have serious consequences. It is increasingly and widely appreciated that laboratory animals experience significant and repeated stress, which is unavoidable and is caused by many aspects of laboratory life, such as captivity, transport, noise, handling, restraint and other procedures, as well as the experimental procedures applied to them. Such stress is difficult to mitigate, and lack of significant desensitisation/habituation can result in considerable psychological and physiological welfare problems, which are mediated by the activation of various neuroendocrine networks that have numerous and pervasive effects. Psychological damage can be reflected in stereotypical behaviours, including repetitive pacing and circling, and even self-harm. Physical consequences include adverse effects on immune function, inflammatory responses, metabolism, and disease susceptibility and progression. Further, some of these effects are epigenetic, and are therefore potentially transgenerational: the biology of animals whose parents/grandparents were wild-caught and/or have experienced chronic stress in laboratories could be altered, as compared to free-living individuals. It is argued that these effects must have consequences for the reliability of experimental data and their extrapolation to humans, and this may not be recognised sufficiently among those who use animals in experiments.

Inflammatory markers and their possible effects on cognitive function in women with posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) has been associated with increased inflammation, albeit with some controversy. Another key feature of PTSD is compromised function in wide-ranging cognitive domains. Increased peripheral inflammation can contribute to cognitive dysfunction, although this relationship has not been studied in patients with PTSD. Here, we examined blood inflammatory markers in adult patients with PTSD compared to healthy controls taking account of potentially confounding effects of childhood maltreatment and comorbid major depressive disorder (MDD), and explored the association between inflammation and cognition. We enrolled 40 women with PTSD, most of whom developed the disorder after interpersonal violence during adulthood, and 65 healthy control women. Diagnoses were made based on DSM-IV. History of childhood maltreatment was assessed using the Childhood Trauma Questionnaire (CTQ). Cognitive function was assessed using the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). Blood samples were collected for the measurement of 5 inflammatory markers including interleukin-6 (IL-6), soluble IL-6 receptor, interleukin-1β, high-sensitivity tumor necrosis factor-α, and high-sensitivity C-reactive protein. Compared to controls, patients with PTSD showed significantly higher IL-6 levels (p = 0.009) and lower scores on all RBANS domains (all p < 0.01). IL-6 levels in patients were not significantly associated with the presence/absence of comorbid MDD or CTQ scores. IL-6 levels in patients were significantly negatively correlated with RBANS visuospatial construction (p = 0.046), language (p = 0.008), attention (p = 0.036) and total score (p = 0.008). These results suggest that elevated IL-6 is associated with PTSD and that the lower cognitive function in PTSD may be due at least partly to increased inflammation.

Early life stress perturbs the function of microglia in the developing rodent brain: New insights and future challenges

The role of the innate immune system in mediating some of the consequences of childhood abuse and neglect has received increasing attention in recent years. Most of the work to date has focused on the role that neuroinflammation plays in the long-term adult psychiatric and medical complications associated with childhood maltreatment. The effects of stress-induced neuroinflammation on neurodevelopment have received little attention because until recently this issue has not been studied systematically in animal models of early life stress. The primary goal of this review is to explore the hypothesis that elevated corticosterone during the first weeks of life in mice exposed to brief daily separation (BDS), which is a mouse model of early life stress, disrupts microglial function during a critical period of brain development. We propose that perturbations of microglial function lead to abnormal maturation of several neuronal and non-neuronal cellular processes resulting in behavioral abnormalities that emerge during the juvenile period and persist in adulthood. Here, we highlight recent work demonstrating that exposure to BDS alters microglial cell number, morphology, phagocytic activity, and gene expression in the developing hippocampus in a manner that extends into the juvenile period. These changes in microglial function are associated with abnormalities in developmental processes mediated by microglia including synaptogenesis, synaptic pruning, axonal growth, and myelination. We examine the changes in microglial gene expression in the context of previous work demonstrating developmental and behavioral abnormalities in BDS mice and in other animal models of early life stress. The possible utility of these findings for developing novel PET imaging to assess microglial function in individuals exposed to childhood maltreatment is also discussed.

Amygdala hyper-connectivity in a mouse model of unpredictable early life stress

Childhood maltreatment is associated with a wide range of psychopathologies including anxiety that emerge in childhood and in many cases persist in adulthood. Increased amygdala activation in response to threat and abnormal amygdala connectivity with frontolimbic brain regions, such as the hippocampus and the prefrontal cortex, are some of the most consistent findings seen in individuals exposed to childhood maltreatment. The underlying mechanisms responsible for these changes are difficult to study in humans but can be elucidated using animal models of early-life stress. Such studies are especially powerful in the mouse where precise control of the genetic background and the stress paradigm can be coupled with resting-state fMRI (rsfMRI) to map abnormal connectivity in circuits that regulate anxiety. To address this issue we first compared the effects of two models of early-life stress, limited bedding (LB) and unpredictable postnatal stress (UPS), on anxiety-like behavior in juvenile and adult mice. We found that UPS, but not LB, causes a robust increase in anxiety in juvenile and adult male mice. Next, we used rsfMRI to compare frontolimbic connectivity in control and UPS adult male mice. We found increased amygdala-prefrontal cortex and amygdala-hippocampus connectivity in UPS. The strength of the amygdala-hippocampal and amygdala-prefrontal cortex connectivity was highly correlated with anxiety-like behavior in the open-field test and elevated plus maze. These findings are the first to link hyperconnectivity in frontolimbic circuits and increased anxiety in a mouse model of early-life stress, allowing for more mechanistic understanding of parallel findings in humans.

Glucocorticoid programming of neuroimmune function

Throughout life physiological systems strive to maintain homeostasis and these systems are susceptible to exposure to maternal or environmental perturbations, particularly during embryonic development. In some cases, these perturbations may influence genetic and physiological processes that permanently alter the functioning of these physiological systems; a process known as developmental programming. In recent years, the neuroimmune system has garnered attention for its fundamental interactions with key hormonal systems, such as the hypothalamic pituitary adrenal (HPA) axis. The ultimate product of this axis, the glucocorticoid hormones, play a key role in modulating immune responses within the periphery and the CNS as part of the physiological stress response. It is well-established that elevated glucocorticoids induced by developmental stress exert profound short and long-term physiological effects, yet there is relatively little information of how these effects are manifested within the neuroimmune system. Pre and post-natal periods are prime candidates for manipulation in order to uncover the physiological mechanisms that underlie glucocorticoid programming of neuroimmune responses. Understanding the potential programming role of glucocorticoids may be key in uncovering vulnerable windows of CNS susceptibility to stressful experiences during embryonic development and improve our use of glucocorticoids as therapeutics in the treatment of neurodegenerative diseases.

Neuroimmune Interactions: From the Brain to the Immune System and Vice Versa

Because of the compartmentalization of disciplines that shaped the academic landscape of biology and biomedical sciences in the past, physiological systems have long been studied in isolation from each other. This has particularly been the case for the immune system. As a consequence of its ties with pathology and microbiology, immunology as a discipline has largely grown independently of physiology. Accordingly, it has taken a long time for immunologists to accept the concept that the immune system is not self-regulated but functions in close association with the nervous system. These associations are present at different levels of organization. At the local level, there is clear evidence for the production and use of immune factors by the central nervous system and for the production and use of neuroendocrine mediators by the immune system. Short-range interactions between immune cells and peripheral nerve endings innervating immune organs allow the immune system to recruit local neuronal elements for fine tuning of the immune response. Reciprocally, immune cells and mediators play a regulatory role in the nervous system and participate in the elimination and plasticity of synapses during development as well as in synaptic plasticity at adulthood. At the whole organism level, long-range interactions between immune cells and the central nervous system allow the immune system to engage the rest of the body in the fight against infection from pathogenic microorganisms and permit the nervous system to regulate immune functioning. Alterations in communication pathways between the immune system and the nervous system can account for many pathological conditions that were initially attributed to strict organ dysfunction. This applies in particular to psychiatric disorders and several immune-mediated diseases. This review will show how our understanding of this balance between long-range and short-range interactions between the immune system and the central nervous system has evolved over time, since the first demonstrations of immune influences on brain functions. The necessary complementarity of these two modes of communication will then be discussed. Finally, a few examples will illustrate how dysfunction in these communication pathways results in what was formerly considered in psychiatry and immunology to be strict organ pathologies.

Neuroimmune Impacts of Early-Life Stress on Development and Psychopathology

Maltreatment and trauma in childhood, termed early-life stress (ELS), has long-term effects on the immune system. ELS impacts immune signaling at the time of exposure but also disrupts the developmental trajectory of certain immunological processes, both in the periphery and in the brain. One consequence of these early alterations is a heightened immune response to stressors later in life. However, chronic and sustained inflammatory response can also lead to excitotoxicity and prevent typical brain development. In this chapter, we discuss current progress toward understanding the contribution of neuroimmune signaling to ELS-attributable dysfunction or maladaptation with a focus on postnatal experiences. To do so we first present an operational definition of ELS. Then, we offer a brief overview of the immune system and neuroimmune development, followed by a section discussing the interaction between immunity, childhood trauma, and mental disorders in humans. We present evidence from animal models about immune alterations after ELS and discuss the ways in which ELS-induced immune changes ultimately affect brain and behavior, as well as the importance of individual differences and future directions in this field. Taken together, we submit that when encountered with ELS, some core brain circuits could develop differently via various mechanisms involving dysfunctional immune reprograming. However, given the remarkable plasticity of both the brain and the immune system, many of the deleterious effects of ELS may be mitigated with interventions that account for sex and target neuroimmune interactions over the lifespan.

Microglia under psychosocial stressors along the aging trajectory: Consequences on neuronal circuits, behavior, and brain diseases

Mounting evidence indicates the importance of microglia for proper brain development and function, as well as in complex stress-related neuropsychiatric disorders and cognitive decline along the aging trajectory. Considering that microglia are resident immune cells of the brain, a homeostatic maintenance of their effector functions that impact neuronal circuitry, such as phagocytosis and secretion of inflammatory factors, is critical to prevent the onset and progression of these pathological conditions. However, the molecular mechanisms by which microglial functions can be properly regulated under healthy and pathological conditions are still largely unknown. We aim to summarize recent progress regarding the effects of psychosocial stress and oxidative stress on microglial phenotypes, leading to neuroinflammation and impaired microglia-synapse interactions, notably through our own studies of inbred mouse strains, and most importantly, to discuss about promising therapeutic strategies that take advantage of microglial functions to tackle such brain disorders in the context of adult psychosocial stress or aging-induced oxidative stress.

Psychosocial stress on neuroinflammation and cognitive dysfunctions in Alzheimer's disease: the emerging role for microglia?

Chronic psychosocial stress is increasingly recognized as a risk factor for late-onset Alzheimer's disease (LOAD) and associated cognitive deficits. Chronic stress also primes microglia and induces inflammatory responses in the adult brain, thereby compromising synapse-supportive roles of microglia and deteriorating cognitive functions during aging. Substantial evidence demonstrates that failure of microglia to clear abnormally accumulating amyloid-beta (Aβ) peptide contributes to neuroinflammation and neurodegeneration in AD. Moreover, genome-wide association studies have linked variants in several immune genes, such as TREM2 and CD33, the expression of which in the brain is restricted to microglia, with cognitive dysfunctions in LOAD. Thus, inflammation-promoting chronic stress may create a vicious cycle of aggravated microglial dysfunction accompanied by increased Aβ accumulation, collectively exacerbating neurodegeneration. Surprisingly, however, little is known about whether and how chronic stress contributes to microglia-mediated neuroinflammation that may underlie cognitive impairments in AD. This review aims to summarize the currently available clinical and preclinical data and outline potential molecular mechanisms linking stress, microglia and neurodegeneration, to foster future research in this field.

Psychoneuroimmunology of Early-Life Stress: The Hidden Wounds of Childhood Trauma?

The brain and the immune system are not fully formed at birth, but rather continue to mature in response to the postnatal environment. The two-way interaction between the brain and the immune system makes it possible for childhood psychosocial stressors to affect immune system development, which in turn can affect brain development and its long-term functioning. Drawing from experimental animal models and observational human studies, we propose that the psychoneuroimmunology of early-life stress can offer an innovative framework to understand and treat psychopathology linked to childhood trauma. Early-life stress predicts later inflammation, and there are striking analogies between the neurobiological correlates of early-life stress and of inflammation. Furthermore, there are overlapping trans-diagnostic patterns of association of childhood trauma and inflammation with clinical outcomes. These findings suggest new strategies to remediate the effect of childhood trauma before the onset of clinical symptoms, such as anti-inflammatory interventions and potentiation of adaptive immunity. Similar strategies might be used to ameliorate the unfavorable treatment response described in psychiatric patients with a history of childhood trauma.

Inflammation, Glutamate, and Glia: A Trio of Trouble in Mood Disorders

Increasing data indicate that inflammation and alterations in glutamate neurotransmission are two novel pathways to pathophysiology in mood disorders. The primary goal of this review is to illustrate how these two pathways may converge at the level of the glia to contribute to neuropsychiatric disease. We propose that a combination of failed clearance and exaggerated release of glutamate by glial cells during immune activation leads to glutamate increases and promotes aberrant extrasynaptic signaling through ionotropic and metabotropic glutamate receptors, ultimately resulting in synaptic dysfunction and loss. Furthermore, glutamate diffusion outside the synapse can lead to the loss of synaptic fidelity and specificity of neurotransmission, contributing to circuit dysfunction and behavioral pathology. This review examines the fundamental role of glia in the regulation of glutamate, followed by a description of the impact of inflammation on glial glutamate regulation at the cellular, molecular, and metabolic level. In addition, the role of these effects of inflammation on glia and glutamate in mood disorders will be discussed along with their translational implications.

Early life stress perturbs the maturation of microglia in the developing hippocampus

Children exposed to abuse or neglect show abnormal hippocampal development and similar findings have been reported in rodent models. Using brief daily separation (BDS), a mouse model of early life stress, we previously showed that exposure to BDS impairs hippocampal function in adulthood and perturbs synaptic maturation, synaptic pruning, axonal growth and myelination in the developing hippocampus. Given that microglia are involved in these developmental processes, we tested whether BDS impairs microglial activity in the hippocampus of 14 (during BDS) and 28-day old mice (one week after BDS). We found that BDS increased the density and altered the morphology of microglia in the hippocampus of 14-day old pups, effects that were no longer present on postnatal day (PND) 28. Despite the normal cell number and morphology seen at PND28, the molecular signature of hippocampal microglia, assessed using the NanoString immune panel, was altered at both ages. We showed that during normal hippocampal development, microglia undergo significant changes between PND14 and PND28, including reduced cell density, decreased ex vivo phagocytic activity, and an increase in the expression of genes involved in inflammation and cell migration. However, microglia harvested from the hippocampus of 28-day old BDS mice showed an increase in phagocytic activity and reduced expression of genes that normally increase across development. Promoter analysis indicated that alteration in the transcriptional activity of PU.1, Creb1, Sp1, and RelA accounted for most of the transcriptional changes seen during normal microglia development and for most of the BDS-induced changes at PND14 and PND28. These findings are the first to demonstrate that early life stress dysregulates microglial function in the developing hippocampus and to identify key transcription factors that are likely to mediate these changes.

Hidden Wounds? Inflammatory Links Between Childhood Trauma and Psychopathology

Childhood trauma is a key risk factor for psychopathology. However, little is known about how exposure to childhood trauma is translated into biological risk for psychopathology. Observational human studies and experimental animal models suggest that childhood exposure to stress can trigger an enduring systemic inflammatory response not unlike the bodily response to physical injury. In turn, these "hidden wounds" of childhood trauma can affect brain development, key behavioral domains (e.g., cognition, positive valence systems, negative valence systems), reactivity to subsequent stressors, and, ultimately, risk for psychopathology. Further research is needed to better characterize the inflammatory links between childhood trauma and psychopathology. Detecting and healing these hidden wounds may help prevent and treat psychopathology emerging after childhood trauma.

The role of the hippocampus in the pathogenesis of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) is a severe acquired illness characterized by a profound sensation of fatigue, not ameliorated by rest and resulting in a substantial decrease in the amount and quality of occupational, social and recreational activities. Despite intense research, the aetiology and pathogenesis of ME/CFS is still unknown and no conclusive biological markers have been found. As a consequence, an accepted curative treatment is still lacking and rehabilitation programmes are not very effective, as few patients recover. Increased knowledge of the mechanisms leading to the emergence and maintenance of the illness is called for. In this study, I will put forth an alternative hypothesis to explain some of the pathologies associated with ME/CFS, by concentrating on one of the major strategic organs of the brain, the hippocampus. I will show that the ME/CFS triggering factors also impact the hippocampus, leading to neurocognitive deficits and disturbances in the regulation of the stress system and pain perception. These deficits lead to a substantial decrease in activity and to sleep disorders, which, in turn, impact the hippocampus and initiate a vicious circle of increased disability.

Early-Life Stress Perturbs Key Cellular Programs in the Developing Mouse Hippocampus

Conflicting reports are available with regard to the effects of childhood abuse and neglect on hippocampal function in children. While earlier imaging studies and some animal work have suggested that the effects of early-life stress (ELS) manifest only in adulthood, more recent studies have documented impaired hippocampal function in maltreated children and adolescents. Additional work using animal modes is needed to clarify the effects of ELS on hippocampal development. In this regard, genomic, proteomic, and molecular tools uniquely available in the mouse make it a particularly attractive model system to study this issue. However, very little work has been done so far to characterize the effects of ELS on hippocampal development in the mouse. To address this issue, we examined the effects of brief daily separation (BDS), a mouse model of ELS that impairs hippocampal-dependent memory in adulthood, on hippocampal development in 28-day-old juvenile mice. This age was chosen because it corresponds to the developmental period in which human imaging studies have revealed abnormal hippocampal development in maltreated children. Exposure to BDS caused a significant decrease in the total protein content of synaptosomes harvested from the hippocampus of 28-day-old male and female mice, suggesting that BDS impairs normal synaptic development in the juvenile hippocampus. Using a novel liquid chromatography multiple reaction monitoring mass spectrometry (LC-MRM) assay, we found decreased expression of many synaptic proteins, as well as proteins involved in axonal growth, myelination, and mitochondrial activity. Golgi staining in 28-day-old BDS mice showed an increase in the number of immature and abnormally shaped spines and a decrease in the number of mature spines in CA1 neurons, consistent with defects in synaptic maturation and synaptic pruning at this age. In 14-day-old pups, BDS deceased the expression of proteins involved in axonal growth and myelination, but did not affect the total protein content of synaptosomes harvested from the hippocampus, or protein levels of other synaptic markers. These results add two important findings to previous work in the field. First, our findings demonstrate that in 28-day-old juvenile mice, BDS impairs synaptic maturation and reduces the expression of proteins that are necessary for axonal growth, myelination, and mitochondrial function. Second, the results suggest a sequential model in which BDS impairs normal axonal growth and myelination before it disrupts synaptic maturation in the juvenile hippocampus.

The interplay of early-life stress, nutrition, and immune activation programs adult hippocampal structure and function

Early-life adversity increases the vulnerability to develop psychopathologies and cognitive decline later in life. This association is supported by clinical and preclinical studies. Remarkably, experiences of stress during this sensitive period, in the form of abuse or neglect but also early malnutrition or an early immune challenge elicit very similar long-term effects on brain structure and function. During early-life, both exogenous factors like nutrition and maternal care, as well as endogenous modulators, including stress hormones and mediator of immunological activity affect brain development. The interplay of these key elements and their underlying molecular mechanisms are not fully understood. We discuss here the hypothesis that exposure to early-life adversity (specifically stress, under/malnutrition and infection) leads to life-long alterations in hippocampal-related cognitive functions, at least partly via changes in hippocampal neurogenesis. We further discuss how these different key elements of the early-life environment interact and affect one another and suggest that it is a synergistic action of these elements that shapes cognition throughout life. Finally, we consider different intervention studies aiming to prevent these early-life adversity induced consequences. The emerging evidence for the intriguing interplay of stress, nutrition, and immune activity in the early-life programming calls for a more in depth understanding of the interaction of these elements and the underlying mechanisms. This knowledge will help to develop intervention strategies that will converge on a more complete set of changes induced by early-life adversity.

Early life stress inhibits expression of ribosomal RNA in the developing hippocampus

Children that are exposed to abuse or neglect show abnormal hippocampal function. However, the developmental mechanisms by which early life stress (ELS) impairs normal hippocampal development have not been elucidated. Here we propose that exposure to ELS blunts normal hippocampal growth by inhibiting the availability of ribosomal RNA (rRNA). In support of this hypothesis, we show that the normal mouse hippocampus undergoes a growth-spurt during the second week of life, followed by a gradual decrease in DNA and RNA content that persists into adulthood. This developmental pattern is associated with accelerated ribosomal RNA (rRNA) synthesis during the second week of life, followed by a gradual decline in rRNA levels that continue into adulthood. Levels of DNA methylation at the ribosomal DNA (rDNA) promoter are lower during the second week of life compared to earlier development or adulthood. Exposure to brief daily separation (BDS), a mouse model of early life stress, increased DNA methylation at the ribosomal DNA promoter, decreased rRNA levels, and blunted hippocampal growth during the second week of life. Exposure to acute (3 hrs) maternal separation decreased rRNA and increased DNA methylation at the rDNA proximal promoter, suggesting that exposure to stress early in life can rapidly regulate the availability of rRNA levels in the developing hippocampus. Given the critical role that rRNA plays in supporting normal growth and development, these findings suggest a novel molecular mechanism to explain how stress early in life impairs hippocampus development in the mouse.

Broken or maladaptive? Altered trajectories in neuroinflammation and behavior after early life adversity

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This paper reviews how early life adversity alters neuroimmune mechanisms.

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Neuroimmune sensitization from early life adversity impacts circuitry at discrete life stages.

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Neuroimmune and neurodevelopmental influences can impact behavior and vulnerability.

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Sexual dimorphism in immune and brain development yield distinct effects of early life adversity.

Exposure to adversity and stress early in development yields vulnerability to mental illnesses throughout the lifespan. Growing evidence suggests that this vulnerability has mechanistic origins involving aberrant development of both neurocircuitry and neuro-immune activity. Here we review the current understanding of when and how stress exposure initiates neuroinflammatory events that interact with brain development. We first review how early life adversity has been associated with various psychopathologies, and how neuroinflammation plays a role in these pathologies. We then summarize data and resultant hypotheses describing how early life adversity may particularly alter neuro-immune development with psychiatric consequences. Finally, we review how sex differences contribute to individualistic vulnerabilities across the lifespan. We submit the importance of understanding how stress during early development might cause outright neural or glial damage, as well as experience-dependent plasticity that may insufficiently prepare an individual for sex-specific or life-stage specific challenges.

Modulation of adult hippocampal neurogenesis by early-life environmental challenges triggering immune activation

The immune system plays an important role in the communication between the human body and the environment, in early development as well as in adulthood. Per se, research has shown that factors such as maternal stress and nutrition as well as maternal infections can activate the immune system in the infant. A rising number of research studies have shown that activation of the immune system in early life can augment the risk of some psychiatric disorders in adulthood, such as schizophrenia and depression. The mechanisms of such a developmental programming effect are unknown; however some preliminary evidence is emerging in the literature, which suggests that adult hippocampal neurogenesis may be involved. A growing number of studies have shown that pre- and postnatal exposure to an inflammatory stimulus can modulate the number of proliferating and differentiating neural progenitors in the adult hippocampus, and this can have an effect on behaviours of relevance to psychiatric disorders. This review provides a summary of these studies and highlights the evidence supporting a neurogenic hypothesis of immune developmental programming.

Assessment of plasma C-reactive protein as a biomarker of posttraumatic stress disorder risk

IMPORTANCE

Posttraumatic stress disorder (PTSD) has been associated in cross-sectional studies with peripheral inflammation. It is not known whether this observed association is the result of PTSD predisposing to inflammation (as sometimes postulated) or to inflammation predisposing to PTSD.

OBJECTIVE

To determine whether plasma concentration of the inflammatory marker C-reactive protein (CRP) helps predict PTSD symptoms.

DESIGN, SETTING, AND PARTICIPANTS

The Marine Resiliency Study, a prospective study of approximately 2600 war zone-deployed Marines, evaluated PTSD symptoms and various physiological and psychological parameters before deployment and at approximately 3 and 6 months following a 7-month deployment. Participants were recruited from 4 all-male infantry battalions imminently deploying to a war zone. Participation was requested of 2978 individuals; 2610 people (87.6%) consented and 2555 (85.8%) were included in the present analysis. Postdeployment data on combat-related trauma were included for 2208 participants (86.4% of the 2555 included) and on PTSD symptoms at 3 and 6 months after deployment for 1861 (72.8%) and 1617 (63.3%) participants, respectively.

MAIN OUTCOMES AND MEASURES

Severity of PTSD symptoms 3 months after deployment assessed by the Clinician-Administered PTSD Scale (CAPS).

RESULTS

We determined the effects of baseline plasma CRP concentration on postdeployment CAPS using zero-inflated negative binomial regression (ZINBR), a procedure designed for distributions, such as CAPS in this study, that have an excess of zeroes in addition to being positively skewed. Adjusting for the baseline CAPS score, trauma exposure, and other relevant covariates, we found baseline plasma CRP concentration to be a highly significant overall predictor of postdeployment CAPS scores (P = .002): each 10-fold increment in CRP concentration was associated with an odds ratio of nonzero outcome (presence vs absence of any PTSD symptoms) of 1.51 (95% CI, 1.15-1.97; P = .003) and a fold increase in outcome with a nonzero value (extent of symptoms when present) of 1.06 (95% CI, 0.99-1.14; P = .09).

CONCLUSIONS

AND RELEVANCE A marker of peripheral inflammation, plasma CRP may be prospectively associated with PTSD symptom emergence, suggesting that inflammation may predispose to PTSD.

Disease prevention in the age of convergence - the need for a wider, long ranging and collaborative vision

It is time to bring our imagination, creativity and passion to the fore in solving the global challenges of our age. Our global health crisis and the pandemic of noncommunicable diseases (NCDs) is clearly rooted in complex modern societal and environmental changes, many of which have effects on developing immune and metabolic responses. It is intimately related to wider environmental challenges. And it is unsurprising that many NCDs share similar risk factors and that many are associated with a rising predisposition for inflammation. Allergy is one of the earliest signs of environmental impact on these biological pathways, and may also offer an early barometer to assess the effects of early interventions. There is dawning awareness of how changing microbial diversity, nutritional patterns, sedentary indoor behaviours and modern pollutants adversely affect early metabolic and immune development, but still much to understand the complexity of these interactions. Even when we do harness the science and technology, these will not provide solutions unless we also address the wider social, cultural and economic determinants of health - addressing the interconnections between human health and the health of our environment. Now more than ever, we need a wider vision and a greater sense of collective responsibility. We need long-range approaches that aim for life long benefits of a 'healthier start to life', and stronger cross-sectoral collaborations to prevent disease. We need to give both our hearts and our minds to solving these global issues.

Psychiatric disturbances regulate the innate immune system in CSF of conscious mice

Environment may affect brain activity through cerebrospinal fluid (CSF) only if there are regulatory molecules or cascades in CSF that are sensitive to external stimuli. This study was designed to identify regulatory activity present in CSF, better elucidating environmental regulation of brain function. By using cannulation-based sequential CSF sampling coupled with mass spectrometry-based identification and quantification of proteins, we show that the naive mouse CSF harbors, among 22 other pathways, the innate immune system as a main pathway, which was downregulated and upregulated, respectively, by acute stressor (AS) and acute cocaine (AC) administrations. Among novel processes and molecular functions, AS also regulated schizophrenia-associated proteins. Furthermore, AC upregulated exosome-related proteins with a false discovery rate of 1.0 × 10(-)(16). These results suggest that psychiatric disturbances regulate the neuroimmune system and brain disorder-related proteins, presenting a sensitive approach to investigating extracellular mechanisms in conscious and various mouse models of psychiatric disorders.