Neuropathological Consequences of Prenatal Infection
Prenatal exposure to infectious pathogens and/or inflammatory processes are increasingly recognized to play an important etiological role in neuropsychiatric and neurological disorders with neurodevelopmental components. Human epidemiological studies have revealed significant associations between prenatal infection and increased risk of various brain disorders, including schizophrenia, autism, bipolar disorder, and cerebral palsy. Besides direct trans-placental viral intrusion, cytokine-associated inflammatory events and downstream pathophysiological effects such as oxidative stress and (temporary) macronutrient and micronutrient deficiencies seem to be critical in mediating the adverse effects of maternal infection on the fetal system (Figure 1). These pathological processes can affect somatic cell development and change the offspring’s neurodevelopmental trajectories, which in turn can lead to the emergence of behavioral and cognitive disturbances later in life.
Figure 1. Possible mechanisms mediating the pathological effects of maternal infection on the developing organism in utero. Maternal infection during pregnancy induces a number of pathophysiological responses in the maternal host, including production of soluble immune factors, such as cytokines, reactive oxygen species, and soluble endocrine factors, such as stress hormones. Some of these factors might cross the placental barrier and enter the fetal environment, thereby causing fetal inflammation and oxidative stress. Abnormal fetal expression of these factors might impair the normal development of peripheral and central organs by modifying the differentiation, proliferation, and/or migration of target cells. These processes may involve alterations in gene expression via epigenetic modifications. Maternal infection during pregnancy can further induce inflammatory response in the placenta and cause placental insufficiency, which, in turn can cause fetal hypoxemia. In addition, infection can cause (temporary) states of macronutrient and micronutrient deficiencies, which limit the fetal availability of essential nutrients necessary for normal fetal development and growth. Finally, maternal infection during pregnancy can modify the microbial composition of the placenta, which might alter the development of the offspring's microbiome and thus predispose the developing organism to dysbiosis and other microbiome-associated abnormalities. Adapted from: Labouesse et al., Am J Physiol Regul Integr Comp Physiol 2015, 309:R1-R12.
Based on the initial findings provided by human epidemiological studies, our group has established several translational animal models of prenatal immune activation that can be used to study the pathological consequences of prenatal infection on an experimental basis. One such model is based on prenatal treatment with the viral mimetic poly(I:C) (polyriboinosinic-polyribocytidilic acid) in mice. Poly(I:C) is a synthetic analog of double-stranded RNA, which induces a cytokine-associated viral-like acute phase response in maternal and fetal compartments upon maternal gestational administration. Using this model, we explore distinct aspects of the pathological link between prenatal infection and neurodevelopmental abnormalities in mouse models. We adopt a multidisciplinary approach, whereby state-of-the art behavioral and cognitive testing is combined with pharmacological, immunohistochemical, gene transcriptional, and epigenetic techniques to gain novel insights into the developmental and molecular mechanisms mediating the pathological consequences of prenatal infection. The ultimate goal of these research efforts is the establishment and preclinical characterization of early interventions that may help attenuating or even preventing the negative long-term effects of prenatal exposure to infection.
Gene-environment and environment-environment interactions
In spite of its relatively frequent occurrence, maternal exposure to infection seems to have rather modest effect sizes on neurodevelopmental disease risk in large populations. For example, the global incidence of schizophrenia after influenza pandemics only increases marginally (relative risk ratios of 1.2 to 2.5) even though 20 to 50% of the general population is typically infected during influenza pandemics. It has therefore been proposed that early-life immune challenges might unfold their neuropathological impact primarily in genetically predisposed subjects. Another feasible scenario is that initial exposure to a prenatal environmental insult, such as infection, can render the offspring more vulnerable to the pathological effects of postnatal environmental insults. To test these hypotheses, we use several environment-environment and gene-environment interaction models, in which maternal immune activation during pregnancy is combined with specific genetic anomalies or additional environmental advertise such as peripubertal exposure to traumatizing experiences or chronic consumption of drugs of abuse. Our ongoing research suggests that prenatal infection prenatal infection can act as a “disease primer” that increases the vulnerability of the offspring to the detrimental neuropathological effects of other environmental insults or specific genetic abnormalities.
Giovanoli S, Engler H, Engler A, Richetto R, Voget M, Willi R, Winter C, Riva MA, Mortensen PB, Feldon J, Schedlowski M, Meyer U (2013): Stress in puberty unmasks latent neuropathological consequences of prenatal immune activation in mice. Science 339:1095-1099.
Vuillermot S, Joodmardi E, Perlmann T, Oegren SO, Feldon J, Meyer U (2012): Prenatal immune activation interacts with genetic Nurr1 deficiency in the development of attentional impairments. Journal of Neuroscience 32, 436-451.
Meyer U, Murray PJ, Urwyler A, Yee BK, Schedlowski M, Feldon J (2008): Adult behavioral and pharmacological dysfunctions following disruption of the fetal brain balance between pro-inflammatory and IL-10-mediated anti-inflammatory signaling. Molecular Psychiatry 13:208-221.
Epigenetic mechanisms and non-genetic transgenerational inheritance
Epigenetic mechanisms such as DNA methylation, histone modifications, and micro-RNA expression can reprogram genome activity and gene expression without altering the DNA sequence. Epigenetic modifications may be an important molecular mechanism translating in-utero immune challenges into subsequent brain and behavioral pathology. We use various epigenetic analyses, including capture-array bisulfite sequencing, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (EpiTYPER), methylated DNA immunoprecipitation (MeDIP), and chromatin immunoprecipitation (ChIP) assays, to identify epigenetic changes in response to maternal infection during pregnancy. In addition, we perform transgenerational breeding of prenatally infected mouse offspring to explore whether prenatal immune activation induces transgenerational effects on brain and behavior. In support of this hypothesis, we have recently found that prenatal viral-like immune challenge causes transgenerational behavioral effects via the paternal lineage. These effects are stable until the third generation, demonstrating transgenerational non-genetic inheritance of pathological traits following in-utero immune activation. Epigenetic analyses are now underway to examine the role of transgenerational epigenetic modifications in this association.
Labouesse MA, Dong E, Grayson DR, Guidotti A, Meyer U (2015): Maternal immune activation induces GAD1 and GAD2 promoter remodeling in the offspring prefrontal cortex. Epigenetics 10:1143-1155.
Weber-Stadlbauer U, Richetto J, Labouesse MA, Bohacek J, Mansuy IM, Meyer U (in press): Transgenerational transmission and modification of pathological traits induced by prenatal immune activation. Molecular Psychiatry 22:102-112.
Richetto J, Massart R, Weber-Stadlbauer U, Szyf M, Riva MA, Meyer U. (2016) Genome-wide DNA Methylation Changes in a Mouse Model of Infection-Mediated Neurodevelopmental Disorders. Biological psychiatry 81:265-276.
In recent years, there has been growing interest in the potential role of microglia and related neuroinflammatory processes in normal brain development and maturation. Microglia are the major immunocompetent cells residing in the brain parenchyma, which can adopt different morphological functions and secrete various inflammatory factors depending on the prevailing cellular milieu. We use immunohistochemical techniques along with detailed stereological, morphological, and densitometric analyses to investigate to role of abnormal microglia functions in models of prenatal immune activation. In addition, we quantify peripheral and central cytokines and other mediators of inflammation using electrochemiluminescence assay to examine whether persistent inflammation may contribute to the age-dependent emergence of brain and behavioral pathology induced by prenatal infection.
Notter T, Coughlin JM, Gschwind T, Weber-Stadlbauer U, Wang Y, Kassiou M, Vernon AC, Benke D, Pomper MG, Sawa A, Meyer U. Translational evaluation of translocator protein as a marker of neuroinflammation in schizophrenia. Mol Psychiatry. 2017 (Epub ahed of print)
Giovanoli S, Notter T, Richetto J, Labouesse MA, Vuillermot S, Riva MA, Meyer U (2015): Late prenatal immune activation causes hippocampal deficits in the absence of persistent inflammation across aging. Journal of Neuroinflammation 12:221.
Meyer U, Nyffeler M, Engler A, Urwyler A, Schedlowski M, Knuesel I, Yee BK, Feldon J (2006): The time of prenatal immune challenge determines the specificity of inflammation-mediated brain and behavioral pathology. Journal of Neuroscience 26:4752-4762.
Figure 2. Stereoscopical reconstruction of micro-computed tomography scans derived from a mouse fed a fat-rich diet (62.6% of calories from fat) throughout adolescence (right) and from a mouse fed a control diet (12.3% of calories from fat). Subcutaneous and visceral fat is represented in yellow and pink color, respectively; overlays between subcutaneous and visceral fat highlights in orange color. Bone is depicted in white, whereas lean mass is excluded here. Courtesy of Marie A. Labouesse.
Labouesse MA, Stadlbauer U, Langhans W, Meyer U (2013): Chronic high fat diet consumption impairs sensorimotor gating in mice. Psychoneuroendocrinology 38:2562-74.
Gut Feelings: Visceral Modulation of Behavior and Cognition
The central nervous system (CNS) and viscera are engaged in constant bidirectional communication. This functional entity has been conceptualized as the gut–brain axis and allows top-down (CNS to viscera) and bottom-up (viscera to CNS) information flow. One of the key neuronal elements of the gut–brain axis is the vagus nerve, whose afferent fibers are crucial for conveying visceral information to the brain. Vagal afferents may also be key neuronal elements mediating “gut feelings”. Such feelings are typically conceptualized as somatic signals that influence decision making and behavioral responses without explicit awareness of the provoking cues. Disruption of vagal signaling has generally been associated with a failure of the organism to convey gut-derived signals form the viscera to the CNS, and such deficits may also play a role in the precipitation of behavioral abnormalities involving altered mood and affect and certain cognitive impairments. We use a rat model of subdiaphragmatic vagal deafferentation (SDA), one of the most complete and selective vagal deafferentation method existing to date, to examine whether the loss of vagal afferent signaling induces behavioral and cognitive adaptations. Our ongoing research suggests that various emotional and cognitive functions are subjected to visceral modulation through abdominal vagal afferents.
Klarer M, Arnold M, Günther L, Winter C, Langhans W, Meyer U (2014): Gut vagal afferents differentially modulate innate anxiety and learned fear. Journal of Neuroscience 34:7067-7076
Neuropathological Consequences of Adolescent High Fat Feeding
The 2015 US Dietary Guidelines have abrogated the recommendations of limiting daily fat intake. The main reason for this change in policy is that restricted dietary fat consumption does not appear to provide the expected beneficial effects on obesity, metabolic derangements and cardiovascular diseases. These recommendations, however, seem to ignore the negative consequences of excessive fat intake on neuronal functions, which can occur even without overt signs of obesity and its comorbidities. The risk of underestimating central effects of high dietary fat intake may be problematic especially for adolescents. Adolescence is characterized by a partial protection against diet-induced obesity despite the large tendency for hyperphagia. The lack of overt signs of obesity may thus mask latent neuronal effects elicited by excessive high fat consumption. Because adolescence is an important window for normal maturation of the prefrontal cortex, chronic high fat intake during this period may cause particularly pronounced deficits in prefrontal cognitive functions. In order to address these issues, we assess the neuronal and cognitive consequences of chronic high fat consumption in mouse models using a multi-disciplinary approach combining behavioral testing, immunohistochemistry, electrophysiology, high-resolution micro-computed tomography (Figure 2), and various metabolic and molecular techniques. The findings from our ongoing research question the recently recommended repeal of limiting the daily fat intake. Rather, they suggest that a careful nutritional balance during adolescence is pivotal for reaching the full capacity of adult brain functions.
Labouesse MA, Lassalle O, Richetto J, Iafrati J, Weber-Stadlbauer U, Notter T, Gschwind T, Pujadas L, Soriano E, Reichelt AC, Labouesse C, Langhans W, Chavis P, Meyer U. Hypervulnerability of the adolescent prefrontal cortex to nutritional stress via reelin deficiency. Mol Psychiatry. 2016 (Epub ahed of print)