What the Spectrum of Microglial Functions Can Teach us About Fetal Alcohol Spectrum Disorder

Developmental Ethanol Exposure Impacts Purkinje Cells but Not Microglia in the Young Adult Cerebellum

Fetal alcohol spectrum disorders (FASD) caused by developmental ethanol exposure lead to cerebellar impairments, including motor problems, decreased cerebellar weight, and cell death. Alterations in the sole output of the cerebellar cortex, Purkinje cells, and central nervous system immune cells, microglia, have been reported in animal models of FASD. To determine how developmental ethanol exposure affects adult cerebellar microglia and Purkinje cells, we used a human third-trimester binge exposure model in which mice received ethanol or saline from postnatal (P) days 4-9. In adolescence, cerebellar cranial windows were implanted and mice were aged to young adulthood for examination of microglia and Purkinje cells in vivo with two-photon imaging or in fixed tissue. Ethanol had no effect on microglia density, morphology, dynamics, or injury response. However, Purkinje cell linear frequency was reduced by ethanol. Microglia-Purkinje cell interactions in the Purkinje Cell Layer were altered in females compared to males. Overall, developmental ethanol exposure had few effects on cerebellar microglia in young adulthood and Purkinje cells appeared to be more susceptible to its effects.

Interleukin-1β and TNF-α are elevated in the amygdala of adult rats prenatally exposed to ethanol

Affective disorders, including anxiety and depression, developed in adult offspring of the mothers who consumed alcohol during pregnancy could be associated with an imbalance in neuroimmune factors in the amygdala (corpus amygdaloideum) resulted in impaired emotional stimulus processing. The aim of this study was to compare the content of cytokines TNF-α, IL-1α, IL-1β, IL-10, and IL-17 in the amygdala of adult female rats exposed to alcohol in utero and control rats. Cytokine levels were evaluated using a multiplex immunoassay system; mRNA expression was investigated using a real-time reverse transcription-polymerase chain reaction (RT-qPCR) assay. Prenatal alcohol exposure led to the increase in the content of TNF-α and IL-1β without significant changes in the mRNA expression level. Our data suggest that ethanol exposure to the fetus during pregnancy can result in long-term alterations in the content of the key neuroinflammatory factors in the amygdala, which in turn can be a risk factor for affective disorders in the adulthood.

Developmental ethanol exposure has minimal impact on cerebellar microglial dynamics, morphology, and interactions with Purkinje cells during adolescence

Introduction

Fetal alcohol spectrum disorders (FASD) are the most common cause of non-heritable, preventable mental disability, occurring in almost 5% of births in the United States. FASD lead to physical, behavioral, and cognitive impairments, including deficits related to the cerebellum. There is no known cure for FASD and their mechanisms remain poorly understood. To better understand these mechanisms, we examined the cerebellum on a cellular level by studying microglia, the principal immune cells of the central nervous system, and Purkinje cells, the sole output of the cerebellum. Both cell types have been shown to be affected in models of FASD, with increased cell death, immune activation of microglia, and altered firing in Purkinje cells. While ethanol administered in adulthood can acutely depress the dynamics of the microglial process arbor, it is unknown how developmental ethanol exposure impacts microglia dynamics and their interactions with Purkinje cells in the long term.

Methods

To address this question, we used a mouse model of human 3rd trimester exposure, whereby L7cre/Ai9+/-/Cx3cr1G/+ mice (with fluorescently labeled microglia and Purkinje cells) of both sexes were subcutaneously treated with a binge-level dose of ethanol (5.0 g/kg/day) or saline from postnatal days 4-9. Cranial windows were implanted in adolescent mice above the cerebellum to examine the long-term effects of developmental ethanol exposure on cerebellar microglia and Purkinje cell interactions using in vivo two-photon imaging.

Results

We found that cerebellar microglia dynamics and morphology were not affected after developmental ethanol exposure. Microglia dynamics were also largely unaltered with respect to how they interact with Purkinje cells, although subtle changes in these interactions were observed in females in the molecular layer of the cerebellum.

Discussion

This work suggests that there are limited in vivo long-term effects of ethanol exposure on microglia morphology, dynamics, and neuronal interactions, so other avenues of research may be important in elucidating the mechanisms of FASD.

Epigenetics and Neuroinflammation Associated With Neurodevelopmental Disorders: A Microglial Perspective

Neuroinflammation is a cause of neurodevelopmental disorders such as autism spectrum disorders, fetal alcohol syndrome, and cerebral palsy. Converging lines of evidence from basic and clinical sciences suggest that dysregulation of the epigenetic landscape, including DNA methylation and miRNA expression, is associated with neuroinflammation. Genetic and environmental factors can affect the interaction between epigenetics and neuroinflammation, which may cause neurodevelopmental disorders. In this minireview, we focus on neuroinflammation that might be mediated by epigenetic dysregulation in microglia, and compare studies using mammals and zebrafish.

Microglia and astrocytes show limited, acute alterations in morphology and protein expression following a single developmental alcohol exposure

Fetal alcohol spectrum disorders (FASD) are the most common cause of nonheritable, preventable mental disability and are characterized by cognitive, behavioral, and physical impairments. FASD occurs in almost 5% of births in the United States, but despite this prevalence there is no known cure, largely because the biological mechanisms that translate alcohol exposure to neuropathology are not well understood. While the effects of early ethanol exposure on neuronal survival and circuitry have received more attention, glia, the cells most closely tied to initiating and propagating inflammatory events, could be an important target for alcohol in the developing brain. Inflammation is known to alter developmental trajectories, but it has recently been shown that even small changes in both astrocytes and microglia in the absence of full-blown inflammatory signaling can alter brain function long-term. Here, we studied the acute response of astrocytes and microglia to a single exposure to ethanol in development across sexes in a mouse model of human third trimester exposure, in order to understand how these cells may transition from their normal developmental path to a different program that leads to FASD neuropathology. We found that although a single ethanol exposure delivered subcutaneously on postnatal day 4 did not cause large changes in microglial morphology or the expression of AldH1L1 and GFAP in the cortex and hippocampus, subtle effects were observed. These findings suggest that even a single, early ethanol exposure can induce mild acute alterations in glia that could contribute to developmental deficits.

Dynamics of microglia and dendritic spines in early adolescent cortex after developmental alcohol exposure

Fetal alcohol spectrum disorder patients suffer from many cognitive disabilities. These include impaired auditory, visual, and tactile sensory information processing, making it more difficult for these patients to learn to navigate social scenarios. Rodent studies have shown that alcohol exposure during the brain growth spurt (BGS) can lead to acute neuronal apoptosis and an immunological response by microglia in the somatosensory cortex. Since microglia have critical physiological functions, including the support of excitatory synapse remodeling via interactions with dendritic spines, we sought to understand whether BGS alcohol exposure has long-term effects on microglial or dendritic spine dynamics. Using in vivo two-photon microscopy in 4-5 week old mice, we evaluated microglial functions such as process motility, the response to tissue injury, and the dynamics of physical interactions between microglial processes and dendritic spines. We also investigated potential differences in the morphology, density, or dynamics of dendritic spines in layer I/II primary sensory cortex of control and BGS alcohol exposed mice. We found that microglial process motility and contact with dendritic spines were not altered after BGS alcohol exposure. While the response of microglial processes toward tissue injury was not significantly altered by prior alcohol exposure, there was a trend suggesting that alcohol early in life may prime microglia to respond more quickly to secondary injury. Spine density, morphology, stability, and remodeling over time were not perturbed after BGS alcohol exposure. We demonstrate that after BGS alcohol exposure, the physiological functions of microglia and excitatory neurons remain intact in early adolescence.

Little cells of the little brain: microglia in cerebellar development and function

Microglia are long-lived resident macrophages of the brain with diverse roles that span development, adulthood, and aging. Once thought to be a relatively homogeneous population, there is a growing recognition that microglia are highly specialized to suit their specific brain region. Cerebellar microglia represent an example of such specialization, exhibiting a dynamical, transcriptional, and immunological profile that differs from that of other microglial populations. Here we review the evidence that cerebellar microglia shape the cerebellar environment and are in turn shaped by it. We examine the roles microglia play in cerebellar function, development, and aging. The emerging findings on cerebellar microglia may also provide insights into disease processes involving cerebellar dysfunction.

Neuroinflammatory contribution of microglia and astrocytes in fetal alcohol spectrum disorders

Ethanol exposure to the fetus during pregnancy can result in fetal alcohol spectrum disorders (FASD). These disorders vary in severity, can affect multiple organ systems, and can lead to lifelong disabilities. Damage to the central nervous system (CNS) is common in FASD, and can result in altered behavior and cognition. The incidence of FASD is alarmingly high, resulting in significant personal and societal costs. There are no cures for FASD. Alcohol can directly alter the function of neurons in the developing CNS. In addition, ethanol can alter the function of CNS glial cells including microglia and astrocytes which normally maintain homeostasis in the CNS. These glial cells can function as resident immune cells in the CNS to protect against pathogens and other insults. However, activation of glia can also damage CNS cells and lead to aberrant CNS function. Ethanol exposure to the developing brain can result in the activation of glia and neuroinflammation, which may contribute to the pathology associated with FASD. This suggests that anti-inflammatory agents may be effective in the treatment of FASD.

Acute ethanol exposure rapidly alters cerebellar and cortical microglial physiology

Alcohol use is highly prevalent in modern society and ramifications of alcohol abuse pose a large public health concern. Previous work investigating the effects of alcohol exposure on the brain has implicated microglia, the resident immune cells of the central nervous system (CNS), as critical participants in the brain's response to chronic and developmental ethanol (EtOH) exposure. As rapid sensors of their environment, microglia also have the capacity to rapidly respond to alcohol administration and to contribute to acute effects of alcohol on the brain; however, their acute responses have not been assessed. Here, for the first time, we have examined the acute response of microglia to alcohol intoxication in vivo utilizing two-photon microscopy to assess the dynamics of these motile cells in both visual cortex and the cerebellum of mice. We found that microglia respond rapidly to EtOH exposure with fast changes in morphology, motility, parenchyma surveillance, and injury response. However, regional differences between the responses of cerebellar and cortical microglial populations indicate that subtle differences in microglial physiology may alter their vulnerability to acute alcohol intoxication. Our findings suggest that the longer-term effects of repeated EtOH exposure on microglia may result from repeat acute alterations in microglial physiology by single exposure to alcohol which rapidly alter behavior in specific microglial populations.

Early life alcohol exposure primes hypothalamic microglia to later-life hypersensitivity to immune stress: possible epigenetic mechanism

Growing evidence has shown that developmental alcohol exposure induces central nervous system inflammation and microglia activation, which may contribute to long-term health conditions, such as fetal alcohol spectrum disorders. These studies sought to investigate whether neonatal alcohol exposure during postnatal days (PND) 2-6 in rats (third trimester human equivalent) leads to long-term disruption of the neuroimmune response by microglia. Exposure to neonatal alcohol resulted in acute increases in activation and inflammatory gene expression in hypothalamic microglia including tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6). Adults with neonatal alcohol pre-exposure (alcohol fed; AF) animals showed an exaggerated peripheral stress hormonal response to an immune challenge (lipopolysaccharides; LPS). In addition, there were significantly more microglia present in the hypothalamus of adult AF animals, and their hypothalamic microglia showed more cluster of differentiation molecule 11b (Cd11b) activation, TNF-α expression, and IL-6 expression in response to LPS. Interestingly, blocking microglia activation with minocycline treatment during PND 2-6 alcohol exposure ameliorated the hormonal and microglial hypersensitivity to LPS in AF adult animals. Investigation of possible epigenetic programming mechanisms by alcohol revealed neonatal alcohol decreased several repressive regulators of transcription in hypothalamic microglia, while concomitantly increasing histone H3 acetyl lysine 9 (H3K9ac) enrichment at TNF-α and IL-6 promoter regions. Importantly, adult hypothalamic microglia from AF animals showed enduring increases in H3K9ac enrichment of TNF-α and IL-6 promoters both at baseline and after LPS exposure, suggesting a possible epigenetic mechanism for the long-term immune disruption due to hypothalamic microglial priming.

Zebrin II Is Ectopically Expressed in Microglia in the Cerebellum of Neurogenin 2 Null Mice

Zebrin II/aldolase C expression in the normal cerebellum is restricted to a Purkinje cell subset and is the canonical marker for stripes and zones. This spatial restriction has been confirmed in over 30 species of mammals, birds, fish, etc. In a transgenic mouse model in which the Neurogenin 2 gene has been disrupted (Neurog2^-/-), the cerebellum is smaller than normal and Purkinje cell dendrites are disordered, but the basic zone and stripe architecture is preserved. Here, we show that in the Neurog2^-/- mouse, in addition to the normal Purkinje cell expression, zebrin II is also expressed in a population of cells with a morphology characteristic of microglia. This identity was confirmed by double immunohistochemistry for zebrin II and the microglial marker, Iba1. The expression of zebrin II in cerebellar microglia is not restricted by zone or stripe or lamina. A second zone and stripe marker, PLCβ4, does not show the same ectopic expression. When microglia are compared in control vs. Neurog2^-/- mice, no difference is seen in apparent number or distribution, suggesting that the ectopic zebrin II immunoreactivity in Neurog2^-/- cerebellum reflects an ectopic expression rather than the invasion of a new population of microglia from the periphery. This ectopic expression of zebrin II in microglia is unique as it is not seen in numerous other models of cerebellar disruption, such as in Acp2^-/- mice and in human pontocerebellar hypoplasia. The upregulation of zebrin II in microglia is thus specific to the disruption of Neurog2 downstream pathways, rather than a generic response to a cerebellar disruption.

Developmental alcohol exposure impairs synaptic plasticity without overtly altering microglial function in mouse visual cortex

Fetal alcohol spectrum disorder (FASD), caused by gestational ethanol (EtOH) exposure, is one of the most common causes of non-heritable and life-long mental disability worldwide, with no standard treatment or therapy available. While EtOH exposure can alter the function of both neurons and glia, it is still unclear how EtOH influences brain development to cause deficits in sensory and cognitive processing later in life. Microglia play an important role in shaping synaptic function and plasticity during neural circuit development and have been shown to mount an acute immunological response to EtOH exposure in certain brain regions. Therefore, we hypothesized that microglial roles in the healthy brain could be permanently altered by early EtOH exposure leading to deficits in experience-dependent plasticity. We used a mouse model of human third trimester high binge EtOH exposure, administering EtOH twice daily by subcutaneous injections from postnatal day 4 through postnatal day 9 (P4-:P9). Using a monocular deprivation model to assess ocular dominance plasticity, we found an EtOH-induced deficit in this type of visually driven experience-dependent plasticity. However, using a combination of immunohistochemistry, confocal microscopy, and in vivo two-photon microscopy to assay microglial morphology and dynamics, as well as fluorescence activated cell sorting (FACS) and RNA-seq to examine the microglial transcriptome, we found no evidence of microglial dysfunction in early adolescence. We also found no evidence of microglial activation in visual cortex acutely after early ethanol exposure, possibly because we also did not observe EtOH-induced neuronal cell death in this brain region. We conclude that early EtOH exposure caused a deficit in experience-dependent synaptic plasticity in the visual cortex that was independent of changes in microglial phenotype or function. This demonstrates that neural plasticity can remain impaired by developmental ethanol exposure even in a brain region where microglia do not acutely assume nor maintain an activated phenotype.