Article Figures & Data
Figures
- Fig. 1
Brain development stages and microglia. Mesenchymal yolk sac progenitors migrate to neuroectodermal developing neuroprogenitors in the first trimester and become dynamically involved in brain maturation. Although poorly understood, microglia appear to play a key role supporting developing neurons and synapses as well as removing dysfunctional synapses and dying cells. Maturation to adult-like microglia parallels brain maturation, resulting in resident brain-specific innate immune monocyte-like cells, microglia. Embryonic myeloid cells enter the embryonic liver and become hepatic Kupffer monocyte-like cells. Bone marrow hematopoietic stem cells form vascular monocytes in adolescence and adulthood. Thus, microglia are brain-specific monocyte-like cells that mature with neurons, glia, and other brain cells across youth, adolescence, adulthood, and senescence. Microglia can be primed during chronic alcohol exposure, which affects brain development, neurocircuitry, and behavior.
- Fig. 2
Microglial secretions regulate hippocampal neurogenesis. Emerging evidence supports microglial signaling through EVs. Generally free cytokines as followed, but not EV signaling. In vivo, AIE increases adult HMGB1 and proinflammatory genes as well as reducing adult hippocampal neurogenesis (DCX+IR). In ex vivo brain slice cultures, EVs can be isolated from the media, washed, and reapplied to naïve cultures (see Zou et al., 2022). This study found EVs from healthy control slices stimulated hippocampal slice culture neurogenesis, whereas EVs from ethanol exposure cultures reduced neurogenesis and increased proinflammatory cytokines. Far left: Ex vivo studies find microglia release healthy EVs from control cultured brain slices that can be purified and increase neurogenesis (DCX+IR) in recipient slices. This supports healthy microglia as promoting neurogenesis and growth. Far right: Ethanol-treated hippocampal slices release EVs that stimulate formation of TNFα-IL-1β and decrease neurogenesis (DCX+IR), consistent with a change in microglial phenotype reflected in ethanol-induced changes in EV signaling. Middle: Schematic of microglial phenotype shifts from release of healthy EVs (blue) to proinflammatory EVs (red) that regulate neurogenesis. Additional studies are needed to better understand shifts in microglial signaling, innate immune memory and microglial priming.
- Fig. 3
Epigenetic cholinergic gene silencing and shrinkage of cholinergic neurons of cholinergic neurons following AIE. In vivo AIE reduces cholinergic neurons. There is a loss of ChAT and multiple other cholinergic neuron genes, but no loss of neurons. Studies on prevention and restoration of cholinergic neurons using anti-inflammatory treatments support loss of the differentiated cholinergic neuron phenotype and not neuronal death (Vetreno et al., 2020). As illustrated, AIE increases gene silencing through induction of REST that recruits G9a, a histone methylating enzyme that silences transcription of cholinergic genes. Top: Schematic of rat brain cholinergic neuron projections (green, red, yellow) across multiple brain regions. Middle: Immunohistochemistry of choline acetyltransferase (ChAT+IR neurons) cholinergic neurons in basal forebrain. Note that in the left and right panel ChAT+IR neurons are larger, while in the middle panel AIE decreases ChAT+IR cholinergic neurons and causes somal shrinkage of the remaining ChAT+ neurons. Bottom: Schematic of relaxed open chromatin transcribing ChAT and condensed chromatin or NFκB regulating transcription of proinflammatory genes. The left panel shows normal control ChAT transcription and condensed chromatin reflecting no proinflammatory gene transcription. The middle panel schematic of AIE illustrates loss of ChAT+ neurons and projection loss (top middle panel), somal shrinkage (ChAT+ images middle), and silencing of ChAT expression through REST-G9a epigenetic mechanisms (bottom middle panel). Proinflammatory transcription factor NFκB is increased in ChAT+ cells consistent with increased proinflammatory gene induction. AIE-induced adult ChAT loss is reversible (right panel) with treatments blocking HMGB1 proinflammatory signals and gene silencing REST-G9a and restoration of ChAT+ cell number, somal size, and projections.
- Fig. 4
Schematic of neuroimmune targets for treatment of AUD. Several targets are identified from studies of AIE adult neuropathology. Target 1 is HMGB1, and Target 2 is the multiple TLR and other receptors as well as their common NFκB signaling. Target 3 is the epigenetic mechanisms increasing proinflammatory genes and reducing trophic factor and neurotransmitter gene expression. Target 4 is PDE that shifts NFκB/cAMP signaling. Target 5 is agents increasing trophic factor transcription. Target 1 is HMGB1. Acute ethanol releases HMGB1, activating signaling cascades of innate immune genes. HMGB1 and TLR receptors are induced in brain by chronic ethanol exposure, sensitizing multiple TLR responses (Qin and Crews, 2012; Qin et al., 2013, 2021). HMGB1 can activate multiple Toll-like receptors and other innate immune receptors through the formation of heteromers. It is poorly understood which brain cells release HMGB1 in response to ethanol and other immune activators. For example, the HMGB1 antagonist glycyrrhizin, as well as HMGB1-neutralizing antibodies, block both glutamate and TNFα-induced neuronal death in brain slice cultures (Zou, 2015). Further, EVs containing HMGB1 as well as free HMGB1 are released, and how this release and the diverse receptor responses interact is not understood. For example, HMGB1 is shown as a gray molecule to two thick arms representing binding boxes. HMGB1 activates multiple innate immune receptors as shown, some through heteromer-increased potency of other agonists. Examples of HMGB1 signaling include (left) RAGE receptor and HMGB1-Let7 containing EVs (Coleman et al., 2017a), (middle) HMGB1-IL1β heteromers activating IL1R (Coleman et al., 2018), HMGB1 alone activating TLR4 or RAGE, and HMGB1-CXCL12 heteromers activating G protein coupled CXCR4 receptors (Harris et al., 2012). These receptors are each targets (Target 2) and are the epigenetic enzymes altering transcriptional balance (Target 3) as well as PDE induction (Target 4) that decreases cAMP-BDNF transcription (Target 5) and protein kinase A blunting of NFκB transcription increasing proinflammatory genes (Avila et al., 2017). These AIE-identified targets may have indications broader than AUD since many psychiatric and neurologic disorders are also associated with increased neuroimmune gene expression.
In this issue
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- Article
- Abstract
- I. Introduction: Adolescent and Underage Drinking are Common But Differ From AUD
- II. Age of Drinking Onset and Risks for AUD
- III. Adolescent Intermittent Ethanol Exposure Procedure Models Human Underage Weekend Binge Drinking Exposure and Persistent Changes in Adulthood
- IV. Hippocampal Neurogenesis and the Neurogenic Niche: HMGB1 and Shifts in the Trophic/Proinflammatory Balance
- V. AIE and HMGB1 Signaling
- VI. Microglia, HMGB1, and Adolescent Alcohol Exposure
- VII. AIE Causes a Persistent Loss of Forebrain Cholinergic Neurons
- VIII. Reversal of AIE Pathology Identifies New Mechanisms and Potential Therapies
- IX. Summary and Conclusions
- Acknowledgments
- Authorship Contributions
- Footnotes
- Abbreviations
- References
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