Ethanol regulates angiogenic cytokines during neural development: evidence from an in vitro model of mitogen-withdrawal-induced cerebral cortical neuroepithelial differentiation

Alcohol Exposure Induces Nucleolar Stress and Apoptosis in Mouse Neural Stem Cells and Late-Term Fetal Brain

Prenatal alcohol exposure (PAE) is a leading cause of neurodevelopmental disability through its induction of neuronal growth dysfunction through incompletely understood mechanisms. Ribosome biogenesis regulates cell cycle progression through p53 and the nucleolar cell stress response. Whether those processes are targeted by alcohol is unknown. Pregnant C57BL/6J mice received 3 g alcohol/kg daily at E8.5-E17.5. Transcriptome sequencing was performed on the E17.5 fetal cortex. Additionally, primary neural stem cells (NSCs) were isolated from the E14.5 cerebral cortex and exposed to alcohol to evaluate nucleolar stress and p53/MDM2 signaling. Alcohol suppressed KEGG pathways involving ribosome biogenesis (rRNA synthesis/processing and ribosomal proteins) and genes that are mechanistic in ribosomopathies (Polr1d, Rpl11; Rpl35; Nhp2); this was accompanied by nucleolar dissolution and p53 stabilization. In primary NSCs, alcohol reduced rRNA synthesis, caused nucleolar loss, suppressed proliferation, stabilized nuclear p53, and caused apoptosis that was prevented by dominant-negative p53 and MDM2 overexpression. Alcohol's actions were dose-dependent and rapid, and rRNA synthesis was suppressed between 30 and 60 min following alcohol exposure. The alcohol-mediated deficits in ribosomal protein expression were correlated with fetal brain weight reductions. This is the first report describing that pharmacologically relevant alcohol levels suppress ribosome biogenesis, induce nucleolar stress in neuronal populations, and involve the ribosomal/MDM2/p53 pathway to cause growth arrest and apoptosis. This represents a novel mechanism of alcohol-mediated neuronal damage.

Sex differences in the transcriptome of extracellular vesicles secreted by fetal neural stem cells and effects of chronic alcohol exposure

BACKGROUND

Prenatal alcohol (ethanol) exposure (PAE) results in brain growth restriction, in part, by reprogramming self-renewal and maturation of fetal neural stem cells (NSCs) during neurogenesis. We recently showed that ethanol resulted in enrichment of both proteins and pro-maturation microRNAs in sub-200-nm-sized extracellular vesicles (EVs) secreted by fetal NSCs. Moreover, EVs secreted by ethanol-exposed NSCs exhibited diminished efficacy in controlling NSC metabolism and maturation. Here we tested the hypothesis that ethanol may also influence the packaging of RNAs into EVs from cell-of-origin NSCs.

METHODS

Sex-specified fetal murine iso-cortical neuroepithelia from three separate pregnancies were maintained ex vivo, as neurosphere cultures to model the early neurogenic niche. EVs were isolated by ultracentrifugation from NSCs exposed to a dose range of ethanol. RNA from paired EV and cell-of-origin NSC samples was processed for ribosomal RNA-depleted RNA sequencing. Differential expression analysis and exploratory weighted gene co-expression network analysis (WGCNA) identified candidate genes and gene networks that were drivers of alterations to the transcriptome of EVs relative to cells.

RESULTS

The RNA content of EVs differed significantly from cell-of-origin NSCs. Biological sex contributed to unique transcriptome variance in EV samples, where > 75% of the most variant transcripts were also sex-variant in EVs but not in cell-of-origin NSCs. WGCNA analysis also identified sex-dependent enrichment of pathways, including dopamine receptor binding and ectoderm formation in female EVs and cell-substrate adhesion in male EVs, with the top significant DEGs from differential analysis of overall individual gene expressions, i.e., Arhgap15, enriched in female EVs, and Cenpa, enriched in male EVs, also serving as WCGNA hub genes of sex-biased EV WGCNA clusters. In addition to the baseline RNA content differences, ethanol exposure resulted in a significant dose-dependent change in transcript expression in both EVs and cell-of-origin NSCs that predominantly altered sex-invariant RNAs. Moreover, at the highest dose, ~ 73% of significantly altered RNAs were enriched in EVs, but depleted in NSCs.

CONCLUSIONS

The EV transcriptome is distinctly different from, and more sex-variant than, the transcriptome of cell-of-origin NSCs. Ethanol, a common teratogen, results in dose-dependent sorting of RNA transcripts from NSCs to EVs which may reprogram the EV-mediated endocrine environment during neurogenesis.

Dose-related shifts in proteome and function of extracellular vesicles secreted by fetal neural stem cells following chronic alcohol exposure

Accumulating evidence indicates that extracellular vesicles (EVs) mediate endocrine functions and also pathogenic effects of neurodevelopmental perturbagens like ethanol. We performed mass-spectrometry on EVs secreted by fetal murine cerebral cortical neural stem cells (NSCs), cultured ex-vivo as sex-specific neurosphere cultures, to identify overrepresented proteins and signaling pathways in EVs relative to parental NSCs in controls, and following exposure of parental NSCs to a dose range of ethanol. EV proteomes differ substantially from parental NSCs, and though EVs sequester proteins across sub-cellular compartments, they are enriched for distinct morphogenetic signals including the planar cell polarity pathway. Ethanol exposure favored selective protein sequestration in EVs and depletion in parental NSCs, and also resulted in dose-independent overrepresentation of cell-cycle and DNA replication pathways in EVs as well as dose-dependent overrepresentation of rRNA processing and mTor stress pathways. Transfer of untreated EVs to naïve cells resulted in decreased oxidative metabolism and S-phase, while EVs derived from ethanol-treated NSCs exhibited diminished effect. Collectively, these data show that NSCs secrete EVs with a distinct proteome that may have a general growth-inhibitory effect on recipient cells. Moreover, while ethanol results in selective transfer of proteins from NSCs to EVs, the efficacy of these exposure-derived EVs is diminished.

Gag-like proteins: Novel mediators of prenatal alcohol exposure in neural development

Background

We previously showed that ethanol did not kill fetal neural stem cells (NSCs), but that their numbers nevertheless are decreased due to aberrant maturation and loss of self‐renewal. To identify mechanisms that mediate this loss of NSCs, we focused on a family of Gag‐like proteins (GLPs), derived from retroviral gene remnants within mammalian genomes. GLPs are important for fetal development, though their role in brain development is virtually unexplored. Moreover, GLPs may be transferred between cells in extracellular vesicles (EVs) and thereby transfer environmental adaptations between cells. We hypothesized that GLPs may mediate some effects of ethanol in NSCs.

Methods

Sex‐segregated male and female fetal murine cortical NSCs, cultured ex vivo as nonadherent neurospheres, were exposed to a dose range of ethanol and to mitogen‐withdrawal‐induced differentiation. We used siRNAs to assess the effects of NSC‐expressed GLP knockdown on growth, survival, and maturation and in silico GLP knockout, in an in vivo single‐cell RNA‐sequencing dataset, to identify GLP‐mediated developmental pathways that were also ethanol‐sensitive.

Results

PEG10 isoform‐1, isoform‐2, and PNMA2 were identified as dominant GLP species in both NSCs and their EVs. Ethanol‐exposed NSCs exhibited significantly elevated PEG10 isoform‐2 and PNMA2 protein during differentiation. Both PEG10 and PNMA2 were mediated apoptosis resistance and additionally, PEG10 promoted neuronal and astrocyte lineage maturation. Neither GLP influenced metabolism nor cell cycle in NSCs. Virtual PEG10 and PNMA2 knockout identified gene transcription regulation and ubiquitin‐ligation processes as candidate mediators of GLP‐linked prenatal alcohol effects.

Conclusions

Collectively, GLPs present in NSCs and their EVs may confer apoptosis resistance within the NSC niche and contribute to the abnormal maturation induced by ethanol.

Extracellular Vesicles in Premature Aging and Diseases in Adulthood Due to Developmental Exposures

The developmental origins of health and disease (DOHaD) is a paradigm that links prenatal and early life exposures that occur during crucial periods of development to health outcome and risk of disease later in life. Maternal exposures to stress, some psychoactive drugs and alcohol, and environmental chemicals, among others, may result in functional changes in developing fetal tissues, creating a predisposition for disease in the individual as they age. Extracellular vesicles (EVs) may be mediators of both the immediate effects of exposure during development and early childhood as well as the long-term consequences of exposure that lead to increased risk and disease severity later in life. Given the prevalence of diseases with developmental origins, such as cardiovascular disease, neurodegenerative disorders, osteoporosis, metabolic dysfunction, and cancer, it is important to identify persistent mediators of disease risk. In this review, we take this approach, viewing diseases typically associated with aging in light of early life exposures and discuss the potential role of EVs as mediators of lasting consequences.

Toxicant and teratogenic effects of prenatal alcohol

Prenatal alcohol exposure can result in growth, cognitive, and behavioral deficits due to the toxicant and teratogenic effects of alcohol. Alcohol is an unusual toxicant, because, unlike other toxicants, it is consumed and has biological effects in the millimolar range. Cerebral cortical development is particularly vulnerable to both alcohol's acute and long-term reprogramming effects. Recent evidence suggests that neuroinflammation may be a persistent result of prenatal alcohol exposure and that modes of cellular communication capable of carrying miRNAs, such as extracellular vesicles, may be an integral part of long-term changes to cellular communication and inflammation following in utero alcohol exposure.

Ethanol Exposure Increases miR-140 in Extracellular Vesicles: Implications for Fetal Neural Stem Cell Proliferation and Maturation

BACKGROUND

Neural stem cells (NSCs) generate most of the neurons of the adult brain in humans, during the mid-first through second-trimester period. This critical neurogenic window is particularly vulnerable to prenatal alcohol exposure, which can result in diminished brain growth. Previous studies showed that ethanol (EtOH) exposure does not kill NSCs, but, rather, results in their depletion by influencing cell cycle kinetics and promoting aberrant maturation, in part, by altering NSC expression of key neurogenic miRNAs. NSCs reside in a complex microenvironment rich in extracellular vesicles, shown to traffic miRNA cargo between cells.

METHODS

We profiled the miRNA content of extracellular vesicles from control and EtOH-exposed ex vivo neurosphere cultures of fetal NSCs. We subsequently examined the effects of one EtOH-sensitive miRNA, miR-140-3p, on NSC growth, survival, and maturation.

RESULTS

EtOH exposure significantly elevates levels of a subset of miRNAs in secreted extracellular vesicles. Overexpression of one of these elevated miRNAs, miR-140-3p, and its passenger strand relative, miR-140-5p, significantly increased the proportion of S-phase cells while decreasing the proportion of G₀ /G₁ cells compared to controls. In contrast, while miR-140-3p knockdown had minimal effects on the proportion of cells in each phase of the cell cycle, knockdown of miR-140-5p significantly decreased the proportion of cells in G₂ /M phase. Furthermore, miR-140-3p overexpression, during mitogen-withdrawal-induced NSC differentiation, favors astroglial maturation at the expense of neural and oligodendrocyte differentiation.

CONCLUSIONS

Collectively, the dysregulated miRNA content of extracellular vesicles following EtOH exposure may result in aberrant neural progenitor cell growth and maturation, explaining brain growth deficits associated with prenatal alcohol exposure.

Sensitivity of neural stem cell survival, differentiation and neurite outgrowth within 3D hydrogels to environmental heavy metals

We investigated the sensitivity of embryonic murine neural stem cells exposed to 10 pM-10 μM concentrations of three heavy metals (Cd, Hg, Pb), continuously for 14 days within 3D collagen hydrogels. Critical endpoints for neurogenesis such as survival, differentiation and neurite outgrowth were assessed. Results suggest significant compromise in cell viability within the first four days at concentrations ≥10 nM, while lower concentrations induced a more delayed effect. Mercury and lead suppressed neural differentiation at as low as 10 pM concentration within 7 days, while all three metals inhibited neural and glial differentiation by day 14. Neurite outgrowth remained unaffected at lower cadmium or mercury concentrations (≤100 pM), but was completely repressed beyond day 1 at higher concentrations. Higher metal concentrations (≥100 pM) suppressed NSC differentiation to motor or dopaminergic neurons. Cytokines and chemokines released by NSCs, and the sub-cellular mechanisms by which metals induce damage to NSCs have been quantified and correlated to phenotypic data. The observed degree of toxicity in NSC cultures is in the order: lead>mercury>cadmium. Results point to the use of biomimetic 3D culture models to screen the toxic effects of heavy metals during developmental stages, and investigate their underlying mechanistic pathways.

MiR-153 targets the nuclear factor-1 family and protects against teratogenic effects of ethanol exposure in fetal neural stem cells

Ethanol exposure during pregnancy is an established cause of birth defects, including neurodevelopmental defects. Most adult neurons are produced during the second trimester-equivalent period. The fetal neural stem cells (NSCs) that generate these neurons are an important but poorly understood target for teratogenesis. A cohort of miRNAs, including miR-153, may serve as mediators of teratogenesis. We previously showed that ethanol decreased, while nicotine increased miR-153 expression in NSCs. To understand the role of miR-153 in the etiology of teratology, we first screened fetal cortical NSCs cultured ex vivo, by microarray and quantitative RT-PCR analyses, to identify cell-signaling mRNAs and gene networks as important miR-153 targets. Moreover, miR-153 over-expression prevented neuronal differentiation without altering neuroepithelial cell survival or proliferation. Analysis of 3'UTRs and in utero over-expression of pre-miR-153 in fetal mouse brain identified Nfia (nuclear factor-1A) and its paralog, Nfib, as direct targets of miR-153. In utero ethanol exposure resulted in a predicted expansion of Nfia and Nfib expression in the fetal telencephalon. In turn, miR-153 over-expression prevented, and partly reversed, the effects of ethanol exposure on miR-153 target transcripts. Varenicline, a partial nicotinic acetylcholine receptor agonist that, like nicotine, induces miR-153 expression, also prevented and reversed the effects of ethanol exposure. These data collectively provide evidence for a role for miR-153 in preventing premature NSC differentiation. Moreover, they provide the first evidence in a preclinical model that direct or pharmacological manipulation of miRNAs have the potential to prevent or even reverse effects of a teratogen like ethanol on fetal development.

Neuroimmune mechanisms of alcohol and drug addiction

Alcohol and other drugs of abuse have significant impacts on the neuroimmune system. Studies have demonstrated that drugs of abuse interact with the neuroimmune system and alter neuroimmune gene expression and signaling, which in turn contribute to various aspects of addiction. As the key component of the CNS immune system, neuroimmune factors mediate neuroinflammation and modulate a wide range of brain function including neuronal activity, endocrine function, and CNS development. These neuromodulatory properties of immune factors, together with their essential role in neuroinflammation, provide a new framework to understand neuroimmune mechanisms mediating brain functional and behavioral changes contributing to addiction. This chapter highlights recent advances in understanding neuroimmune changes associated with exposure to alcohol and other drugs of abuse, including opiates, marijuana, methamphetamine, and cocaine. It provides a brief overview on what we know about neuroimmune signaling and its role in drug action and addiction.

CD24 expression identifies teratogen-sensitive fetal neural stem cell subpopulations: evidence from developmental ethanol exposure and orthotopic cell transfer models

Background

Ethanol is a potent teratogen. Its adverse neural effects are partly mediated by disrupting fetal neurogenesis. The teratogenic process is poorly understood, and vulnerable neurogenic stages have not been identified. Identifying these is a prerequisite for therapeutic interventions to mitigate effects of teratogen exposures.

Methods

We used flow cytometry and qRT-PCR to screen fetal mouse-derived neurosphere cultures for ethanol-sensitive neural stem cell (NSC) subpopulations, to study NSC renewal and differentiation. The identity of vulnerable NSC populations was validated in vivo, using a maternal ethanol exposure model. Finally, the effect of ethanol exposure on the ability of vulnerable NSC subpopulations to integrate into the fetal neurogenic environment was assessed following ultrasound guided, adoptive transfer.

Results

Ethanol decreased NSC mRNAs for c-kit, Musashi-1and GFAP. The CD24⁺ NSC population, specifically the CD24⁺CD15⁺ double-positive subpopulation, was selectively decreased by ethanol. Maternal ethanol exposure also resulted in decreased fetal forebrain CD24 expression. Ethanol pre-exposed CD24⁺ cells exhibited increased proliferation, and deficits in cell-autonomous and cue-directed neuronal differentiation, and following orthotopic transplantation into naïve fetuses, were unable to integrate into neurogenic niches. CD24^depleted cells retained neurosphere regeneration capacity, but following ethanol exposure, generated increased numbers of CD24⁺ cells relative to controls.

Conclusions

Neuronal lineage committed CD24⁺ cells exhibit specific vulnerability, and ethanol exposure persistently impairs this population’s cell-autonomous differentiation capacity. CD24⁺ cells may additionally serve as quorum sensors within neurogenic niches; their loss, leading to compensatory NSC activation, perhaps depleting renewal capacity. These data collectively advance a mechanistic hypothesis for teratogenesis leading to microencephaly.

Suppression and epigenetic regulation of MiR-9 contributes to ethanol teratology: evidence from zebrafish and murine fetal neural stem cell models

BACKGROUND

Fetal alcohol exposure produces multiorgan defects, making it difficult to identify underlying etiological mechanisms. However, recent evidence for ethanol (EtOH) sensitivity of the miRNA miR-9 suggests one mechanism, whereby EtOH broadly influences development. We hypothesized that loss of miR-9 function recapitulates aspects of EtOH teratology.

METHODS

Zebrafish embryos were exposed to EtOH during gastrulation, or injected with anti-miR-9 or nonsense control morpholinos during the 2-cell stage of development and collected between 24 and 72 hours postfertilization (hpf). We also assessed the expression of developmentally important, and known miR-9 targets, FGFR-1, FOXP2, and the nontargeted transcript, MECP2. Methylation at CpG islands of mammalian miR-9 genes was assessed in fetal murine neural stem cells (mNSCs) by methylation-specific PCR, and miRNA processing assessed by qRT-PCR for pre-miR-9 transcripts.

RESULTS

EtOH treatment and miR-9 knockdown resulted in similar cranial defects including microcephaly. Additionally, EtOH transiently suppressed miR-9, as well as FGFR-1 and FOXP2, and alterations in miR-9 expression were correlated with severity of EtOH-induced teratology. In mNSCs, EtOH increased CpG dinucleotide methylation at the miR-9-2 locus and accumulation of pre-miR-9-3.

CONCLUSIONS

EtOH exerts regulatory control at multiple levels of miR-9 biogenesis. Moreover, early embryonic loss of miR-9 function recapitulated the severe range of teratology associated with developmental EtOH exposure. EtOH also disrupts the relationship between miR-9 and target gene expression, suggesting a nuanced relationship between EtOH and miRNA regulatory networks in the developing embryo. The implications of these data for the expression and function of mature miR-9 warrant further investigation.

Age-related severity of focal ischemia in female rats is associated with impaired astrocyte function

In middle-aged female rats, focal ischemia leads to a larger cortical infarction as compared with younger females. To determine if stroke-induced cytotoxicity in middle-aged females was associated with impaired astrocyte function, astrocytes were harvested and cultured from the ischemic cortex of young and middle-aged female rats. Middle-aged astrocytes cleared significantly less glutamate from media as compared with young female astrocytes. Furthermore, astrocyte-conditioned media from middle-aged female astrocytes induced greater migration of peripheral blood monocyte cells (PBMCs) and expressed higher levels of the chemoattractant macrophage inflammatory protein-1 (MIP-1). Middle-aged astrocytes also induced greater migration of neural progenitor cells (NPCs), however, their ability to promote neuronal differentiation of neural progenitor cells was similar to young astrocytes. In males, where cortical infarct volume is similar in young and middle-aged animals, no age-related impairment was observed in astrocyte function. These studies show that the aging astrocyte may directly contribute to infarct severity by inefficient glutamate clearance and enhanced cytokine production and suggest a cellular target for improved stroke therapy among older females.

Opposing actions of ethanol and nicotine on microRNAs are mediated by nicotinic acetylcholine receptors in fetal cerebral cortical-derived neural progenitor cells

BACKGROUND

Ethanol (EtOH) and nicotine are often co-abused. However, their combined effects on fetal neural development, particularly on fetal neural stem cells (NSCs), which generate most neurons of the adult brain during the second trimester of pregnancy, are poorly understood. We previously showed that EtOH influenced NSC maturation in part, by suppressing the expression of specific microRNAs (miRNAs). Here, we tested in fetal NSCs the extent to which EtOH and nicotine coregulated known EtOH-sensitive (miR-9, miR-21, miR-153, and miR-335), a nicotine-sensitive miRNA (miR-140-3p), and mRNAs for nicotinic acetylcholine receptor (nAChR) subunits. Additionally, we tested the extent to which these effects were nAChR dependent.

METHODS

Gestational day 12.5 mouse fetal murine cerebral cortical-derived neurosphere cultures were exposed to EtOH, nicotine, and mecamylamine, a noncompetitive nAChR antagonist, individually or in combination, for short (24 hour) and long (5 day) periods, to mimic exposure during the in vivo period of neurogenesis. Levels of miRNAs, miRNA-regulated transcripts, and nAChR subunit mRNAs were assessed by quantitative reverse transcription polymerase chain reaction.

RESULTS

EtOH suppressed the expression of known EtOH-sensitive miRNAs and miR-140-3p, while nicotine at concentrations attained by cigarette smokers induced a dose-related increase in these miRNAs. Nicotine's effect was blocked by EtOH and by mecamylamine. Finally, EtOH decreased the expression of nAChR subunit mRNAs and, like mecamylamine, prevented the nicotine-associated increase in α4 and β2 nAChR transcripts.

CONCLUSIONS

EtOH and nicotine exert mutually antagonistic, nAChR-mediated effects on teratogen-sensitive miRNAs in fetal NSCs. These data suggest that concurrent exposure to EtOH and nicotine disrupts miRNA regulatory networks that are important for NSC maturation.

Ethanol exposure during pregnancy persistently attenuates cranially directed blood flow in the developing fetus: evidence from ultrasound imaging in a murine second trimester equivalent model

BACKGROUND

Ethanol (EtOH) consumption during pregnancy can lead to fetal growth retardation, mental retardation, and neurodevelopmental delay. The fetal brain initiates neurogenesis and vasculogenesis during the second trimester, and depends on maternal-fetal circulation for nutrition and growth signals. We used high-resolution in vivo ultrasound imaging to test the hypothesis that EtOH interferes with fetal brain-directed blood flow during this critical developmental period.

METHODS

Pregnant mice were lightly anesthetized on gestational day 12 with an isoflurane/oxygen mixture. We assessed the effect of single and repeated binge-like maternal EtOH exposures at 3 g/kg, administered by intragastric gavage or intraperitoneal injection, on maternal circulation and fetal umbilical, aortic, internal carotid, and middle cerebral arterial circulation.

RESULTS

Binge maternal EtOH exposure, regardless of exposure route, significantly reduced fetal arterial blood acceleration and velocity time integral (VTI), from umbilical to cerebral arteries, without a change in fetal heart rate and resistivity indices. Importantly a single maternal binge EtOH exposure induced persistent suppression of fetal arterial VTI for at least 24 hours. Repeated binge episodes resulted in a continuing and persistent suppression of fetal VTI. Qualitative assessments showed that maternal EtOH exposure induced oscillatory, nondirectional blood flow in fetal cerebral arteries. Maternal cardiac and other physiological parameters remained unaltered.

CONCLUSIONS

These data show that binge-type maternal EtOH exposure results in rapid and persistent loss of blood flow from the umbilical artery to the fetal brain, potentially compromising nutrition and the maternal/fetal endocrine environment during a critical period for neuron formation and angiogenesis in the maturing brain.

Stage-specific changes in the levels of granulocyte-macrophage colony-stimulating factor and its receptor in the biological fluid and organ of mouse fetuses

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a hematopoietic cytokine that has neurotrophic and neuroprotective functions. However, its function in the mid- to late-gestational fetus remains unclear. We used enzyme-linked immunosorbent assay to analyze GM-CSF levels in cerebrospinal fluid (CSF), serum, and amniotic fluid of mouse fetuses. We also examined GM-CSF and receptor α (GM-CSFRα) levels in the fetal brain, liver, and placenta. GM-CSF peaked between embryonic day (E) 14 and E15 in the CSF. GM-CSF level was higher in the fetal serum than in the dam serum on E13 and decreased thereafter. GM-CSF and GM-CSFRα levels peaked between E13 and E15 in the brain. These results suggest that GM-CSF plays stage- and organ-specific roles in fetal development.

Safety issues with ethanol as an excipient in drugs intended for pediatric use

Ethanol is commonly used as an excipient in the manufacture of medicines, including the ones intended for administration in children. However, ethanol cannot be considered an inert substance; on the contrary, its use in pharmaceutical preparations is associated with safety issues. Newborns, infants and children are not able to metabolize ethanol as efficiently as adults; as a consequence, they may be at higher risk of both acute and chronic alcohol-related toxicities.

Ethanol-induced methylation of cell cycle genes in neural stem cells

Ethanol inhibits the proliferation of neural precursors by altering mitogenic and anti-mitogenic growth factor signaling and can affect global methylation activity in the fetus. We tested the hypothesis that epigenetic modification of specific cell cycle genes underlies the ethanol-induced inhibition of growth factor-regulated cell cycle progression. Monolayer cultures of neural stem cells (NSCs) were treated with fibroblast growth factor 2 or transforming growth factor (TGF) β1 in the absence or presence of ethanol. Ethanol increased the total length of the cell cycle by elongating the amount of time spent in the gap 1 (G1) and synthesis (S) phases of the cell cycle. Ethanol induced the hypermethylation of multiple cell cycle genes associated with the G1/S and gap 2/mitotic phase (G2/M) checkpoints and increased the expression and activity of DNA methyltransferases. These changes were most pronounced in the presence of TGFβ1. Epigenetic alterations paralleled the down-regulation of associated transcripts and other checkpoint-related mRNAs both in vitro (NS-5 cell culture) and in vivo (fetal mouse cortex). Ethanol-induced hypermethylation was accompanied by decreases in the proportion of NSCs expressing associated cell cycle proteins. Thus, ethanol disrupts growth factor-related cell cycle progression by inducing checkpoint restriction at the G1/S transition through a feed-forward system involving the methylation of G2/M regulators.

Astrocytes from acyclic female rats exhibit lowered capacity for neuronal differentiation

Astrocytes comprise a large proportion of the central nervous system support cells and play a critical role in neural injury and repair. The present study examined the impact of ovarian aging using an ex vivo model system, where astrocytes were derived from the olfactory bulb of young, reproductively competent females and reproductive senescent females. Cellular morphology and the spatial pattern of laminin deposition was altered in astrocyte cultures derived from reproductive senescent females. Young adult astrocytes had a flattened polygonal shape with actin bundles at the cell edges, while reproductive senescent astrocytes had a contractile appearance with thick stress fibers visible throughout the cell. Moreover, in reproductive senescent astrocytes, BDNF was elevated with a concomitant reduction in expression of the BDNF receptor, TrkB. To examine the ability of astrocytes derived from young adult and reproductive senescent females to promote neuronal differentiation, neural progenitor cells (NPCs) were co-cultured with astrocytes derived from these groups. At day 4 in vitro, MAP-2(+) NPCs were located in smaller clusters when co-cultured with young adult astrocytes and in large clusters when co-cultured with older astrocytes. At days 6 and 10, neuronal differentiation was significantly reduced in reproductive senescent astrocyte-NPC co-cultures, as determined by NeuN(+) cell numbers and MAP-2(+) process lengths. Furthermore, estrogen only enhanced neuronal differentiation in young adult-NPC co-cultures. The ovarian age-related astrocyte phenotype thus limits the ability of this cell to promote neuronal differentiation in NPC populations and suggests that the astrocyte-mediated microenvironment in older acyclic females is less conducive to repair following neurovascular injury.

Modeling the impact of alcohol on cortical development in a dish: strategies from mapping neural stem cell fate

During the second trimester period, neuroepithelial stem cells give birth to millions of new neuroblasts, which migrate away from their germinal zones to populate the developing brain and terminally differentiate into neurons. During this period, large numbers of cells are also eliminated by programmed cell death. Therefore, the second trimester constitutes an important critical period for neuronal proliferation, migration, differentiation and apoptosis. Substantial evidence indicates that teratogens like ethanol can interfere with neuronal maturation. However, there is a paucity of good model systems to study early, second trimester events. In vivo models are inherently interpretatively complex because cell proliferation, migration, differentiation, and death mechanisms occur concurrently in regions like the cerebral cortex. This temporal overlap of multiple developmental critical periods makes it difficult to evaluate the relative vulnerability of any individual critical period. Our laboratory has elected to utilize fetal rodent cerebral cortical-derived neurosphere cultures as an experimental model of the second-trimester ventricular neuroepithelium. This model has enabled us to use flow cytometric approaches to identify neuroepithelial stem cell and progenitor sub-populations and to show that ethanol accelerates the maturation of neural stem cells. We have also developed a simplified mitogen-withdrawal/matrix-adhesion paradigm to model the exit of neuroepithelial cells from the ventricular zone towards the subventricular zone and cortical plate, and their maturation into multipolar neurons. We can treat neurosphere cultures with ethanol to mimic exposure during the period of neuroepithelial proliferation and by using the step-wise maturation model, ask questions about the impact of prior ethanol exposure on the subsequent maturation of neurons as they migrate and undergo terminal differentiation. The combination of neurosphere culture and stepwise maturation models will enable us to dissect out the contributions of specific developmental critical periods to the overall teratology of a drug of abuse like ethanol.

Computational models of ethanol-induced neurodevelopmental toxicity across species: Implications for risk assessment

Computational, systems-based approaches can provide a quantitative construct for evaluating risk in the context of mechanistic data. Previously, we developed computational models for the rat, mouse, rhesus monkey, and human, describing the acquisition of adult neuron number in the neocortex during the key neurodevelopmental processes of neurogenesis and synaptogenesis. Here we apply mechanistic data from the rat describing ethanol-induced toxicity in the developing neocortex to evaluate the utility of these models for analyzing neurodevelopmental toxicity across species. Our model can explain long-term neocortical neuronal loss in the rodent model after in utero exposure to ethanol based on inhibition of proliferation during neurogenesis. Our human model predicts a significant neuronal deficit after daily peak BECs reaching 10-20 mg/dl, which is the approximate BEC reached after drinking one standard drink within one hour. In contrast, peak daily BECs of 100 mg/dl are necessary to predict similar deficits in the rat. Our model prediction of increased sensitivity of primate species to ethanol-induced inhibition of proliferation is based on application of in vivo experimental data from primates showing a prolonged rapid growth period in the primate versus rodent neuronal progenitor population. To place our predictions into a broader context, we evaluate the evidence for functional low-dose effects across rats, monkeys, and humans. Results from this critical evaluation suggest subtle effects are evident at doses causing peak BECs of approximately 20 mg/dl daily, corroborating our model predictions. Our example highlights the utility of a systems-based modeling approach in risk assessment.

Ethanol exposure during neurogenesis induces persistent effects on neural maturation: evidence from an ex vivo model of fetal cerebral cortical neuroepithelial progenitor maturation

Ethanol is a significant neuroteratogen. We previously used fetal cortical-derived neurosphere cultures as an ex vivo model of the second trimester ventricular neuroepithelium, and showed that ethanol directly induced fetal stem and progenitor cell proliferation and maturation without inducing death. However, ethanol is defined as a teratogen because of its capacity to persistently disrupt neural maturation beyond a specific exposure period. We therefore utilized a simplified neuronal maturation paradigm to examine the immediate and persistent changes in neuronal migration following ethanol exposure during the phase of neuroepithelial proliferation. Our data indicate that mRNA transcripts for migration-associated genes RhoA, Paxillin (Pxn), and CDC42 were immediately induced following ethanol exposure, whereas dynein light chain, LC8-type 1 (DYNLL1), and growth-associated protein (Gap)-43 were suppressed. With the exception of Gap43, ethanol did not induce persistent changes in the other mRNAs, suggesting that ethanol had an activational, rather than organizational, impact on migration-associated mRNAs. However, despite this lack of persistent effects on these mRNAs, ethanol exposure during the proliferation period significantly increased subsequent neuronal migration. Moreover, differentiating neurons, pretreated with ethanol during the proliferation phase, exhibited reduced neurite branching and an increased length of primary neurites, indicating a persistent destabilization of neuronal maturation. Collectively, our data indicate that ethanol-exposed proliferating neuroepithelial precursors exhibit subsequent differentiation-associated increases in migratory behavior, independent of mRNA transcript levels. These data help explain the increased incidence of cerebral cortical neuronal heterotopias associated with the fetal alcohol syndrome.

Competing interactions between micro-RNAs determine neural progenitor survival and proliferation after ethanol exposure: evidence from an ex vivo model of the fetal cerebral cortical neuroepithelium

The fetal brain is sensitive to a variety of teratogens, including ethanol. We showed previously that ethanol induced mitosis and stem cell maturation, but not death, in fetal cerebral cortex-derived progenitors. We tested the hypothesis that micro-RNAs (miRNAs) could mediate the teratogenic effects of ethanol in a fetal mouse cerebral cortex-derived neurosphere culture model. Ethanol, at a level attained by alcoholics, significantly suppressed the expression of four miRNAs, miR-21, -335, -9, and -153, whereas a lower ethanol concentration, attainable during social drinking, induced miR-335 expression. A GABA(A) receptor-dependent mechanism mediated miR-21, but not miR-335 suppression, suggesting that divergent mechanisms regulate ethanol-sensitive miRNAs. Antisense-mediated suppression of miR-21 expression resulted in apoptosis, suggesting that miR-21 is an antiapoptotic factor. miR-335 knockdown promoted cell proliferation and prevented death induced by concurrently suppressing miR-21, indicating that miR-335 is a proapoptotic, antimitogenic factor whose actions are antagonistic to miR-21. Computational analyses identified two genes, Jagged-1, a Notch-receptor ligand, and embryonic-lethal abnormal vision, Drosophila-like 2 (ELAVL2), a brain-specific regulator of RNA stability, as presumptive targets of three of four ethanol-sensitive micro-RNAs. Combined knockdown of miR-335, -21, and -153 significantly increased Jagged-1 mRNA. Furthermore, ethanol induced both Jagged-1 and ELAVL2 mRNA. The collective suppression of micro-RNAs is consistent with ethanol induction of cell cycle and neuroepithelial maturation in the absence of apoptosis. These data identify a role for micro-RNAs as epigenetic intermediaries, which permit teratogens to shape complex, divergent developmental processes, and additionally demonstrate that coordinately regulated miRNAs exhibit both functional synergy and antagonism toward each other.