George R. Flentke

Ethanol Triggers Neural Crest Apoptosis through the Selective Activation of a Pertussis Toxin–Sensitive G Protein and a Phospholipase Cβ–Dependent Ca2+ Transient

BACKGROUND Alcohol is a potent neurotoxin that triggers the selective apoptosis of neuronal populations in the developing fetus. For neural crest cells, clinically relevant ethanol levels (0.3%) rapidly elicit a phospholipase C (PLC)-dependent intracellular Ca2+ transient that is sufficient to activate apoptosis. We investigated the biochemical origins of this Ca2+ transient. METHODS Three somite chick embryos (stage 8-) were pretreated with agonists and antagonists of PLC signaling pathways before ethanol challenge. The resulting intracellular Ca2+ release was quantified using Fluo-3; apoptosis was assessed using vital dyes. RESULTS Pretreatment of embryos with PLC antagonists U73122 or ET-18-OCH3 confirmed that a phosphoinositide-specific PLC was required for both the ethanol-dependent Ca2+ transient and subsequent cell death. Ethanol rapidly elevated intracellular inositol-1,4,5-trisphosphate [Ins(1,4,5)P3] levels in the rostral portion of the embryo that contains neural crest progenitors. The Ins(1,4,5)P3 receptor antagonist xestospongin C blocked the appearance of the ethanol-dependent Ca2+ transient. Pretreatment with the pan-Galpha protein antagonist GDPbetaS, but not with the tyrosine kinase antagonist genistein, suppressed ethanols ability to elicit the Ca2+ transient, suggesting that a rise in PLC activity and Ins(1,4,5)P3 concentration originates from stimulation of heterotrimeric G proteins. To probe the identity of this G protein, embryos were treated with G protein antagonists. Pertussis toxin and NF023 suppressed the ethanol-induced Ca2+ transient and subsequent neural crest apoptosis, whereas suramin was weakly inhibitory. C3 exoenzyme was embryolethal over a wide concentration range, consistent with suggestions that Rho family GTPases participate in neural crest development. Galphai2 was identified by immunostaining in the neural crest cells. CONCLUSION We propose a role for Galphai/o protein activation and subsequent interaction of Gbetagamma with PLCbeta in mediating the proapoptotic effects of ethanol upon the developing neural crest.

Calcium‐mediated repression of β‐catenin and its transcriptional signaling mediates neural crest cell death in an avian model of fetal alcohol syndrome

Fetal alcohol syndrome (FAS) is a common birth defect in many societies. Affected individuals have neurodevelopmental disabilities and a distinctive craniofacial dysmorphology. These latter deficits originate during early development from the ethanol-mediated apoptotic depletion of cranial facial progenitors, a population known as the neural crest. We showed previously that this apoptosis is caused because acute ethanol exposure activates G-protein-dependent intracellular calcium within cranial neural crest progenitors, and this calcium transient initiates the cell death. The dysregulated signals that reside downstream of ethanols calcium transient and effect neural crest death are unknown. Here we show that ethanols repression of the transcriptional effector β-catenin causes the neural crest losses. Clinically relevant ethanol concentrations (22-78 mM) rapidly deplete nuclear β-catenin from neural crest progenitors, with accompanying losses of β-catenin transcriptional activity and downstream genes that govern neural crest induction, expansion, and survival. Using forced expression studies, we show that β-catenin loss of function (via dominant-negative T cell transcription factor [TCF]) recapitulates ethanols effects on neural crest apoptosis, whereas β-catenin gain-of-function in ethanols presence preserves neural crest survival. Blockade of ethanols calcium transient using Bapta-AM normalizes β-catenin activity and prevents the neural crest losses, whereas ionomycin treatment is sufficient to destabilize β-catenin. We propose that ethanols repression of β-catenin causes the neural crest losses in this model of FAS. β-Catenin is a novel target for ethanols teratogenicity. β-Catenin/Wnt signals participate in many developmental events and its rapid and persistent dysregulation by ethanol may explain why the latter is such a potent teratogen.

CaMKII activation is a novel effector of alcohol’s neurotoxicity in neural crest stem/progenitor cells

J. Neurochem. (2011) 118, 646–657.

Increased Fibronectin Deposition in Embryonic Hearts of Retinol-Binding Protein–Null Mice

Abstract— Precise regulation of retinoid levels is critical for normal heart development. Retinol-binding protein (RBP), an extracellular retinol transporter, is strongly secreted by cardiogenic endoderm. This study addresses whether RBP gene ablation affects heart development. Despite exhibiting an >85% decrease in serum retinol, adult RBP-null mice are viable, breed, and have normal vision when maintained on a vitamin A–sufficient diet. Comparison of RBP-null with wild-type (WT) hearts from embryos at day 9.0 (E9.0) through E12.5 revealed an RBP-null phenotype similar to that of other retinoid-deficient models. At an early stage, RBP-null hearts display retarded trabecular development, which recovers by E9.5. This is accompanied at E9.5 and E10.5 by precocious differentiation of subepicardial cardiac myocytes. Most remarkably, RBP-null hearts display augmented deposition of fibronectin protein in the cardiac jelly at E9.0 through E10.5 and in the outflow tract at E12.5. This phenomenon, which was detected by immunohistochemistry and Western blotting without increased fibronectin transcript levels, is accompanied by increased numbers of mesenchymal cells in the outflow tract but not in the atrioventricular canal. RBP-null cardiac myocytes, especially in the subepicardial layer, display increased cell proliferation. This phenotype may present a model of subclinical retinoid insufficiency characterized by alteration of an extracellular matrix component and altered cellular differentiation and proliferation, changes that may have functional consequences for adult cardiac function. This murine model may have relevance to fetal development in human populations with inadequate retinoid intake.

CaMKII represses transcriptionally active β‐catenin to mediate acute ethanol neurodegeneration and can phosphorylate β‐catenin

Prenatal ethanol exposure causes persistent neurodevelopmental deficits by inducing apoptosis within neuronal progenitors including the neural crest. The cellular signaling events underlying this apoptosis are unclear. Using an established chick embryo model, we previously identified ethanols activation of calmodulin‐dependent protein kinase II (CaMKII) as a crucial early step in this pathway. Here, we report that CaMKII is pro‐apoptotic because it mediates the loss of transcriptionally active β‐catenin, which normally provides trophic support to these cells. β‐catenin over‐expression normalized cell survival in ethanols presence. CaMKII inhibition similarly restored β‐catenin content and transcriptional activity within ethanol‐treated cells and prevented their cell death. In contrast, inhibition of alternative effectors known to destabilize β‐catenin, including glycogen synthase kinase‐3β, Protein Kinase C, JNK, and calpain, failed to normalize cell survival and β‐catenin activity in ethanols presence. Importantly, we found that purified CaMKII can directly phosphorylate β‐catenin. Using targeted mutagenesis we identified CaMKII phosphorylation sites within human β‐catenin at T332, T472, and S552. This is the first demonstration that β‐catenin is a phosphorylation target of CaMKII and represents a novel mechanism by which calcium signals could regulate β‐catenin‐dependent transcription. These results inform ethanols neurotoxicity and offer unexpected insights into other neurodevelopmental and neurodegenerative disorders having dysregulated calcium or β‐catenin signaling.

Altered cardiac function and ventricular septal defect in avian embryos exposed to low-dose trichloroethylene.

Trichloroethylene (TCE) is the most frequently reported organic groundwater contaminant in the United States. It is controversial whether gestational TCE exposure causes congenital heart defects. The basis for TCEs proposed cardiac teratogenicity is not well understood. We previously showed that chick embryos exposed to 8 ppb TCE during cardiac morphogenesis have reduced cardiac output and increased mortality. To further investigate TCEs cardioteratogenic potential, we exposed in ovo chick embryos to TCE and evaluated the heart thereafter. Significant mortality was observed following TCE exposures of 8-400 ppb during a narrow developmental period (Hamburger-Hamilton [HH] stages 15-20, embryo day ED2.3-3.5) that is characterized by myocardial expansion, secondary heart looping, and endocardial cushion formation. Of the embryos that died, most did so between ED5.5 and ED6.5. Echocardiography of embryos at ED5.5 found that TCE-exposed hearts displayed significant functional and morphological heterogeneity affecting heart rate, left ventricular mass, and wall thickness. Individual embryos were identified with cardiac hypertrophy as well as with hypoplasia. Chick embryos exposed to 8 ppb TCE at HH17 that survived to hatch exhibited a high incidence (38%, p < 0.01, n = 16) of muscular ventricular septal defects (VSDs) as detected by echocardiography and confirmed by gross dissection; no VSDs were found in controls (n = 14). The TCE-induced VSDs may be secondary to functional impairments that alter cardiac hemodynamics and subsequent ventricular foramen closure, an interpretation consistent with recent demonstrations that TCE impairs calcium handling in cardiomyocytes. These data demonstrate that TCE is a cardiac teratogen for chick.

Adequacy of maternal iron status protects against behavioral, neuroanatomical, and growth deficits in fetal alcohol spectrum disorders.

Fetal alcohol spectrum disorders (FASD) are the leading non-genetic cause of neurodevelopmental disability in children. Although alcohol is clearly teratogenic, environmental factors such as gravidity and socioeconomic status significantly modify individual FASD risk despite equivalent alcohol intake. An explanation for this variability could inform FASD prevention. Here we show that the most common nutritional deficiency of pregnancy, iron deficiency without anemia (ID), is a potent and synergistic modifier of FASD risk. Using an established rat model of third trimester-equivalent binge drinking, we show that ID significantly interacts with alcohol to impair postnatal somatic growth, associative learning, and white matter formation, as compared with either insult separately. For the associative learning and myelination deficits, the ID-alcohol interaction was synergistic and the deficits persisted even after the offsprings’ iron status had normalized. Importantly, the observed deficits in the ID-alcohol animals comprise key diagnostic criteria of FASD. Other neurobehaviors were normal, showing the ID-alcohol interaction was selective and did not reflect a generalized malnutrition. Importantly ID worsened FASD outcome even though the mothers lacked overt anemia; thus diagnostics that emphasize hematological markers will not identify pregnancies at-risk. This is the first direct demonstration that, as suggested by clinical studies, maternal iron status has a unique influence upon FASD outcome. While alcohol is unquestionably teratogenic, this ID-alcohol interaction likely represents a significant portion of FASD diagnoses because ID is more common in alcohol-abusing pregnancies than generally appreciated. Iron status may also underlie the associations between FASD and parity or socioeconomic status. We propose that increased attention to normalizing maternal iron status will substantially improve FASD outcome, even if maternal alcohol abuse continues. These findings offer novel insights into how alcohol damages the developing brain.

An Evolutionarily Conserved Mechanism of Calcium-Dependent Neurotoxicity in a Zebrafish Model of Fetal Alcohol Spectrum Disorders

BACKGROUND Fetal alcohol spectrum disorders (FASD) are a leading cause of neurodevelopmental disability. Nonhuman animal models offer novel insights into its underlying mechanisms. Although the developing zebrafish has great promise for FASD research, a significant challenge to its wider adoption is the paucity of clear, mechanistic parallels between its ethanol (EtOH) responses and those of nonpiscine, established models. Inconsistencies in the published pharmacodynamics for EtOH-exposed zebrafish, alongside the use of comparatively high EtOH doses, challenge the interpretation of this models clinical relevance. METHODS To address these limitations, we developed a binge, single-exposure model of EtOH exposure in the early zebrafish embryo. RESULTS Brief (3-hour) EtOH exposure is sufficient to cause significant neural crest losses and craniofacial alterations, with peak vulnerability during neurogenesis and early somitogenesis. These losses are apoptotic, documented using TUNEL assay and secA5-YFP-reporter fish. Apoptosis is dose dependent with an EC50 = 56.2 ± 14.3 mM EtOHint , a clinically relevant value within the range producing apoptosis in chick and mouse neural crest. This apoptosis requires the calcium-dependent activation of CaMKII and recapitulates the well-described EtOH signaling mechanism in avian neural crest. Importantly, we resolve the existing confusion regarding zebrafish EtOH kinetics. We show that steady-state EtOH concentrations within both chorion-intact and dechorionated embryos are maintained at 35.7 ± 2.8% of EtOHext levels across the range from 50 to 300 mM EtOHext , a value consistent with several published reports. Equilibrium is rapid and complete within 5 minutes of EtOH addition. CONCLUSIONS The calcium/CaMKII mechanism of EtOHs neurotoxicity is shared between an amniote (chick) and teleost fish, indicating that this mechanism is evolutionarily conserved. Our data suggest that EtOHext concentrations >2% (v/v) for chorion-intact embryos and 1.5% (v/v) for dechorionated embryos have limited clinical relevance. The strong parallels with established models endorse the zebrafishs relevance for mechanistic studies of EtOHs developmental neurotoxicity.

A ryanodine receptor-dependent Ca i 2+ asymmetry at Hensen's node mediates avian lateral identity

In mouse, the establishment of left-right (LR) asymmetry requires intracellular calcium (Cai2+) enrichment on the left of the node. The use of Cai2+ asymmetry by other vertebrates, and its origins and relationship to other laterality effectors are largely unknown. Additionally, the architecture of Hensens node raises doubts as to whether Cai2+ asymmetry is a broadly conserved mechanism to achieve laterality. We report here that the avian embryo uses a left-side enriched Cai2+ asymmetry across Hensens node to govern its lateral identity. Elevated Cai2+ was first detected along the anterior node at early HH4, and its emergence and left-side enrichment by HH5 required both ryanodine receptor (RyR) activity and extracellular calcium, implicating calcium-induced calcium release (CICR) as the novel source of the Cai2+. Targeted manipulation of node Cai2+ randomized heart laterality and affected nodal expression. Bifurcation of the Cai2+ field by the emerging prechordal plate may permit the independent regulation of LR Cai2+ levels. To the left of the node, RyR/CICR and H+V-ATPase activity sustained elevated Cai2+. On the right, Cai2+ levels were actively repressed through the activities of H+K+ ATPase and serotonin-dependent signaling, thus identifying a novel mechanism for the known effects of serotonin on laterality. Vitamin A-deficient quail have a high incidence of situs inversus hearts and had a reversed calcium asymmetry. Thus, Cai2+ asymmetry across the node represents a more broadly conserved mechanism for laterality among amniotes than had been previously believed.

Microarray analysis of retinoid-dependent gene activity during rat embryogenesis: increased collagen fibril production in a model of retinoid insufficiency.

Retinoic acid (RA) is an essential mediator of embryogenesis. Some, but not all, of its targets have been identified. We previously developed a rat model of gestational retinoid deficiency (RAD; Power et al. [ 1999 ] Dev. Dyn. 216:469–480) and generated embryos with developmental impairments that closely resemble genetic and dietary models of retinoid insufficiency. Here, we used microarray analysis and expression profiling to identify 88 transcripts whose abundance was altered under conditions of retinoid insufficiency, as compared with normal embryos. Among these, the induction by RAD of genes involved in collagen I synthesis (COL1A1, IA2 and VA2, prolyl‐4‐hydroxylase‐α1) and protein galactosylation (galactokinase, ABO galactosyltransferase, UDP‐galactose transporter‐related protein) was especially noteworthy because extracellular matrix regulates many developmental events. We also identified several genes involved with stress responses (cathepsin H, UBC2E, IGFBP3, smoothelin). Real‐time polymerase chain reaction analysis of selected candidates revealed excellent agreement with the array findings. Further validation came from the demonstration that these genes were similarly dysregulated in two genetic models of retinoid insufficiency, the retinol binding protein null‐mutant embryo and the Raldh2 null‐mutant embryo. In situ hybridization of RAD embryos found increased collagen IA1 and IGFBP3 mRNA within the connective mesenchyme and vasculature, respectively, and a failure to repress the growth factor midkine within the RAD neural tube. Many of the identified genes were not known previously to respond to retinoid status and will provide new insights to retinoid roles and to the consequences of retinoid insufficiency. Developmental Dynamics 229:886–898, 2004.