Chronic ethanol exposure in the embryonic chick heart: effect on myocardial function and structure

Prenatal ethanol exposure impairs the conduction delay at the atrioventricular junction in the looping heart

The etiology of ethanol-related congenital heart defects has been the focus of much study, but most research has concentrated on cellular and molecular mechanisms. We have shown with optical coherence tomography (OCT) that ethanol exposure led to increased retrograde flow and smaller atrioventricular (AV) cushions compared with controls. Since AV cushions play a role in patterning the conduction delay at the atrioventricular junction (AVJ), this study aims to investigate whether ethanol exposure alters the AVJ conduction in early looping hearts and whether this alteration is related to the decreased cushion size. Quail embryos were exposed to a single dose of ethanol at gastrulation, and Hamburger-Hamilton stage 19-20 hearts were dissected for imaging. Cardiac conduction was measured using an optical mapping microscope and we imaged the endocardial cushions using OCT. Our results showed that, compared with controls, ethanol-exposed embryos exhibited abnormally fast AVJ conduction and reduced cushion size. However, this increased conduction velocity (CV) did not strictly correlate with decreased cushion volume and thickness. By matching the CV map to the cushion-size map along the inflow heart tube, we found that the slowest conduction location was consistently at the atrial side of the AVJ, which had the thinner cushions, not at the thickest cushion location at the ventricular side as expected. Our findings reveal regional differences in the AVJ myocardium even at this early stage in heart development. These findings reveal the early steps leading to the heterogeneity and complexity of conduction at the mature AVJ, a site where arrhythmias can be initiated.NEW & NOTEWORTHY To the best of our knowledge, this is the first study investigating the impact of ethanol exposure on the early cardiac conduction system. Our results showed that ethanol-exposed embryos exhibited abnormally fast atrioventricular conduction. In addition, our findings, in CV measurements and endocardial cushion thickness, reveal regional differences in the AVJ myocardium even at this early stage in heart development, suggesting that the differentiation and maturation at this site are complex and warrant further studies.

The avian embryo as a model for fetal alcohol spectrum disorder

Prenatal alcohol exposure (PAE) remains a leading preventable cause of structural birth defects and permanent neurodevelopmental disability. The chicken (Gallus gallus domesticus) is a powerful embryological research model, and was possibly the first in which the teratogenicity of alcohol was demonstrated. Pharmacologically relevant exposure to alcohol in the range of 20-70 mmol/L (20-80 mg/egg) disrupt the growth of chicken embryos, morphogenesis, and behavior, and the resulting phenotypes strongly parallel those of mammalian models. The avian embryo's direct accessibility has enabled novel insights into the teratogenic mechanisms of alcohol. These include the contribution of IGF1 signaling to growth suppression, the altered flow dynamics that reshape valvuloseptal morphogenesis and mediate its cardiac teratogenicity, and the suppression of Wnt and Shh signals thereby disrupting the migration, expansion, and survival of the neural crest, and underlie its characteristic craniofacial deficits. The genetic diversity within commercial avian strains has enabled the identification of unique loci, such as ribosome biogenesis, that modify vulnerability to alcohol. This venerable research model is equally relevant for the future, as the application of technological advances including CRISPR, optogenetics, and biophotonics to the embryo's ready accessibility creates a unique model in which investigators can manipulate and monitor the embryo in real-time to investigate the effect of alcohol on cell fate.

Connecting teratogen-induced congenital heart defects to neural crest cells and their effect on cardiac function

Neural crest cells play many key roles in embryonic development, as demonstrated by the abnormalities that result from their specific absence or dysfunction. Unfortunately, these key cells are particularly sensitive to abnormalities in various intrinsic and extrinsic factors, such as genetic deletions or ethanol-exposure that lead to morbidity and mortality for organisms. This review discusses the role identified for a segment of neural crest in regulating the morphogenesis of the heart and associated great vessels. The paradox is that their derivatives constitute a small proportion of cells to the cardiovascular system. Findings supporting that these cells impact early cardiac function raises the interesting possibility that they indirectly control cardiovascular development at least partially through regulating function. Making connections between insults to the neural crest, cardiac function, and morphogenesis is more approachable with technological advances. Expanding our understanding of early functional consequences could be useful in improving diagnosis and testing therapies.

Ethanol exposure alters early cardiac function in the looping heart: a mechanism for congenital heart defects?

Alcohol-induced congenital heart defects are frequently among the most life threatening and require surgical correction in newborns. The etiology of these defects, collectively known as fetal alcohol syndrome, has been the focus of much study, particularly involving cellular and molecular mechanisms. Few studies have addressed the influential role of altered cardiac function in early embryogenesis because of a lack of tools with the capability to assay tiny beating hearts. To overcome this gap in our understanding, we used optical coherence tomography (OCT), a nondestructive imaging modality capable of micrometer-scale resolution imaging, to rapidly and accurately map cardiovascular structure and hemodynamics in real time under physiological conditions. In this study, we exposed avian embryos to a single dose of alcohol/ethanol at gastrulation when the embryo is sensitive to the induction of birth defects. Late-stage hearts were analyzed using standard histological analysis with a focus on the atrio-ventricular valves. Early cardiac function was assayed using Doppler OCT, and structural analysis of the cardiac cushions was performed using OCT imaging. Our results indicated that ethanol-exposed embryos developed late-stage valvuloseptal defects. At early stages, they exhibited increased regurgitant flow and developed smaller atrio-ventricular cardiac cushions, compared with controls (uninjected and saline-injected embryos). The embryos also exhibited abnormal flexion/torsion of the body. Our evidence suggests that ethanol-induced alterations in early cardiac function have the potential to contribute to late-stage valve and septal defects, thus demonstrating that functional parameters may serve as early and sensitive gauges of cardiac normalcy and abnormalities.

Supraphysiological acetaldehyde levels suppress growth in chicken embryos

Although exposure to ethanol is known to cause growth inhibition in a developing embryo, the contributing effect of acetaldehyde on growth is not as well documented. In this study, we measured acetaldehyde-induced growth suppression in three different chicken strains: Peterson x Hubbard, HY x Hubbard, and W36 Ginther White Leghorn. The chicken embryo provides a useful model for studying fetal alcohol syndrome (FAS) and has been used extensively in our laboratory. The current study was undertaken to determine whether the chicken embryo could serve as a model for studying the effects of acetaldehyde on growth. Acetaldehyde caused a significant reduction in embryonic weights only at the higher acetaldehyde concentrations. Torso-to-head ratios were unchanged at every acetaldehyde dose for all strains, supporting the suggestion that acetaldehyde-induced growth suppression was generalized in all tissues, rather than being exhibited as a selective decrease of neuronal tissue. All strains experienced a significant decrease in viability only at higher acetaldehyde concentrations, but differences in viability were evident among the strains. These results support findings obtained from previous work done on ethanol-induced differences among chicken strains by supporting the suggestion that the strain of chicken is important when studying the effects of teratogens on growth and viability. More importantly, the supraphysiological concentrations of acetaldehyde necessary to induce growth suppression seem to indicate that the chicken embryo may not be a viable model of FAS for studying the direct effects of acetaldehyde on embryonic growth.

Maternal issues affecting the fetus

Maternal disorders and exposures that affect fetal cardiac structure and function are reviewed, emphasizing fetal echocardiographic diagnosis and monitoring, and approaches for in utero therapy. Maternal diabetes, hyperthyroidism, lupus erythematosis, epilepsy, congenital heart disease, infections, and drug exposures are considered.

Ethanol differentially affects metabolic and mitotic processes in chick embryonic cells

Our laboratory has been investigating the mechanisms by which ethanol-induced growth inhibition occurs in a developing embryo, and our studies have focused on disruption of cellular signaling pathways. Previous work on ethanol-induced changes in signaling systems that regulate ornithine decarboxylase activity indicated that the pathways containing protein kinase A, protein kinase C (PKC), and insulin-dependent tyrosine kinase were important for the control of ornithine decarboxylase in chick embryonic cells. Herein, we report ethanol's effect on the regulation of glucose uptake and thymidine uptake by these same kinase pathways. A pronounced increase in glucose uptake was associated with PKC downregulation in both vehicle- and ethanol-exposed cells, with the larger increase occurring in ethanol-exposed cells. An increase in thymidine uptake was associated with an activation of all three kinases, as well as with downregulation of PKC. Because previous work on signaling pathways has looked for changes in the insulin signaling pathway, the work herein focuses on the signaling pathways involving protein kinase A and PKC. cAMP levels were increased by ethanol treatment, but the increase was relatively small. Analysis of changes in PKC activity induced by ethanol exposure showed a significant suppression of PKC activity in the ethanol-treated cells and suggested that, overall, ethanol treatment affects the regulation of glucose uptake in embryonic cells predominantly by PKC downregulation.

Ethyl alcohol reduces cardiac output, stroke volume, and end diastolic volume in the embryonic chick

It has been established that ethanol causes both human congenital cardiac malformations and structural intracardiac abnormalities in the embryonic chick. In view of a theory that reduced embryonic tissue hemodynamics are associated with the development of malformations, we attempted to determine whether or not a) ethanol altered cardiac blood flow and b) altered hemodynamics were a function of ethanol dose in the chick embryo. Cardiac function in Hamburger-Hamilton stage 19 chick embryos was recorded on videotape before and up to 10 hours after exposure to graded doses of ethanol. Parameters of cardiac function, including cardiac output, were determined from videotaped images by means of computer assistance. Cardiac output decreased in a linear fashion with dose for up to 3 hours after exposure to ethanol. The maximum relative percent decrease in cardiac output was directly related to the dose of ethanol administered. Furthermore, the time required after ethanol treatment for mean cardiac output to return to pretreatment and control values was also dose-dependent--lower doses of ethanol required less time for mean cardiac output to return to pretreatment and control values. Although relatively high doses of ethanol depress cardiac rate, we attribute the significant decrease in cardiac output primarily to parallel dose-dependent decreases in both stroke volume and end diastolic volume. Our data are consistent with the hypothesis that reduced embryonic cardiac blood flow during cardiogenesis is associated with the development of ethanol-induced intracardiac defects in chick embryos.

Relation of early hemodynamic changes to final cardiac phenotype and survival after neural crest ablation in chick embryos

BACKGROUND

Microcinephotography was used to study a model of persistent truncus arteriosus created in chick embryos by ablation of premigratory neural crest destined for the third and fourth aortic arch arteries as well as the septum of the cardiac outflow tract.

METHODS AND RESULTS

Twenty-five control embryos and 105 of 202 experimental embryos were filmed on day 3 of incubation and then reincubated. The remaining 97 experimental embryos were not filmed because of twisting of the embryos, but they were reincubated. There was no difference in either the survival rate (p greater than 0.23) from day 3 to day 11 of incubation or the incidence of persistent truncus arteriosus (p greater than 0.08) between the filmed and the nonfilmed embryos. Incomplete looping of the cardiac tube observed in experimental embryos during early cardiogenesis correlated with a right ventricular origin of the outflow vessels in the definitive heart. Hemodynamic measurements indicated that there was no difference in heart rate, ejection fraction, systolic and diastolic areas, stroke volume, and cardiac output between controls and the experimental group as a whole. However, embryos that did not survive to day 11 had decreased stroke volume (p less than 0.001) and cardiac output (p less than 0.001), whereas embryos that survived to day 11 with cardiac malformations had increased stroke volume and cardiac output in early embryogenesis.

CONCLUSIONS

Increased stroke volume and cardiac output may be necessary factors for survival in embryos with cardiac dysmorphogenesis and probably are associated with dilation of the ventricular portion of the cardiac tube, which leads to malalignment of the outflow vessel or vessels.

Microcinephotography of the developing heart in neural crest-ablated chick embryos

Microcinephotography was used to study heart development in a neural crest model of heart defects, that is, persistent truncus arteriosus, interrupted aortic arch, double outlet right ventricle, or single ventricle and tricuspid valve anomalies. These defects were created in chick embryos by ablation of premigratory neural crest destined for the aorticopulmonary and truncal septa, as well as the third and fourth aortic arch arteries. When embryogenesis reached the looped cardiac tube stage of development (Hamburger-Hamilton stage 18), 19 experimental and 15 control embryos were filmed at 100 frames per second under controlled environmental conditions. Analysis of the microcinephotography films showed the following significant distinguishing characteristics of the developing heart in the experimental embryos: altered conotruncal shape in 100%, depressed contractility and dilation of the primitive ventricle in 84%, decreased emptying of the bulbus cordis in 79%, incompetent truncal cushions in 68%, incomplete looping of the cardiac tube in 58%, and fourth right aortic arch artery without blood flow and third right aortic arch artery with increased flow in 53%. These abnormal characteristics suggested that there were functional and morphological changes in the developing heart of experimental embryos before the time when the predicted structural heart defects would be apparent. It is proposed that the primitive ventricle might attempt to compensate for depressed contractility by ventricular dilation. The incompetent truncal cushions could be secondary to the depressed contractility or secondary to the neural crest ablation that is known to cause persistent truncus arteriosus, an interrupted aortic arch, or both. The absence of blood flow in the right fourth aortic arch artery that will become the definitive aorta correlates with the expected incidence of interrupted aortic arches in this neural crest-ablation model of heart defects. It is speculated that the incomplete looping of the cardiac tube might hinder normal developmental alignment of the outflow and inflow tracts, producing a spectrum of lesions of maldevelopment of the tricuspid valve and dextroposition of the aorta.

Correlation of ventricular area, perimeter, and conotruncal diameter with ventricular mass and function in the chick embryo from stages 12 to 24

Ventricular form and function are interrelated during cardiovascular development. The study of muscle mechanics requires the real-time measurement of length, area, or volume. Because volume measures are not currently possible in the embryonic heart, we tested the hypothesis that end-diastolic (ED) and end-systolic (ES) ventricular perimeter, area, and conotruncal diameter correlate with ventricular mass and function in the stage 12 to stage 24 white Leghorn chick embryo. Video images of the contracting heart were recorded at 60 Hz on 1/2" videotape and studied with a custom image-analysis workstation. ED and ES video fields were selected by maximum and minimum ventricular area and were planimetered for epicardial ventricular perimeter, area, and conotruncal diameter. Data are reported as (mean +/- SEM, n greater than or equal to 8) and were tested by analysis of variance and regression analysis. Heart rate calculated from cycle length increased from 78 +/- 6 beats/min at stage 12 to 162 +/- 5 beats/min at stage 24. ED and ES area increased geometrically versus stage (y = 0.53 - 0.08x + 0.004x2, r = 0.96, p less than 0.001; and y = 0.60 - 0.09x + 0.004x2, r = 0.98, p less than 0.001, respectively). ED and ES perimeter and conotruncal diameter increased linearly versus stage (r = 0.95, p less than 0.001; r = 0.96, p less than 0.001; and r = 0.93, p less than 0.001; r = 0.93, p less than 0.001, respectively). Shortening fraction for each measurement increased from stage 12 to 16 or 18 then decreased.(ABSTRACT TRUNCATED AT 250 WORDS)