Neural crest development in fetal alcohol syndrome

Abstract

Fetal alcohol spectrum disorder (FASD) is a leading cause of neurodevelopmental disability. Some affected individuals possess distinctive craniofacial deficits, but many more lack overt facial changes. An understanding of the mechanisms underlying these deficits would inform their diagnostic utility. Our understanding of these mechanisms is challenged because ethanol lacks a single receptor when redirecting cellular activity. This review summarizes our current understanding of how ethanol alters neural crest development. Ample evidence shows that ethanol causes the "classic" fetal alcohol syndrome (FAS) face (short palpebral fissures, elongated upper lip, deficient philtrum) because it suppresses prechordal plate outgrowth, thereby reducing neuroectoderm and neural crest induction and causing holoprosencephaly. Prenatal alcohol exposure (PAE) at premigratory stages elicits a different facial appearance, indicating FASD may represent a spectrum of facial outcomes. PAE at this premigratory period initiates a calcium transient that activates CaMKII and destabilizes transcriptionally active β-catenin, thereby initiating apoptosis within neural crest populations. Contributing to neural crest vulnerability are their low antioxidant responses. Ethanol-treated neural crest produce reactive oxygen species and free radical scavengers attenuate their production and prevent apoptosis. Ethanol also significantly impairs neural crest migration, causing cytoskeletal rearrangements that destabilize focal adhesion formation; their directional migratory capacity is also lost. Genetic factors further modify vulnerability to ethanol-induced craniofacial dysmorphology and include genes important for neural crest development, including shh signaling, PDFGA, vangl2, and ribosomal biogenesis. Because facial and brain development are mechanistically and functionally linked, research into ethanol's effects on neural crest also informs our understanding of ethanol's CNS pathologies.

Keywords: apoptosis; calcium signaling; craniofacial; fetal alcohol spectrum disorders; neural crest; sonic hedgehog.

Figure 1

Effects of ethanol upon neural crest development. Although chick embryos are depicted, these changes have been documented in diverse vertebrate species including mouse, zebrafish, and Xenopus. The left-half of each embryo depicts normal development, and each right-half contrasts the consequences of ethanol exposure. Details of these changes are presented in the text. Purple, shh; teal, neural crest; blue, chondrogenesis; red dots, apoptosis; pink stars, ROS; teal squiggles, migrating neural crest. Cont, control; EtOH, ethanol; NC, neural crest; PCM, prechordal plate; ROS, reactive oxygen species.

Figure 2

Key events in ethanol-induced apoptosis of chick cranial neural crest. The green box on the left panel indicates the headfold region where calcium is imaged in the 3-somite chick embryo. Exposure to 52mM ethanol instigates the mobilization of intracellular calcium stores (*) within the early head fold (boxed region) as quantified using Fura2. This selectively activates CaMKII within the anterior neural folds including neural crest (arrows) as detected using antibody directed against phospho-CaMKII (green signal, arrows). A dorsal view of the headfolds is depicted. Among other targets, CaMKII phosphorylates and destabilizes β-catenin protein (green signal at green arrows in boxed region) within the dorsal neural folds enriched in neural crest (D). Shown is a transverse section, dorsal at top, through the headfold of embryos having 3 somites; blue indicates DAPI-stained nuclei. Subsequently, there is significant apoptosis (red signal) within ethanol-exposed dorsal neuroprogenitors of the hindbrain including neural crest (strong green signal), detected using antibody against the neural crest marker snail2. The saline-treated control hindbrain displays little cell death. Shown is a transverse section through rhombomere 4, which normally lacks appreciable cell death, of embryos having 16–18 somites. Chick embryos normally have a low-level green autofluorescent background.