Analysis of crosstalk between retinoic acid and sonic hedgehog pathways following ethanol exposure in embryonic zebrafish

Pharmacological activation of the Sonic hedgehog pathway with a Smoothened small molecule agonist ameliorates the severity of alcohol-induced morphological and behavioral birth defects in a zebrafish model of fetal alcohol spectrum disorder

Ethanol exposure during the early stages of embryonic development can lead to a range of morphological and behavioral differences termed fetal alcohol spectrum disorders (FASDs). In a zebrafish model, we have shown that acute ethanol exposure at 8-10 hr postfertilization (hpf), a critical time of development, produces birth defects similar to those clinically characterized in FASD. Dysregulation of the Sonic hedgehog (Shh) pathway has been implicated as a molecular basis for many of the birth defects caused by prenatal alcohol exposure. We observed in zebrafish embryos that shh expression was significantly decreased by ethanol exposure at 8-10 hpf, while smo expression was much less affected. Treatment of zebrafish embryos with SAG or purmorphamine, small molecule Smoothened agonists that activate Shh signaling, ameliorated the severity of ethanol-induced developmental malformations including altered eye size and midline brain development. Furthermore, this rescue effect of Smo activation was dose dependent and occurred primarily when treatment was given after ethanol exposure. Markers of Shh signaling (gli1/2) and eye development (pax6a) were restored in embryos treated with SAG post-ethanol exposure. Since embryonic ethanol exposure has been shown to produce later-life neurobehavioral impairments, juvenile zebrafish were examined in the novel tank diving test. Our results further demonstrated that in zebrafish embryos exposed to ethanol, SAG treatment was able to mitigate long-term neurodevelopmental impairments related to anxiety and risk-taking behavior. Our results indicate that pharmacological activation of the Shh pathway at specific developmental timing markedly diminishes the severity of alcohol-induced birth defects.

Zebrafish models of fetal alcohol spectrum disorders

Fetal alcohol spectrum disorder (FASD) describes a wide range of structural deficits and cognitive impairments. FASD impacts up to 5% of children born in the United States each year, making ethanol one of the most common teratogens. Due to limitations and ethical concerns, studies in humans are limited in their ability to study FASD. Animal models have proven critical in identifying and characterizing the mechanisms underlying FASD. In this review, we will focus on the attributes of zebrafish that make it a strong model in which to study ethanol-induced developmental defects. Zebrafish have several attributes that make it an ideal model in which to study FASD. Zebrafish produced large numbers of externally fertilized, translucent embryos. With a high degree of genetic amenability, zebrafish are at the forefront of identifying and characterizing the gene-ethanol interactions that underlie FASD. Work from multiple labs has shown that embryonic ethanol exposures result in defects in craniofacial, cardiac, ocular, and neural development. In addition to structural defects, ethanol-induced cognitive and behavioral impairments have been studied in zebrafish. Building upon these studies, work has identified ethanol-sensitive loci that underlie the developmental defects. However, analyses show there is still much to be learned of these gene-ethanol interactions. The zebrafish is ideally suited to expand our understanding of gene-ethanol interactions and their impact on FASD. Because of the conservation of gene function between zebrafish and humans, these studies will directly translate to studies of candidate genes in human populations and allow for better diagnosis and treatment of FASD.

Zebrafish as an Animal Model for Ocular Toxicity Testing: A Review of Ocular Anatomy and Functional Assays

Xenobiotics make their way into organisms from diverse sources including diet, medication, and pollution. Our understanding of ocular toxicities from xenobiotics in humans, livestock, and wildlife is growing thanks to laboratory animal models. Anatomy and physiology are conserved among vertebrate eyes, and studies with common mammalian preclinical species (rodent, dog) can predict human ocular toxicity. However, since the eye is susceptible to toxicities that may not involve a histological correlate, and these species rely heavily on smell and hearing to navigate their world, discovering visual deficits can be challenging with traditional animal models. Alternative models capable of identifying functional impacts on vision and requiring minimal amounts of chemical are valuable assets to toxicology. Human and zebrafish eyes are anatomically and functionally similar, and it has been reported that several common human ocular toxicants cause comparable toxicity in zebrafish. Vision develops rapidly in zebrafish; the tiny larvae rely on visual cues as early as 4 days, and behavioral responses to those cues can be monitored in high-throughput fashion. This article describes the comparative anatomy of the zebrafish eye, the notable differences from the mammalian eye, and presents practical applications of this underutilized model for assessment of ocular toxicity.

Multifactorial Genetic and Environmental Hedgehog Pathway Disruption Sensitizes Embryos to Alcohol-Induced Craniofacial Defects

BACKGROUND

Prenatal alcohol exposure (PAE) is perhaps the most common environmental cause of human birth defects. These exposures cause a range of structural and neurological defects, including facial dysmorphologies, collectively known as fetal alcohol spectrum disorders (FASD). While PAE causes FASD, phenotypic outcomes vary widely. It is thought that multifactorial genetic and environmental interactions modify the effects of PAE. However, little is known of the nature of these modifiers. Disruption of the Hedgehog (Hh) signaling pathway has been suggested as a modifier of ethanol teratogenicity. In addition to regulating the morphogenesis of craniofacial tissues commonly disrupted in FASD, a core member of the Hh pathway, Smoothened, is susceptible to modulation by structurally diverse chemicals. These include environmentally prevalent teratogens like piperonyl butoxide (PBO), a synergist found in thousands of pesticide formulations.

METHODS

Here, we characterize multifactorial genetic and environmental interactions using a zebrafish model of craniofacial development.

RESULTS

We show that loss of a single allele of shha sensitized embryos to both alcohol- and PBO-induced facial defects. Co-exposure of PBO and alcohol synergized to cause more frequent and severe defects. The effects of this co-exposure were even more profound in the genetically susceptible shha heterozygotes.

CONCLUSIONS

Together, these findings shed light on the multifactorial basis of alcohol-induced craniofacial defects. In addition to further implicating genetic disruption of the Hh pathway in alcohol teratogenicity, our findings suggest that co-exposure to environmental chemicals that perturb Hh signaling may be important variables in FASD and related craniofacial disorders.

Animal models of gene-alcohol interactions

Most birth defects arise from complex interactions between multiple genetic and environmental factors. However, our current understanding of how these interactions and their contributions affect birth defects remains incomplete. Human studies are limited in their ability to identify the fundamental causes of birth defects due to ethical and practical limitations. Animal models provide a great number of resources not available to human studies and they have been critical in advancing our understanding of birth defects and the complex interactions that underlie them. In this review, we discuss the use of animal models in the context of gene-environment interactions that underlie birth defects. We focus on alcohol which is the most common environmental factor associated with birth defects. Prenatal alcohol exposure leads to a wide range of cognitive impairments and structural deficits broadly termed fetal alcohol spectrum disorders (FASD). We discuss the broad impact of prenatal alcohol exposure on the developing embryo and elaborate on the current state of gene-alcohol interactions. Additionally, we discuss how animal models have informed our understanding of the genetics of FASD. Ultimately, these topics will provide insight into the use of animal models in understanding gene-environment interactions and their subsequent impact on birth defects.

Diving into the world of alcohol teratogenesis: a review of zebrafish models of fetal alcohol spectrum disorder

The term fetal alcohol spectrum disorder (FASD) refers to the entire suite of deleterious outcomes resulting from embryonic exposure to alcohol. Along with other reviews in this special issue, we provide insight into how animal models, specifically the zebrafish, have informed our understanding of FASD. We first provide a brief introduction to FASD. We discuss the zebrafish as a model organism and its strengths for alcohol research. We detail how zebrafish has been used to model some of the major defects present in FASD. These include behavioral defects, such as social behavior as well as learning and memory, and structural defects, disrupting organs such as the brain, sensory organs, heart, and craniofacial skeleton. We provide insights into how zebrafish research has aided in our understanding of the mechanisms of ethanol teratogenesis. We end by providing some relatively recent advances that zebrafish has provided in characterizing gene-ethanol interactions that may underlie FASD.

Differences in neural crest sensitivity to ethanol account for the infrequency of anterior segment defects in the eye compared with craniofacial anomalies in a zebrafish model of fetal alcohol syndrome

BACKGROUND

Ethanol (ETOH) exposure during pregnancy is associated with craniofacial and neurologic abnormalities, but infrequently disrupts the anterior segment of the eye. In these studies, we used zebrafish to investigate differences in the teratogenic effect of ETOH on craniofacial, periocular, and ocular neural crest.

METHODS

Zebrafish eye and neural crest development was analyzed by means of live imaging, TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay, immunostaining, detection of reactive oxygen species, and in situ hybridization.

RESULTS

Our studies demonstrated that foxd3-positive neural crest cells in the periocular mesenchyme and developing eye were less sensitive to ETOH than sox10-positive craniofacial neural crest cells that form the pharyngeal arches and jaw. ETOH increased apoptosis in the retina, but did not affect survival of periocular and ocular neural crest cells. ETOH also did not increase reactive oxygen species within the eye. In contrast, ETOH increased ventral neural crest apoptosis and reactive oxygen species production in the facial mesenchyme. In the eye and craniofacial region, sod2 showed high levels of expression in the anterior segment and in the setting of Sod2 knockdown, low levels of ETOH decreased migration of foxd3-positive neural crest cells into the developing eye. However, ETOH had minimal effect on the periocular and ocular expression of transcription factors (pitx2 and foxc1) that regulate anterior segment development.

CONCLUSION

Neural crest cells contributing to the anterior segment of the eye exhibit increased ability to withstand ETOH-induced oxidative stress and apoptosis. These studies explain the rarity of anterior segment dysgenesis despite the frequent craniofacial abnormalities in fetal alcohol syndrome. Birth Defects Research 109:1212-1227, 2017. © 2017 Wiley Periodicals, Inc.

Zebrafish Models of Human Disease: Gaining Insight into Human Disease at ZFIN

The Zebrafish Model Organism Database (ZFIN; https://zfin.org) is the central resource for genetic, genomic, and phenotypic data for zebrafish (Danio rerio) research. ZFIN continuously assesses trends in zebrafish research, adding new data types and providing data repositories and tools that members of the research community can use to navigate data. The many research advantages and flexibility of manipulation of zebrafish have made them an increasingly attractive animal to model and study human disease.To facilitate disease-related research, ZFIN developed support to provide human disease information as well as annotation of zebrafish models of human disease. Human disease term pages at ZFIN provide information about disease names, synonyms, and references to other databases as well as a list of publications reporting studies of human diseases in which zebrafish were used. Zebrafish orthologs of human genes that are implicated in human disease etiology are routinely studied to provide an understanding of the molecular basis of disease. Therefore, a list of human genes involved in the disease with their corresponding zebrafish ortholog is displayed on the disease page, with links to additional information regarding the genes and existing mutations. Studying human disease often requires the use of models that recapitulate some or all of the pathologies observed in human diseases. Access to information regarding existing and published models can be critical, because they provide a tractable way to gain insight into the phenotypic outcomes of the disease. ZFIN annotates zebrafish models of human disease and supports retrieval of these published models by listing zebrafish models on the disease term page as well as by providing search interfaces and data download files to access the data. The improvements ZFIN has made to annotate, display, and search data related to human disease, especially zebrafish models for disease and disease-associated gene information, should be helpful to researchers and clinicians considering the use of zebrafish to study human disease.

Comparison of molecular marker expression in early zebrafish brain development following chronic ethanol or morpholino treatment

This study was undertaken to ascertain whether defined markers of early zebrafish brain development are affected by chronic ethanol exposure or morpholino knockdown of agrin, sonic hedgehog, retinoic acid, and fibroblast growth factors, four signaling molecules that are suggested to be ethanol sensitive. Zebrafish embryos were exposed to 2% ethanol from 6 to 24 hpf or injected with agrin, shha, aldh1a3, or fgf8a morpholinos. In situ hybridization was employed to analyze otx2, pax6a, epha4a, krx20, pax2a, fgf8a, wnt1, and eng2b expression during early brain development. Our results showed that pax6a mRNA expression was decreased in eye, forebrain, and hindbrain of both chronic ethanol exposed and select MO treatments. Epha4a expression in rhombomere R1 boundary was decreased in chronic ethanol exposure and aldh1a3 morphants, lost in fgf8a morphants, but largely unaffected in agrin and shha morphants. Ectopic pax6a and epha4a expression in midbrain was only found in fgf8a morphants. These results suggest that while chronic ethanol induces obvious morphological change in brain architecture, many molecular markers of these brain structures are relatively unaffected by ethanol exposure.

Long-term behavioral change as a result of acute ethanol exposure in zebrafish: Evidence for a role for sonic hedgehog but not retinoic acid signaling

BACKGROUND

Developmental exposure to ethanol is recognized to produce long-term neurobehavioral impairment in multiple animal models. However, the molecular mechanisms underlying these deficits remain poorly understood. The present study was undertaken to ascertain whether two well-characterized targets of prenatal alcohol exposure, sonic hedgehog (Shh) and retinoic acid (RA), that induce the hallmark morphological phenotypes of fetal alcohol spectrum disorders (FASD), are involved in the generation of behavioral alterations as a result of alcohol exposure.

METHODS

Zebrafish embryos were exposed to ethanol (0%, 1%, 3%) at either 8-10 or 24-27h post-fertilization (hpf) and then evaluated during adolescence in the novel tank dive test to assess anxiety and risk-taking behavior. Overt signs of dysmorphogenesis were also scored and behavioral and morphological changes were compared for embryos treated with alcohol alone or in combination with subthreshold doses of shh or alhh1a3 morpholinos (MOs).

RESULTS

Ethanol treated fish displayed altered tank diving behavior that was not exacerbated by combined MO treatment. While treatment of embryos with either shha mRNA or RA prior to ethanol exposure only ameliorated the altered tank diving response in the case of shha mRNA overexpression, dysmorphogenesis was rescued by both treatments.

CONCLUSION

These results suggest that the effects of ethanol exposure on changes in anxiety and risk-taking behavior in adolescent zebrafish is manifested by a blunting of Shh, but not RA, signaling during early development.

Gene-environment interactions in development and disease

Developmental geneticists continue to make substantial jumps in our understanding of the genetic pathways that regulate development. This understanding stems predominantly from analyses of genetically tractable model organisms developing in laboratory environments. This environment is vastly different from that in which human development occurs. As such, most causes of developmental defects in humans are thought to involve multifactorial gene-gene and gene-environment interactions. In this review, we discuss how gene-environment interactions with environmental teratogens may predispose embryos to structural malformations. We elaborate on the growing number of gene-ethanol interactions that might underlie susceptibility to fetal alcohol spectrum disorders. WIREs Dev Biol 2017, 6:e247. doi: 10.1002/wdev.247 For further resources related to this article, please visit the WIREs website.

Fishing for Fetal Alcohol Spectrum Disorders: Zebrafish as a Model for Ethanol Teratogenesis

Fetal Alcohol Spectrum Disorders (FASD) describes a wide array of ethanol-induced developmental defects, including craniofacial dysmorphology and cognitive impairments. It affects ∼1 in 100 children born in the United States each year. Due to the pleiotropic effects of ethanol, animal models have proven critical in characterizing the mechanisms of ethanol teratogenesis. In this review, we focus on the utility of zebrafish in characterizing ethanol-induced developmental defects. A growing number of laboratories have focused on using zebrafish to examine ethanol-induced defects in craniofacial, cardiac, ocular, and neural development, as well as cognitive and behavioral impairments. Growing evidence supports that genetic predisposition plays a role in these ethanol-induced defects, yet little is understood about these gene-ethanol interactions. With a high degree of genetic amenability, zebrafish is at the forefront of identifying and characterizing the gene-ethanol interactions that underlie FASD. Because of the conservation of gene function between zebrafish and humans, these studies will directly translate to studies of candidate genes in human populations and allow for better diagnosis and treatment of FASD.

The Genetics of Fetal Alcohol Spectrum Disorders

The term "fetal alcohol spectrum disorders" (FASD) defines the full range of ethanol (EtOH)-induced birth defects. Numerous variables influence the phenotypic outcomes of embryonic EtOH exposure. Among these variables, genetics appears to play an important role, yet our understanding of the genetic predisposition to FASD is still in its infancy. We review the current literature that relates to the genetics of FASD susceptibility and gene-EtOH interactions. Where possible, we comment on potential mechanisms of reported gene-EtOH interactions. Early indications of genetic sensitivity to FASD came from human and animal studies using twins or inbred strains, respectively. These analyses prompted searches for susceptibility loci involved in EtOH metabolism and analyses of candidate loci, based on phenotypes observed in FASD. More recently, genetic screens in animal models have provided an additional insight into the genetics of FASD. Understanding FASD requires that we understand the many factors influencing phenotypic outcome following embryonic EtOH exposure. We are gaining ground on understanding some of the genetics behind FASD, yet much work remains to be carried out. Coordinated analyses using human patients and animal models are likely to be highly fruitful in uncovering the genetics behind FASD.