Zebrafish raptor mutation inhibits the activity of mTORC1, inducing craniofacial defects due to autophagy-induced neural crest cell death

The mechanistic target of rapamycin (mTOR) coordinates metabolism and cell growth with environmental inputs. mTOR forms two functional complexes: mTORC1 and mTORC2. Proper development requires both complexes but mTORC1 has unique roles in numerous cellular processes, including cell growth, survival and autophagy. Here, we investigate the function of mTORC1 in craniofacial development. We created a zebrafish raptor mutant via CRISPR/Cas9, to specifically disrupt mTORC1. The entire craniofacial skeleton and eyes were reduced in size in mutants; however, overall body length and developmental timing were not affected. The craniofacial phenotype associates with decreased chondrocyte size and increased neural crest cell death. We found that autophagy is elevated in raptor mutants. Chemical inhibition of autophagy reduced cell death and improved craniofacial phenotypes in raptor mutants. Genetic inhibition of autophagy, via mutation of the autophagy gene atg7, improved facial phenotypes in atg7;raptor double mutants, relative to raptor single mutants. We conclude that finely regulated levels of autophagy, via mTORC1, are crucial for craniofacial development.

[WITHDRAWN] Overlapping Targets of Shh and Gata3 during Craniofacial Development

The full text of this preprint has been withdrawn by the authors while they make corrections to the work. Therefore, the authors do not wish this work to be cited as a reference. Questions should be directed to the corresponding author.

Loss of Nicotinamide nucleotide transhydrogenase sensitizes embryos to ethanol-induced neural crest and neural apoptosis via generation of reactive oxygen species

Fetal alcohol spectrum disorders (FASD) are a continuum of birth defects caused by prenatal alcohol exposure. FASD are the most common environmentally induced birth defect and are highly variable. The genetics of an individual influence the severity of their FASD phenotype. However, the genes that sensitize an individual to ethanol-induced birth defects are largely unknown. The ethanol-sensitive mouse substrain, C57/B6J, carries several known mutations including one in Nicotinamide nucleotide transhydrogenase (Nnt). Nnt is a mitochondrial transhydrogenase thought to have an important role in detoxifying reactive oxygen species (ROS) and ROS has been implicated in ethanol teratogenesis. To directly test the role of Nnt in ethanol teratogenesis, we generated zebrafish nnt mutants via CRISPR/Cas9. Zebrafish embryos were dosed with varying concentrations of ethanol across different timepoints and assessed for craniofacial malformations. We utilized a ROS assay to determine if this could be a contributing factor of these malformations. We found that exposed and unexposed mutants had higher levels of ROS compared to their wildtype counterparts. When treated with ethanol, nnt mutants experienced elevated apoptosis in the brain and neural crest, a defect that was rescued by administration of the antioxidant, N-acetyl cysteine (NAC). NAC treatment also rescued most craniofacial malformations. Altogether this research demonstrates that ethanol-induced oxidative stress leads to craniofacial and neural defects due to apoptosis in nnt mutants. This research further supports the growing body of evidence implicating oxidative stress in ethanol teratogenesis. These findings suggest that antioxidants can be used as a potential therapeutic in the treatment of FASD.

Embryonic ethanol exposure disrupts craniofacial neuromuscular integration in zebrafish larvae

Forming a vertebrate head involves the meticulous integration of multiple tissue types during development. Prenatal alcohol exposure is known to cause a variety of birth defects, especially to tissues in the vertebrate head. However, a systematic analysis of coordinated defects across tissues in the head is lacking. Here, we delineate the effects of ethanol on individual tissue types and their integration during craniofacial development. We found that exposure to 1% ethanol induced ectopic cranial muscle and nerve defects with only slight effects on skeletal pattern. Ectopic muscles were, however, unaccompanied by ectopic tendons and could be partially rescued by anesthetizing the larvae before muscle fibers appeared. This finding suggests that the ectopic muscles result from fiber detachment and are not due to an underlying muscle patterning defect. Interestingly, immobilization did not rescue the nerve defects, thus ethanol has an independent effect on each tissue even though they are linked in developmental time and space. Time-course experiments demonstrated an increase in nerve defects with ethanol exposure between 48hpf-4dpf. Time-lapse imaging confirmed the absence of nerve pathfinding or misrouting defects until 48hpf. These results indicate that ethanol-induced nerve defects occur at the time of muscle innervation and after musculoskeletal patterning. Further, we investigated the effect of ethanol on the neuromuscular junctions of the craniofacial muscles and found a reduced number of postsynaptic receptors with no significant effect on the presynaptic terminals. Our study shows that craniofacial soft tissues are particularly susceptible to ethanol-induced damage and that these defects appear independent from one another. Thus, the effects of ethanol on the vertebrate head appear highly pleiotropic.

High-throughput detection of craniofacial defects in fluorescent zebrafish

Losses and malformations of cranial neural crest cell (cNCC) derivatives are a hallmark of several common brain and face malformations. Nevertheless, the etiology of these cNCC defects remains unknown for many cases, suggesting a complex basis involving interactions between genetic and/or environmental factors. However, the sheer number of possible factors (thousands of genes and hundreds of thousands of toxicants) has hindered identification of specific interactions. Here, we develop a high-throughput analysis that will enable faster identification of multifactorial interactions in the genesis of craniofacial defects. Zebrafish embryos expressing a fluorescent marker of cNCCs (fli1:EGFP) were exposed to a pathway inhibitor standard or environmental toxicant, and resulting changes in fluorescence were measured in high-throughput using a fluorescent microplate reader to approximate cNCC losses. Embryos exposed to the environmental Hedgehog pathway inhibitor piperonyl butoxide (PBO), a Hedgehog pathway inhibitor standard, or alcohol (ethanol) exhibited reduced fli1:EGFP fluorescence at one day post fertilization, which corresponded with craniofacial defects at five days post fertilization. Combining PBO and alcohol in a co-exposure paradigm synergistically reduced fluorescence, demonstrating a multifactorial interaction. Using pathway reporter transgenics, we show that the plate reader assay is sensitive at detecting alterations in Hedgehog signaling, a critical regulator of craniofacial development. We go on to demonstrate that this technique readily detects defects in other important cell types, namely neurons. Together, these findings demonstrate this novel in vivo platform can predict developmental abnormalities and multifactorial interactions in high-throughput.

Predicting Modifiers of Genotype-Phenotype Correlations in Craniofacial Development

Most human birth defects are phenotypically variable even when they share a common genetic basis. Our understanding of the mechanisms of this variation is limited, but they are thought to be due to complex gene-environment interactions. Loss of the transcription factor Gata3 associates with the highly variable human birth defects HDR syndrome and microsomia, and can lead to disruption of the neural crest-derived facial skeleton. We have demonstrated that zebrafish gata3 mutants model the variability seen in humans, with genetic background and candidate pathways modifying the resulting phenotype. In this study, we sought to use an unbiased bioinformatic approach to identify environmental modifiers of gata3 mutant craniofacial phenotypes. The LINCs L1000 dataset identifies chemicals that generate differential gene expression that either positively or negatively correlates with an input gene list. These chemicals are predicted to worsen or lessen the mutant phenotype, respectively. We performed RNA-seq on neural crest cells isolated from zebrafish across control, Gata3 loss-of-function, and Gata3 rescue groups. Differential expression analyses revealed 551 potential targets of gata3. We queried the LINCs database with the 100 most upregulated and 100 most downregulated genes. We tested the top eight available chemicals predicted to worsen the mutant phenotype and the top eight predicted to lessen the phenotype. Of these, we found that vinblastine, a microtubule inhibitor, and clofibric acid, a PPAR-alpha agonist, did indeed worsen the gata3 phenotype. The Topoisomerase II and RNA-pol II inhibitors daunorubicin and triptolide, respectively, lessened the phenotype. GO analysis identified Wnt signaling and RNA polymerase function as being enriched in our RNA-seq data, consistent with the mechanism of action of some of the chemicals. Our study illustrates multiple potential pathways for Gata3 function, and demonstrates a systematic, unbiased process to identify modifiers of genotype-phenotype correlations.

Gene-alcohol interactions in birth defects

Most human birth defects are thought to result from complex interactions between combinations of genetic and environmental factors. This is true even for conditions that, at face value, may appear simple and straightforward, like fetal alcohol spectrum disorders (FASD). FASD describe the full range of structural and neurological disruptions that result from prenatal alcohol exposure. While FASD require alcohol exposure, evidence from human and animal model studies demonstrate that additional genetic and/or environmental factors can influence the embryo's susceptibility to alcohol. Only a limited number of alcohol interactions in birth defects have been identified, with many sensitizing genetic and environmental factors likely yet to be identified. Because of this, while unsatisfying, there is no definitively "safe" dose of alcohol for all pregnancies. Determining these other factors, as well as mechanistically characterizing known interactions, is critical for better understanding and preventing FASD and requires combined scrutiny of human and model organism studies.

Embryonic Nicotine Exposure Disrupts Adult Social Behavior and Craniofacial Development in Zebrafish

Cigarette smoking remains the leading cause of preventable death and morbidity worldwide. Smoking during pregnancy is associated with numerous adverse birth outcomes, including craniofacial and behavioral abnormalities. Although tobacco smoke contains more than 4000 toxic substances, nicotine is addictive and is likely the most teratogenic substance in cigarette smoke. However, much remains to be determined about the effects of embryonic nicotine exposure on behavior and craniofacial development. Therefore, this study evaluated adult social behavior in zebrafish, craniofacial defects, and nicotine metabolism in embryos after embryonic nicotine exposure. Zebrafish embryos were exposed to different doses of nicotine beginning at 6 h post fertilization. To evaluate craniofacial defects, the embryos were collected at 4 days post fertilization and stained with Alizarin Red and Alcian Blue. For behavioral testing, embryos were reared to adulthood. To evaluate nicotine metabolism, cotinine levels were analyzed at various time points. Our findings demonstrate that embryonic exposure to nicotine modifies social behavior in adulthood, causes craniofacial defects with reduced size of craniofacial cartilages, and that zebrafish metabolize nicotine to cotinine, as in humans. Together, our data suggest that zebrafish are useful as a model for studying nicotine-related diseases.

Divergent cis-regulatory evolution underlies the convergent loss of sodium channel expression in electric fish

South American and African weakly electric fish independently evolved electric organs from muscle. In both groups, a voltage-gated sodium channel gene independently lost expression from muscle and gained it in the electric organ, allowing the channel to become specialized for generating electric signals. It is unknown how this voltage-gated sodium channel gene is targeted to muscle in any vertebrate. We describe an enhancer that selectively targets sodium channel expression to muscle. Next, we demonstrate how the loss of this enhancer, but not trans-activating factors, drove the loss of sodium channel gene expression from muscle in South American electric fish. While this enhancer is also altered in African electric fish, key transcription factor binding sites and enhancer activity are retained, suggesting that the convergent loss of sodium channel expression from muscle in these two electric fish lineages occurred via different processes.

Loss of tumor protein 53 protects against alcohol-induced facial malformations in mice and zebrafish

BACKGROUND

Alcohol exposure during the gastrulation stage of development causes the craniofacial and brain malformations that define fetal alcohol syndrome. These malformations, such as a deficient philtrum, are exemplified by a loss of midline tissue and correspond, at least in part, to regionally selective cell death in the embryo. The tumor suppressor protein Tp53 is an important mechanism for cell death, but the role of Tp53 in the consequences of alcohol exposure during the gastrulation stage has yet to be examined. The current studies used mice and zebrafish to test whether genetic loss of Tp53 is a conserved mechanism to protect against the effects of early developmental stage alcohol exposure.

METHODS

Female mice, heterozygous for a mutation in the Tp53 gene, were mated with Tp53 heterozygous males, and the resulting embryos were exposed during gastrulation on gestational day 7 (GD 7) to alcohol (two maternal injections of 2.9 g/kg, i.p., 4 h apart) or a vehicle control. Zebrafish mutants or heterozygotes for the tp53^zdf1  M214K mutation and their wild-type controls were exposed to alcohol (1.5% or 2%) beginning 6 h postfertilization (hpf), the onset of gastrulation.

RESULTS

Examination of GD 17 mice revealed that eye defects were the most common phenotype among alcohol-exposed fetuses, occurring in nearly 75% of the alcohol-exposed wild-type fetuses. Tp53 gene deletion reduced the incidence of eye defects in both the heterozygous and mutant fetuses (to about 35% and 20% of fetuses, respectively) and completely protected against alcohol-induced facial malformations. Zebrafish (4 days postfertilization) also demonstrated alcohol-induced reductions of eye size and trabeculae length that were less common and less severe in tp53 mutants, indicating a protective effect of tp53 deletion.

CONCLUSIONS

These results identify an evolutionarily conserved role of Tp53 as a pathogenic mechanism for alcohol-induced teratogenesis.

Exposure to ethanol leads to midfacial hypoplasia in a zebrafish model of FASD via indirect interactions with the Shh pathway

Background

Gene-environment interactions are likely to underlie most human birth defects. The most common known environmental contributor to birth defects is prenatal alcohol exposure. Fetal alcohol spectrum disorders (FASD) describe the full range of defects that result from prenatal alcohol exposure. Gene-ethanol interactions underlie susceptibility to FASD, but we lack a mechanistic understanding of these interactions. Here, we leverage the genetic tractability of zebrafish to address this problem.

Results

We first show that vangl2, a member of the Wnt/planar cell polarity (Wnt/PCP) pathway that mediates convergent extension movements, strongly interacts with ethanol during late blastula and early gastrula stages. Embryos mutant or heterozygous for vangl2 are sensitized to ethanol-induced midfacial hypoplasia. We performed single-embryo RNA-seq during early embryonic stages to assess individual variation in the transcriptional response to ethanol and determine the mechanism of the vangl2-ethanol interaction. To identify the pathway(s) that are disrupted by ethanol, we used these global changes in gene expression to identify small molecules that mimic the effects of ethanol via the Library of Integrated Network-based Cellular Signatures (LINCS L1000) dataset. Surprisingly, this dataset predicted that the Sonic Hedgehog (Shh) pathway inhibitor, cyclopamine, would mimic the effects of ethanol, despite ethanol not altering the expression levels of direct targets of Shh signaling. Indeed, we found that ethanol and cyclopamine strongly, but indirectly, interact to disrupt midfacial development. Ethanol also interacts with another Wnt/PCP pathway member, gpc4, and a chemical inhibitor of the Wnt/PCP pathway, blebbistatin, phenocopies the effect of ethanol. By characterizing membrane protrusions, we demonstrate that ethanol synergistically interacts with the loss of vangl2 to disrupt cell polarity required for convergent extension movements.

Conclusions

Our results show that the midfacial defects in ethanol-exposed vangl2 mutants are likely due to an indirect interaction between ethanol and the Shh pathway. Vangl2 functions as part of a signaling pathway that regulates coordinated cell movements during midfacial development. Ethanol exposure alters the position of a critical source of Shh signaling that separates the developing eye field into bilateral eyes, allowing the expansion of the midface. Collectively, our results shed light on the mechanism by which the most common teratogen can disrupt development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01062-9.

Variation in phenotypes from a Bmp-Gata3 genetic pathway is modulated by Shh signaling

We sought to understand how perturbation of signaling pathways and their targets generates variable phenotypes. In humans, GATA3 associates with highly variable defects, such as HDR syndrome, microsomia and choanal atresia. We previously characterized a zebrafish point mutation in gata3 with highly variable craniofacial defects to the posterior palate. This variability could be due to residual Gata3 function, however, we observe the same phenotypic variability in gata3 null mutants. Using hsp:GATA3-GFP transgenics, we demonstrate that Gata3 function is required between 24 and 30 hpf. At this time maxillary neural crest cells fated to generate the palate express gata3. Transplantation experiments show that neural crest cells require Gata3 function for palatal development. Via a candidate approach, we determined if Bmp signaling was upstream of gata3 and if this pathway explained the mutant's phenotypic variation. Using BRE:d2EGFP transgenics, we demonstrate that maxillary neural crest cells are Bmp responsive by 24 hpf. We find that gata3 expression in maxillary neural crest requires Bmp signaling and that blocking Bmp signaling, in hsp:DN-Bmpr1a-GFP embryos, can phenocopy gata3 mutants. Palatal defects are rescued in hsp:DN-Bmpr1a-GFP;hsp:GATA3-GFP double transgenic embryos, collectively demonstrating that gata3 is downstream of Bmp signaling. However, Bmp attenuation does not alter phenotypic variability in gata3 loss-of-function embryos, implicating a different pathway. Due to phenotypes observed in hypomorphic shha mutants, the Sonic Hedgehog (Shh) pathway was a promising candidate for this pathway. Small molecule activators and inhibitors of the Shh pathway lessen and exacerbate, respectively, the phenotypic severity of gata3 mutants. Importantly, inhibition of Shh can cause gata3 haploinsufficiency, as observed in humans. We find that gata3 mutants in a less expressive genetic background have a compensatory upregulation of Shh signaling. These results demonstrate that the level of Shh signaling can modulate the phenotypes observed in gata3 mutants.

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.

Visible Light Mediated Bidirectional Control over Carbonic Anhydrase Activity in Cells and in Vivo Using Azobenzenesulfonamides

Two azobenzenesulfonamide molecules with thermally stable cis configurations resulting from fluorination of positions ortho to the azo group are reported that can differentially regulate the activity of carbonic anhydrase in the trans and cis configurations. These fluorinated probes each use two distinct visible wavelengths (520 and 410 or 460 nm) for isomerization with high photoconversion efficiency. Correspondingly, the cis isomer of these systems is highly stable and persistent (as evidenced by structural studies in solid and solution state), permitting regulation of metalloenzyme activity without continuous irradiation. Herein, we use these probes to demonstrate the visible light mediated bidirectional control over the activity of zinc-dependent carbonic anhydrase in solution as an isolated protein, in intact live cells and in vivo in zebrafish during embryo development.

Novel Ethanol-Sensitive Mutants Identified in an F3 Forward Genetic Screen

Background

Fetal alcohol spectrum disorders (FASD) collectively refer to all deleterious outcomes due to prenatal alcohol exposures. Alterations to the face are common phenotypes in FASD. While alcohol exposure is the underlying cause of FASD, many variables modify the outcomes of such exposures. Genetic risk is one such variable, yet we still have a limited understanding of the nature of the genetic loci mediating susceptibility to FASD.

Methods

We employed ENU‐based random mutagenesis in zebrafish to identify mutations that enhanced the teratogenicity of ethanol (EtOH). F3 embryos obtained from 126 inbred F2 families were exposed to 1% EtOH in the medium (approximately 41 mM tissue levels). Zebrafish stained with Alcian Blue and Alizarin Red were screened for qualitative alterations to the craniofacial skeleton between 4 and 7 days postfertilization (dpf).

Results

In all, we recovered 6 EtOH‐sensitive mutants, 5 from the genetic screen itself and one as a background mutation in one of our wild‐type lines. Each mutant has a unique EtOH‐induced phenotype relative to the other mutant lines. All but 1 mutation appears to be recessive in nature, and only 1 mutant, au29, has apparent craniofacial defects in the absence of EtOH. To validate the genetic screen, we genetically mapped au29 and found that it carries a mutation in a previously uncharacterized gene, si:dkey‐88l16.3.

Conclusions

The phenotypes of these EtOH‐sensitive mutants differ from those in previous characterizations of gene–EtOH interactions. Thus, each mutant is likely to provide novel insights into EtOH teratogenesis. Given that most of these mutants only have craniofacial defects in the presence of EtOH and our mapping of au29, it is also likely that many of the mutants will be previously uncharacterized. Collectively, our findings point to the importance of unbiased genetic screens in the identification, and eventual characterization, of risk alleles for FASD.

Social deficits following embryonic ethanol exposure arise in post-larval zebrafish

Prenatal alcohol exposure is the leading cause of birth defects, collectively termed fetal alcohol spectrum disorders (FASD). In the United States and Canada, 1 in 100 children will be born with FASD. Some of the most commonly debilitating defects of FASD are in social behavior. Zebrafish are highly social animals, and embryonic ethanol exposure from 24 to 26 hours post-fertilization disrupts this social (shoaling) response in adult zebrafish. Recent findings have suggested that social behaviors are present in zebrafish larvae as young as 3 weeks, but how they relate to adult shoaling is unclear. We tested the same ethanol-exposed zebrafish for social impairments at 3 weeks then again at 16 weeks. At both ages, live conspecifics were used to elicit a social response. We did not find alcohol-induced differences in behavior in 3-week-old fish when they were able to see conspecifics. We do find evidence that control zebrafish are able to use nonvisual stimuli to detect conspecifics, and this behavior is disrupted in the alcohol-exposed fish. As adults, these fish displayed a significant decrease in social behavior when conspecifics are visible. This surprising finding demonstrates that the adult and larval social behaviors are, at least partly, separable. Future work will investigate the nature of these nonvisual cues and how the neurocircuitry differs between the larval and adult social behaviors.

Differentially sensitive neuronal subpopulations in the central nervous system and the formation of hindbrain heterotopias in ethanol-exposed zebrafish

BACKGROUND

A cardinal feature of prenatal ethanol exposure is CNS damage, resulting in a continuum of neurological and behavioral impairments that are described by the term fetal alcohol spectrum disorders (FASD). FASDs are variable and depend on several factors, including the amount, timing, and duration of prenatal ethanol exposure. To enhance interventions for CNS dysfunction, it is necessary to identify ethanol-sensitive neuronal populations and expand the understanding of factors that modify ethanol teratogenesis.

METHODS

To investigate the susceptibility of different neuronal subtypes, we exposed transgenic zebrafish (Danio rerio) to several ethanol concentrations (0.25, 0.5, 1.0, 1.5, or 2.0%), at different hours post fertilization (hpf; 0, 6, or 24 hpf), for various durations (0-24, 0-48, 4-24, 6-24, 6-48,or 24-48 hpf). Following exposure, embryo survival rates were determined, and CNS neurogenesis, differentiation, and patterning were assessed.

RESULTS

Embryo survival rates decrease as ethanol concentrations increase and drastically decline when exposed from 0-24 hpf compared to 4-24 hpf. Abnormal tangential migration of facial motor neurons is observed in isl1:gfp embryos exposed to ethanol concentrations as low as 0.25%, and the formation of IVth ventricle heterotopias are revealed by embryos exposed to ≥1.0% ethanol. Whereas, expression of olig2:dsred and ptf1a:gfp in the cerebellum and spinal cord are largely unaffected. While levels of etv4 mRNA are overtly resistant to ethanol, we observe significant reductions in ptch2 mRNA levels.

CONCLUSIONS

These data show differentially sensitive CNS neuron subpopulations with susceptibility to low levels of ethanol. In addition, these data reveal the formation of ethanol-induced hindbrain heterotopias.

Social behavioral phenotyping of the zebrafish casper mutant following embryonic alcohol exposure

The term Fetal Alcohol Spectrum Disorder (FASD) describes all the deleterious consequences of prenatal alcohol exposure. Impaired social behavior is a common symptom of FASD. The zebrafish has emerged as a powerful model organism with which to examine the effects of embryonic alcohol exposure on social behavior due to an innate strong behavior, called shoaling. The relative transparency of the embryo also makes zebrafish powerful for cellular analyses, such as characterizing neural circuitry. However, as zebrafish develop, pigmentation begins to obscure the brain and other tissues. Due to mutations disrupting pigmentation, the casper zebrafish strain remains relatively transparent throughout adulthood, potentially permitting researchers to image neural circuits in vivo, via epifluorescence, confocal and light sheet microscopy. Currently, however the behavioral profile of casper zebrafish post embryonic alcohol exposure has not been completed. We report that exposure to 1% alcohol from either 6 to 24, or 24 to 26 h postfertilization reduces the social behavior of adult casper zebrafish. Our findings set the stage for the use of this important zebrafish resource in studies of FASD.

PDGFRA gene, maternal binge drinking and obstructive heart defects

Obstructive heart defects (OHDs) are a major health concern worldwide. The platelet-derived growth factor (PDGF) genes are known to have regulatory functions that are essential for proper heart development. In a zebrafish model, Pdgfra was further demonstrated to interact with ethanol during craniofacial development. In this article, we investigated interactions between variants in PDGF genes and periconceptional alcohol exposure on the risk of OHDs by applying log-linear models to 806 OHD case and 995 control families enrolled in the National Birth Defects Prevention Study. The interactions between four variants in PDGFA and maternal binge drinking reached a nominal significance level. The maternal T allele of rs869978 was estimated to increase OHD risk among women who binge drink, while infant genotypes of rs2291591, rs2228230, rs1547904, and rs869978 may reduce the risk. Although none of these associations remain statistically significant after multiple testing adjustment and the estimated maternal effect may be influenced by unknown confounding factors, such as maternal smoking, these findings are consistent with previous animal studies supporting potential interactions between the PDGFRA gene and maternal alcohol exposure. Replication studies with larger sample sizes are needed to further elucidate this potential interplay and its influence on OHD risks.

In vivo zebrafish morphogenesis shows Cyp26b1 promotes tendon condensation and musculoskeletal patterning in the embryonic jaw

Integrated development of diverse tissues gives rise to a functional, mobile vertebrate musculoskeletal system. However, the genetics and cellular interactions that drive the integration of muscle, tendon, and skeleton are poorly understood. In the vertebrate head, neural crest cells, from which cranial tendons derive, pattern developing muscles just as tendons have been shown to in limb and trunk tissue, yet the mechanisms of this patterning are unknown. From a forward genetic screen, we determined that cyp26b1 is critical for musculoskeletal integration in the ventral pharyngeal arches, particularly in the mandibulohyoid junction where first and second arch muscles interconnect. Using time-lapse confocal analyses, we detail musculoskeletal integration in wild-type and cyp26b1 mutant zebrafish. In wild-type fish, tenoblasts are present in apposition to elongating muscles and condense in discrete muscle attachment sites. In the absence of cyp26b1, tenoblasts are generated in normal numbers but fail to condense into nascent tendons within the ventral arches and, subsequently, muscles project into ectopic locales. These ectopic muscle fibers eventually associate with ectopic tendon marker expression. Genetic mosaic analysis demonstrates that neural crest cells require Cyp26b1 function for proper musculoskeletal development. Using an inhibitor, we find that Cyp26 function is required in a short time window that overlaps the dynamic window of tenoblast condensation. However, cyp26b1 expression is largely restricted to regions between tenoblast condensations during this time. Our results suggest that degradation of RA by this previously undescribed population of neural crest cells is critical to promote condensation of adjacent scxa-expressing tenoblasts and that these condensations are subsequently required for proper musculoskeletal integration.

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.

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.

Bmp signaling mediates endoderm pouch morphogenesis by regulating Fgf signaling in zebrafish

The endodermal pouches are a series of reiterated structures that segment the pharyngeal arches and help pattern the vertebrate face. Multiple pathways regulate the complex process of endodermal development, including the Bone morphogenetic protein (Bmp) pathway. However, the role of Bmp signaling in pouch morphogenesis is poorly understood. Using genetic and chemical inhibitor approaches, we show that pouch morphogenesis requires Bmp signaling from 10-18 h post-fertilization, immediately following gastrulation. Blocking Bmp signaling during this window results in morphological defects to the pouches and craniofacial skeleton. Using genetic chimeras we show that Bmp signals directly to the endoderm for proper morphogenesis. Time-lapse imaging and analysis of reporter transgenics show that Bmp signaling is necessary for pouch outpocketing via the Fibroblast growth factor (Fgf) pathway. Double loss-of-function analyses demonstrate that Bmp and Fgf signaling interact synergistically in craniofacial development. Collectively, our analyses shed light on the tissue and signaling interactions that regulate development of the vertebrate face.

An Fgf-Shh signaling hierarchy regulates early specification of the zebrafish skull

The neurocranium generates most of the craniofacial skeleton and consists of prechordal and postchordal regions. Although development of the prechordal is well studied, little is known of the postchordal region. Here we characterize a signaling hierarchy necessary for postchordal neurocranial development involving Fibroblast growth factor (Fgf) signaling for early specification of mesodermally-derived progenitor cells. The expression of hyaluron synthetase 2 (has2) in the cephalic mesoderm requires Fgf signaling and Has2 function, in turn, is required for postchordal neurocranial development. While Hedgehog (Hh)-deficient embryos also lack a postchordal neurocranium, this appears primarily due to a later defect in chondrocyte differentiation. Inhibitor studies demonstrate that postchordal neurocranial development requires early Fgf and later Hh signaling. Collectively, our results provide a mechanistic understanding of early postchordal neurocranial development and demonstrate a hierarchy of signaling between Fgf and Hh in the development of this structure.

Pdgfra and Pdgfrb genetically interact during craniofacial development

BACKGROUND

One of the most prevalent congenital birth defects is cleft palate. The palatal skeleton is derived from the cranial neural crest and platelet-derived growth factors (Pdgf) are critical in palatogenesis. Of the two Pdgf receptors, pdgfra is required for neural crest migration and palatogenesis. However, the role pdgfrb plays in the neural crest, or whether pdgfra and pdgfrb interact during palatogenesis is unclear.

RESULTS

We find that pdgfrb is dispensable for craniofacial development in zebrafish. However, the palatal defect in pdgfra;pdgfrb double mutants is significantly more severe than in pdgfra single mutants. Data in mouse suggest this interaction is conserved and that neural crest requires both genes. In zebrafish, pdgfra and pdgfrb are both expressed by neural crest within the pharyngeal arches, and pharmacological analyses demonstrate Pdgf signaling is required at these times. While neither proliferation nor cell death appears affected, time-lapsed confocal analysis of pdgfra;pdgfrb mutants shows a failure of proper neural crest condensation during palatogenesis.

CONCLUSIONS

We provide data showing that pdgfra and pdgfrb interact during palatogenesis in both zebrafish and mouse. In zebrafish, this interaction affects proper condensation of maxillary neural crest cells, revealing a previously unknown interaction between Pdgfra and Pdgfrb during palate formation. Developmental Dynamics 245:641-652, 2016. © 2016 Wiley Periodicals, Inc.

Commentary: catching a conserved mechanism of ethanol teratogenicity

BACKGROUND

Due to its profound impact on human development, ethanol (EtOH) teratogenicity is a field of intense study. The complexity of variables that influence the outcomes of embryonic or prenatal EtOH exposure compels the use of animal models in which these variables can be isolated.

METHODS

Numerous model systems have been used in these studies. The zebrafish is a powerful model system, which has seen a recent increase in usage for EtOH studies.

RESULTS

Those using zebrafish for alcohol studies often face 2 questions: (i) How physiologically relevant are the doses of EtOH administered to zebrafish embryos? and (ii) Will the mechanisms of EtOH teratogenesis be conserved to other model systems and human?

CONCLUSIONS

The current article by Flentke and colleagues () helps to shed important light on these questions and clearly demonstrates that the zebrafish will be a valuable model system with which to understand EtOH teratogenicity.

Developmental age strengthens barriers to ethanol accumulation in zebrafish

Fetal Alcohol Spectrum Disorders (FASD) describes a wide range of phenotypic defects affecting facial and neurological development associated with ethanol teratogenicity. It affects approximately 1 in 100 children born in the United States each year. Genetic predisposition along with timing and dosage of ethanol exposure are critical in understanding the prevalence and variability of FASD. The zebrafish attributes of external fertilization, genetic tractability, and high fecundity make it a powerful tool for FASD studies. However, a lack of consensus of ethanol treatment paradigms has limited the interpretation of these various studies. Here we address this concern by examining ethanol tissue concentrations across timing and genetic background. We utilize headspace gas chromatography to determine ethanol concentration in the AB, fli1:EGFP, and Tu backgrounds. In addition, we treated these embryos with ethanol over two different developmental time windows, 6-24 h post fertilization (hpf) and 24-48 hpf. Our analysis demonstrates that embryos rapidly equilibrate to a sub-media level of ethanol. Embryos then maintain this level of ethanol for the duration of exposure. The ethanol tissue concentration level is independent of genetic background, but is timing-dependent. Embryos exposed from 6 to 24 hpf were 2.7-4.2-fold lower than media levels, while embryos were 5.7-6.2-fold lower at 48 hpf. This suggests that embryos strengthen one or more barriers to ethanol as they develop. In addition, both the embryo and, to a lesser extent, the chorion, surrounding the embryo are barriers to ethanol. Overall, this work will help tighten ethanol treatment regimens and strengthen zebrafish as a model of FASD.

A visualizable chain-terminating inhibitor of glycosaminoglycan biosynthesis in developing zebrafish

Heparan sulfate (HS) and chondroitin sulfate (CS) glycosaminoglycans (GAG) are proteoglycan-associated polysaccharides with essential functions in animals. They have been studied extensively by genetic manipulation of biosynthetic enzymes, but chemical tools for probing GAG function are limited. HS and CS possess a conserved xylose residue that links the polysaccharide chain to a protein backbone. Here we report that, in zebrafish embryos, the peptide-proximal xylose residue can be metabolically replaced with a chain-terminating 4-azido-4-deoxyxylose (4-XylAz) residue by administration of UDP-4-azido-4-deoxyxylose (UDP-4-XylAz). UDP-4-XylAz disrupted both HS and CS biosynthesis and caused developmental abnormalities reminiscent of GAG biosynthesis and laminin mutants. The azide substituent of protein-bound 4-XylAz allowed for rapid visualization of the organismal sites of chain termination in vivo through bioorthogonal reaction with fluorescent cyclooctyne probes. UDP-4-XylAz therefore complements genetic tools for studies of GAG function in zebrafish embryogenesis.

Analyzing craniofacial morphogenesis in zebrafish using 4D confocal microscopy

Time-lapse imaging is a technique that allows for the direct observation of the process of morphogenesis, or the generation of shape. Due to their optical clarity and amenability to genetic manipulation, the zebrafish embryo has become a popular model organism with which to perform time-lapse analysis of morphogenesis in living embryos. Confocal imaging of a live zebrafish embryo requires that a tissue of interest is persistently labeled with a fluorescent marker, such as a transgene or injected dye. The process demands that the embryo is anesthetized and held in place in such a way that healthy development proceeds normally. Parameters for imaging must be set to account for three-dimensional growth and to balance the demands of resolving individual cells while getting quick snapshots of development. Our results demonstrate the ability to perform long-term in vivo imaging of fluorescence-labeled zebrafish embryos and to detect varied tissue behaviors in the cranial neural crest that cause craniofacial abnormalities. Developmental delays caused by anesthesia and mounting are minimal, and embryos are unharmed by the process. Time-lapse imaged embryos can be returned to liquid medium and subsequently imaged or fixed at later points in development. With an increasing abundance of transgenic zebrafish lines and well-characterized fate mapping and transplantation techniques, imaging any desired tissue is possible. As such, time-lapse in vivo imaging combines powerfully with zebrafish genetic methods, including analyses of mutant and microinjected embryos.

A screen of zebrafish mutants identifies ethanol-sensitive genetic loci

BACKGROUND

Fetal alcohol spectrum disorders (FASD) are a highly variable set of phenotypes caused by fetal alcohol exposure. Numerous factors influence FASD phenotypes, including genetics. The zebrafish is a powerful vertebrate model system with which to identify these genetic factors. Many zebrafish mutants are housed at the Zebrafish International Resource Center (ZIRC). These mutants are readily accessible and an excellent source to screen for ethanol (EtOH)-sensitive developmental structural mutants.

METHODS

We screened mutants obtained from ZIRC for sensitivity to EtOH teratogenesis. Embryos were treated with 1% EtOH (41 mM tissue levels) from 6 hours postfertilization onward. Levels of apoptosis were evaluated at 24 hours postfertilization. At 4 days postfertilization, the craniofacial skeleton, peripheral axon projections, and sensory neurons of neuromasts were examined. Fish were genotyped to determine whether there were phenotype/genotype correlations.

RESULTS

Five of 20 loci interacted with EtOH. Notable among these was that vangl2, involved in convergent extension movements of the embryonic axis, interacted strongly with EtOH. Untreated vangl2 mutants had normal craniofacial morphology, while severe midfacial defects including synophthalmia and narrowing of the palatal skeleton were found in all EtOH-treated mutants and a low percentage of heterozygotes. The cell cycle gene, plk1, also interacted strongly with EtOH. Untreated mutants have slightly elevated levels of apoptosis and loss of ventral craniofacial elements. Exposure to EtOH results in extensive apoptosis along with loss of neural tissue and the entire craniofacial skeleton. Phenotypes of hinfp, mars, and foxi1 mutants were also exacerbated by EtOH.

CONCLUSIONS

Our results provide insight into the gene-EtOH interactions that may underlie EtOH teratogenesis. They support previous findings that EtOH disrupts elongation of the embryonic axis. Importantly, these results show that the zebrafish is an efficient model with which to test for gene-EtOH interactions. Understanding these interactions will be crucial to understanding of the FASD variation.

Pdgfra protects against ethanol-induced craniofacial defects in a zebrafish model of FASD

Human birth defects are highly variable and this phenotypic variability can be influenced by both the environment and genetics. However, the synergistic interactions between these two variables are not well understood. Fetal alcohol spectrum disorders (FASD) is the umbrella term used to describe the wide range of deleterious outcomes following prenatal alcohol exposure. Although FASD are caused by prenatal ethanol exposure, FASD are thought to be genetically modulated, although the genes regulating sensitivity to ethanol teratogenesis are largely unknown. To identify potential ethanol-sensitive genes, we tested five known craniofacial mutants for ethanol sensitivity: cyp26b1, gata3, pdgfra, smad5 and smoothened. We found that only platelet-derived growth factor receptor alpha (pdgfra) interacted with ethanol during zebrafish craniofacial development. Analysis of the PDGF family in a human FASD genome-wide dataset links PDGFRA to craniofacial phenotypes in FASD, prompting a mechanistic understanding of this interaction. In zebrafish, untreated pdgfra mutants have cleft palate due to defective neural crest cell migration, whereas pdgfra heterozygotes develop normally. Ethanol-exposed pdgfra mutants have profound craniofacial defects that include the loss of the palatal skeleton and hypoplasia of the pharyngeal skeleton. Furthermore, ethanol treatment revealed latent haploinsufficiency, causing palatal defects in ∼62% of pdgfra heterozygotes. Neural crest apoptosis partially underlies these ethanol-induced defects in pdgfra mutants, demonstrating a protective role for Pdgfra. This protective role is mediated by the PI3K/mTOR pathway. Collectively, our results suggest a model where combined genetic and environmental inhibition of PI3K/mTOR signaling leads to variability within FASD.

Ahsa1 and Hsp90 activity confers more severe craniofacial phenotypes in a zebrafish model of hypoparathyroidism, sensorineural deafness and renal dysplasia (HDR)

The severity of most human birth defects is highly variable. Our ability to diagnose, treat and prevent defects relies on our understanding of this variability. Mutation of the transcription factor GATA3 in humans causes the highly variable hypoparathyroidism, sensorineural deafness and renal dysplasia (HDR) syndrome. Although named for a triad of defects, individuals with HDR can also exhibit craniofacial defects. Through a forward genetic screen for craniofacial mutants, we isolated a zebrafish mutant in which the first cysteine of the second zinc finger of Gata3 is mutated. Because mutation of the homologous cysteine causes HDR in humans, these zebrafish mutants could be a quick and effective animal model for understanding the role of gata3 in the HDR disease spectrum. We demonstrate that, unexpectedly, the chaperone proteins Ahsa1 and Hsp90 promote severe craniofacial phenotypes in our zebrafish model of HDR syndrome. The strengths of the zebrafish system, including rapid development, genetic tractability and live imaging, make this an important model for variability.

Bmp and Shh signaling mediate the expression of satb2 in the pharyngeal arches

In human, mutation of the transcription factor SATB2 causes severe defects to the palate and jaw. The expression and sequence of SATB2 is highly conserved across vertebrate species, including zebrafish. We sought to understand the regulation of satb2 using the zebrafish model system. Due to the normal expression domains of satb2, we analyzed satb2 expression in mutants with disrupted Hh signaling or defective ventral patterning. While satb2 expression appears independent of Edn1 signaling, appropriate expression requires Shha, Smo, Smad5 and Hand2 function. Transplantation experiments show that neural crest cells receive both Bmp and Hh signaling to induce satb2 expression. Dorsomorphin- and cyclopamine-mediated inhibition of Bmp and Hh signaling, respectively, suggests that proper satb2 expression requires a relatively earlier Bmp signal and a later Hh signal. We propose that Bmp signaling establishes competence for the neural crest to respond to Hh signaling, thus inducing satb2 expression.

Hh signaling regulates patterning and morphogenesis of the pharyngeal arch-derived skeleton

The proper function of the craniofacial skeleton requires the proper shaping of many individual skeletal elements. Neural crest cells generate much of the craniofacial skeleton and morphogenesis of skeletal elements occurs in transient, reiterated structures termed pharyngeal arches. The shape of individual elements depends upon intrinsic patterning within the neural crest as well as extrinsic signals to the neural crest from adjacent tissues within the arches. Hedgehog (Hh) signaling is known to play roles in craniofacial development, yet its involvement in intrinsic and extrinsic patterning of the craniofacial skeleton is still not well understood. Here, we show that morphogenetic movements of the pharyngeal arches and patterning of the neural crest require Hh signaling. Loss of Hh signaling, in smoothened (smo) mutants, disrupts the expression of some Dlx genes as well as other markers of dorsal/ventral patterning of the neural crest. Transplantation of wild-type neural crest cells into smo mutants rescues this defect, demonstrating that the neural crest requires reception of Hh signals for proper patterning. Despite the rescue, morphogenesis of the facial skeleton is not fully recovered. Through transplant analyses, we find two additional requirements for Hh signaling. The endoderm requires the reception of Hh signals for proper morphogenetic movements of the pharyngeal arches and the neural crest require the reception of Hh signaling for the activity of a reverse signal that maintains sonic hedgehog expression in the endoderm. Collectively, these results demonstrate that Hh signaling is essential to establish intrinsic and extrinsic patterning information for the craniofacial skeleton.

Examination of a palatogenic gene program in zebrafish

Human palatal clefting is debilitating and difficult to rectify surgically. Animal models enhance our understanding of palatogenesis and are essential in strategies designed to ameliorate palatal malformations in humans. Recent studies have shown that the zebrafish palate, or anterior neurocranium, is under similar genetic control to the amniote palatal skeleton. We extensively analyzed palatogenesis in zebrafish to determine the similarity of gene expression and function across vertebrates. By 36 hours postfertilization (hpf) palatogenic cranial neural crest cells reside in homologous regions of the developing face compared with amniote species. Transcription factors and signaling molecules regulating mouse palatogenesis are expressed in similar domains during palatogenesis in zebrafish. Functional investigation of a subset of these genes, fgf10a, tgfb2, pax9, and smad5 revealed their necessity in zebrafish palatogenesis. Collectively, these results suggest that the gene regulatory networks regulating palatogenesis may be conserved across vertebrate species, demonstrating the utility of zebrafish as a model for palatogenesis.

miR-196 regulates axial patterning and pectoral appendage initiation

Vertebrate Hox clusters contain protein-coding genes that regulate body axis development and microRNA (miRNA) genes whose functions are not yet well understood. We overexpressed the Hox cluster microRNA miR-196 in zebrafish embryos and found four specific, viable phenotypes: failure of pectoral fin bud initiation, deletion of the 6th pharyngeal arch, homeotic aberration and loss of rostral vertebrae, and reduced number of ribs and somites. Reciprocally, miR-196 knockdown evoked an extra pharyngeal arch, extra ribs, and extra somites, confirming endogenous roles of miR-196. miR-196 injection altered expression of hox genes and the signaling of retinoic acid through the retinoic acid receptor gene rarab. Knocking down rarab mimicked the pectoral fin phenotype of miR-196 overexpression, and reporter constructs tested in tissue culture and in embryos showed that the rarab 3'UTR is a miR-196 target for pectoral fin bud initiation. These results show that a Hox cluster microRNA modulates development of axial patterning similar to nearby protein-coding Hox genes, and acts on appendicular patterning at least in part by modulating retinoic acid signaling.

A cross-species analysis of Satb2 expression suggests deep conservation across vertebrate lineages

Mutation of SATB2 causes cleft palate in humans. To understand the role of SATB2 function in palatogenesis, SATB2 analyses in vertebrate model systems will be essential. To facilitate these analyses, we have performed a cross-species comparison of SATB2 structure and function across three vertebrate model systems: mouse, chick, and zebrafish. We find that the SATB2 transcript is highly conserved across human, mouse, chick, and zebrafish, especially within the Satb2 functional domains. Furthermore, our expression analyses demonstrate that SATB2 is likely to have similar functions in vertebrate model organisms and humans during development of the facial processes and secondary palate. Together, these data suggest an evolutionary conserved role for SATB2 during development of the face and palate across vertebrates. Moreover, expression of zebrafish satb2 in the anterior neurocranium supports the utility of the anterior neurocranium as a simplified model of amniote palatogenesis.

Uhrf1 and Dnmt1 are required for development and maintenance of the zebrafish lens

DNA methylation is one of the key mechanisms underlying the epigenetic regulation of gene expression. During DNA replication, the methylation pattern of the parent strand is maintained on the replicated strand through the action of Dnmt1 (DNA Methyltransferase 1). In mammals, Dnmt1 is recruited to hemimethylated replication foci by Uhrf1 (Ubiquitin-like, Containing PHD and RING Finger Domains 1). Here we show that Uhrf1 is required for DNA methylation in vivo during zebrafish embryogenesis. Due in part to the early embryonic lethality of Dnmt1 and Uhrf1 knockout mice, roles for these proteins during lens development have yet to be reported. We show that zebrafish mutants in uhrf1 and dnmt1 have defects in lens development and maintenance. uhrf1 and dnmt1 are expressed in the lens epithelium, and in the absence of Uhrf1 or of catalytically active Dnmt1, lens epithelial cells have altered gene expression and reduced proliferation in both mutant backgrounds. This is correlated with a wave of apoptosis in the epithelial layer, which is followed by apoptosis and unraveling of secondary lens fibers. Despite these disruptions in the lens fiber region, lens fibers express appropriate differentiation markers. The results of lens transplant experiments demonstrate that Uhrf1 and Dnmt1 functions are required lens-autonomously, but perhaps not cell-autonomously, during lens development in zebrafish. These data provide the first evidence that Uhrf1 and Dnmt1 function is required for vertebrate lens development and maintenance.

MicroRNAs and micromanaging the skeleton in disease, development and evolution

MicroRNAs (miRNAs) are short, non-protein-encoding RNAs that effect post-transcriptional gene regulation by targeting messenger RNAs. miRNAs are associated with specific human diseases and help regulate development. Here we review recent advances in understanding the roles of miRNAs in skeletal malformations, including cleft palate, and in the evolution of skeletal morphologies. We propose the hypothesis that evolutionary variation in miRNA expression patterns or structural variation in miRNA binding sites in messenger RNAs can help explain the evolution of craniofacial variation among species, the development of human craniofacial disease and physiological changes leading to osteopenia that increases with ageing.

The midline, oral ectoderm, and the arch-0 problem

In most versions of theories of the segmentation of the vertebrate head, a premandibular segment is present rostral to the jaw-forming mandibular segment. These theories posit that in ancient fishes this segment included a gill and a gill-supporting skeleton, which then was modified to support the anterior brain. However, we find no recent evidence for existence of such a premandibular segment. Rather, new findings from studies of fate mapping and gene expression show that the "premandibular" territory is in fact the maxillary region of the mandibular arch. A signaling cascade, beginning with dorsal midline mesoderm in the gastrula and relayed through neural ectoderm and then oral ectoderm, greatly expands the skeletal derivatives of maxillary neural crest in a manner fully consistent with the Gans-Northcutt theory of the vertebrate new head.

RAD marker microarrays enable rapid mapping of zebrafish mutations

A RAD marker microarray was constructed to facilitate rapid genetic mapping of zebrafish mutations and used to localize previously unmapped mutations to genomic regions just a few centiMorgans in length.

We constructed a restriction site associated DNA (RAD) marker microarray to facilitate rapid genetic mapping of zebrafish mutations. Using these microarrays with a bulk segregant approach, we localized previously unmapped mutations to genomic regions just a few centiMorgans in length. Furthermore, we developed an approach to assay individual RAD markers in pooled populations and refined one region. The RAD approach is highly effective for genetic mapping in zebrafish and is an attractive option for mapping in other organisms.

mef2ca is required in cranial neural crest to effect Endothelin1 signaling in zebrafish

Mef2 genes encode highly conserved transcription factors involved in somitic and cardiac mesoderm development in diverse bilaterians. Vertebrates have multiple mef2 genes. In mice, mef2c is required for heart and vascular development. We show that a zebrafish mef2c gene (mef2ca) is required in cranial neural crest (CNC) for proper head skeletal patterning. mef2ca mutants have head skeletal phenotypes resembling those seen upon partial loss-of-function of endothelin1 (edn1). Furthermore, mef2ca interacts genetically with edn1, arguing that mef2ca functions within the edn1 pathway. mef2ca is expressed in CNC and this expression does not require edn1 signaling. Mosaic analyses reveal that mef2ca is required in CNC for pharyngeal skeletal morphogenesis. Proper expression of many edn1-dependent target genes including hand2, bapx1, and gsc, depends upon mef2ca function. mef2ca plays a critical role in establishing the proper nested expression patterns of dlx genes. dlx5a and dlx6a, known Edn1 targets, are downregulated in mef2ca mutant pharyngeal arch CNC. Surprisingly, dlx4b and dlx3b are oppositely affected in mef2ca mutants. dlx4b expression is abolished while the edn1-dependent dlx3b is ectopically expressed in more dorsal CNC. Together our results support a model in which CNC cells require mef2ca downstream of edn1 signaling for proper craniofacial development.

phospholipase C, beta 3 is required for Endothelin1 regulation of pharyngeal arch patterning in zebrafish

Genetic and pharmacological studies demonstrate that Endothelin1 (Edn1) is a key signaling molecule for patterning the facial skeleton in fish, chicks, and mice. When Edn1 function is reduced early in development the ventral lower jaw and supporting structures are reduced in size and often fused to their dorsal upper jaw counterparts. We show that schmerle (she) encodes a zebrafish ortholog of Phospholipase C, beta 3 (Plcbeta3) required in cranial neural crest cells for Edn1 regulation of pharyngeal arch patterning. Sequencing and co-segregation demonstrates that two independent she (plcbeta3) alleles have missense mutations in conserved residues within the catalytic domains of Plcbeta3. Homozygous plcbeta3 mutants are phenotypically similar to edn1 mutants and exhibit a strong arch expression defect in Edn1-dependent Distalless (Dlx) genes as well as expression defects in several Edn1-dependent intermediate and ventral arch domain transcription factors. plcbeta3 also genetically interacts with edn1, supporting a model in which Edn1 signals through a G protein-coupled receptor to activate Plcbeta3. Mild skeletal defects occur in plcbeta3 heterozygotes, showing the plcbeta3 mutations are partially dominant. Through a morpholino-mediated deletion in the N-terminal PH domain of Plcbeta3, we observe a partial rescue of facial skeletal defects in homozygous plcbeta3 mutants, supporting a hypothesis that an intact PH domain is necessary for the partial dominance we observe. In addition, through mosaic analyses, we show that wild-type neural crest cells can efficiently rescue facial skeletal defects in homozygous plcbeta3 mutants, demonstrating that Plcbeta3 function is required in neural crest cells and not other cell types to pattern the facial skeleton.

Moz-dependent Hox expression controls segment-specific fate maps of skeletal precursors in the face

Development of the facial skeleton depends on interactions between intrinsic factors in the skeletal precursors and extrinsic signals in the facial environment. Hox genes have been proposed to act cell-intrinsically in skeletogenic cranial neural crest cells (CNC) for skeletal pattern. However,Hox genes are also expressed in other facial tissues, such as the ectoderm and endoderm, suggesting that Hox genes could also regulate extrinsic signalling from non-CNC tissues. Here we study moz mutant zebrafish in which hoxa2b and hoxb2a expression is lost and the support skeleton of the second pharyngeal segment is transformed into a duplicate of the first-segment-derived jaw skeleton. By performing tissue mosaic experiments between moz- and wild-type embryos, we show that Moz and Hox genes function in CNC, but not in the ectoderm or endoderm,to specify the support skeleton. How then does Hox expression within CNC specify a support skeleton at the cellular level? Our fate map analysis of skeletal precursors reveals that Moz specifies a second-segment fate map in part by regulating the interaction of CNC with the first endodermal pouch(p1). Removal of p1, either by laser ablation or in the itga5b926 mutant, reveals that p1 epithelium is required for development of the wild-type support but not the moz-duplicate jaw-like skeleton. We present a model in which Moz-dependent Hox expression in CNC shapes the normal support skeleton by instructing second-segment CNC to undergo skeletogenesis in response to local extrinsic signals.