Germline mutations in Ras pathway components are associated with a large class of human developmental abnormalities, known as RASopathies, that are characterized by a range of structural and functional phenotypes, including cardiac defects and neurocognitive delays. Although it is generally believed that RASopathies are caused by altered levels of pathway activation, the signaling changes in developing tissues remain largely unknown. We used assays with spatiotemporal resolution in Drosophila melanogaster (fruit fly) and Danio rerio (zebrafish) to quantify signaling changes caused by mutations in MAP2K1 (encoding MEK), a core component of the Ras pathway that is mutated in both RASopathies and cancers in humans. Surprisingly, we discovered that intrinsically active MEK variants can both increase and reduce the levels of pathway activation in vivo. The sign of the effect depends on cellular context, implying that some of the emerging phenotypes in RASopathies may be caused by increased, as well as attenuated, levels of Ras signaling.
Germ-line mutations in components of the Ras/MAPK pathway result in developmental disorders called RASopathies, affecting about 1/1,000 human births. Rapid advances in genome sequencing make it possible to identify multiple disease-related mutations, but there is currently no systematic framework for translating this information into patient-specific predictions of disease progression. As a first step toward addressing this issue, we developed a quantitative, inexpensive, and rapid framework that relies on the early zebrafish embryo to assess mutational effects on a common scale. Using this assay, we assessed 16 mutations reported in MEK1, a MAPK kinase, and provide a robust ranking of these mutations. We find that mutations found in cancer are more severe than those found in both RASopathies and cancer, which, in turn, are generally more severe than those found only in RASopathies. Moreover, this rank is conserved in other zebrafish embryonic assays and Drosophila-specific embryonic and adult assays, suggesting that our ranking reflects the intrinsic property of the mutant molecule. Furthermore, this rank is predictive of the drug dose needed to correct the defects. This assay can be readily used to test the strengths of existing and newly found mutations in MEK1 and other pathway components, providing the first step in the development of rational guidelines for patient-specific diagnostics and treatment of RASopathies.
Cardiac development is a dynamic process regulated by spatial and temporal cues that are integrated to effect molecular, cellular, and tissue-level events that form the adult heart. Disruption of these highly orchestrated events can be devastating for cardiac form and function. Aberrations in heart development result in congenital heart defects (CHDs), which affect 1 in 100 infants in the United States each year. Zebrafish have proven informative as a model organism to understand both heart development and the mechanisms associated with CHDs due to the similarities in heart morphogenesis among vertebrates, as well as their genetic tractability and amenability to live imaging. In this review, we discuss the mechanisms of zebrafish heart development and the utility of zebrafish for understanding syndromic CHDs, those cardiac abnormalities that occur in the context of multisystem disorders. We conclude with avenues of zebrafish research that will potentially inform future therapeutic approaches for the treatment of CHDs.
Left-right (L-R) asymmetry of the internal organs of vertebrates is presaged by domains of asymmetric gene expression in the lateral plate mesoderm (LPM) during somitogenesis. Ciliated L-R coordinators (LRCs) are critical for biasing the initiation of asymmetrically expressed genes, such as nodal and pitx2, to the left LPM. Other midline structures, including the notochord and floorplate, are then required to maintain these asymmetries. Here we report an unexpected role for the zebrafish EGF-CFC gene one-eyed pinhead (oep) in the midline to promote pitx2 expression in the LPM. Late zygotic oep (LZoep) mutants have strongly reduced or absent pitx2 expression in the LPM, but this expression can be rescued to strong levels by restoring oep in midline structures only. Furthermore, removing midline structures from LZoep embryos can rescue pitx2 expression in the LPM, suggesting the midline is a source of an LPM pitx2 repressor that is itself inhibited by oep Reducing lefty1 activity in LZoep embryos mimics removal of the midline, implicating lefty1 in the midline-derived repression. Together, this suggests a model where Oep in the midline functions to overcome a midline-derived repressor, involving lefty1, to allow for the expression of left side-specific genes in the LPM.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.
Cilia are microtubule-based projections that function in the movement of extracellular fluid. This requires cilia to be: (1) motile and driven by dynein complexes and (2) correctly polarized on the surface of cells, which requires planar cell polarity (PCP). Few factors that regulate both processes have been discovered. We reveal that C21orf59/Kurly (Kur), a cytoplasmic protein with some enrichment at the base of cilia, is needed for motility; zebrafish mutants exhibit characteristic developmental abnormalities and dynein arm defects. kur was also required for proper cilia polarization in the zebrafish kidney and the larval skin of Xenopus laevis. CRISPR/Cas9 coupled with homologous recombination to disrupt the endogenous kur locus in Xenopus resulted in the asymmetric localization of the PCP protein Prickle2 being lost in mutant multiciliated cells. Kur also makes interactions with other PCP components, including Disheveled. This supports a model wherein Kur plays a dual role in cilia motility and polarization.
RASopathies are developmental disorders caused by germline mutations in the Ras-MAPK pathway, and are characterized by a broad spectrum of functional and morphological abnormalities. The high incidence of these disorders (∼1/1000 births) motivates the development of systematic approaches for their efficient diagnosis and potential treatment. Recent advances in genome sequencing have greatly facilitated the genotyping and discovery of mutations in affected individuals, but establishing the causal relationships between molecules and disease phenotypes is non-trivial and presents both technical and conceptual challenges. Here, we discuss how these challenges could be addressed using genetically modified model organisms that have been instrumental in delineating the Ras-MAPK pathway and its roles during development. Focusing on studies in mice, zebrafish and Drosophila, we provide an up-to-date review of animal models of RASopathies at the molecular and functional level. We also discuss how increasingly sophisticated techniques of genetic engineering can be used to rigorously connect changes in specific components of the Ras-MAPK pathway with observed functional and morphological phenotypes. Establishing these connections is essential for advancing our understanding of RASopathies and for devising rational strategies for their management and treatment.
A diverse family of cytoskeletal dynein motors powers various cellular transport systems, including axonemal dyneins generating the force for ciliary and flagellar beating essential to movement of extracellular fluids and of cells through fluid. Multisubunit outer dynein arm (ODA) motor complexes, produced and preassembled in the cytosol, are transported to the ciliary or flagellar compartment and anchored into the axonemal microtubular scaffold via the ODA docking complex (ODA-DC) system. In humans, defects in ODA assembly are the major cause of primary ciliary dyskinesia (PCD), an inherited disorder of ciliary and flagellar dysmotility characterized by chronic upper and lower respiratory infections and defects in laterality. Here, by combined high-throughput mapping and sequencing, we identified CCDC151 loss-of-function mutations in five affected individuals from three independent families whose cilia showed a complete loss of ODAs and severely impaired ciliary beating. Consistent with the laterality defects observed in these individuals, we found Ccdc151 expressed in vertebrate left-right organizers. Homozygous zebrafish ccdc151(ts272a) and mouse Ccdc151(Snbl) mutants display a spectrum of situs defects associated with complex heart defects. We demonstrate that CCDC151 encodes an axonemal coiled coil protein, mutations in which abolish assembly of CCDC151 into respiratory cilia and cause a failure in axonemal assembly of the ODA component DNAH5 and the ODA-DC-associated components CCDC114 and ARMC4. CCDC151-deficient zebrafish, planaria, and mice also display ciliary dysmotility accompanied by ODA loss. Furthermore, CCDC151 coimmunoprecipitates CCDC114 and thus appears to be a highly evolutionarily conserved ODA-DC-related protein involved in mediating assembly of both ODAs and their axonemal docking machinery onto ciliary microtubules.
Larval zebrafish offer the potential for large-scale optical imaging of neural activity throughout the central nervous system; however, several barriers challenge their utility. First, ~panneuronal probe expression has to date only been demonstrated at early larval stages up to 7 days post-fertilization (dpf), precluding imaging at later time points when circuits are more mature. Second, nuclear exclusion of genetically-encoded calcium indicators (GECIs) limits the resolution of functional fluorescence signals collected during imaging. Here, we report the creation of transgenic zebrafish strains exhibiting robust, nuclearly targeted expression of GCaMP3 across the brain up to at least 14 dpf utilizing a previously described optimized Gal4-UAS system. We confirmed both nuclear targeting and functionality of the modified probe in vitro and measured its kinetics in response to action potentials (APs). We then demonstrated in vivo functionality of nuclear-localized GCaMP3 in transgenic zebrafish strains by identifying eye position-sensitive fluorescence fluctuations in caudal hindbrain neurons during spontaneous eye movements. Our methodological approach will facilitate studies of larval zebrafish circuitry by both improving resolution of functional Ca(2+) signals and by allowing brain-wide expression of improved GECIs, or potentially any probe, further into development.
Failure to properly establish the left-right (L/R) axis is a major cause of congenital heart defects in humans, but how L/R patterning of the embryo leads to asymmetric cardiac morphogenesis is still unclear. We find that asymmetric Nodal signaling on the left and Bmp signaling act in parallel to establish zebrafish cardiac laterality by modulating cell migration velocities across the L/R axis. Moreover, we demonstrate that Nodal plays the crucial role in generating asymmetry in the heart and that Bmp signaling via Bmp4 is dispensable in the presence of asymmetric Nodal signaling. In addition, we identify a previously unappreciated role for the Nodal-transcription factor FoxH1 in mediating cell responsiveness to Bmp, further linking the control of these two pathways in the heart. The interplay between these TGFβ pathways is complex, with Nodal signaling potentially acting to limit the response to Bmp pathway activation and the dosage of Bmp signals being critical to limit migration rates. These findings have implications for understanding the complex genetic interactions that lead to congenital heart disease in humans.
A key challenge in genetics is identifying the functional roles of genes in pathways. Numerous functional genomics techniques (e.g. machine learning) that predict protein function have been developed to address this question. These methods generally build from existing annotations of genes to pathways and thus are often unable to identify additional genes participating in processes that are not already well studied. Many of these processes are well studied in some organism, but not necessarily in an investigator's organism of interest. Sequence-based search methods (e.g. BLAST) have been used to transfer such annotation information between organisms. We demonstrate that functional genomics can complement traditional sequence similarity to improve the transfer of gene annotations between organisms. Our method transfers annotations only when functionally appropriate as determined by genomic data and can be used with any prediction algorithm to combine transferred gene function knowledge with organism-specific high-throughput data to enable accurate function prediction. We show that diverse state-of-art machine learning algorithms leveraging functional knowledge transfer (FKT) dramatically improve their accuracy in predicting gene-pathway membership, particularly for processes with little experimental knowledge in an organism. We also show that our method compares favorably to annotation transfer by sequence similarity. Next, we deploy FKT with state-of-the-art SVM classifier to predict novel genes to 11,000 biological processes across six diverse organisms and expand the coverage of accurate function predictions to processes that are often ignored because of a dearth of annotated genes in an organism. Finally, we perform in vivo experimental investigation in Danio rerio and confirm the regulatory role of our top predicted novel gene, wnt5b, in leftward cell migration during heart development. FKT is immediately applicable to many bioinformatics techniques and will help biologists systematically integrate prior knowledge from diverse systems to direct targeted experiments in their organism of study.
The satellite symposium on 'Making and breaking the left-right axis: implications of laterality in development and disease' was held in June 2013 in conjunction with the 17th International Society for Developmental Biology meeting in Cancún, Mexico. As we summarize here, leaders in the field gathered at the symposium to discuss recent advances in understanding how left-right asymmetry is generated and utilized across the animal kingdom.
Histological techniques are critical for observing tissue and cellular morphology. In this paper, we outline our protocol for embedding, serial sectioning, staining and visualizing zebrafish embryos embedded in JB-4 plastic resin-a glycol methacrylate-based medium that results in excellent preservation of tissue morphology. In addition, we describe our procedures for staining plastic sections with toluidine blue or hematoxylin and eosin, and show how to couple these stains with whole-mount RNA in situ hybridization. We also describe how to maintain and visualize immunofluorescence and EGFP signals in JB-4 resin. The protocol we outline-from embryo preparation, embedding, sectioning and staining to visualization-can be accomplished in 3 d. Overall, we reinforce that plastic embedding can provide higher resolution of cellular details and is a valuable tool for cellular and morphological studies in zebrafish.
Vertebrate mesendoderm specification requires the Nodal signaling pathway and its transcriptional effector FoxH1. However, loss of FoxH1 in several species does not reliably cause the full range of loss-of-Nodal phenotypes, indicating that Nodal signals through additional transcription factors during early development. We investigated the FoxH1-dependent and -independent roles of Nodal signaling during mesendoderm patterning using a novel recessive zebrafish FoxH1 mutation called midway, which produces a C-terminally truncated FoxH1 protein lacking the Smad-interaction domain but retaining DNA-binding capability. Using a combination of gel shift assays, Nodal overexpression experiments, and genetic epistasis analyses, we demonstrate that midway more accurately represents a complete loss of FoxH1-dependent Nodal signaling than the existing zebrafish FoxH1 mutant schmalspur. Maternal-zygotic midway mutants lack notochords, in agreement with FoxH1 loss in other organisms, but retain near wild-type expression of markers of endoderm and various nonaxial mesoderm fates, including paraxial and intermediate mesoderm and blood precursors. We found that the activity of the T-box transcription factor Eomesodermin accounts for specification of these tissues in midway embryos. Inhibition of Eomesodermin in midway mutants severely reduces the specification of these tissues and effectively phenocopies the defects seen upon complete loss of Nodal signaling. Our results indicate that the specific combinations of transcription factors available for signal transduction play critical and separable roles in determining Nodal pathway output during mesendoderm patterning. Our findings also offer novel insights into the co-evolution of the Nodal signaling pathway, the notochord specification program, and the chordate branch of the deuterostome family of animals.
The advent of methods for optical imaging of large-scale neural activity at cellular resolution in behaving animals presents the problem of identifying behavior-encoding cells within the resulting image time series. Rapid and precise identification of cells with particular neural encoding would facilitate targeted activity measurements and perturbations useful in characterizing the operating principles of neural circuits. Here we report a regression-based approach to semiautomatically identify neurons that is based on the correlation of fluorescence time series with quantitative measurements of behavior. The approach is illustrated with a novel preparation allowing synchronous eye tracking and two-photon laser scanning fluorescence imaging of calcium changes in populations of hindbrain neurons during spontaneous eye movement in the larval zebrafish. Putative velocity-to-position oculomotor integrator neurons were identified that showed a broad spatial distribution and diversity of encoding. Optical identification of integrator neurons was confirmed with targeted loose-patch electrical recording and laser ablation. The general regression-based approach we demonstrate should be widely applicable to calcium imaging time series in behaving animals.
Left-right (L/R) patterning is crucial for the proper development of all vertebrates and requires asymmetric expression of nodal in the lateral plate mesoderm (LPM). The mechanisms governing asymmetric initiation of nodal have been studied extensively, but because Nodal is a potent activator of its own transcription, it is also crucial to understand the regulation required to maintain this asymmetry once it is established. The 'midline barrier', consisting of lefty1 expression, is a conserved mechanism for restricting Nodal activity to the left. However, the anterior and posterior extremes of the LPM are competent to respond to Nodal signals yet are not adjacent to this barrier, suggesting that lefty1 is not the only mechanism preventing ectopic Nodal activation. Here, we demonstrate the existence of two additional midline barriers. The first is a 'posterior barrier' mediated by Bmp signaling that prevents nodal propagation through the posterior LPM. In contrast to previous reports, we find that Bmp represses Nodal signaling independently of lefty1 expression and through the activity of a ligand other than Bmp4. The 'anterior barrier' is mediated by lefty2 expression in the left cardiac field and prevents Nodal activation from traveling across the anterior limit of the notochord and propagating down the right LPM. Both barriers appear to be conserved across model systems and are thus likely to be present in all vertebrates.
The categorical data set is an important data class in experimental biology and contains data separable into several mutually exclusive categories. Unlike measurement of a continuous variable, categorical data cannot be analyzed with methods such as the Student's t-test. Thus, these data require a different method of analysis to aid in interpretation. In this article, we will review issues related to categorical data, such as how to plot them in a graph, how to integrate results from different experiments, how to calculate the error bar/region, and how to perform significance tests. In addition, we illustrate analysis of categorical data using experimental results from developmental biology and virology studies.
Research focused on cilia as extremely important cellular organelles has flourished in recent years. A thorough understanding of cilia regulation and function is critical, as disruptions of cilia structure and/or function have been linked to numerous human diseases and disorders. The tropical freshwater zebrafish is an excellent model organism in which to study cilia structure and function. We can readily image cilia and their motility in embryonic structures including Kupffer's vesicle during somite stages and the pronephros from 1 day postfertilization onward. Here, we describe how to image cilia by whole-mount immunofluorescence, transverse cryosection/immunohistochemistry, and transmission electron microscopy. We also describe how to obtain videos of cilia motility in living embryos.
Zebrafish are an attractive model for studying the earliest cellular defects occurring during renal cyst formation because its kidney (the pronephros) is simple and genes that cause cystic kidney diseases (CKD) in humans, cause pronephric dilations in zebrafish. By comparing phenotypes in three different mutants, locke, swt and kurly, we find that dilations occur prior to 48 hpf in the medial tubules, a location similar to where cysts form in some mammalian diseases. We demonstrate that the first observable phenotypes associated with dilation include cilia motility and luminal remodeling defects. Importantly, we show that some phenotypes common to human CKD, such as an increased number of cells, are secondary consequences of dilation. Despite having differences in cilia motility, locke, swt and kurly share similar cystic phenotypes, suggesting that they function in a common pathway. To begin to understand the molecular mechanisms involved in cyst formation, we have cloned the swt mutation and find that it encodes a novel leucine rich repeat containing protein (LRRC50), which is thought to function in correct dynein assembly in cilia. Finally, we show that knock-down of polycystic kidney disease 2 (pkd2) specifically causes glomerular cysts and does not affect cilia motility, suggesting multiple mechanisms exist for cyst formation.
The Nodal signaling pathway plays a conserved role in determining left-sided identity in vertebrates with this early left-right (L/R) patterning influencing the asymmetric development and placement of visceral organs. We have studied the role of Nodal signaling in asymmetric cardiac morphogenesis in zebrafish and describe two distinct rotations occurring within the heart. The first is driven by an asymmetric migration of myocardial cells during cardiac jogging, resulting in the conversion of the L/R axis to the dorsal-ventral (D/V) axis of the linear heart. This first rotation is directly influenced by the laterality of asymmetric gene expression. The second rotation occurs before cardiac looping and positions the original left cells exposed to Nodal signaling back to the left of the wild-type (WT) heart by 48 hours postfertilization (hpf). The direction of this second rotation is determined by the laterality of cardiac jogging and is not directly influenced by asymmetric gene expression. Finally, we have identified a role for Nodal signaling in biasing the location of the inner ventricular and outer atrial curvature formations. These results suggest that Nodal signaling directs asymmetric cardiac morphogenesis through establishing and subsequently reinforcing laterality information over the course of cardiac development.
Work in mouse has implicated cilia motility and leftward nodal flow as the mechanism for breaking left-right symmetry. In zebrafish, it is assumed that Kupffer's vesicle is analogous to the mouse node. However, its architecture is different and the fluid dynamics inside Kupffer's vesicle is not completely understood. We show that cells lining both the dorsal roof and the ventral floor of Kupffer's vesicle possess posteriorly pointed cilia that rotate clockwise when viewed apically. Analysis of bead movements within Kupffer's vesicle shows a net circular flow but the local flow differs in direction depending on the location within the vesicle. Histological analysis suggests that the orientation of the cells at anterior-dorsal region likely direct net flow in the vesicle. Our data suggest that the plane of the circular net flow is tilted with respect to the D-V axis, which may be converted to a local leftward flow in the anterior-dorsal region of the vesicle.
The zebrafish mutation curly up (cup) affects the zebrafish ortholog of polycystic kidney disease 2, a gene that encodes the Ca(2+)-activated non-specific cation channel, Polycystin 2. We have characterized two alleles of cup, both of which display defects in organ positioning that resemble human heterotaxia, as well as abnormalities in asymmetric gene expression in the lateral plate mesoderm (LPM) and dorsal diencephalon of the brain. Interestingly, mouse and zebrafish pkd2(-/-) mutants have disparate effects on nodal expression. In the majority of cup embryos, the zebrafish nodal gene southpaw (spaw) is activated bilaterally in LPM, as opposed to the complete absence of Nodal reported in the LPM of the Pkd2-null mouse. The mouse data indicate that Pkd2 is responsible for an asymmetric calcium transient that is upstream of Nodal activation. In zebrafish, it appears that pkd2 is not responsible for the activation of spaw transcription, but is required for a mechanism to restrict spaw expression to the left half of the embryo. pkd2 also appears to play a role in the propagation of Nodal signals in the LPM. Based on morpholino studies, we propose an additional role for maternal pkd2 in general mesendoderm patterning.
The relationship between structural and functional asymmetries in the brain remains unclear. A recent report describes a zebrafish mutant that provides us with some enticing clues about this relationship.
The embryonic kidney of the zebrafish is the pronephros. The ease of genetic analysis and experimentation in zebrafish, coupled with the simplicity of the pronephros, make the zebrafish an ideal model system for studying kidney development and function. Several mutations have been isolated in zebrafish genetic screens that result in cyst formation in the pronephros. Cloning and characterization of these mutations will provide insight into kidney development but may also provide understanding of the molecular basis of cystic kidney diseases. In this review, we focus on the zebrafish as a model for understanding cystic kidney disease and the links between cystic kidney disease and left-right patterning.