Mutations affecting cell fates and cellular rearrangements during gastrulation in zebrafish

CRIMP: a CRISPR/Cas9 insertional mutagenesis protocol and toolkit

Site-directed insertion is a powerful approach for generating mutant alleles, but low efficiency and the need for customisation for each target has limited its application. To overcome this, we developed a highly efficient targeted insertional mutagenesis system, CRIMP, and an associated plasmid toolkit, CRIMPkit, that disrupts native gene expression by inducing complete transcriptional termination, generating null mutant alleles without inducing genetic compensation. The protocol results in a high frequency of integration events and can generate very early targeted insertions, during the first cell division, producing embryos with expression in one or both halves of the body plan. Fluorescent readout of integration events facilitates selection of successfully mutagenized fish and, subsequently, visual identification of heterozygous and mutant animals. Together, these advances greatly improve the efficacy of generating and studying mutant lines. The CRIMPkit contains 24 ready-to-use plasmid vectors to allow easy and complete mutagenesis of any gene in any reading frame without requiring custom sequences, modification, or subcloning.

Genetic requirement of dact1/2 to regulate noncanonical Wnt signaling and calpain 8 during embryonic convergent extension and craniofacial morphogenesis

Wnt signaling plays a crucial role in the early embryonic patterning and development, to regulate convergent extension during gastrulation and the establishment of the dorsal axis. Further, Wnt signaling is a crucial regulator of craniofacial morphogenesis. The adapter proteins Dact1 and Dact2 modulate the Wnt signaling pathway through binding to Disheveled. However, the distinct relative functions of Dact1 and Dact2 during embryogenesis remain unclear. We found that dact1 and dact2 genes have dynamic spatiotemporal expression domains that are reciprocal to one another and to wnt11f2, that suggest distinct functions during zebrafish embryogenesis. We found that both dact1 and dact2 contribute to axis extension, with compound mutants exhibiting a similar convergent extension defect and craniofacial phenotype to the wnt11f2 mutant. Utilizing single-cell RNAseq and gpc4 mutant that disrupts noncanonical Wnt signaling, we identified dact1/2 specific roles during early development. Comparative whole transcriptome analysis between wildtype, gpc4 and dact1/2 mutants revealed a novel role for dact1/2 in regulating the mRNA expression of the classical calpain capn8. Over-expression of capn8 phenocopies dact1/2 craniofacial dysmorphology. These results identify a previously unappreciated role of capn8 and calcium-dependent proteolysis during embryogenesis. Taken together, our findings highlight the distinct and overlapping roles of dact1 and dact2 in embryonic craniofacial development, providing new insights into the multifaceted regulation of Wnt signaling.

Telencephalic eversion in embryos and early larvae of four teleost species

The telencephalon of ray-finned fishes undergoes eversion, which is very different to the evagination that occurs in most other vertebrates. Ventricle morphogenesis is key to build an everted telencephalon. Thus, here we use the apical marker zona occludens 1 to understand ventricle morphology, extension of the tela choroidea and the eversion process during early telencephalon development of four teleost species: giant danio (Devario aequipinnatus), blind cavefish (Astyanax mexicanus), medaka (Oryzias latipes), and paradise fish (Macroposus opercularis). In addition, by using immunohistochemistry against tubulin and calcium-binding proteins, we analyze the general morphology of the telencephalon, showing changes in the location and extension of the olfactory bulb and other telencephalic regions from 2 to 5 days of development. We also analyze the impact of abnormal eye and telencephalon morphogenesis on eversion, showing that cyclops mutants do undergo eversion despite very dramatic abnormal eye morphology. We discuss how the formation of the telencephalic ventricle in teleost fish, with its characteristic shape, is a crucial event during eversion.

Hilde Mangold: Original microscope slides and records of the gastrula organizer experiments

The discovery of the amphibian gastrula organizer and its publication by Hans Spemann and Hilde Mangold in 1924 is a foundation of experimental embryology, and has shaped our understanding of embryonic induction and pattern formation in vertebrates until today. The original publication is a piece of scientific art, characterized by the meticulous hand drawings by Hilde Mangold, as well as the text that develops mechanistic concepts of modern embryology. While historic microphotographs of specimens got lost, the original microscope slides and Hilde Mangold's laboratory notebook have been secured in embryological collections until today. Here, we make the original data of the six embryonic specimens reported in 1924, as well as the laboratory notebook, available in an accessible digital format. Together, these data shed light on the scientific process that led to the discovery, and should help to make the experiments on the most important signalling center in early vertebrate development transparent for generations of embryologists to come.

Rbm8a deficiency causes hematopoietic defects by modulating Wnt/PCP signaling

Defects in blood development frequently occur among syndromic congenital anomalies. Thrombocytopenia-Absent Radius (TAR) syndrome is a rare congenital condition with reduced platelets (hypomegakaryocytic thrombocytopenia) and forelimb anomalies, concurrent with more variable heart and kidney defects. TAR syndrome associates with hypomorphic gene function for RBM8A/Y14 that encodes a component of the exon junction complex involved in mRNA splicing, transport, and nonsense-mediated decay. How perturbing a general mRNA-processing factor causes the selective TAR Syndrome phenotypes remains unknown. Here, we connect zebrafish rbm8a perturbation to early hematopoietic defects via attenuated non-canonical Wnt/Planar Cell Polarity (PCP) signaling that controls developmental cell re-arrangements. In hypomorphic rbm8a zebrafish, we observe a significant reduction of cd41-positive thrombocytes. rbm8a-mutant zebrafish embryos accumulate mRNAs with individual retained introns, a hallmark of defective nonsense-mediated decay; affected mRNAs include transcripts for non-canonical Wnt/PCP pathway components. We establish that rbm8a-mutant embryos show convergent extension defects and that reduced rbm8a function interacts with perturbations in non-canonical Wnt/PCP pathway genes wnt5b, wnt11f2, fzd7a, and vangl2. Using live-imaging, we found reduced rbm8a function impairs the architecture of the lateral plate mesoderm (LPM) that forms hematopoietic, cardiovascular, kidney, and forelimb skeleton progenitors as affected in TAR Syndrome. Both mutants for rbm8a and for the PCP gene vangl2 feature impaired expression of early hematopoietic/endothelial genes including runx1 and the megakaryocyte regulator gfi1aa. Together, our data propose aberrant LPM patterning and hematopoietic defects as consequence of attenuated non-canonical Wnt/PCP signaling upon reduced rbm8a function. These results also link TAR Syndrome to a potential LPM origin and a developmental mechanism.

Exploring hematopoiesis in zebrafish using forward genetic screening

Zebrafish have emerged as a powerful animal model for investigating the genetic basis of hematopoiesis. Owing to its close genetic and developmental similarities to humans, combined with its rapid reproduction and extensive genomic resources, zebrafish have become a versatile and efficient platform for genetic studies. In particular, the forward genetic screening approach has enabled the unbiased identification of novel genes and pathways related to blood development, from hematopoietic stem cell formation to terminal differentiation. Recent advances in mutant gene mapping have further expanded the scope of forward genetic screening, facilitating the identification of previously unknown genes and pathways relevant to hematopoiesis. In this review, we provide an overview of the zebrafish forward screening approach for hematopoietic gene discovery and highlight the key genes and pathways identified using this method. This review emphasizes the importance of zebrafish as a model system for understanding the genetic basis of hematopoiesis and its associated disorders.

Functional analysis of germline VANGL2 variants using rescue assays of vangl2 knockout zebrafish

Developmental studies have shown that the evolutionarily conserved Wnt Planar Cell Polarity (PCP) pathway is essential for the development of a diverse range of tissues and organs including the brain, spinal cord, heart and sensory organs, as well as establishment of the left-right body axis. Germline mutations in the highly conserved PCP gene VANGL2 in humans have only been associated with central nervous system malformations, and functional testing to understand variant impact has not been performed. Here we report three new families with missense variants in VANGL2 associated with heterotaxy and congenital heart disease p.(Arg169His), non-syndromic hearing loss p.(Glu465Ala) and congenital heart disease with brain defects p.(Arg135Trp). To test the in vivo impact of these and previously described variants, we have established clinically-relevant assays using mRNA rescue of the vangl2 mutant zebrafish. We show that all variants disrupt Vangl2 function, although to different extents and depending on the developmental process. We also begin to identify that different VANGL2 missense variants may be haploinsufficient and discuss evidence in support of pathogenicity. Together, this study demonstrates that zebrafish present a suitable pipeline to investigate variants of unknown significance and suggests new avenues for investigation of the different developmental contexts of VANGL2 function that are clinically meaningful.

Actin-mediated endocytosis in the E-YSL helps drive epiboly in zebrafish

In zebrafish, a punctate band of F-actin is reported to develop in the external yolk syncytial layer (E-YSL) during the latter part of epiboly in zebrafish embryos. Here, electron microscopy (EM) and fluorescence confocal microscopy were conducted to investigate dynamic changes in the E-YSL membrane during epiboly. Using scanning EM, we report that the surface of the E-YSL is highly convoluted, consisting of a complex interwoven network of branching membrane surface microvilli-like protrusions. The region of membrane surface protrusions was relatively wide at 30% epiboly but narrowed as epiboly progressed. This narrowing was coincident with the formation of the punctate actin band. We also demonstrated using immunogold transmission EM that actin clusters were localized at the membrane surface mainly within the protrusions as well as in deeper locations of the E-YSL. Furthermore, during the latter part of epiboly, the punctate actin band was coincident with a region of highly dynamic endocytosis. Treatment with cytochalasin B led to the disruption of the punctate actin band and the membrane surface protrusions, as well as the attenuation of endocytosis. Together, our data suggest that, in the E-YSL, the region encompassing the membrane surface protrusions and its associated punctate actin band are likely to be an integral part of the localized endocytosis, which is important for the progression of epiboly in zebrafish embryos.

Vangl2 deficient zebrafish exhibit hallmarks of neural tube closure defects

Neural tube defects (NTDs) are among the most devastating and common congenital anomalies worldwide, and the ability to model these conditions in vivo is essential for identifying causative genetic and environmental factors. Although zebrafish are ideal for rapid candidate testing, their neural tubes develop primarily via a solid neural keel rather that the fold-and-fuse method employed by mammals, raising questions about their suitability as an NTD model. Here, we demonstrate that despite outward differences, zebrafish anterior neurulation closely resembles that of mammals. For the first time, we directly observe fusion of the bilateral neural folds to enclose a lumen in zebrafish embryos. The neural folds fuse by zippering between multiple distinct but contiguous closure sites. Embryos lacking vangl2, a core planar cell polarity and NTD risk gene, exhibit delayed neural fold fusion and abnormal neural groove formation, yielding distinct openings and midline bifurcations in the developing neural tube. These data provide direct evidence for fold-and-fuse neurulation in zebrafish and its disruption upon loss of an NTD risk gene, highlighting conservation of vertebrate neurulation and the utility of zebrafish for modeling NTDs.

Significance of Premature Vertebral Mineralization in Zebrafish Models in Mechanistic and Pharmaceutical Research on Hereditary Multisystem Diseases

Zebrafish are increasingly becoming an important model organism for studying the pathophysiological mechanisms of human diseases and investigating how these mechanisms can be effectively targeted using compounds that may open avenues to novel treatments for patients. The zebrafish skeleton has been particularly instrumental in modeling bone diseases as-contrary to other model organisms-the lower load on the skeleton of an aquatic animal enables mutants to survive to early adulthood. In this respect, the axial skeletons of zebrafish have been a good read-out for congenital spinal deformities such as scoliosis and degenerative disorders such as osteoporosis and osteoarthritis, in which aberrant mineralization in humans is reflected in the respective zebrafish models. Interestingly, there have been several reports of hereditary multisystemic diseases that do not affect the vertebral column in human patients, while the corresponding zebrafish models systematically show anomalies in mineralization and morphology of the spine as their leading or, in some cases, only phenotype. In this review, we describe such examples, highlighting the underlying mechanisms, the already-used or potential power of these models to help us understand and amend the mineralization process, and the outstanding questions on how and why this specific axial type of aberrant mineralization occurs in these disease models.

Intergenerational plasticity to cycling high temperature and hypoxia affects offspring stress responsiveness and tolerance in zebrafish

Predicted climate change-induced increases in heat waves and hypoxic events will have profound effects on fishes, yet the capacity of parents to alter offspring phenotype via non-genetic inheritance and buffer against these combined stressors is not clear. This study tested how prolonged adult zebrafish exposure to combined diel cycles of thermal stress and hypoxia affect offspring early survival and development, parental investment of cortisol and heat shock proteins (HSPs), larval offspring stress responses, and both parental and offspring heat and hypoxia tolerance. Parental exposure to the combined stressor did not affect fecundity, but increased mortality, produced smaller embryos and delayed hatching. The combined treatment also reduced maternal deposition of cortisol and increased embryo hsf1, hsp70a, HSP70, hsp90aa and HSP90 levels. In larvae, basal cortisol levels did not differ between treatments, but acute exposure to combined heat stress and hypoxia increased cortisol levels in control larvae with no effect on larvae from exposed parents. In contrast, whereas larval basal hsf1, hsp70a and hsp90aa levels differed between parental treatments, the combined acute stressor elicited similar transcriptional responses across treatments. Moreover, the combined acute stressor only induced a marked increase in HSP47 levels in the larvae derived from exposed parents. Finally, combined hypoxia and elevated temperatures increased both thermal and hypoxia tolerance in adults and conferred an increase in offspring thermal but not hypoxia tolerance. These results demonstrate that intergenerational acclimation to combined thermal stress and hypoxia elicit complex carryover effects on stress responsiveness and offspring tolerance with potential consequences for resilience.

Functional comparison of human ACVR1 and zebrafish Acvr1l FOP-associated variants in embryonic zebrafish

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP), a rare disease characterized by progressive heterotopic ossification of muscle and connective tissues, is caused by autosomal dominant activating mutations in the type I receptor, ACVR1/ALK2. The classic human FOP variant, ACVR1R206H , shows increased bone morphogenetic protein (BMP) signaling and activation by activins.

RESULTS

Here, we performed in vivo functional characterization of human ACVR1R206H and orthologous zebrafish Acvr1lR203H using early embryonic zebrafish dorsoventral patterning as a phenotypic readout for receptor activity. Our results showed that human ACVR1R206H and zebrafish Acvr1lR203H exhibit functional differences in early embryonic zebrafish, and that human ACVR1R206H retained its signaling activity in the absence of a ligand-binding domain (LBD). We also showed, for the first time, that zebrafish Acvr2ba/Acvr2bb receptors are required for human ACVR1R206H signaling in early embryonic zebrafish.

CONCLUSIONS

Together, these data provide new insight into ACVR1R206H signaling pathways that may facilitate the design of new and effective therapies for FOP patients.

The non-canonical Wnt receptor Ror2 is required for cartilage cell polarity and morphogenesis of the craniofacial skeleton in zebrafish

Non-canonical/β-catenin-independent Wnt signaling plays crucial roles in tissue/cell polarity in epithelia, but its functions have been less well studied in mesenchymal tissues, such as the skeleton. Mutations in non-canonical Wnt signaling pathway genes cause human skeletal diseases such as Robinow syndrome and Brachydactyly Type B1, which disrupt bone growth throughout the endochondral skeleton. Ror2 is one of several non-canonical Wnt receptor/co-receptors. Here, we show that ror2-/- mutant zebrafish have craniofacial skeletal defects, including disruptions of chondrocyte polarity. ror1-/- mutants appear to be phenotypically wild type, but loss of both ror1 and ror2 leads to more severe cartilage defects, indicating partial redundancy. Skeletal defects in ror1/2 double mutants resemble those of wnt5b-/- mutants, suggesting that Wnt5b is the primary Ror ligand in zebrafish. Surprisingly, the proline-rich domain of Ror2, but not its kinase domain, is required to rescue its function in mosaic transgenic experiments in ror2-/- mutants. These results suggest that endochondral bone defects in ROR-related human syndromes reflect defects in cartilage polarity and morphogenesis.

Sequential action of JNK genes establishes the embryonic left-right axis

The establishment of the left-right axis is crucial for the placement, morphogenesis and function of internal organs. Left-right specification is proposed to be dependent on cilia-driven fluid flow in the embryonic node. Planar cell polarity (PCP) signalling is crucial for patterning of nodal cilia, yet downstream effectors driving this process remain elusive. We have examined the role of the JNK gene family, a proposed downstream component of PCP signalling, in the development and function of the zebrafish node. We show jnk1 and jnk2 specify length of nodal cilia, generate flow in the node and restrict southpaw to the left lateral plate mesoderm. Moreover, loss of asymmetric southpaw expression does not result in disturbances to asymmetric organ placement, supporting a model in which nodal flow may be dispensable for organ laterality. Later, jnk3 is required to restrict pitx2c expression to the left side and permit correct endodermal organ placement. This work uncovers multiple roles for the JNK gene family acting at different points during left-right axis establishment. It highlights extensive redundancy and indicates JNK activity is distinct from the PCP signalling pathway.

CARMIL3 is important for cell migration and morphogenesis during early development in zebrafish

Cell migration is important during early animal embryogenesis. Cell migration and cell shape are controlled by actin assembly and dynamics, which depend on capping proteins, including the barbed-end heterodimeric actin capping protein (CP). CP activity can be regulated by capping-protein-interacting (CPI) motif proteins, including CARMIL (capping protein Arp2/3 myosin-I linker) family proteins. Previous studies of CARMIL3, one of the three highly conserved CARMIL genes in vertebrates, have largely been limited to cells in culture. Towards understanding CARMIL function during embryogenesis in vivo, we analyzed zebrafish lines carrying mutations of carmil3. Maternal-zygotic mutants showed impaired endodermal migration during gastrulation, along with defects in dorsal forerunner cell (DFC) cluster formation, which affected the morphogenesis of Kupffer's vesicle (KV). Mutant KVs were smaller, contained fewer cells and displayed decreased numbers of cilia, leading to defects in left/right (L/R) patterning with variable penetrance and expressivity. The penetrance and expressivity of the KV phenotype in carmil3 mutants correlated well with the L/R heart positioning defect at the end of embryogenesis. This in vivo animal study of CARMIL3 reveals its new role during morphogenesis of the vertebrate embryo. This role involves migration of endodermal cells and DFCs, along with subsequent morphogenesis of the KV and L/R asymmetry.

The zebrafish issue: 25 years on

In the 1990s, labs on both sides of the Atlantic performed the largest genetic mutagenesis screen at that time using an emerging model organism: the zebrafish. Led by Christiane Nüsslein-Volhard in Tübingen, Germany, and Wolfgang Driever in Boston, USA, these colossal screens culminated in 1996 with the publication of 37 articles in a special issue of Development, which remains the journal's largest issue to this day. To celebrate the anniversary of the zebrafish issue and reflect on the 25 years since its publication, five zebrafish researchers share what the issue means to them, how it has contributed to their career and its impact on the zebrafish community.

Core pathway proteins and the molecular basis of planar polarity in the zebrafish gastrula

The planar polarization of cells and subcellular structures is critical for embryonic development. Coordination of this polarity can provide cells a sense of direction in relation to the anterior-posterior and dorsal-ventral body axes. Fly epithelia use a core pathway comprised of transmembrane (Van Gogh/Strabismus, Frizzled, and Flamingo/Starry night) and cytoplasmic (Prickle or Spiny-legs, Dishevelled, and Diego) proteins to communicate directional information between cells and thereby promote the uniform orientation of structures such as hairs. In the zebrafish gastrula, planar polarity underlies complex cellular processes, including directed migration and intercalation, that are required to shape the embryo body. Like other vertebrates, the zebrafish genome encodes homologs of each core protein, and it is well-established that polarized gastrula cell behaviors are regulated by some of them. However, it is unknown whether a conserved six-member core protein pathway regulates planar polarity during zebrafish gastrulation. Here, we review our current understanding of core protein function as it relates to two specific examples of planar polarity, the dorsal convergence of lateral gastrula cells and the mediolateral intercalation of midline cells. We consider the hallmarks of fly planar polarity and discuss data regarding asymmetric protein localization and function, and the intercellular communication of polarity information.

Dissecting Organismal Morphogenesis by Bridging Genetics and Biophysics

Multicellular organisms develop complex shapes from much simpler, single-celled zygotes through a process commonly called morphogenesis. Morphogenesis involves an interplay between several factors, ranging from the gene regulatory networks determining cell fate and differentiation to the mechanical processes underlying cell and tissue shape changes. Thus, the study of morphogenesis has historically been based on multidisciplinary approaches at the interface of biology with physics and mathematics. Recent technological advances have further improved our ability to study morphogenesis by bridging the gap between the genetic and biophysical factors through the development of new tools for visualizing, analyzing, and perturbing these factors and their biochemical intermediaries. Here, we review how a combination of genetic, microscopic, biophysical, and biochemical approaches has aided our attempts to understand morphogenesis and discuss potential approaches that may be beneficial to such an inquiry in the future. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Planar polarization of cilia in the zebrafish floor-plate involves Par3-mediated posterior localization of highly motile basal bodies

Epithelial cilia, whether motile or primary, often display an off-center planar localization within the apical cell surface. This form of planar cell polarity (PCP) involves the asymmetric positioning of the ciliary basal body (BB). Using the monociliated epithelium of the embryonic zebrafish floor-plate, we investigated the dynamics and mechanisms of BB polarization by live imaging. BBs were highly motile, making back-and-forth movements along the antero-posterior (AP) axis and contacting both the anterior and posterior membranes. Contacts exclusively occurred at junctional Par3 patches and were often preceded by membrane digitations extending towards the BB, suggesting focused cortical pulling forces. Accordingly, BBs and Par3 patches were linked by dynamic microtubules. Later, BBs became less motile and eventually settled at posterior apical junctions enriched in Par3. BB posterior positioning followed Par3 posterior enrichment and was impaired upon Par3 depletion or disorganization of Par3 patches. In the PCP mutant vangl2, BBs were still motile but displayed poorly oriented membrane contacts that correlated with Par3 patch fragmentation and lateral spreading. Thus, we propose an unexpected function for posterior Par3 enrichment in controlling BB positioning downstream of the PCP pathway.

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.

Mechanical control of tissue shape and morphogenetic flows during vertebrate body axis elongation

Shaping embryonic tissues into their functional morphologies requires cells to control the physical state of the tissue in space and time. While regional variations in cellular forces or cell proliferation have been typically assumed to be the main physical factors controlling tissue morphogenesis, recent experiments have revealed that spatial variations in the tissue physical (fluid/solid) state play a key role in shaping embryonic tissues. Here we theoretically study how the regional control of fluid and solid tissue states guides morphogenetic flows to shape the extending vertebrate body axis. Our results show that both the existence of a fluid-to-solid tissue transition along the anteroposterior axis and the tissue surface tension determine the shape of the tissue and its ability to elongate unidirectionally, with large tissue tensions preventing unidirectional elongation and promoting blob-like tissue expansions. We predict both the tissue morphogenetic flows and stresses that enable unidirectional axis elongation. Our results show the existence of a sharp transition in the structure of morphogenetic flows, from a flow with no vortices to a flow with two counter-rotating vortices, caused by a transition in the number and location of topological defects in the flow field. Finally, comparing the theoretical predictions to quantitative measurements of both tissue flows and shape during zebrafish body axis elongation, we show that the observed morphogenetic events can be explained by the existence of a fluid-to-solid tissue transition along the anteroposterior axis. These results highlight the role of spatiotemporally-controlled fluid-to-solid transitions in the tissue state as a physical mechanism of embryonic morphogenesis.

Maternal contributions to gastrulation in zebrafish

Gastrulation is a critical early morphogenetic process of animal development, during which the three germ layers; mesoderm, endoderm and ectoderm, are rearranged by internalization movements. Concurrent epiboly movements spread and thin the germ layers while convergence and extension movements shape them into an anteroposteriorly elongated body with head, trunk, tail and organ rudiments. In zebrafish, gastrulation follows the proliferative and inductive events that establish the embryonic and extraembryonic tissues and the embryonic axis. Specification of these tissues and embryonic axes are controlled by the maternal gene products deposited in the egg. These early maternally controlled processes need to generate sufficient cell numbers and establish the embryonic polarity to ensure normal gastrulation. Subsequently, after activation of the zygotic genome, the zygotic gene products govern mesoderm and endoderm induction and germ layer patterning. Gastrulation is initiated during the maternal-to-zygotic transition, a process that entails both activation of the zygotic genome and downregulation of the maternal transcripts. Genomic studies indicate that gastrulation is largely controlled by the zygotic genome. Nonetheless, genetic studies that investigate the relative contributions of maternal and zygotic gene function by comparing zygotic, maternal and maternal zygotic mutant phenotypes, reveal significant contribution of maternal gene products, transcripts and/or proteins, that persist through gastrulation, to the control of gastrulation movements. Therefore, in zebrafish, the maternally expressed gene products not only set the stage for, but they also actively participate in gastrulation morphogenesis.

Dorsal convergence of gastrula cells requires Vangl2 and an adhesion protein-dependent change in protrusive activity

Lateral zebrafish hypoblast cells initiate dorsal convergence near mid-gastrulation and exhibit non-polarized morphologies, limited cell-cell contact and indirect migration trajectories. By late gastrulation, mesodermal cells become packed as they engage in planar cell polarity (PCP)-dependent movement. Here, we aimed to understand this transition in cell behavior by examining the relationship between protrusion dynamics and establishment of PCP and directed migration. We found that wild-type cells undergo a reduction in bleb protrusions near late gastrulation accompanied by a VANGL planar cell polarity protein 2 (Vangl2)-regulated increase in filopodia number and polarization. Manipulation of blebs is sufficient to interfere with PCP and directed migration. We show that Vangl2, fibronectin and cadherin 2 function to suppress blebbing. Vangl2 maintains ezrin b (Ezrb) protein levels and higher Ezrb activation rescues defective mediolateral cell alignment and migration paths in vangl2 mutant embryos. Transplantation experiments show that loss of vangl2 disrupts protrusion formation cell-autonomously while fibronectin acts non-autonomously. We propose that dorsal convergence requires the coordinated action of Vangl2, Ezrb and cell-adhesion proteins to inhibit blebs and promote polarized actin-rich protrusive activity and PCP.

Zebrafish Wtx is a negative regulator of Wnt signaling but is dispensable for embryonic development and organ homeostasis

BACKGROUND

The X-chromosomally linked gene WTX is a human disease gene and a member of the AMER family. Mutations in WTX are found in Wilms tumor, a form of pediatric kidney cancer and in patients suffering from OSCS (Osteopathia striata with cranial sclerosis), a sclerosing bone disorder. Functional data suggest WTX to be an inhibitor of the Wnt/β-catenin signaling pathway. Deletion of Wtx in mouse leads to perinatal death, impeding the analysis of its physiological role.

RESULTS

To gain insights into the function of Wtx in development and homeostasis we have used zebrafish as a model and performed both knockdown and knockout studies using morpholinos and transcription activator-like effector nucleases (TALENs), respectively. Wtx knockdown led to increased Wnt activity and embryonic dorsalization. Also, wtx mutants showed a transient upregulation of Wnt target genes in the context of caudal fin regeneration. Surprisingly, however, wtx as well as wtx/amer2/amer3 triple mutants developed normally, were fertile and did not show any anomalies in organ maintenance.

CONCLUSIONS

Our data show that members of the zebrafish wtx/amer gene family, while sharing a partially overlapping expression pattern do not compensate for each other. This observation demonstrates a remarkable robustness during development and regeneration in zebrafish.

FishNET: An automated relational database for zebrafish colony management

The zebrafish Danio rerio is a powerful model system to study the genetics of development and disease. However, maintenance of zebrafish husbandry records is both time intensive and laborious, and a standardized way to manage and track the large amount of unique lines in a given laboratory or centralized facility has not been embraced by the field. Here, we present FishNET, an intuitive, open-source, relational database for managing data and information related to zebrafish husbandry and maintenance. By creating a "virtual facility," FishNET enables users to remotely inspect the rooms, racks, tanks, and lines within a given facility. Importantly, FishNET scales from one laboratory to an entire facility with several laboratories to multiple facilities, generating a cohesive laboratory and community-based platform. Automated data entry eliminates confusion regarding line nomenclature and streamlines maintenance of individual lines, while flexible query forms allow researchers to retrieve database records based on user-defined criteria. FishNET also links associated embryonic and adult biological samples with data, such as genotyping results or confocal images, to enable robust and efficient colony management and storage of laboratory information. A shared calendar function with email notifications and automated reminders for line turnover, automated tank counts, and census reports promote communication with both end users and administrators. The expected benefits of FishNET are improved vivaria efficiency, increased quality control for experimental numbers, and flexible data reporting and retrieval. FishNET's easy, intuitive record management and open-source, end-user-modifiable architecture provides an efficient solution to real-time zebrafish colony management for users throughout a facility and institution and, in some cases, across entire research hubs.

Identification of regulatory elements recapitulating early expression of L-plastin in the zebrafish enveloping layer and embryonic periderm

We have cloned and characterized an intronic fragment of zebrafish lymphocyte cytosolic protein 1 (lcp1, also called L-plastin) that drives expression to the zebrafish enveloping layer (EVL). L-plastin is a calcium-dependent actin-bundling protein belonging to the plastin/fimbrin family of proteins, and is necessary for the proper migration and attachment of several adult cell types, including leukocytes and osteoclasts. However, in zebrafish lcp1 is abundantly expressed much earlier, during differentiation of the EVL. The cells of this epithelial layer migrate collectively, spreading vegetally over the yolk. L-plastin expression persists into the larval periderm, a transient epithelial tissue that forms the first larval skin. This finding establishes that L-plastin is activated in two different embryonic waves, with a distinct regulatory switch between the early EVL and the later leukocyte. To better study L-plastin expressing cells we attempted CRISPR/Cas9 homology-driven recombination (HDR) to insert a self-cleaving peptide (Cre-P2A-EGFP-CAAX) downstream of the native lcp1 promoter. This produced a stable zebrafish line expressing Cre recombinase in EVL nuclei and green fluorescence in EVL cell membranes. In vivo tracking of these labeled cells provided enhanced views of EVL migration behavior, membrane extensions, and mitotic events. Finally, we experimentally dissected key elements of the targeted lcp1 locus, discovering a ∼300 bp intronic sequence sufficient to drive EVL expression. The lcp1: Cre-P2A-EGFP-CAAX zebrafish should be useful for studying enveloping layer specification, gastrulation movements and periderm development in this widely used vertebrate model. In addition, the conserved regulatory sequences we have isolated predict that L-plastin orthologs may have a similar early expression pattern in other vertebrate embryos.

VANGL2 protein stability is regulated by integrin αv and the extracellular matrix

Vang-like 2 (VANGL2) is a four-pass transmembrane protein required for a variety of polarized cell behaviors underlying embryonic development. Recent data show human VANGL2 interacts with integrin αv to control cell adhesion to extracellular matrix proteins. The goal of this study was to further define the functional relationship between integrin αv and VANGL2. We demonstrate integrin αv regulates VANGL2 protein levels both in vitro and in the zebrafish embryo. While integrin αv knockdown reduces VANGL2 expression at membrane compartments, it does not affect VANGL2 transcription. Knockdown of integrin β5, but not β1 or β3, also decreases VANGL2 protein levels. Inhibition of protein translation using cycloheximide demonstrates that integrin αv knockdown cells have increased VANGL2 degradation while interference with either proteasome or lysosome function restores VANGL2. We further show integrin activation and stimulation of cell-matrix adhesion using MnCl₂ fails to influence VANGL2. However, MnCl₂ treatment stabilizes VANGL2 protein expression levels in the presence of cycloheximide. In the converse experiment, blockage of integrin-mediated cell-matrix adhesion using a cyclic RGD peptide causes a reduction in VANGL2 protein levels. Together, our findings support a model where integrin αv and cellular interactions with the extracellular matrix are required to maintain VANGL2 protein levels and thus function at the plasma membrane.

Vangl2-dependent regulation of membrane protrusions and directed migration requires a fibronectin extracellular matrix

During zebrafish gastrulation the planar cell polarity (PCP) protein Vang-like 2 (Vangl2) regulates the polarization of cells that are engaged in directed migration. However, it is unclear whether Vangl2 influences membrane-protrusive activities in migrating gastrula cells and whether these processes require the fibronectin extracellular matrix. Here, we report that Vangl2 modulates the formation and polarization of actin-rich filopodia-like and large lamellipodia-like protrusions in ectodermal cells. By contrast, disrupted Glypican4/PCP signaling affects protrusion polarity but not protrusion number or directed migration. Analysis of fluorescent fusion protein expression suggests that there is widespread Vangl2 symmetry in migrating cells, but there is enrichment at membrane domains that are developing large protrusions compared with non-protrusive domains. We show that the fibronectin extracellular matrix is essential for cell-surface Vangl2 expression, membrane-protrusive activity and directed migration. Manipulation of fibronectin protein levels rescues protrusion and directed migration phenotypes in vangl2 mutant embryos, but it is not sufficient to restore either PCP, or convergence and extension movements. Together, our findings identify distinct roles for Vangl2 and Glypican4/PCP signaling during membrane protrusion formation and demonstrate that cell-matrix interactions underlie Vangl2-dependent regulation of protrusive activities in migrating gastrula cells.

Myosin1D is an evolutionarily conserved regulator of animal left-right asymmetry

The establishment of left-right (LR) asymmetry is fundamental to animal development, but the identification of a unifying mechanism establishing laterality across different phyla has remained elusive. A cilia-driven, directional fluid flow is important for symmetry breaking in numerous vertebrates, including zebrafish. Alternatively, LR asymmetry can be established independently of cilia, notably through the intrinsic chirality of the acto-myosin cytoskeleton. Here, we show that Myosin1D (Myo1D), a previously identified regulator of Drosophila LR asymmetry, is essential for the formation and function of the zebrafish LR organizer (LRO), Kupffer's vesicle (KV). Myo1D controls the orientation of LRO cilia and interacts functionally with the planar cell polarity (PCP) pathway component VanGogh-like2 (Vangl2), to shape a productive LRO flow. Our findings identify Myo1D as an evolutionarily conserved regulator of animal LR asymmetry, and show that functional interactions between Myo1D and PCP are central to the establishment of animal LR asymmetry.

Directional selectivity of afferent neurons in zebrafish neuromasts is regulated by Emx2 in presynaptic hair cells

The orientation of hair bundles on top of sensory hair cells (HCs) in neuromasts of the lateral line system allows fish to detect direction of water flow. Each neuromast shows hair bundles arranged in two opposing directions and each afferent neuron innervates only HCs of the same orientation. Previously, we showed that this opposition is established by expression of Emx2 in half of the HCs, where it mediates hair bundle reversal (Jiang et al., 2017). Here, we show that Emx2 also regulates neuronal selection: afferent neurons innervate either Emx2-positive or negative HCs. In emx2 knockout and gain-of-function neuromasts, all HCs are unidirectional and the innervation patterns and physiological responses of the afferent neurons are dependent on the presence or absence of Emx2. Our results indicate that Emx2 mediates the directional selectivity of neuromasts by two distinct processes: regulating hair bundle orientation in HCs and selecting afferent neuronal targets.

Gon4l regulates notochord boundary formation and cell polarity underlying axis extension by repressing adhesion genes

Anteroposterior (AP) axis extension during gastrulation requires embryonic patterning and morphogenesis to be spatiotemporally coordinated, but the underlying genetic mechanisms remain poorly understood. Here we define a role for the conserved chromatin factor Gon4l, encoded by ugly duckling (udu), in coordinating tissue patterning and axis extension during zebrafish gastrulation through direct positive and negative regulation of gene expression. Although identified as a recessive enhancer of impaired axis extension in planar cell polarity (PCP) mutants, udu functions in a genetically independent, partially overlapping fashion with PCP signaling to regulate mediolateral cell polarity underlying axis extension in part by promoting notochord boundary formation. Gon4l limits expression of the cell–cell and cell–matrix adhesion molecules EpCAM and Integrinα3b, excesses of which perturb the notochord boundary via tension-dependent and -independent mechanisms, respectively. By promoting formation of this AP-aligned boundary and associated cell polarity, Gon4l cooperates with PCP signaling to coordinate morphogenesis along the AP embryonic axis.

Anteroposterior axis extension during gastrulation is dynamically coordinated, but how this is regulated at a molecular level is unclear. Here, the authors show in zebrafish that the chromatin factor Gon4l, encoded by ugly duckling, coordinates axis extension by modulating EpCAM and Integrinα3b expression.

Leucine repeat adaptor protein 1 interacts with Dishevelled to regulate gastrulation cell movements in zebrafish

Gastrulation is a fundamental morphogenetic event that requires polarised cell behaviours for coordinated asymmetric cell movements. Wnt/PCP signalling plays a critical role in this process. Dishevelled is an important conserved scaffold protein that relays Wnt/PCP signals from membrane receptors to the modulation of cytoskeleton organisation. However, it remains unclear how its activity is regulated for the activation of downstream effectors. Here, we report that Lurap1 is a Dishevelled-interacting protein that regulates Wnt/PCP signalling in convergence and extension movements during vertebrate gastrulation. Its loss-of-function leads to enhanced Dishevelled membrane localisation and increased JNK activity. In maternal-zygotic lurap1 mutant zebrafish embryos, cell polarity and directional movement are disrupted. Time-lapse analyses indicate that Lurap1, Dishevelled, and JNK functionally interact to orchestrate polarised cellular protrusive activity, and Lurap1 is required for coordinated centriole/MTOC positioning in movement cells. These findings demonstrate that Lurap1 functions to regulate cellular polarisation and motile behaviours during gastrulation movements.

Gastrulation is an early morphogenic event driven by coordinated asymmetric/polarised cell movements. Here, the authors show in zebrafish that Lurap1, a protein that interacts with Dishevelled, regulates Wnt and planar cell polarity, coordinating centriole positioning during convergence and extension.

VANGL2 interacts with integrin αv to regulate matrix metalloproteinase activity and cell adhesion to the extracellular matrix

Planar cell polarity (PCP) proteins are implicated in a variety of morphogenetic processes including embryonic cell migration and potentially cancer progression. During zebrafish gastrulation, the transmembrane protein Vang-like 2 (VANGL2) is required for PCP and directed cell migration. These cell behaviors occur in the context of a fibrillar extracellular matrix (ECM). While it is thought that interactions with the ECM regulate cell migration, it is unclear how PCP proteins such as VANGL2 influence these events. Using an in vitro cell culture model system, we previously showed that human VANGL2 negatively regulates membrane type-1 matrix metalloproteinase (MMP14) and activation of secreted matrix metalloproteinase 2 (MMP2). Here, we investigated the functional relationship between VANGL2, integrin αvβ3, and MMP2 activation. We provide evidence that VANGL2 regulates cell surface integrin αvβ3 expression and adhesion to fibronectin, laminin, and vitronectin. Inhibition of MMP14/MMP2 activity suppressed the cell adhesion defect in VANGL2 knockdown cells. Furthermore, our data show that MMP14 and integrin αv are required for increased proteolysis by VANGL2 knockdown cells. Lastly, we have identified integrin αvβ3 as a novel VANGL2 binding partner. Together, these findings begin to dissect the molecular underpinnings of how VANGL2 regulates MMP activity and cell adhesion to the ECM.

LITTLE FISH, BIG DATA: ZEBRAFISH AS A MODEL FOR CARDIOVASCULAR AND METABOLIC DISEASE

The burden of cardiovascular and metabolic diseases worldwide is staggering. The emergence of systems approaches in biology promises new therapies, faster and cheaper diagnostics, and personalized medicine. However, a profound understanding of pathogenic mechanisms at the cellular and molecular levels remains a fundamental requirement for discovery and therapeutics. Animal models of human disease are cornerstones of drug discovery as they allow identification of novel pharmacological targets by linking gene function with pathogenesis. The zebrafish model has been used for decades to study development and pathophysiology. More than ever, the specific strengths of the zebrafish model make it a prime partner in an age of discovery transformed by big-data approaches to genomics and disease. Zebrafish share a largely conserved physiology and anatomy with mammals. They allow a wide range of genetic manipulations, including the latest genome engineering approaches. They can be bred and studied with remarkable speed, enabling a range of large-scale phenotypic screens. Finally, zebrafish demonstrate an impressive regenerative capacity scientists hope to unlock in humans. Here, we provide a comprehensive guide on applications of zebrafish to investigate cardiovascular and metabolic diseases. We delineate advantages and limitations of zebrafish models of human disease and summarize their most significant contributions to understanding disease progression to date.

A role for planar cell polarity during early endoderm morphogenesis

The zebrafish endoderm begins to develop at gastrulation stages as a monolayer of cells. The behaviour of the endoderm during gastrulation stages is well understood. However, knowledge of the morphogenic movements of the endoderm during somitogenesis stages, as it forms a mesenchymal rod, is lacking. Here we characterise endodermal development during somitogenesis stages, and describe the morphogenic movements as the endoderm transitions from a monolayer of cells into a mesenchymal endodermal rod. We demonstrate that, unlike the overlying mesoderm, endodermal cells are not polarised during their migration to the midline at early somitogenesis stages. Specifically, we describe the stage at which endodermal cells begin to leave the monolayer, a process we have termed 'midline aggregation'. The planar cell polarity (PCP) signalling pathway is known to regulate mesodermal and ectodermal cell convergence towards the dorsal midline. However, a role for PCP signalling in endoderm migration to the midline during somitogenesis stages has not been established. In this report, we investigate the role for PCP signalling in multiple phases of endoderm development during somitogenesis stages. Our data exclude involvement of PCP signalling in endodermal cells as they leave the monolayer.

Imaging early embryonic calcium activity with GCaMP6s transgenic zebrafish

Intracellular Ca²⁺ signaling regulates cellular activities during embryogenesis and in adult organisms. We generated stable Tg[βactin2:GCaMP6s]^stl351 and Tg[ubi:GCaMP6s]^stl352 transgenic lines that combine the ubiquitously-expressed Ca²⁺ indicator GCaMP6s with the transparent characteristics of zebrafish embryos to achieve superior in vivo Ca²⁺ imaging. Using the Tg[βactin2:GCaMP6s]^stl351 line featuring strong GCaMP6s expression from cleavage through gastrula stages, we detected higher frequency of Ca²⁺ transients in the superficial blastomeres during the blastula stages preceding the midblastula transition. Additionally, GCaMP6s also revealed that dorsal-biased Ca²⁺ signaling that follows the midblastula transition persisted longer during gastrulation, compared with earlier studies. We observed that dorsal-biased Ca²⁺ signaling is diminished in ventralized ichabod/β-catenin2 mutant embryos and ectopically induced in embryos dorsalized by excess β-catenin. During gastrulation, we directly visualized Ca²⁺ signaling in the dorsal forerunner cells, which form in a Nodal signaling dependent manner and later give rise to the laterality organ. We found that excess Nodal increases the number and the duration of Ca²⁺ transients specifically in the dorsal forerunner cells. The GCaMP6s transgenic lines described here enable unprecedented visualization of dynamic Ca²⁺ events from embryogenesis through adulthood, augmenting the zebrafish toolbox.

Behavioral alterations of zebrafish larvae after early embryonic exposure to ketamine

RATIONALE

Ketamine has been associated with pediatric risks that include neurocognitive impairment and long-term behavioral disorders. However, the neurobehavioral effects of ketamine exposure in early development remain uncertain.

OBJECTIVES

This study aimed to test stage- and dose-dependent effects of ketamine exposure on certain brain functions by evaluating alterations in locomotion, anxiety-like and avoidance behaviors, as well as socialization.

METHODS

Embryos were exposed to different concentrations of ketamine (0, 0.2, 0.4, and 0.8 mg mL^-1) for 20 min during the 256-cell (2.5 h post fertilization-hpf), 50% epiboly (5.5 hpf), and 1-4 somites (10.5 hpf) stages. General exploratory activities, natural escape-like responses, and social interactions were analyzed under continuous light or under a moving light stimulus.

RESULTS

A dose-dependent decrease in the overall mean speed was perceived in the embryos exposed during the 256-cell stage. These results were related to previously observed head and eye malformations, following ketamine exposure at this stage and may indicate possible neurobehavioral disorders when ketamine exposure is performed at this stage. Results also showed that ketamine exposure during the 50% epiboly and 1-4 somites stages induced a significant increment of the anxiety-like behavior and a decrease in avoidance behavior in all exposed groups.

CONCLUSIONS

Overall, the results validate the neurodevelopmental risks of early-life exposure to ketamine.

Optogenetic Control of Protein Function: From Intracellular Processes to Tissue Morphogenesis

Optogenetics is an emerging and powerful technique that allows the control of protein activity with light. The possibility of inhibiting or stimulating protein activity with the spatial and temporal precision of a pulse of laser light is opening new frontiers for the investigation of developmental pathways and cell biological bases underlying organismal development. With this powerful technique in hand, it will be possible to address old and novel questions about how cells, tissues, and organisms form. In this review, we focus on the applications of existing optogenetic tools for addressing issues in animal morphogenesis.

Optogenetics allows the manipulation of gene expression and cell behaviors in individual cells or group of cells in the context of intact organisms.

Precise spatiotemporal and quantitative perturbation of signaling systems using optogenetics provides a powerful new approach to study morphogenesis of multicellular systems.

The possibility to combine optogenetics with genetically encoded biosensors (e.g., tension sensors) or chemical probes enables the perturbation and monitoring of biochemical reactions of interest in vivo.

By allowing the control of key behaviors, such as cell motility, polarity, and proliferation, optogenetics will be instrumental for characterizing the mechanisms underlying organ regeneration and cancer growth/invasion in living organisms.

Intracellular Golgi Complex organization reveals tissue specific polarity during zebrafish embryogenesis

BACKGROUND

Cell polarity is essential for directed migration of mesenchymal cells and morphogenesis of epithelial tissues. Studies in cultured cells indicate that a condensed Golgi Complex (GC) is essential for directed protein trafficking to establish cell polarity underlying directed cell migration. Dynamic changes of the GC intracellular organization during early vertebrate development remain to be investigated.

RESULTS

We used antibody labeling and fusion proteins in vivo to study the organization and intracellular placement of the GC during early zebrafish embryogenesis. We found that the GC was dispersed into several puncta containing cis- and trans-Golgi Complex proteins, presumably ministacks, until the end of the gastrula period. By early segmentation stages, the GC condensed in cells of the notochord, adaxial mesoderm, and neural plate, and its intracellular position became markedly polarized away from borders between these tissues.

CONCLUSIONS

We find that GC is dispersed in early zebrafish cells, even when cells are engaged in massive gastrulation movements. The GC accumulates into patches in a stage and cell-type specific manner, and becomes polarized away from borders between the embryonic tissues. With respect to tissue borders, intracellular GC polarity in notochord is independent of mature apical/basal polarity, Wnt/PCP, or signals from adaxial mesoderm. Developmental Dynamics 245:678-691, 2016. © 2016 Wiley Periodicals, Inc.

Ccdc11 is a novel centriolar satellite protein essential for ciliogenesis and establishment of left-right asymmetry

Mutations in CCDC11 cause aberrant placement of internal organs and congenital heart disease in humans. Ccdc11 is a novel component of centriolar satellites and plays a critical role in motile and sensory ciliogenesis. The results implicate centriolar satellites in the pathology of left–right patterning and heart disease.

The establishment of left–right (L-R) asymmetry in vertebrates is dependent on the sensory and motile functions of cilia during embryogenesis. Mutations in CCDC11 disrupt L-R asymmetry and cause congenital heart disease in humans, yet the molecular and cellular functions of the protein remain unknown. Here we demonstrate that Ccdc11 is a novel component of centriolar satellites—cytoplasmic granules that serve as recruitment sites for proteins destined for the centrosome and cilium. Ccdc11 interacts with core components of satellites, and its loss disrupts the subcellular organization of satellite proteins and perturbs primary cilium assembly. Ccdc11 colocalizes with satellite proteins in human multiciliated tracheal epithelia, and its loss inhibits motile ciliogenesis. Similarly, depletion of CCDC11 in Xenopus embryos causes defective assembly and motility of cilia in multiciliated epidermal cells. To determine the role of CCDC11 during vertebrate development, we generated mutant alleles in zebrafish. Loss of CCDC11 leads to defective ciliogenesis in the pronephros and within the Kupffer’s vesicle and results in aberrant L-R axis determination. Our results highlight a critical role for Ccdc11 in the assembly and function of motile cilia and implicate centriolar satellite–associated proteins as a new class of proteins in the pathology of L-R patterning and congenital heart disease.

A role of glypican4 and wnt5b in chondrocyte stacking underlying craniofacial cartilage morphogenesis

The Wnt/Planar Cell Polarity (PCP) pathway controls cell morphology and behavior during animal development. Several zebrafish mutants were identified as having perturbed Wnt/PCP signaling. Many of these mutants have defects in craniofacial formation. To better understand the role that Wnt/PCP plays in craniofacial development we set out to identify which of the mutants, known to be associated with the Wnt/PCP pathway, perturb head cartilage formation by disrupting chondrocyte morphology. Here we demonstrate that while vang-like 2 (vangl2), wnt11 and scribbled (scrib) mutants have severe craniofacial morphogenesis defects they do not display the chondrocyte stacking and intercalation problems seen in glypican 4 (gpc4) and wnt5b mutants. The function of Gpc4 or Wnt5b appears to be important for chondrocyte organization, as the neural crest in both mutants is specified, undergoes migration, and differentiates into the same number of cells to compose the craniofacial cartilage elements. We demonstrate that Gpc4 activity is required cell autonomously in the chondrocytes and that the phenotype of single heterozygous mutants is slightly enhanced in embryos double heterozygous for wnt5b and gpc4. This data suggests a novel mechanism for Wnt5b and Gpc4 regulation of chondrocyte behavior that is independent of the core Wnt/PCP molecules and differs from their collaborative action of controlling cell movements during gastrulation.

Glypican4 promotes cardiac specification and differentiation by attenuating canonical Wnt and Bmp signaling

Glypicans are heparan sulphate proteoglycans (HSPGs) attached to the cell membrane by a glycosylphosphatidylinositol (GPI) anchor, and interact with various extracellular growth factors and receptors. The Drosophila division abnormal delayed (dally) was the first glypican loss-of-function mutant described that displays disrupted cell divisions in the eye and morphological defects in the wing. In human, as in most vertebrates, six glypican-encoding genes have been identified (GPC1-6), and mutations in several glypican genes cause multiple malformations including congenital heart defects. To understand better the role of glypicans during heart development, we studied the zebrafish knypek mutant, which is deficient for Gpc4. Our results demonstrate that knypek/gpc4 mutant embryos display severe cardiac defects, most apparent by a strong reduction in cardiomyocyte numbers. Cell-tracing experiments, using photoconvertable fluorescent proteins and genetic labeling, demonstrate that Gpc4 'Knypek' is required for specification of cardiac progenitor cells and their differentiation into cardiomyocytes. Mechanistically, we show that Bmp signaling is enhanced in the anterior lateral plate mesoderm of knypek/gpc4 mutants and that genetic inhibition of Bmp signaling rescues the cardiomyocyte differentiation defect observed in knypek/gpc4 embryos. In addition, canonical Wnt signaling is upregulated in knypek/gpc4 embryos, and inhibiting canonical Wnt signaling in knypek/gpc4 embryos by overexpression of the Wnt inhibitor Dkk1 restores normal cardiomyocyte numbers. Therefore, we conclude that Gpc4 is required to attenuate both canonical Wnt and Bmp signaling in the anterior lateral plate mesoderm to allow cardiac progenitor cells to specify and differentiate into cardiomyocytes. This provides a possible explanation for how congenital heart defects arise in glypican-deficient patients.

Paraquat exposure induces behavioral deficits in larval zebrafish during the window of dopamine neurogenesis

Exposure to environmental risk factors such as herbicides in early life has been proposed to play important roles in the development of neurodegenerative disorders in adult life. To test this hypothesis, we used a zebrafish model to link the herbicide paraquat (PQ) to disease etiology. Strikingly, treatment of 18 hpf embryonic zebrafish with low-dose PQ treatment (0.04 ppm, lower than the accepted human daily exposure) resulted in 50% display of neurodegenerative phenotypes and motor deficits at various developmental stages (segmentation to larval stage). Wide arrays of biomarkers have been employed to delineate the toxic responses which include lipid peroxidation, glutathione (GSH) and apoptosis studies. A decrease in the GSH levels, increase in lipid peroxidation and apoptosis, respectively, were observed at various developmental stages. Unexpectedly, we show that the exposure to paraquat during the window of dopamine neurogenesis causes Parkinsonian like motor defects in later life by perturbing cholinergic system due to oxidative stress.

The TORC2 component, Sin1, controls migration of anterior mesendoderm during zebrafish gastrulation

TORC2 is a serine-threonine kinase complex conserved through evolution that recently emerged as a new regulator of actin dynamics and cell migration. However, knockout in mice of its core components Sin1 and Rictor is embryonic lethal, which has limited in vivo analyses. Here, we analysed TORC2 function during early zebrafish development, using a morpholino-mediated loss of function of sin1. Sin1 appears required during gastrulation for migration of the prechordal plate, the anterior most mesoderm. In absence of Sin1, cells migrate both slower and less persistently, which can be correlated to a reduction in actin-rich protrusions and a randomisation of the remaining protrusions. These results demonstrate that, as established in vitro, the TORC2 component Sin1 controls actin dynamics and cell migration in vivo. We furthermore establish that Sin1 is required for protrusion formation downstream of PI3K, and is acting upstream of the GTPase Rac1, since expression of an activated form of Rac1 is sufficient to rescue sin1 loss of function.

Global identification of Smad2 and Eomesodermin targets in zebrafish identifies a conserved transcriptional network in mesendoderm and a novel role for Eomesodermin in repression of ectodermal gene expression

Background

Nodal signalling is an absolute requirement for normal mesoderm and endoderm formation in vertebrate embryos, yet the transcriptional networks acting directly downstream of Nodal and the extent to which they are conserved is largely unexplored, particularly in vivo. Eomesodermin also plays a role in patterning mesoderm and endoderm in vertebrates, but its mechanisms of action and how it interacts with the Nodal signalling pathway are still unclear.

Results

Using a combination of expression analysis and chromatin immunoprecipitation with deep sequencing (ChIP-seq) we identify direct targets of Smad2, the effector of Nodal signalling in blastula stage zebrafish embryos, including many novel target genes. Through comparison of these data with published ChIP-seq data in human, mouse and Xenopus we show that the transcriptional network driven by Smad2 in mesoderm and endoderm is conserved in these vertebrate species. We also show that Smad2 and zebrafish Eomesodermin a (Eomesa) bind common genomic regions proximal to genes involved in mesoderm and endoderm formation, suggesting Eomesa forms a general component of the Smad2 signalling complex in zebrafish. Combinatorial perturbation of Eomesa and Smad2-interacting factor Foxh1 results in loss of both mesoderm and endoderm markers, confirming the role of Eomesa in endoderm formation and its functional interaction with Foxh1 for correct Nodal signalling. Finally, we uncover a novel role for Eomesa in repressing ectodermal genes in the early blastula.

Conclusions

Our data demonstrate that evolutionarily conserved developmental functions of Nodal signalling occur through maintenance of the transcriptional network directed by Smad2. This network is modulated by Eomesa in zebrafish which acts to promote mesoderm and endoderm formation in combination with Nodal signalling, whilst Eomesa also opposes ectoderm gene expression. Eomesa, therefore, regulates the formation of all three germ layers in the early zebrafish embryo.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-014-0081-5) contains supplementary material, which is available to authorized users.