Convergence of distinct pathways to heart patterning revealed by the small molecule concentramide and the mutation heart-and-soul

PRC1 Stabilizes Cardiac Contraction by Regulating Cardiac Sarcomere Assembly and Cardiac Conduction System Construction

Cardiac development is a complex process that is strictly controlled by various factors, including PcG protein complexes. Several studies have reported the critical role of PRC2 in cardiogenesis. However, little is known about the regulation mechanism of PRC1 in embryonic heart development. To gain more insight into the mechanistic role of PRC1 in cardiogenesis, we generated a PRC1 loss-of-function zebrafish line by using the CRISPR/Cas9 system targeting rnf2, a gene encoding the core subunit shared by all PRC1 subfamilies. Our results revealed that Rnf2 is not involved in cardiomyocyte differentiation and heart tube formation, but that it is crucial to maintaining regular cardiac contraction. Further analysis suggested that Rnf2 loss-of-function disrupted cardiac sarcomere assembly through the ectopic activation of non-cardiac sarcomere genes in the developing heart. Meanwhile, Rnf2 deficiency disrupts the construction of the atrioventricular canal and the sinoatrial node by modulating the expression of bmp4 and other atrioventricular canal marker genes, leading to an impaired cardiac conduction system. The disorganized cardiac sarcomere and defective cardiac conduction system together contribute to defective cardiac contraction. Our results emphasize the critical role of PRC1 in the cardiac development.

Knock-in tagging in zebrafish facilitated by insertion into non-coding regions

Zebrafish provide an excellent model for in vivo cell biology studies because of their amenability to live imaging. Protein visualization in zebrafish has traditionally relied on overexpression of fluorescently tagged proteins from heterologous promoters, making it difficult to recapitulate endogenous expression patterns and protein function. One way to circumvent this problem is to tag the proteins by modifying their endogenous genomic loci. Such an approach is not widely available to zebrafish researchers because of inefficient homologous recombination and the error-prone nature of targeted integration in zebrafish. Here, we report a simple approach for tagging proteins in zebrafish on their N or C termini with fluorescent proteins by inserting PCR-generated donor amplicons into non-coding regions of the corresponding genes. Using this approach, we generated endogenously tagged alleles for several genes that are crucial for epithelial biology and organ development, including the tight junction components ZO-1 and Cldn15la, the trafficking effector Rab11a, the apical polarity protein aPKC and the ECM receptor Integrin β1b. Our approach facilitates the generation of knock-in lines in zebrafish, opening the way for accurate quantitative imaging studies.

Towards spatio-temporally resolved developmental cardiac gene regulatory networks in zebrafish

Heart formation in the zebrafish involves a rapid, complex series of morphogenetic events in three-dimensional space that spans cardiac lineage specification through to chamber formation and maturation. This process is tightly orchestrated by a cardiac gene regulatory network (GRN), which ensures the precise spatio-temporal deployment of genes critical for heart formation. Alterations of the timing or spatial localisation of gene expression can have a significant impact in cardiac ontogeny and may lead to heart malformations. Hence, a better understanding of the cellular and molecular basis of congenital heart disease relies on understanding the behaviour of cardiac GRNs with precise spatiotemporal resolution. Here, we review the recent technical advances that have expanded our capacity to interrogate the cardiac GRN in zebrafish. In particular, we focus on studies utilising high-throughput technologies to systematically dissect gene expression patterns, both temporally and spatially during heart development.

A countermeasure development pipeline

We have developed an integrated pipeline for countermeasure discovery that, under the auspices of the National Institutes of Health Countermeasures Against Chemical Threats network, is one of the few efforts within academia that by design spans the spectrum from discovery to phase I. The successful implementation of this approach for cyanide would enable efficient proof-of-concept studies that would lay the foundation for a generalizable strategy for parallel mechanistic studies and accelerated countermeasure development in the face of new and emerging chemical threats.

Zebrafish: A Model for the Study of Toxicants Affecting Muscle Development and Function

The rapid progress in medicine, agriculture, and allied sciences has enabled the development of a large amount of potentially useful bioactive compounds, such as drugs and pesticides. However, there is another side of this phenomenon, which includes side effects and environmental pollution. To avoid or minimize the uncontrollable consequences of using the newly developed compounds, researchers seek a quick and effective means of their evaluation. In achieving this goal, the zebrafish (Danio rerio) has proven to be a highly useful tool, mostly because of its fast growth and development, as well as the ability to absorb the molecules diluted in water through its skin and gills. In this review, we focus on the reports concerning the application of zebrafish as a model for assessing the impact of toxicants on skeletal muscles, which share many structural and functional similarities among vertebrates, including zebrafish and humans.

Strategies for analyzing cardiac phenotypes in the zebrafish embryo

The molecular mechanisms underlying cardiogenesis are of critical biomedical importance due to the high prevalence of cardiac birth defects. Over the past two decades, the zebrafish has served as a powerful model organism for investigating heart development, facilitated by its powerful combination of optical access to the embryonic heart and plentiful opportunities for genetic analysis. Work in zebrafish has identified numerous factors that are required for various aspects of heart formation, including the specification and differentiation of cardiac progenitor cells, the morphogenesis of the heart tube, cardiac chambers, and atrioventricular canal, and the establishment of proper cardiac function. However, our current roster of regulators of cardiogenesis is by no means complete. It is therefore valuable for ongoing studies to continue pursuit of additional genes and pathways that control the size, shape, and function of the zebrafish heart. An extensive arsenal of techniques is available to distinguish whether particular mutations, morpholinos, or small molecules disrupt specific processes during heart development. In this chapter, we provide a guide to the experimental strategies that are especially effective for the characterization of cardiac phenotypes in the zebrafish embryo.

Therapeutic Potential of Differentiated Mesenchymal Stem Cells for Treatment of Osteoarthritis

Osteoarthritis (OA) is a chronic, progressive, and irreversible degenerative joint disease. Conventional OA treatments often result in complications such as pain and limited activity. However, transplantation of mesenchymal stem cells (MSCs) has several beneficial effects such as paracrine effects, anti-inflammatory activity, and immunomodulatory capacity. In addition, MSCs can be differentiated into several cell types, including chondrocytes, osteocytes, endothelia, and adipocytes. Thus, transplantation of MSCs is a suggested therapeutic tool for treatment of OA. However, transplanted naïve MSCs can cause problems such as heterogeneous populations including differentiated MSCs and undifferentiated cells. To overcome this problem, new strategies for inducing differentiation of MSCs are needed. One possibility is the application of microRNA (miRNA) and small molecules, which regulate multiple molecular pathways and cellular processes such as differentiation. Here, we provide insight into possible strategies for cartilage regeneration by transplantation of differentiated MSCs to treat OA patients.

Isolation of a small molecule with anti-MRSA activity from a mangrove symbiont Streptomyces sp. PVRK-1 and its biomedical studies in Zebrafish embryos

OBJECTIVE

The aim of the present study was to isolate the anti-MRSA (Methicillin Resistant Staphylococcus aureus) molecule from the Mangrove symbiont Streptomyces and its biomedical studies in Zebrafish embryos.

METHODS

MRSA was isolated from the pus samples of Colachal hospitals and confirmed by amplification of mecA gene. Anti-MRSA molecule producing strain was identified by 16s rRNA gene sequencing. Anti-MRSA compound production was optimized by Solid State Fermentation (SSF) and the purification of the active molecule was carried out by TLC and RP-HPLC. The inhibitory concentration and LC50 were calculated using Statistical software SPSS. The Biomedical studies including the cardiac assay and organ toxicity assessment were carried out in Zebrafish.

RESULTS

The bioactive anti-MRSA small molecule A2 was purified by TLC with Rf value of 0.37 with 1.389 retention time at RP-HPLC. The Inhibitory Concentration of the purified molecule A2 was 30 µg/mL but, the inhibitory concentration of the MRSA in the infected embryo was 32-34 µg/mL for TLC purified molecule A2 with LC50 mean value was 61.504 µg/mL. Zebrafish toxicity was assessed in 48-60 µg/mL by observing the physiological deformities and the heart beat rates (HBR) of embryos for anti MRSA molecule showed the mean of 41.33-41.67 HBR/15 seconds for 40 µg/mL and control was 42.33-42.67 for 15 seconds which significantly showed that the anti-MRSA molecule A2 did not affected the HBR.

CONCLUSIONS

Anti-MRSA molecule from Streptomyces sp PVRK-1 was isolated and biomedical studies in Zebrafish model assessed that the molecule was non toxic at the minimal inhibitory concentration of MRSA.

1,2,3-Dithiazoles – new reversible melanin synthesis inhibitors: a chemical genomics study

1,2,3-Dithiazolimines show potent and reversible inhibition of melanin synthesis in Xenopus laevis embryos.

Zebrafish pronephros tubulogenesis and epithelial identity maintenance are reliant on the polarity proteins Prkc iota and zeta

The zebrafish pronephros provides an excellent in vivo system to study the mechanisms of vertebrate nephron development. When and how renal progenitors in the zebrafish embryo undergo tubulogenesis to form nephrons is poorly understood, but is known to involve a mesenchymal to epithelial transition (MET) and the acquisition of polarity. Here, we determined the precise timing of these events in pronephros tubulogenesis. As the ternary polarity complex is an essential regulator of epithelial cell polarity across tissues, we performed gene knockdown studies to assess the roles of the related factors atypical protein kinase C iota and zeta (prkcι, prkcζ). We found that prkcι and prkcζ serve partially redundant functions to establish pronephros tubule epithelium polarity. Further, the loss of prkcι or the combined knockdown of prkcι/ζ disrupted proximal tubule morphogenesis and podocyte migration due to cardiac defects that prevented normal fluid flow to the kidney. Surprisingly, tubule cells in prkcι/ζ morphants displayed ectopic expression of the transcription factor pax2a and the podocyte-associated genes wt1a, wt1b, and podxl, suggesting that prkcι/ζ are needed to maintain renal epithelial identity. Knockdown of genes essential for cardiac contractility and vascular flow to the kidney, such as tnnt2a, or elimination of pronephros fluid output through knockdown of the intraflagellar transport gene ift88, was not associated with ectopic pronephros gene expression, thus suggesting a unique role for prkcι/ζ in maintaining tubule epithelial identity separate from the consequence of disruptions to renal fluid flow. Interestingly, knockdown of pax2a, but not wt1a, was sufficient to rescue ectopic tubule gene expression in prkcι/ζ morphants. These data suggest a model in which the redundant activities of prkcι and prkcζ are essential to establish tubule epithelial polarity and also serve to maintain proper epithelial cell type identity in the tubule by inhibiting pax2a expression. These studies provide a valuable foundation for further analysis of MET during nephrogenesis, and have implications for understanding the pathways that affect nephron epithelial cells during kidney disease and regeneration.

The Par-PrkC polarity complex is required for cilia growth in zebrafish photoreceptors

Specification and development of the apical membrane in epithelial cells requires the function of polarity proteins, including Pard3 and an atypical protein kinase C (PrkC). Many epithelial cells possess microtubule-based organelles, known as cilia, that project from their apical surface and the membrane surrounding the cilium is contiguous with the apical cell membrane. Although cilia formation in cultured cells required Pard3, the in vivo requirement for Pard3 in cilia development remains unknown. The vertebrate photoreceptor outer segment represents a highly specialized cilia structure in which to identify factors necessary for apical and ciliary membrane formation. Pard3 and PrkC localized to distinct domains within vertebrate photoreceptors. Using partial morpholino knockdown, photo-morpholinos, and pharmacological approaches, the function of Pard3 and PrkC were found to be required for the formation of both the apical and ciliary membrane of vertebrate photoreceptors. Inhibition of Pard3 or PrkC activity significantly reduced the size of photoreceptor outer segments and resulted in mislocalization of rhodopsin. Suppression of Pard3 or PrkC also led to a reduction in cilia size and cilia number in Kupffer's Vesicle, which resulted in left-right asymmetry defects. Thus, the Par-PrkC complex functions in cilia formation in vivo and this likely reflects a general role in specifying non-ciliary and ciliary compartments of the apical domain.

Profiling of biomarkers for the exposure of polycyclic aromatic hydrocarbons: lamin-A/C isoform 3, poly[ADP-ribose] polymerase 1, and mitochondria copy number are identified as universal biomarkers

This study investigated the profiling of polycyclic aromatic hydrocarbon- (PAH-) induced genotoxicity in cell lines and zebrafish. Each type of cells displayed different proportionality of apoptosis. Mitochondrial DNA (mtDNA) copy number was dramatically elevated after 5-day treatment of fluoranthene and pyrene. The notable deregulated proteins for PAHs exposure were displayed as follows: lamin-A/C isoform 3 and annexin A1 for benzopyrene; lamin-A/C isoform 3 and DNA topoisomerase 2-alpha for pentacene; poly[ADP-ribose] polymerase 1 (PARP-1) for fluoranthene; and talin-1 and DNA topoisomerase 2-alpha for pyrene. Among them, lamin-A/C isoform 3 and PARP-1 were further confirmed using mRNA and protein expression study. Obvious morphological abnormalities including curved backbone and cardiomegaly in zebrafish were observed in the 54 hpf with more than 400 nM of benzopyrene. In conclusion, the change of mitochondrial genome (increased mtDNA copy number) was closely associated with PAH exposure in cell lines and mesenchymal stem cells. Lamin-A/C isoform 3, talin-1, and annexin A1 were identified as universal biomarkers for PAHs exposure. Zebrafish, specifically at embryo stage, showed suitable in vivo model for monitoring PAHs exposure to hematopoietic tissue and other organs.

Discovery of therapeutic targets by phenotype-based zebrafish screens

The easy identification of phenotypes in the transparent zebrafish embryo has enabled numerous genetic, antisense morpholino oligonucleotide, and small molecule screens. Can zebrafish screens also be used for unbiased discovery of novel drug targets?:

Developmental toxicity and estrogenic potency of zearalenone in zebrafish (Danio rerio)

Zearalenone (ZEA, F2) is one of the most common mycotoxins and the only known mycoestrogen. It enters the food and feed chain from contaminated cereals and infiltrates into sewage or natural waters posing potential threat to exposed livestock, wildlife and humans. Therefore evaluation of its biological effects is of international importance. We performed toxicological tests on zebrafish (Danio rerio) larvae and adults. Developmental toxicity was assessed by an extended (5 days) fish embryo toxicity test (FET). Effects of early ZEA exposure were concentration-dependent with LC50 and LC10 values of 893 and 335 μg/L. In larvae exposed to 500 μg/L and above, ZEA induced similar phenotype to has (heart-and soul) showing defects in heart and eye development and upward curvature of the body axis. From 250 μg/L at 72 hpf the gap in the melanophore streak at the base of the tail fin was missing and the fin fold was abnormal, suggesting disturbance in the development of the adult tail fin primordium. Estrogenic potency was measured on the basis of Vitellogenin (Vtg) protein (adults) levels and relative abundance of vitellogenin-1 mRNA (vtg-1) (larvae and adults). qRT-PCR in larvae proved to be sufficient substitute to adult tests and sensitive enough to detect ZEA in 0.1 μg/L concentrations, that is close to levels observed in wastewaters. Developmental defects reveal that besides direct estrogenic effects, zearalenone might interact with other ontogenic pathways.

Integrated compound profiling screens identify the mitochondrial electron transport chain as the molecular target of the natural products manassantin, sesquicillin, and arctigenin

Phenotypic compound screens can be used to identify novel targets in signaling pathways and disease processes, but the usefulness of these screens depends on the ability to quickly determine the target and mechanism of action of the molecules identified as hits. One fast route to discovering the mechanism of action of a compound is to profile its properties and to match this profile with those of compounds of known mechanism of action. In this work, the Novartis collection of over 12,000 pure natural products was screened for effects on early zebrafish development. The largest phenotypic class of hits, which caused developmental arrest without necrosis, contained known electron transport chain inhibitors and many compounds of unknown mechanism of action. High-throughput transcriptional profiling revealed that these compounds are mechanistically related to one another. Metabolic and biochemical assays confirmed that all of the molecules that induced developmental arrest without necrosis inhibited the electron transport chain. These experiments demonstrate that the electron transport chain is the target of the natural products manassantin, sesquicillin, and arctigenin. The overlap between the zebrafish and transcriptional profiling screens was not perfect, indicating that multiple profiling screens are necessary to fully characterize molecules of unknown function. Together, zebrafish screening and transcriptional profiling represent sensitive and scalable approaches for identifying bioactive compounds and elucidating their mechanism of action.

Drug Screening to Treat Early-Onset Eye Diseases: Can Zebrafish Expedite the Discovery?

The molecular basis of many early-onset eye diseases has been uncovered, but the number of available drug treatments for improving deteriorated vision is still scarce. Consequently, there is a high demand for new drugs to treat these diseases. This review first provides a brief synopsis of the use of zebrafish model for screening drugs with vision benefits. In particular, visual-motor response, the activity response of larvae to a change in light stimuli, is proposed to serve as a simple and efficient tool for screening drugs that may improve vision in various zebrafish visual mutants. The second part of the review discusses the identification of novel drug candidates, with particular emphasis on naturally derived chemicals including traditional Chinese medicines and nutritional therapies on retinal degenerative diseases. Many of these chemicals have been used in neuroprotection and/or have been consumed by many populations for good health and vision; thus, the screening of these chemicals with various zebrafish visual mutants would expedite the development of novel drugs for treating early-onset eye diseases.

Uncovering the molecular and cellular mechanisms of heart development using the zebrafish

Over the past 20 years, the zebrafish has emerged as a powerful model organism for studying cardiac development. Its ability to survive without an active circulation and amenability to forward genetics has led to the identification of numerous mutants whose study has helped elucidate new mechanisms in cardiac development. Furthermore, its transparent, externally developing embryos have allowed detailed cellular analyses of heart development. In this review, we discuss the molecular and cellular processes involved in zebrafish heart development from progenitor specification to development of the valve and the conduction system. We focus on imaging studies that have uncovered the cellular bases of heart development and on zebrafish mutants with cardiac abnormalities whose study has revealed novel molecular pathways in cardiac cell specification and tissue morphogenesis.

In vivo conditions to identify Prkci phosphorylation targets using the analog-sensitive kinase method in zebrafish

Protein kinase C iota is required for various cell biological processes including epithelial tissue polarity and organ morphogenesis. To gain mechanistic insight into different roles of this kinase, it is essential to identify specific substrate proteins in their cellular context. The analog-sensitive kinase method provides a powerful tool for the identification of kinase substrates under in vivo conditions. However, it has remained a major challenge to establish screens based on this method in multicellular model organisms. Here, we report the methodology for in vivo conditions using the analog-sensitive kinase method in a genetically-tractable vertebrate model organism, the zebrafish. With this approach, kinase substrates can uniquely be labeled in the developing zebrafish embryo using bulky ATPγS analogs which results in the thiophosphorylation of substrates. The labeling of kinase substrates with a thiophosphoester epitope differs from phosphoesters that are generated by all other kinases and allows for an enrichment of thiophosphopeptides by immunoaffinity purification. This study provides the foundation for using the analog-sensitive kinase method in the context of complex vertebrate development, physiology, or disease.

Zebrafish models in cardiac development and congenital heart birth defects

The zebrafish has become an ideal vertebrate animal system for investigating cardiac development due to its genetic tractability, external fertilization, early optical clarity and ability to survive without a functional cardiovascular system during development. In particular, recent advances in imaging techniques and the creation of zebrafish transgenics now permit the in vivo analysis of the dynamic cellular events that transpire during cardiac morphogenesis. As a result, the combination of these salient features provides detailed insight as to how specific genes may influence cardiac development at the cellular level. In this review, we will highlight how the zebrafish has been utilized to elucidate not only the underlying mechanisms of cardiac development and human congenital heart diseases (CHDs), but also potential pathways that may modulate cardiac regeneration. Thus, we have organized this review based on the major categories of CHDs-structural heart, functional heart, and vascular/great vessel defects, and will conclude with how the zebrafish may be further used to contribute to our understanding of specific human CHDs in the future.

Cardiosulfa induces heart deformation in zebrafish through the AhR-mediated, CYP1A-independent pathway

Heart development is a complicated and elaborate biological process. To study this and similar complicated process and diseases, the discovery and use of small molecules for probing biological events is invaluable. As part of such an investigation, we have identified cardiosulfa, a small molecule that induces severely impaired heart morphology and function in zebrafish. The results of the present study show that cardiosulfa-promoted heart deformation is protected by negative regulators of the aryl hydrocarbon receptor (AhR) signaling pathway, such as the AhR antagonist CH-223191 and an AhR2-morpholino antisense oligonucleotide, zfahr2-MO. However, the toxic effect of cardiosulfa is not alleviated by zfcyp1a-MO, a morpholino antisense oligo for cytochrome P450 1A (CYP1A), which is the most well-characterized gene of the AhR pathway. Similar results were obtained for the known AhR agonist PCB126. These observations suggest that cardiosulfa causes heart deformation in zebrafish through the AhR-mediated, CYP1A-independent pathway. Our results indicate that cardiosulfa has potential as a novel type of a biological probe to investigate the AhR pathway.

Quercetin-4'-O-β-D-glucopyranoside (QODG) inhibits angiogenesis by suppressing VEGFR2-mediated signaling in zebrafish and endothelial cells

BACKGROUND

Angiogenesis plays an important role in many physiological and pathological processes. Identification of small molecules that block angiogenesis and are safe and affordable has been a challenge in drug development. Hypericum attenuatum Choisy is a Chinese herb medicine commonly used for treating hemorrhagic diseases. The present study investigates the anti-angiogenic effects of quercetin-4'-O-β-D-glucopyranoside (QODG), a flavonoid isolated from Hypericum attenuatum Choisy, in vivo and in vitro, and clarifies the underlying mechanism of the activity.

METHODOLOGY/PRINCIPAL FINDINGS

Tg(fli1:EGFP) transgenic zebrafish embryos were treated with different concentrations of quercetin-4'-O-β-D-glucopyranoside (QODG) (20, 60, 180 µM) from 6 hours post fertilisation (hpf) to 72 hpf, and adult zebrafish were allowed to recover in different concentrations of QODG (20, 60, 180 µM) for 7 days post amputation (dpa) prior morphological observation and angiogenesis phenotypes assessment. Human umbilical vein endothelial cells (HUVECs) were treated with or without VEGF and different concentrations of QODG (5, 20, 60, 180 µM), then tested for cell viability, cell migration, tube formation and apoptosis. The role of VEGFR2-mediated signaling pathway in QODG-inhibited angiogenesis was evaluated using quantitative real-time PCR (qRT-PCR) and Western blotting.

CONCLUSION/SIGNIFICANCE

Quercetin-4'-O-β-D-glucopyranoside (QODG) was shown to inhibit angiogenesis in human umbilical vein endothelial cells (HUVECs) in vitro and zebrafish in vivo via suppressing VEGF-induced phosphorylation of VEGFR2. Our results further indicate that QODG inhibits angiogenesis via inhibition of VEGFR2-mediated signaling with the involvement of some key kinases such as c-Src, FAK, ERK, AKT, mTOR and S6K and induction of apoptosis. Together, this study reveals, for the first time, that QODG acts as a potent VEGFR2 kinase inhibitor, and exerts the anti-angiogenic activity at least in part through VEGFR2-mediated signaling pathway.

Sodium pump activity in the yolk syncytial layer regulates zebrafish heart tube morphogenesis

Na(+),K(+) ATPase pumps Na(+) out of and K(+) into the cytosol, maintaining a resting potential that is essential for the function of excitable tissues like cardiac muscle. In addition to its well-characterized physiological role in the heart, Na(+),K(+) ATPase also regulates the morphogenesis of the embryonic zebrafish heart via an as yet unknown mechanism. Here, we describe a novel non-cell autonomous function of Na(+),K(+) ATPase/Atp1a1 in the elongation of the zebrafish heart tube. Embryos lacking Atp1a1 function exhibit abnormal migration behavior of cardiac precursors, defects in the elongation of the heart tube, and a severe reduction in ECM/Fibronectin deposition around the myocardium, despite the presence of normal cell polarity and junctions in the myocardial epithelium prior to the timeframe of heart tube elongation. Interestingly, we found that Atp1a1 is not present in the myocardium at the time when cardiac morphogenesis defects first become apparent, but is expressed in an extra-embryonic tissue, the yolk syncytial layer (YSL), at earlier stages. Knockdown of Atp1a1 activity specifically in the YSL using morpholino oligonucleotides produced heart tube elongation defects like those found in atp1a1 mutants, indicating that Atp1a1 function in the YSL is necessary for heart tube elongation. Furthermore, atp1a1 expression in the YSL was regulated by the homeobox transcription factor mxtx1. Together, these data reveal a new non-cell autonomous role for Atp1a1 in cardiac morphogenesis and establish Na(+),K(+) ATPase as a major player in the genetic pathway by which the YSL regulates embryonic ECM deposition.

Systematic approaches to toxicology in the zebrafish

As the current paradigms of drug discovery evolve, it has become clear that a more comprehensive understanding of the interactions between small molecules and organismal biology will be vital. The zebrafish is emerging as a complement to existing in vitro technologies and established preclinical in vivo models that can be scaled for high-throughput. In this review, we highlight the current status of zebrafish toxicology studies, identify potential future niches for the model in the drug development pipeline, and define the hurdles that must be overcome as zebrafish technologies are refined for systematic toxicology.

Signaling pathways in early cardiac development

Cardiomyocyte differentiation is a complex multistep process requiring the proper temporal and spatial integration of multiple signaling pathways. Previous embryological and genetic studies have identified a number of signaling pathways that are critical to mediate the initial formation of the mesoderm and its allocation to the cardiomyocyte lineage. It has become clear that some of these signaling networks work autonomously, in differentiating myocardial cells whereas others work non-autonomously, in neighboring tissues, to regulate cardiac differentiation indirectly. Here, we provide an overview of three signaling networks that mediate cardiomyocyte specification and review recent insights into their specific roles in heart development. In addition, we demonstrate how systems level, 'omic approaches' and other high-throughput techniques such as small molecules screens are beginning to impact our understanding of cardiomyocyte specification and, to identify novel signaling pathways involved in this process. In particular, it now seems clear that at least one chemokine receptor CXCR4 is an important marker for cardiomyocyte progenitors and may play a functional role in their differentiation. Finally, we discuss some gaps in our current understanding of early lineage selection that could be addressed by various types of omic analysis.

Zebrafish as a model to study cardiac development and human cardiac disease

Over the last decade, the zebrafish has entered the field of cardiovascular research as a new model organism. This is largely due to a number of highly successful small- and large-scale forward genetic screens, which have led to the identification of zebrafish mutants with cardiovascular defects. Genetic mapping and identification of the affected genes have resulted in novel insights into the molecular regulation of vertebrate cardiac development. More recently, the zebrafish has become an attractive model to study the effect of genetic variations identified in patients with cardiovascular defects by candidate gene or whole-genome-association studies. Thanks to an almost entirely sequenced genome and high conservation of gene function compared with humans, the zebrafish has proved highly informative to express and study human disease-related gene variants, providing novel insights into human cardiovascular disease mechanisms, and highlighting the suitability of the zebrafish as an excellent model to study human cardiovascular diseases. In this review, I discuss recent discoveries in the field of cardiac development and specific cases in which the zebrafish has been used to model human congenital and acquired cardiac diseases.

Claudin5 genes encoding tight junction proteins are required for Xenopus heart formation

Claudin proteins are the major components of tight junctions connecting adjacent cells, where they regulate a variety of cellular activities. In the present paper we identified two Xenopus claudin5 genes (cldn5a and 5b), which are expressed early in the developing cardiac region. Precocious cldn5 expression was observed in explants of non-heart-forming mesoderm under inhibition of the canonical Wnt pathway. Cardiogenesis was severely perturbed by antisense oligonucleotides against cldn5 or by Cldn5 proteins lacking the cytoplasmic domain. Results of light- and electron-microscopic observations suggested that cldn5a and 5b are required for Xenopus heart tube formation through epithelialization of the precardiac mesoderm.

Par6B and atypical PKC regulate mitotic spindle orientation during epithelial morphogenesis

Cdc42 plays an evolutionarily conserved role in promoting cell polarity and is indispensable during epithelial morphogenesis. To further investigate the role of Cdc42, we have used a three-dimensional matrigel model, in which single Caco-2 cells develop to form polarized cysts. Using this system, we previously reported that Cdc42 controls mitotic spindle orientation during cell division to correctly position the apical surface in a growing epithelial structure. In the present study, we have investigated the specific downstream effectors through which Cdc42 controls this process. Here, we report that Par6B and its binding partner, atypical protein kinase C (aPKC), are required to regulate Caco-2 morphogenesis. Depletion or inhibition of Par6B or aPKC phenocopies the loss of Cdc42, inducing misorientation of the mitotic spindle, mispositioning of the nascent apical surface, and ultimately, the formation of aberrant cysts with multiple lumens. Mechanistically, Par6B and aPKC function interdependently in this context. Par6B localizes to the apical surface of Caco-2 cysts and is required to recruit aPKC to this compartment. Conversely, aPKC protects Par6B from proteasomal degradation, in a kinase-independent manner. In addition, we report that depletion or inhibition of aPKC induces robust apoptotic cell death in Caco-2 cells, significantly reducing both cyst size and number. Cell survival and apical positioning depend upon different thresholds of aPKC expression, suggesting that they are controlled by distinct downstream pathways. We conclude that Par6B and aPKC control mitotic spindle orientation in polarized epithelia and, furthermore, that aPKC coordinately regulates multiple processes to promote morphogenesis.

A guide to analysis of cardiac phenotypes in the zebrafish embryo

The zebrafish is an ideal model organism for investigating the molecular mechanisms underlying cardiogenesis, due to the powerful combination of optical access to the embryonic heart and plentiful opportunities for genetic analysis. A continually increasing number of studies are uncovering mutations, morpholinos, and small molecules that cause striking cardiac defects and disrupt blood circulation in the zebrafish embryo. Such defects can result from a wide variety of origins including defects in the specification or differentiation of cardiac progenitor cells; errors in the morphogenesis of the heart tube, the cardiac chambers, or the atrioventricular canal or problems with establishing proper cardiac function. An extensive arsenal of techniques is available to distinguish between these possibilities and thereby decipher the roots of cardiac defects. In this chapter, we provide a guide to the experimental strategies that are particularly effective for the characterization of cardiac phenotypes in the zebrafish embryo.

Dissection of cardiovascular development and disease pathways in zebrafish

The use of animal models in medicine has contributed significantly to the development of drug treatments and surgical procedures for the last century, in particular for cardiovascular disease. In order to model human disease in an animal, an appreciation of the strengths and limitations of the system are required to interpret results and design the logical sequence of steps toward clinical translation. As the world's population ages, cardiovascular disease will become even more prominent and further progress will be essential to stave off what seems destined to become a massive public health issue. Future treatments will require the imaginative application of current models as well as the generation of new ones. In this review, we discuss the resources available for modeling cardiovascular disease in zebrafish and the varied attributes of this system. We then discuss current zebrafish disease models and their potential that has yet to be exploited.

The role of crumbs genes in the vertebrate cornea

PURPOSE

To evaluate the role of crumbs genes and related epithelial polarity loci in the vertebrate cornea.

METHODS

The authors used histologic analysis and electron microscopy to evaluate the corneas of zebrafish mutant for a crumbs locus oko meduzy (ome) and in mutants of four other loci, nagie oko (nok), heart and soul (has), mosaic eyes (moe), and ncad (formerly glass onion), that function in the same or related genetic pathways. In parallel, they performed an evaluation of corneas in human carriers of a crumbs gene, CRB1, and mutations using topography and biomicroscopy. The expression of the CRB1 gene in the normal human cornea was examined by polymerase chain reaction (PCR) and immunohistochemical staining.

RESULTS

The corneas of zebrafish mutants display severe abnormalities of the epithelial and stromal layers. The epithelial cells do not properly adhere to each other, and fluid-filled spaces form between them. In addition, the layering of the corneal stroma is poorly formed or absent. The corneas of human carriers of CRB1 mutations display shape deviations compared with what has been observed in normal individuals. A PCR product of the correct size was obtained from normal human corneal samples. Sequence analyses confirmed its identity to be the human CRB1 gene. Immunohistochemical staining using anti-CRB1 yielded positive brown deposits in the human cornea.

CONCLUSIONS

crumbs genes play a role in the differentiation of the vertebrate cornea. Corneal defects associated with crumbs gene mutations are very severe in the zebrafish model and, in comparison, appear clinically less pronounced in the human eye.

Hand2 ensures an appropriate environment for cardiac fusion by limiting Fibronectin function

Heart formation requires the fusion of bilateral cardiomyocyte populations as they move towards the embryonic midline. The bHLH transcription factor Hand2 is essential for cardiac fusion; however, the effector genes that execute this function of Hand2 are unknown. Here, we provide in zebrafish the first evidence for a downstream component of the Hand2 pathway that mediates cardiac morphogenesis. Although hand2 is expressed in cardiomyocytes, mosaic analysis demonstrates that it plays a non-autonomous role in regulating cardiomyocyte movement. Gene expression profiles reveal heightened expression of fibronectin 1 (fn1) in hand2 mutant embryos. Reciprocally, overexpression of hand2 leads to decreased Fibronectin levels. Furthermore, reduction of fn1 function enables rescue of cardiac fusion in hand2 mutants: bilateral cardiomyocyte populations merge and exhibit improved tissue architecture, albeit without major changes in apicobasal polarity. Together, our data provide a novel example of a tissue creating a favorable environment for its morphogenesis: the Hand2 pathway establishes an appropriate environment for cardiac fusion through negative modulation of Fn1 levels.

Chemical screening for hair cell loss and protection in the zebrafish lateral line

In humans, most hearing loss results from death of hair cells, the mechanosensory receptors of the inner ear. Two goals of current hearing research are to protect hair cells from degeneration and to regenerate new hair cells, replacing those that are lost due to aging, disease, or environmental challenges. One limitation of research in the auditory field has been the relative inaccessibility of the mechanosensory systems in the inner ear. Zebrafish possess hair cells in both their inner ear and their lateral line system that are morphologically and functionally similar to human hair cells. The external location of the mechanosensory hair cells in the lateral line and the ease of in vivo labeling and imaging make the zebrafish lateral line a unique system for the study of hair cell toxicity, protection, and regeneration. This review focuses on the lateral line system as a model for understanding loss and protection of mechanosensory hair cells. We discuss chemical screens to identify compounds that induce hair cell loss and others that protect hair cells from known toxins and the potential application of these screens to human medicine.

Mutations in N-cadherin and a Stardust homolog, Nagie oko, affect cell-cycle exit in zebrafish retina

It has been reported that the loss of apicobasal cell polarity and the disruption of adherens junctions induce hyperplasia in the mouse developing brain. However, it is not fully understood whether hyperplasia is caused by an enhanced cell proliferation, an inhibited neurogenesis, or both. In this study, we found that the ratio of the number of proliferating progenitor cells to the total number of retinal cells increases in the neurogenic stages in zebrafish n-cadherin (ncad) and nagie oko (nok) mutants, in which the apicobasal cell polarity and adherens junctions in the retinal epithelium are disrupted. The cell-cycle progression was not altered in the ncad and nok mutants. Rather, the ratio of the number of cells undergoing neurogenic cell division to the total number of cells undergoing mitosis decreased in the ncad and nok mutant retinas, suggesting that the switching from proliferative cell division to neurogenic cell division was compromised in these mutant retinas. These findings suggest that the inhibition of neurogenesis is a primary defect that causes hyperplasia in the ncad and nok mutant retinas. The Hedgehog-protein kinase A signaling pathway and the Notch signaling pathway regulate retinal neurogenesis in zebrafish. We found that both signaling pathways are involved in the generation of neurogenic defects in the ncad and nok mutant retinas. Taken together, these findings suggest that apicobasal cell polarity and epithelial integrity are essential for retinal neurogenesis in zebrafish.

A key role for the integrin alpha2beta1 in experimental and developmental angiogenesis

The alpha2beta1 integrin receptor plays a key role in angiogenesis. Here we investigated the effects of small molecule inhibitors (SMIs) designed to disrupt integrin alpha2 I or beta1 I-like domain function on angiogenesis. In unchallenged endothelial cells, fibrillar collagen induced robust capillary morphogenesis. In contrast, tube formation was significantly reduced by SMI496, a beta1 I-like domain inhibitor and by function-blocking anti-alpha2beta1 but not -alpha1beta1 antibodies. Endothelial cells bound fluorescein-labeled collagen I fibrils, an interaction specifically inhibited by SMI496. Moreover, SMI496 caused cell retraction and cytoskeletal collapse of endothelial cells as well as delayed endothelial cell wound healing. SMI activities were examined in vivo by supplementing the growth medium of zebrafish embryos expressing green fluorescent protein under the control of the vascular endothelial growth factor receptor-2 promoter. SMI496, but not a control compound, interfered with angiogenesis in vivo by reversibly inhibiting sprouting from the axial vessels. We further characterized zebrafish alpha2 integrin and discovered that this integrin is highly conserved, especially the I domain. Notably, a similar vascular phenotype was induced by morpholino-mediated knockdown of the integrin alpha2 subunit. By live videomicroscopy, we confirmed that the vessels were largely nonfunctional in the absence of alpha2beta1 integrin. Collectively, our results provide strong biochemical and genetic evidence of a central role for alpha2beta1 integrin in experimental and developmental angiogenesis.

Distinct populations of quiescent and proliferative pancreatic beta-cells identified by HOTcre mediated labeling

Pancreatic beta-cells are critical regulators of glucose homeostasis, and they vary dramatically in their glucose stimulated metabolic response and levels of insulin secretion. It is unclear whether these parameters are influenced by the developmental origin of individual beta-cells. Using HOTcre, a Cre-based genetic switch that uses heat-induction to precisely control the temporal expression of transgenes, we labeled two populations of beta-cells within the developing zebrafish pancreas. These populations originate in distinct pancreatic buds and exhibit gene expression profiles suggesting distinct functions during development. We find that the dorsal bud derived beta-cells are quiescent and exhibit a marked decrease in insulin expression postembryonically. In contrast, ventral bud derived beta-cells proliferate actively, and maintain high levels of insulin expression compared with dorsal bud derived beta-cells. Therapeutic strategies to regulate beta-cell proliferation and function are required to cure pathological states that result from excessive beta-cell proliferation (e.g., insulinoma) or insufficient beta-cell mass (e.g., diabetes mellitus). Our data reveal the existence of distinct populations of beta-cells in vivo and should help develop better strategies to regulate beta-cell differentiation and proliferation.

Improvements in histological quality and signal retention following in situ hybridization in early chick embryos using plastic resin and recolorization

We describe a novel method that allows reliable detection of in situ hybridization signals in thin sections of plastic embedded embryos. Sections from plastic embedded embryos are thinner and have superior histological quality compared to paraffin, gelatin, agarose embedded sections or cryosections; however, plastic resin traditionally has not been used as an embedding medium following in situ hybridization because of loss of signal. When signal is detected with alkaline phosphatase and NBT/BCIP, the resulting colored precipitate is subject to fading when samples are exposed to organic compounds. The colored precipitate can be redeposited by repeating the NBT/BCIP reaction following plastic sectioning. This recolorization shows no loss of specificity, because signal is detected only where the anti-digoxigenin/alkaline phosphatase conjugated antibody is bound to the riboprobe. Strong signals can be detected without recolorization; however, weaker signals require the recolorization step. This novel method of re-depositing colored precipitate after processing and sectioning allows accurate determination of the location of gene expression and study of this expression in high quality histological sections of early chick embryos.

Shaping the zebrafish heart: from left-right axis specification to epithelial tissue morphogenesis

Although vertebrates appear bilaterally symmetric on the outside, various internal organs, including the heart, are asymmetric with respect to their position and/or their orientation based on the left/right (L/R) axis. The L/R axis is determined during embryo development. Determination of the L/R axis is fundamentally different from the determination of the anterior-posterior or the dorsal-ventral axis. In all vertebrates a ciliated organ has been described that induces a left-sided gene expression program, which includes Nodal expression in the left lateral plate mesoderm. To have a better understanding of organ laterality it is important to understand how L/R patterning induces cellular responses during organogenesis. In this review, we discuss the current understanding of the mechanisms of L/R patterning during zebrafish development and focus on how this affects cardiac morphogenesis. Several recent studies have provided unprecedented insights into the intimate link between L/R signaling and the cellular responses that drive morphogenesis of this organ.

A chemical genomic approach identifies matrix metalloproteinases as playing an essential and specific role in Xenopus melanophore migration

To dissect the function of matrix metalloproteinases (MMPs) involved in cellular migration in vivo, we undertook both a forward chemical genomic screen and a functional approach to discover modulators of melanophore (pigment cell) migration in Xenopus laevis. We identified the 8-quinolinol derivative NSC 84093 as affecting melanophore migration in the developing embryo and have shown it to act as a MMP inhibitor. Potential targets of NSC 84093 investigated include MMP-14 and MMP-2. MMP-14 is expressed in migrating neural crest cells from which melanophores are derived. MMP-2 is expressed at the relevant time of development and in a pattern that suggests it contributes to melanophore migration. Morpholino-mediated knockdown of both MMPs demonstrates they play a key role in melanophore migration and partially phenocopy the effect of NSC 84093.

Apical membrane maturation and cellular rosette formation during morphogenesis of the zebrafish lateral line

Tissue morphogenesis and cell sorting are major forces during organ development. Here, we characterize the process of tissue morphogenesis within the zebrafish lateral line primordium, a migratory sheet of cells that gives rise to the neuromasts of the posterior lateral line organ. We find that cells within this epithelial tissue constrict actin-rich membranes and enrich apical junction proteins at apical focal points. The coordinated apical membrane constriction in single Delta D-positive hair cell progenitors and in their neighbouring prospective support cells generates cellular rosettes. Live imaging reveals that cellular rosettes subsequently separate from each other and give rise to individual neuromasts. Genetic analysis uncovers an involvement of Lethal giant larvae proteins in the maturation of apical junction belts during cellular rosette formation. Our findings suggest that apical constriction of cell membranes spatially confines regions of strong cell-cell adhesion and restricts the number of tightly interconnected cells into cellular rosettes, which ensures the correct deposition of neuromasts during morphogenesis of the posterior lateral line organ.

AHR-dependent misregulation of Wnt signaling disrupts tissue regeneration

The origins of molecular toxicology can be traced to understanding the interactions between halogenated aromatic hydrocarbons and the aryl hydrocarbon receptor (AHR). The physiological consequences of activation of the aryl hydrocarbon receptor are diverse, and we are just beginning to understand the importance of the AHR signal transduction pathway in homeostasis and disease. The many downstream targets that mediate these biological responses remain undefined. Studies have exploited the power of the zebrafish model to elucidate the mechanisms by which AHR activation disrupts biological signaling. Recent genomic analysis performed in a zebrafish tissue regeneration model revealed functional cross talk between AHR and the well-established Wnt/beta-catenin signal transduction pathway. This review focuses on the development of the zebrafish model of AHR biology and the application of in vivo toxicogenomics to unravel molecular mechanisms.

Regulation of neurocoel morphogenesis by Pard6 gamma b

The Par3/Par6/aPKC protein complex plays a key role in the establishment and maintenance of apicobasal polarity, a cellular characteristic essential for tissue and organ morphogenesis, differentiation and homeostasis. During a forward genetic screen for liver and pancreas mutants, we identified a pard6gammab mutant, representing the first known pard6 mutant in a vertebrate organism. pard6gammab mutants exhibit defects in epithelial tissue development as well as multiple lumens in the neural tube. Analyses of the cells lining the neural tube cavity, or neurocoel, in wildtype and pard6gammab mutant embryos show that lack of Pard6gammab function leads to defects in mitotic spindle orientation during neurulation. We also found that the PB1 (aPKC-binding) and CRIB (Cdc-42-binding) domains and the KPLG amino acid sequence within the PDZ domain (Pals1-and Crumbs binding) are not required for Pard6gammab localization but are essential for its function in neurocoel morphogenesis. Apical membranes are reduced, but not completely absent, in mutants lacking the zygotic, or both the maternal and zygotic, function of pard6gammab, leading us to examine the localization and function of the three additional zebrafish Pard6 proteins. We found that Pard6alpha, but not Pard6beta or Pard6gammaa, could partially rescue the pard6gammab(s441) mutant phenotypes. Altogether, these data indicate a previously unappreciated functional diversity and complexity within the vertebrate pard6 gene family.

Divergent polarization mechanisms during vertebrate epithelial development mediated by the Crumbs complex protein Nagie oko

The zebrafish MAGUK protein Nagie oko is a member of the evolutionarily conserved Crumbs protein complex and functions as a scaffolding protein involved in the stabilization of multi-protein assemblies at the tight junction. During zebrafish embryogenesis, mutations in nagie oko cause defects in both epithelial polarity and cardiac morphogenesis. We used deletion constructs of Nagie oko in functional rescue experiments to define domains essential for cell polarity, maintenance of epithelial integrity and cardiac morphogenesis. Inability of Nagie oko to interact with Crumbs proteins upon deletion of the PDZ domain recreates all aspects of the nagie oko mutant phenotype. Consistent with this observation, apical localization of Nagie oko within the myocardium and neural tube is dependent on Oko meduzy/Crumbs2a. Disruption of direct interactions with Patj or Lin-7, two other members of the Crumbs protein complex, via the bipartite L27 domains produces only partial nagie oko mutant phenotypes and does not impair correct junctional localization of the truncated Nagie oko deletion protein within myocardial cells. Similarly, loss of the evolutionarily conserved region 1 domain, which mediates binding to Par6, causes only a subset of the nagie oko mutant epithelial phenotypes. Finally, deletion of the C-terminus, including the entire guanylate kinase and the SH3 domains, renders the truncated Nagie oko protein inactive and recreates all features of the nagie oko mutant phenotype when tested in functional complementation assays. Our observations reveal a previously unknown diversity of alternative multi-protein assembly compositions of the Crumbs-Nagie-oko and Par6-aPKC protein complexes that are highly dependent on the developmental context.

Nuclear localization of the zebrafish tight junction protein nagie oko

The tight junctions-associated MAGUK protein nagie oko is closely related to Drosophila Stardust, mouse protein associated with lin-seven 1 (Pals1), and human MAGUK p55 subfamily member 5 (Mpp5). As a component of the evolutionarily conserved Crumbs protein complex, nagie oko is essential for the maintenance of epithelial cell polarity. Here, we show that nagie oko contains a predicted nuclear export and two conserved nuclear localization signals. We find that loss of the predicted nuclear export signal results in nuclear protein accumulation. We show that nagie oko nuclear import is redundantly controlled by the two nuclear localization signals and the evolutionarily conserved region 1 (ECR1), which links nagie oko with Par6-aPKC. Finally, deletion forms of nagie oko that lack nuclear import and export signals complement several nagie oko mutant defects in cell polarity and epithelial integrity. This finding provides an entry point to potentially novel and unknown roles of this important cell polarity regulator.

The teleost retina as a model for developmental and regeneration biology

Retinal development in teleosts can broadly be divided into three epochs. The first is the specification of cellular domains in the larval forebrain that give rise to the retinal primordia and undergo early morphogenetic movements. The second is the neurogenic events within the retina proper-proliferation, cell fate determination, and pattern formation-that establish neuronal identities and form retinal laminae and cellular mosaics. The third, which is unique to teleosts and occurs in the functioning eye, is stretching of the retina and persistent neurogenesis that allows the growth of the retina to keep pace with the growth of the eye and other tissues. The first two events are rapid, complete by about 3 days postfertilization in the zebrafish embryo. The third is life-long and accounts for the bulk of retinal growth and the vast majority of adult retinal neurons. In addition, but clearly related to the retina's developmental history, lesions that kill retinal neurons elicit robust neuronal regeneration that originates from cells intrinsic to the retina. This paper reviews recent studies of retinal development in teleosts, focusing on those that shed light on the genetic and molecular regulation of retinal specification and morphogenesis in the embryo, retinal neurogenesis in larvae and adults, and injury-induced neuronal regeneration.