Cardiac patterning and morphogenesis in zebrafish

Developmental Toxicity and Cardiotoxicity of N, N-Dimethylaniline in Zebrafish Embryos

N, N-Dimethylaniline is an important chemical intermediate and an important metabolite of the pesticide Fenaminosulf. It is widely used in chemical production, but there is an extreme paucity of environmental risk assessments for N, N-dimethylaniline.: In this study, the cardiotoxicity of continuous exposure to N, N-dimethylaniline (20, 40, and 80 μg/mL) for 72 h was evaluated using zebrafish embryos.: The study found that N, N-dimethylaniline not only exhibits developmental toxicity to zebrafish embryos, leading to abnormalities such as pericardial edema, yolk sac edema, and spinal curvature, but also induces oxidative stress, lipid accumulation, and apoptosis, particularly affecting the heart region. Cardiac function indicators such as pericardial area, sinus venosus (SV) and bulbar artery (BA) distance, heart rate, and red blood cell (RBC) rate were all significantly altered due to exposure to N, N-dimethylaniline, with impaired cardiac morphology and structure and the downregulation of gene expression related to heart development and function (myl7, vmhc, myh6, bmp4, tbx2b, and has2).: The research findings suggest that the heart may be the potential target organ for the toxic effects of N, N-dimethylaniline, providing a scientific basis for the rational use of this compound and environmental protection. Furthermore, it enhances public awareness of the safety of substances that may degrade to produce N, N-dimethylaniline during their use.

Identification of teleost tnnc1a enhancers for specific pan-cardiac transcription

Troponin C regulates muscle contraction by forming the troponin complex with troponin I and troponin T. Different muscle types express different troponin C genes. The mechanisms of such differential transcription are not fully understood. The Zebrafish tnnc1a gene is restrictively expressed in cardiac muscles. We here identify the enhancers and promoters of the zebrafish and medaka tnnc1a genes, including intronic enhancers in zebrafish and medaka and an upstream enhancer in the medaka. The intronic and upstream enhancers are likely functionally redundant. The GFP transgenic reporter driven by these enhancers is expressed more strongly in the ventricle than in the atrium, recapitulating the expression pattern of the endogenous zebrafish tnnc1a gene. Our study identifies a new set of enhancers for cardiac-specific transgenic expression in zebrafish. These enhancers can serve as tools for future identification of transcription factor networks that drive cardiac-specific gene transcription.

Single-shot Mueller-matrix imaging of zebrafish tissues: In vivo analysis of developmental and pathological features

Zebrafish is a well-established animal model for developmental and disease studies. Its optical transparency at early developmental stages allows in vivo tissues visualization. Interaction of polarized light with these tissues provides information on their structure and properties. This approach is effective for muscle tissue analysis due to its birefringence. To enable real-time Mueller-matrix characterization of unanesthetized fish, we assembled a microscope for single-shot Mueller-matrix imaging. First, we performed a continuous observation of 48 species within the period of 2 to 96 hpf and measured temporal dependencies of the polarization features in different tissues. These measurements show that hatching was accompanied by a sharp change in the angle and degree of linearly polarized light after interaction with muscles. Second, we analyzed nine species with skeletal disorders and demonstrated that the spatial distribution of light depolarization features clearly indicated them. Obtained results demonstrated that real-time Mueller-matrix imaging is a powerful tool for label-free monitoring zebrafish embryos.

Mef2c factors are required for early but not late addition of cardiomyocytes to the ventricle

During heart formation, the heart grows and undergoes dramatic morphogenesis to achieve efficient embryonic function. Both in fish and amniotes, much of the growth occurring after initial heart tube formation arises from second heart field (SHF)-derived progenitor cell addition to the arterial pole, allowing chamber formation. In zebrafish, this process has been extensively studied during embryonic life, but it is unclear how larval cardiac growth occurs beyond 3 days post-fertilisation (dpf). By quantifying zebrafish myocardial growth using live imaging of GFP-labelled myocardium we show that the heart grows extensively between 3 and 5 dpf. Using methods to assess cell division, cellular development timing assay and Kaede photoconversion, we demonstrate that proliferation, CM addition, and hypertrophy contribute to ventricle growth. Mechanistically, we show that reduction in Mef2c activity (mef2ca^+/-;mef2cb^-/-), downstream or in parallel with Nkx2.5 and upstream of Ltbp3, prevents some CM addition and differentiation, resulting in a significantly smaller ventricle by 3 dpf. After 3 dpf, however, CM addition in mef2ca^+/-;mef2cb^-/- mutants recovers to a normal pace, and the heart size gap between mutants and their siblings diminishes into adulthood. Thus, as in mice, there is an early time window when SHF contribution to the myocardium is particularly sensitive to loss of Mef2c activity.

miR-182-5p is an evolutionarily conserved Tbx5 effector that impacts cardiac development and electrical activity in zebrafish

To dissect the TBX5 regulatory circuit, we focused on microRNAs (miRNAs) that collectively contribute to make TBX5 a pivotal cardiac regulator. We profiled miRNAs in hearts isolated from wild-type, CRE, Tbx5lox/+and Tbx5del/+ mice using a Next Generation Sequencing (NGS) approach. TBX5 deficiency in cardiomyocytes increased the expression of the miR-183 cluster family that is controlled by Kruppel-like factor 4, a transcription factor repressed by TBX5. MiR-182-5p, the most highly expressed miRNA of this family, was functionally analyzed in zebrafish. Transient overexpression of miR-182-5p affected heart morphology, calcium handling and the onset of arrhythmias as detected by ECG tracings. Accordingly, several calcium channel proteins identified as putative miR-182-5p targets were downregulated in miR-182-5p overexpressing hearts. In stable zebrafish transgenic lines, we demonstrated that selective miRNA-182-5p upregulation contributes to arrhythmias. Moreover, cardiac-specific down-regulation of miR-182-5p rescued cardiac defects in a zebrafish model of Holt-Oram syndrome. In conclusion, miR-182-5p exerts an evolutionarily conserved role as a TBX5 effector in the onset of cardiac propensity for arrhythmia, and constitutes a relevant target for mediating the relationship between TBX5, arrhythmia and heart development.

Zebrafish Vestigial Like Family Member 4b Is Required for Valvulogenesis Through Sequestration of Transcription Factor Myocyte Enhancer Factor 2c

A variety of cardiac transcription factors/cofactors, signaling pathways, and downstream structural genes integrate to form the regulatory hierarchies to ensure proper cardiogenesis in vertebrate. Major interaction proteins of the transcription cofactor vestigial like family member 4 (VGLL4) include myocyte enhancer factor 2 (MEF2) and TEA domain transcription factors (TEAD), both of which play important roles in embryonic cardiac development and in adulthood. In this study, we identified that the deficiency of zebrafish vgll4b paralog, a unique family member expressed in developing heart, led to an impaired valve development. Mechanistically, in vgll4b mutant embryos the disruption of Vgll4b-Mef2c complex, rather than that of Vgll4b-Tead complex, resulted in an aberrant expression of krüppel-like factor 2a (klf2a) in endocardium. Such misexpression of klf2a eventually evoked the valvulogenesis defects. Our findings suggest that zebrafish Vgll4b plays an important role in modulating the transcription activity of Mef2c on klf2a during valve development in a blood-flow-independent manner.

Teleost Metamorphosis: The Role of Thyroid Hormone

In most teleosts, metamorphosis encompasses a dramatic post-natal developmental process where the free-swimming larvae undergo a series of morphological, cellular and physiological changes that enable the larvae to become a fully formed, albeit sexually immature, juvenile fish. In all teleosts studied to date thyroid hormones (TH) drive metamorphosis, being the necessary and sufficient factors behind this developmental transition. During metamorphosis, negative regulation of thyrotropin by thyroxine (T4) is relaxed allowing higher whole-body levels of T4 that enable specific responses at the tissue/cellular level. Higher local thyroid cellular signaling leads to cell-specific responses that bring about localized developmental events. TH orchestrate in a spatial-temporal manner all local developmental changes so that in the end a fully functional organism arises. In bilateral teleost species, the most evident metamorphic morphological change underlies a transition to a more streamlined body. In the pleuronectiform lineage (flatfishes), these metamorphic morphological changes are more dramatic. The most evident is the migration of one eye to the opposite side of the head and the symmetric pelagic larva development into an asymmetric benthic juvenile. This transition encompasses a dramatic loss of the embryonic derived dorsal-ventral and left-right axis. The embryonic dorsal-ventral axis becomes the left-right axis, whereas the embryonic left-right axis becomes, irrespectively, the dorsal-ventral axis of the juvenile animal. This event is an unparalleled morphological change in vertebrate development and a remarkable display of the capacity of TH-signaling in shaping adaptation and evolution in teleosts. Notwithstanding all this knowledge, there are still fundamental questions in teleost metamorphosis left unanswered: how the central regulation of metamorphosis is achieved and the neuroendocrine network involved is unclear; the detailed cellular and molecular events that give rise to the developmental processes occurring during teleost metamorphosis are still mostly unknown. Also in flatfish, comparatively little is still known about the developmental processes behind asymmetric development. This review summarizes the current knowledge on teleost metamorphosis and explores the gaps that still need to be challenged.

Ioxynil and diethylstilbestrol disrupt vascular and heart development in zebrafish

BACKGROUND

Endocrine disruption is one of the consequences of industrialization and chemicals released into the environment have a profound impact on organisms. Waterborne micromolar concentrations of ioxynil (IOX) and diethylstilbestrol (DES) in fish affect the development of the heart, vasculature and thyroid gland.

OBJECTIVES

The present study aimed to determine how IOX and DES disrupt the crosstalk between the developing thyroid gland and cardio-vascular system in zebrafish.

METHODS

Twelve hours post fertilization (hpf) wild type, Tg(fli1:GFP) or Tg(cmalc2:GFPCaaX) zebrafish embryos were exposed to 0.1 μM IOX or DES for 36 h (up until 48 hpf) or 60 h (up until 72 hpf). Embryos were used for vascular endothelial cell sorting, whole-mount immunohistochemistry, tissue selective transcriptomics, selected gene expression analysis by quantitative real-time polymerase chain reaction analysis and determination of heart rate by live imaging.

RESULTS

Exposure of zebrafish embryos to IOX and DES (0.1 μM) increased heart beat frequency and reduced ventricle volume and aorta diameter. The transcriptome of endothelial cells from blood vessels of hypertrophic, dilated and arrhythmogenic right ventricular cardiomyopathy was significantly changed and compound-specific toxic effects were found in IOX and DES exposed embryos. Both DES and IOX directly affected vascular and heart development and this indirectly impaired thyroid gland development in zebrafish. Even though the toxicity end-point of the two chemicals was similar, their action seemed to be via different gene regulatory pathways and physiological mechanisms.

CONCLUSION

IOX and DES directly disrupt cardiovascular development and there is an associated disruption of thyroid tissue that most likely has long term consequences for this endocrine axis.

Aconitum alkaloids induce cardiotoxicity and apoptosis in embryonic zebrafish by influencing the expression of cardiovascular relative genes

Aconitine (AC) and mesaconitine (MA) are major bioactive diterpenoid alkaloids derived from herbal aconitum plants. Emerging evidence indicates that AC plays a pivotal role in the cardiotoxicity for aconite poisoning. However, the cardiotoxicity data of MA, especially those on the difference between AC and MA are quite limited. Zebrafish embryos were used in this study for toxicological screening, and the cardiac morphology and function were observed. Embryos were analyzed by means of high-performance liquid chromatography (HPLC) after exposure and pharmacokinetic behaviors were also investigated. Results showed that 1.5% of the aconitum alkaloids penetrated into the zebrafish embryos. 2.5 μg/L AC and 20 μg/L MA caused a deficient cardiovascular system with yolk sac hemorrhage and early cardiac dysfunctions were observed in 96 h post-fertilization. AC showed greater cardiotoxicity than MA by comparing the EC₅₀ of pericardium edema. Aconitum alkaloids exposure also resulted in a significant decrease in the expression of cardiac genes (Tbx5, Gata4, and Nkx2.5) from an early stage (12-24 hpf), which may partly explained that the death caused by aconitum is most likely to occur within the first 24 h. In addition, a high percentage of apoptotic cells was observed in the brain region, which identified another potential target of the DDA action in zebrafish embryos.

Rapamycin attenuates pathological hypertrophy caused by an absence of trabecular formation

Cardiac trabeculae are mesh-like muscular structures within ventricular walls. Subtle perturbations in trabeculation are associated with many congenital heart diseases (CHDs), and complete failure to form trabeculae leads to embryonic lethality. Despite the severe consequence of an absence of trabecular formation, the exact function of trabeculae remains unclear. Since ErbB2 signaling plays a direct and essential role in trabecular initiation, in this study, we utilized the erbb2 zebrafish mutant as a model to address the function of trabeculae in the heart. Intriguingly, we found that the trabeculae-deficient erbb2 mutant develops a hypertrophic-like (HL) phenotype that can be suppressed by inhibition of Target of Rapamycin (TOR) signaling in a similar fashion to adult mammalian hearts subjected to mechanical overload. Further, cell transplantation experiments demonstrated that erbb2 mutant cells in an otherwise wildtype heart did not undergo hypertrophy, indicating that erbb2 mutant HL phenotypes are due to a loss of trabeculae. Together, we propose that trabeculae serve to enhance contractility and that defects in this process lead to wall-stress induced hypertrophic remodeling.

Multi-organ toxicity induced by fine particulate matter PM2.5 in zebrafish (Danio rerio) model

The fine particulate matter (PM_2.5) in air pollution is a major public health concern and now known to contribute to severe diseases, therefore, a comprehensive understanding of PM_2.5-induced adverse effects in living organisms is needed urgently. This study was aimed to evaluate the toxicity of PM_2.5 on multi-organ systems in a zebrafish (Danio rerio) model. The embryonic toxicity induced by PM_2.5 was demonstrated by an increase in mortality and inhibition of hatching rate, in a dose- and time-dependent manner. PM_2.5 caused the pericardial edema, as well as reducing heart rate and cardiac output. The area of sub-intestinal vessels (SIVs) was significant reduced in Tg(fli-1:EGFP) transgenic zebrafish lines. Morphological defects and yolk sac retention were associated with hepatocyte injury. In addition, PM_2.5 disrupted the axonal integrity, altering of axon length and pattern in Tg(NBT:EGFP) transgenic lines. Genes involved in cardiac function (spaw, supt6h, cmlc1), angiogenesis (vegfr2a, vegfr2b), and neural function (gabrd, chrna3, npy8br) were markedly down-regulated; while genes linked to hepatic metabolism (cyp1a, cyp1b1, cyp1c1) were significantly up-regulated by PM_2.5. In summary, our data showed that PM_2.5 induced the cardiovascular toxicity, hepatotoxicity and neurotoxicity in zebrafish, suggested that PM_2.5 could cause multi-organ toxicity in aquatic organism.

Spatial Allocation and Specification of Cardiomyocytes during Zebrafish Embryogenesis

Incomplete development and severe malformation of the heart result in miscarriage of embryos because of its malfunction as a pump for circulation. During cardiogenesis, development of the heart is precisely coordinated by the genetically-primed program that is revealed by the sequential expression of transcription factors. It is important to investigate how spatial allocation of the heart containing cardiomyocytes and other mesoderm-derived cells is determined. In addition, the molecular mechanism underlying cardiomyocyte differentiation still remains elusive. The location of ectoderm-, mesoderm-, and endoderm-derived organs is determined by their initial allocation and subsequent mutual cell-cell interactions or paracrine-based regulation. In the present work, we provide an overview of cardiac development controlled by the germ layers and discuss the points that should be uncovered in future for understanding cardiogenesis.

Cardiac contraction activates endocardial Notch signaling to modulate chamber maturation in zebrafish

Congenital heart disease often features structural abnormalities that emerge during development. Accumulating evidence indicates a crucial role for cardiac contraction and the resulting fluid forces in shaping the heart, yet the molecular basis of this function is largely unknown. Using the zebrafish as a model of early heart development, we investigated the role of cardiac contraction in chamber maturation, focusing on the formation of muscular protrusions called trabeculae. By genetic and pharmacological ablation of cardiac contraction, we showed that cardiac contraction is required for trabeculation through its role in regulating notch1b transcription in the ventricular endocardium. We also showed that Notch1 activation induces expression of ephrin b2a (efnb2a) and neuregulin 1 (nrg1) in the endocardium to promote trabeculation and that forced Notch activation in the absence of cardiac contraction rescues efnb2a and nrg1 expression. Using in vitro and in vivo systems, we showed that primary cilia are important mediators of fluid flow to stimulate Notch expression. Together, our findings describe an essential role for cardiac contraction-responsive transcriptional changes in endocardial cells to regulate cardiac chamber maturation.

Pbx4 is Required for the Temporal Onset of Zebrafish Myocardial Differentiation

Proper control of the temporal onset of cellular differentiation is critical for regulating cell lineage decisions and morphogenesis during development. Pbx homeodomain transcription factors have emerged as important regulators of cellular differentiation. We previously showed, by using antisense morpholino knockdown, that Pbx factors are needed for the timely activation of myocardial differentiation in zebrafish. In order to gain further insight into the roles of Pbx factors in heart development, we show here that zebrafish pbx4 mutant embryos exhibit delayed onset of myocardial differentiation, such as delayed activation of tnnt2a expression in early cardiomyocytes in the anterior lateral plate mesoderm. We also observe delayed myocardial morphogenesis and dysmorphic patterning of the ventricle and atrium, consistent with our previous Pbx knock-down studies. In addition, we find that pbx4 mutant larvae have aberrant outflow tracts and defective expression of the proepicardial marker tbx18. Finally, we present evidence for Pbx expression in cardiomyocyte precursors as well as heterogeneous Pbx expression among the pan-cytokeratin-expressing proepicardial cells near the developing ventricle. In summary, our data show that Pbx4 is required for the proper temporal activation of myocardial differentiation and establish a basis for studying additional roles of Pbx factors in heart development.

Valvular and Mural Endocardiosis in Aging Zebrafish (Danio rerio)

Endocardiosis or myxomatous degeneration of the cardiac valves is a well-described age-related change in humans and dogs. Lesions consist of polypoid nodular proliferations of loose extracellular matrix and valvular interstitial cells, most commonly affecting the mitral valve. This entity has not been previously described in fish. Herein we report the appearance, location, and occurrence of valvular and mural endocardiosis in a retrospective survey of aging laboratory zebrafish. Endocardiosis was present in 59 of 777 fish (7.59%), most commonly affecting the sinoatrial (34 fish; 57.6%) and atrioventricular (33 fish; 55.9%) valves. Lesions were more common in fish raised in recirculating water systems and fed commercial diets (52/230 fish; 22.6%) versus flow-through systems with fish fed semi-purified diets (4/234; 1.71%). Lesions were overrepresented in fish heterozygous for a mutant smoothened allele (34/61 fish, 55.7% vs 17/168, 10.1% wild type). There was no association between endocardiosis and intestinal carcinoids. Valvular endocardiosis is a significant age- and husbandry-related background finding in zebrafish and should be considered in the design and interpretation of research studies.

microRNA and Cardiac Regeneration

Heart diseases are a very common health problem in developed as well as developing countries. In particular, ischemic heart disease and heart failure represent a plague for the patients and for the society. Loss of cardiac tissue after myocardial infarction or dysfunctioning tissue in nonischemic cardiomyopathies may result in cardiac failure. Despite great advancements in the treatment of these diseases, there is a substantial unmet need for novel therapies, ideally addressing repair and regeneration of the damaged or lost myocardium. Along this line, cardiac cell based therapies have gained substantial attention. Three main approaches are currently under investigation: stem cell therapy with either embryonic or adult stem cells; generation of patient-specific induced pluripotent stem cells; stimulation of endogenous regeneration trough direct reprogramming of fibroblasts into cardiomyocytes, activation of resident cardiac stem cells or induction of native resident cardiomyocytes to reenter the cell cycle. All these strategies need to be optimized since their efficiency is low.It has recently become clear that cardiac signaling and transcriptional pathways are intimately intertwined with microRNA molecules which act as modulators of cardiac development, function, and disease. Moreover, miRNA also regulates stem cell differentiation. Here we describe how miRNA may circumvent hurdles that hamper the field of cardiac regeneration and stem cell therapy, and how miRNA may result as the most suitable solution for the damaged heart.

Block the function of nonmuscle myosin II by blebbistatin induces zebrafish embryo cardia bifida

Nonmuscle myosin II (NM II) is the name given to the multi-subunit protein product of three genes encoding different nonmuscle myosin heavy chains including NM II-A, NM II-B, and NM II-C. Blebbistatin is a small molecule that has been shown to be a relatively specific inhibitor of NM II. Blocking the function of NM II by blebbistatin induces zebrafish embryo cardia bifida at a dose-dependent manner. In situ hybridization analysis with ventricular marker ventricular myosin heavy chain (vmhc) and atrial marker atrial myosin heavy chain (amhc) showed each of the heart contained both distinct atria and ventricle. However, the cardia bifida embryos had highly variable distance between two separate ventricles. We also provided evidence that time window from 12 to 20 h post fertilization (hpf) is necessary and sufficient for cardia bifida formation caused by blebbistatin treatment. Expression of spinster homolog 2 (spns2) was decreased in blebbistatin-treated embryos, suggesting the cardia bifida phenotype caused by NM II inhibition was relevant to precardiac mesoderm migration defects. Through in situ hybridization analysis, we showed that foxa1 was expressed in endoderm of blebbistatin-treated embryos at 24-hpf stage, suggesting the endoderm formation is normal in cardia bifida embryos caused by blebbistatin treatment. In addition, we demonstrated that blebbistatin treatment resulted in morphology alteration of zebrafish cardiomyocytes in vivo and neonatal mouse cardiomyocytes in vitro.

Cardiotoxicity evaluation of anthracyclines in zebrafish (Danio rerio)

Drug-induced cardiotoxicity is a leading factor for drug withdrawals, and limits drug efficacy and clinical use. Therefore, new alternative animal models and methods for drug safety evaluation have been given great attention. Anthracyclines (ANTs) are widely prescribed anticancer agents that have a cumulative dose relationship with cardiotoxicity. We performed experiments to study the toxicity of ANTs in early developing zebrafish embryos, especially their effects on the heart. LC50 values for daunorubicin, pirarubicin, doxorubicin (DOX), epirubicin and DOX-liposome at 72 h post-fertilization were 122.7 μM, 111.9 μM, 31.2 μM, 108.3 μM and 55.8 μM, respectively. At the same time, zebrafish embryos were exposed to ANTs in three exposure stages and induced incomplete looping of the heart tube, pericardia edema and bradycardia in a dose-dependent manner, eventually leading to death. DOX caused the greatest heart defects in the treatment stages and its liposome reduced the effects on the heart, while daunorubicin produced the least toxicity. Genes and proteins related to heart development were also identified to be sensitive to ANT exposure and downregulated by ANTs. It revealed ANTs could disturb the heart formation and development. ANTs induced cardiotoxicity in zebrafish has similar effects in mammalian models, indicating that zebrafish may have a potential value for assessment of drug-induced developmental cardiotoxicity.

Zebrafish heart development is regulated via glutaredoxin 2 dependent migration and survival of neural crest cells

Glutaredoxin 2 is a vertebrate specific oxidoreductase of the thioredoxin family of proteins modulating the intracellular thiol pool. Thereby, glutaredoxin 2 is important for specific redox signaling and regulates embryonic development of brain and vasculature via reversible oxidative posttranslational thiol modifications. Here, we describe that glutaredoxin 2 is also required for successful heart formation. Knock-down of glutaredoxin 2 in zebrafish embryos inhibits the invasion of cardiac neural crest cells into the primary heart field. This leads to impaired heart looping and subsequent obstructed blood flow. Glutaredoxin 2 specificity of the observed phenotype was confirmed by rescue experiments. Active site variants of glutaredoxin 2 revealed that the (de)-glutathionylation activity is required for proper heart formation. Our data suggest that actin might be one target during glutaredoxin 2 regulated cardiac neural crest cell migration and embryonic heart development. In summary, this work represents further evidence for the general importance of redox signaling in embryonic development and highlights additionally the importance of glutaredoxin 2 during embryogenesis.

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Reversible redox regulation, S-glutathionylation, regulates heart formation.

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Glutaredoxin 2 controls migration of neural crest cells.

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Loss of glutaredoxin 2 impairs heart looping and subsequently heart functionality.

Noonan and LEOPARD syndrome Shp2 variants induce heart displacement defects in zebrafish

Germline mutations in PTPN11, encoding Shp2, cause Noonan syndrome (NS) and LEOPARD syndrome (LS), two developmental disorders that are characterized by multiple overlapping symptoms. Interestingly, Shp2 catalytic activity is enhanced by NS mutations and reduced by LS mutations. Defective cardiac development is a prominent symptom of both NS and LS, but how the Shp2 variants affect cardiac development is unclear. Here, we have expressed the most common NS and LS Shp2-variants in zebrafish embryos to investigate their role in cardiac development in vivo. Heart function was impaired in embryos expressing NS and LS variants of Shp2. The cardiac anomalies first occurred during elongation of the heart tube and consisted of reduced cardiomyocyte migration, coupled with impaired leftward heart displacement. Expression of specific laterality markers was randomized in embryos expressing NS and LS variants of Shp2. Ciliogenesis and cilia function in Kupffer's vesicle was impaired, likely accounting for the left/right asymmetry defects. Mitogen-activated protein kinase (MAPK) signaling was activated to a similar extent in embryos expressing NS and LS Shp2 variants. Interestingly, inhibition of MAPK signaling prior to gastrulation rescued cilia length and heart laterality defects. These results suggest that NS and LS Shp2 variant-mediated hyperactivation of MAPK signaling leads to impaired cilia function in Kupffer's vesicle, causing left-right asymmetry defects and defective early cardiac development.

Gene miles-apart is required for formation of otic vesicle and hair cells in zebrafish

Hearing loss is a serious burden to physical and mental health worldwide. Aberrant development and damage of hearing organs are recognized as the causes of hearing loss, the molecular mechanisms underlining these pathological processes remain elusive. Investigation of new molecular mechanisms involved in proliferation, differentiation, migration and maintenance of neuromast primordium and hair cells will contribute to better understanding of hearing loss pathology. This knowledge will enable the development of protective agents and mechanism study of drug ototoxicity. In this study, we demonstrate that the zebrafish gene miles-apart, a homolog of sphingosine-1-phosphate receptor 2 (s1pr2) in mammals, has an important role in the development of otic vesicle, neuromasts and survival of hair cells. Whole-mount in situ hybridization of embryos showed that miles-apart expression occurred mainly in the encephalic region and the somites at 24 h.p.f. (hour post fertilization), in the midbrain/hindbrain boundary, the brainstem and the pre-neuromast of lateral line at 48 h.p.f. in a strict spatiotemporal regulation. Both up- and downregulation of miles-apart led to abnormal otoliths and semicircular canals, excess or few hair cells and neuromasts, and their disarranged depositions in the lateral lines. Miles-apart (Mil) dysregulation also caused abnormal expression of hearing-associated genes, including hmx2, fgf3, fgf8a, foxi1, otop1, pax2.1 and tmieb during zebrafish organogenesis. Moreover, in larvae miles-apart gene knockdown significantly upregulated proapoptotic gene zBax2 and downregulated prosurvival gene zMcl1b; in contrast, the level of zBax2 was decreased and of zMcl1b enhanced by miles-apart overexpression. Collectively, Mil activity is linked to organization and number decision of hair cells within a neuromast, also to deposition of neuromasts and formation of otic vesicle during zebrafish organogenesis. At the larva stage, Mil as an upstream regulator of bcl-2 gene family has a role in protection of hair cells against apoptosis by promoting expression of prosurvival gene zMcl1b and suppressing proapoptotic gene zBax2.

DiGeorge syndrome gene tbx1 functions through wnt11r to regulate heart looping and differentiation

DiGeorge syndrome (DGS) is the most common microdeletion syndrome, and is characterized by congenital cardiac, craniofacial and immune system abnormalities. The cardiac defects in DGS patients include conotruncal and ventricular septal defects. Although the etiology of DGS is critically regulated by TBX1 gene, the molecular pathways underpinning TBX1's role in heart development are not fully understood. In this study, we characterized heart defects and downstream signaling in the zebrafish tbx1^−/− mutant, which has craniofacial and immune defects similar to DGS patients. We show that tbx1^−/− mutants have defective heart looping, morphology and function. Defective heart looping is accompanied by failure of cardiomyocytes to differentiate normally and failure to change shape from isotropic to anisotropic morphology in the outer curvatures of the heart. This is the first demonstration of tbx1's role in regulating heart looping, cardiomyocyte shape and differentiation, and may explain how Tbx1 regulates conotruncal development in humans. Next we elucidated tbx1's molecular signaling pathway guided by the cardiac phenotype of tbx1^−/− mutants. We show for the first time that wnt11r (wnt11 related), a member of the non-canonical Wnt pathway, and its downstream effector gene alcama (activated leukocyte cell adhesion molecule a) regulate heart looping and differentiation similarly to tbx1. Expression of both wnt11r and alcama are downregulated in tbx1^−/− mutants. In addition, both wnt11r^−/− mutants and alcama morphants have heart looping and differentiation defects similar to tbx1^−/− mutants. Strikingly, heart looping and differentiation in tbx1^−/− mutants can be partially rescued by ectopic expression of wnt11r or alcama, supporting a model whereby heart looping and differentiation are regulated by tbx1 in a linear pathway through wnt11r and alcama. This is the first study linking tbx1 and non-canonical Wnt signaling and extends our understanding of DGS and heart development.

Yes-associated protein (yap) is required for early embryonic development in zebrafish (danio rerio)

The hippo (Hpo) signaling pathway plays a critical role in regulation of organ size. The kinase cascade ultimately antagonizes the transcriptional co-activator Yki/YAP, which is a key regulator of cell proliferation and apoptosis. In this study, we performed a knocking down study using antisense morpholino (MO) reagents and found that zebrafish YAP, a key transcriptional co-activator of Hpo pathway, plays a critical role in early embryonic development. At the cellular level, yap inhibition increases apoptosis and decreases cell proliferation. Reduction of yap function severely delays several developmental events, including gastrulation, cardiogenesis and hematopoiesis. Knockdown of yap showed some evidence of ventralization, including reduction of dorsally expressed marker goosecoid (gsc), expansion of ventral marker gata2, disruption of the somites, and reduction in head size. Finally, we performed a preliminary analysis with real-time polymerase chain reaction (qPCR) for the candidate targets of zebrafish Hpo pathway. In conclusion, our results revealed that zebrafish yap coordinately regulates cell proliferation and apoptosis and is required for dorsoventral axis formation, gastrulation, cardiogenesis, hematopoiesis, and somitogenesis.

S1pr2/Gα13 signaling controls myocardial migration by regulating endoderm convergence

A key process during vertebrate heart development is the migration of bilateral populations of myocardial precursors towards the midline to form the primitive heart tube. In zebrafish, signaling mediated by sphingosine-1-phosphate (S1P) and its cognate G protein-coupled receptor (S1pr2/Mil) is essential for myocardial migration, but the underlying mechanisms remain undefined. Here, we show that suppression of Gα(13) signaling disrupts myocardial migration, leading to the formation of two bilaterally located hearts (cardia bifida). Genetic studies indicate that Gα(13) acts downstream of S1pr2 to regulate myocardial migration through a RhoGEF-dependent pathway. Furthermore, disrupting any component of the S1pr2/Gα(13)/RhoGEF pathway impairs endoderm convergence during segmentation, and the endodermal defects correlate with the extent of cardia bifida. Moreover, endoderm transplantation reveals that the presence of wild-type anterior endodermal cells in Gα(13)-deficient embryos is sufficient to rescue the endoderm convergence defect and cardia bifida, and, conversely, that the presence of anterior endodermal cells defective for S1pr2 or Gα(13) in wild-type embryos causes such defects. Thus, S1pr2/Gα(13) signaling probably acts in the endoderm to regulate myocardial migration. In support of this notion, cardiac-specific expression of Gα(13) fails to rescue cardia bifida in the context of global Gα(13) inhibition. Our data demonstrate for the first time that the Gα(13)/RhoGEF-dependent pathway functions downstream of S1pr2 to regulate convergent movement of the endoderm, an event that is crucial for coordinating myocardial migration.

Ion flux dependent and independent functions of ion channels in the vertebrate heart: lessons learned from zebrafish

Ion channels orchestrate directed flux of ions through membranes and are essential for a wide range of physiological processes including depolarization and repolarization of biomechanical activity of cells. Besides their electrophysiological functions in the heart, recent findings have demonstrated that ion channels also feature ion flux independent functions during heart development and morphogenesis. The zebrafish is a well-established animal model to decipher the genetics of cardiovascular development and disease of vertebrates. In large scale forward genetics screens, hundreds of mutant lines have been isolated with defects in cardiovascular structure and function. Detailed phenotyping of these lines and identification of the causative genetic defects revealed new insights into ion flux dependent and independent functions of various cardiac ion channels.

Embryo exposure to elevated cortisol level leads to cardiac performance dysfunction in zebrafish

In zebrafish (Danio rerio), de novo cortisol synthesis commences only after hatching, providing an interesting model to study the effects of maternal stress and abnormal cortisol deposition on embryo development and performance. We hypothesized that elevated cortisol levels during pre-hatch embryogenesis compromise cardiac performance in developing zebrafish. Cortisol was microinjected into one-cell embryos to elevate basal cortisol levels during embryogenesis. Elevated embryo cortisol content increased heart deformities, including pericardial edema and malformed chambers, and lowered resting heartbeat post-hatch. This phenotype coincided with suppression of key cardiac genes, including nkx2.5, cardiac myosin light chain 1, cardiac troponin type T2A, and calcium transporting ATPase, underpinning a mechanistic link to heart malformation. The attenuation of the heartbeat response to a secondary stressor post-hatch also confirms a functional reduction in cardiac performance. Altogether, high cortisol content during embryogenesis, mimicking increased deposition due to maternal stress, decreases cardiac performance and may reduce zebrafish offspring survival.

Knockdown of leptin A expression dramatically alters zebrafish development

Using morpholino antisense oligonucleotide (MO) technology, we blocked leptin A or leptin receptor expression in embryonic zebrafish, and analyzed consequences of leptin A knock-down on fish development. Embryos injected with leptin A or leptin receptor MOs (leptin A or leptin receptor morphants) had smaller bodies and eyes, undeveloped inner ear, enlarged pericardial cavity, curved body and/or tail and larger yolk compared to control embryos of the same stages. The defects persisted in 6-9 days old larvae. We found that blocking leptin A function had little effect on the development of early brain (1 day old), but differentiation of both the morphant dorsal brain and retinal cells was severely disrupted in older (2 days old) embryos. Despite the enlarged pericardial cavity, differentiation of cardiac cells appeared to be similar to control embryos. Formation of the morphants' inner ear is also severely disrupted, which corroborates existing reports of leptin receptor expression in inner ear of both zebrafish and mammals. Co-injection of leptin A MO and recombinant leptin results in partial rescue of the wild-type phenotype. Our results suggest that leptin A plays distinct roles in zebrafish development.

Progesterone and adipoQ receptor 11 links ras signaling to cardiac development in zebrafish

OBJECTIVE

Progesterone and adipoQ receptor (PAQR) 10 and PAQR11 are 2 highly homologous genes involved in compartmentalized Ras signaling in the Golgi apparatus. The aim of this study was to investigate the physiological functions of PAQR10 and PAQR11.

METHODS AND RESULTS

We used zebrafish as a model system to analyze the potential function of PAQR10/PAQR11. The expression profiles of PAQR10 and PAQR11 in zebrafish embryos are overlapping in many areas, but only PAQR11 is expressed in the developing heart. Knockdown of PAQR11 but not PAQR10 in zebrafish embryos causes cardiac developmental defects, including dilation of cardiac chambers, abnormal heart looping, disruption of atrioventricular cushion formation, heart edema, and blood regurgitation. PAQR11 knockdown markedly reduces the number and proliferation rate of cardiomyocytes and alters the morphology of myocardial cells during early heart development. The cardiac defects caused by PAQR11 knockdown can be phenocopied by MEK inhibitors and a dominant negative Ras. Furthermore, constitutively active Ras and especially a Golgi-localized but not a plasma membrane-localized Ras are able to rescue the cardiac defects caused by PAQR11 knockdown.

CONCLUSIONS

This study not only provides in vivo evidence that PAQR11 plays a critical role in heart morphogenesis but also pinpoints the importance of compartmentalized Ras signaling during development.

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.

Cardiac developmental toxicity

Congenital heart disease (CHD) is a highly prevalent problem with mostly unknown origins. Many cases of CHD likely involve an environmental exposure coupled with genetic susceptibility, but practical and ethical considerations make nongenetic causes of CHD difficult to assess in humans. The development of the heart is highly conserved across all vertebrate species, making animal models an excellent option for screening potential cardiac teratogens. This review will discuss exposures known to cause cardiac defects, stages of heart development that are most sensitive to teratogen exposure, benefits and limitations of animal models of cardiac development, and future considerations for cardiac developmental toxicity research.

microRNAs in cardiovascular development

Heart development requires precise temporal-spatial regulation of gene expression, in which the highly conserved modulation networks of transcription factors accurately control the signaling pathways required for normal cardiovascular development. Even slight perturbation of such programming during cardiogenesis can cause congenital heart defects and late neonatal or adult heart disease. microRNAs (miRNAs), a class of "small" non-coding RNAs, have recently drawn a lot of attention for their "big" impact on cardiovascular development and diseases. miRNAs negatively regulate the expression of their target genes in most biological organisms through post-transcriptional processes. Here, we review the roles of miRNAs in cardiovascular development and function, looking inside the molecular mechanisms by which miRNAs act as "fine tuners" and/or "safeguards" to maintain the homeostasis of cardiovascular system. We also propose new directions for therapeutic potential of these tiny molecules.

Nephronectin regulates atrioventricular canal differentiation via Bmp4-Has2 signaling in zebrafish

The extracellular matrix is crucial for organogenesis. It is a complex and dynamic component that regulates cell behavior by modulating the activity, bioavailability and presentation of growth factors to cell surface receptors. Here, we determined the role of the extracellular matrix protein Nephronectin (Npnt) in heart development using the zebrafish model system. The vertebrate heart is formed as a linear tube in which myocardium and endocardium are separated by a layer of extracellular matrix termed the cardiac jelly. During heart development, the cardiac jelly swells at the atrioventricular (AV) canal, which precedes valve formation. Here, we show that Npnt expression correlates with this process. Morpholino-mediated knockdown of Npnt prevents proper valve leaflet formation and trabeculation and results in greater than 85% lethality at 7 days post-fertilization. The earliest observed phenotype is an extended tube-like structure at the AV boundary. In addition, the expression of myocardial genes involved in cardiac valve formation (cspg2, fibulin 1, tbx2b, bmp4) is expanded and endocardial cells along the extended tube-like structure exhibit characteristics of AV cells (has2, notch1b and Alcam expression, cuboidal cell shape). Inhibition of has2 in npnt morphants rescues the endocardial, but not the myocardial, expansion. By contrast, reduction of BMP signaling in npnt morphants reduces the ectopic expression of myocardial and endocardial AV markers. Taken together, our results identify Npnt as a novel upstream regulator of Bmp4-Has2 signaling that plays a crucial role in AV canal differentiation.

Evolution of the vertebrate pth2 (tip39) gene family and the regulation of PTH type 2 receptor (pth2r) and its endogenous ligand pth2 by hedgehog signaling in zebrafish development

In mammals, parathyroid hormone (PTH), secreted by parathyroid glands, increases calcium levels in the blood from reservoirs in bone. While mammals have two PTH receptor genes, PTH1R and PTH2R, zebrafish has three receptors, pth1r, pth2r, and pth3r. PTH can activate all three zebrafish Pthrs while PTH2 (alias tuberoinfundibular peptide 39, TIP39) preferentially activates zebrafish and mammalian PTH2Rs. We know little about the roles of the PTH2/PTH2R system in the development of any animal. To determine the roles of PTH2 and PTH2R during vertebrate development, we evaluated their expression patterns in developing zebrafish, observed their phylogenetic and conserved synteny relationships with humans, and described the genomic organization of pth2, pth2r, and pth2r splice variants. Expression studies showed that pth2 is expressed in cells adjacent to the ventral part of the posterior tuberculum in the diencephalon, whereas pth2r is robustly expressed throughout the central nervous system. Otic vesicles express both pth2 and pth2r, but heart expresses only pth2. Analysis of mutants showed that hedgehog (Hh) signaling regulates the expression of pth2 transcripts more than that of nearby gnrh2-expressing cells. Genomic analysis showed that a lizard, chicken, and zebra finch lack a PTH2 gene, which is associated with an inversion breakpoint. Likewise, chickens lack PTH2R, while humans lack PTH3R, a case of reciprocally missing ohnologs (paralogs derived from a genome duplication). The considerable evolutionary conservation in genomic structure, synteny relationships, and expression of zebrafish pth2 and pth2r provides a foundation for exploring the endocrine roles of this system in developing vertebrate embryos.

β-Lapachone induces heart morphogenetic and functional defects by promoting the death of erythrocytes and the endocardium in zebrafish embryos

Background

β-Lapachone has antitumor and wound healing-promoting activities. To address the potential influences of various chemicals on heart development of zebrafish embryos, we previously treated zebrafish embryos with chemicals from a Sigma LOPAC1280™ library and found several chemicals including β-lapachone that affected heart morphogenesis. In this study, we further evaluated the effects of β-lapachone on zebrafish embryonic heart development.

Methods

Embryos were treated with β-lapachone or dimethyl sulfoxide (DMSO) at 24 or 48 hours post fertilization (hpf) for 4 h at 28°C. Heart looping and valve development was analyzed by whole-mount in situ hybridization and histological analysis. For fractional shortening and wall shear stress analyses, AB and Tg (gata1:DsRed) embryos were recorded for their heart pumping and blood cell circulations via time-lapse fluorescence microscopy. Dextran rhodamine dye injection into the tail reticular cells was used to analyze circulation. Reactive oxygen species (ROS) was analyzed by incubating embryos in 5-(and 6-)-chloromethyl-2',7'-dichloro-dihydrofluorescein diacetate (CM-H₂DCFDA) and recorded using fluorescence microscopy. o-Dianisidine (ODA) staining and whole mount in situ hybridization were used to analyze erythrocytes. TUNEL assay was used to examine DNA fragmentation.

Results

We observed a linear arrangement of the ventricle and atrium, bradycardia arrhythmia, reduced fractional shortening, circulation with a few or no erythrocytes, and pericardial edema in β-lapachone-treated 52-hpf embryos. Abnormal expression patterns of cmlc2, nppa, BMP4, versican, and nfatc1, and histological analyses showed defects in heart-looping and valve development of β-lapachone-treated embryos. ROS production was observed in erythrocytes and DNA fragmentation was detected in both erythrocytes and endocardium of β-lapachone-treated embryos. Reduction in wall shear stress was uncovered in β-lapachone-treated embryos. Co-treatment with the NQO1 inhibitor, dicoumarol, or the calcium chelator, BAPTA-AM, rescued the erythrocyte-deficiency in circulation and heart-looping defect phenotypes in β-lapachone-treated embryos. These results suggest that the induction of apoptosis of endocardium and erythrocytes by β-lapachone is mediated through an NQO1- and calcium-dependent pathway.

Conclusions

The novel finding of this study is that β-lapachone affects heart morphogenesis and function through the induction of apoptosis of endocardium and erythrocytes. In addition, this study further demonstrates the importance of endocardium and hemodynamic forces on heart morphogenesis and contractile performance.

Essential roles of fibronectin in the development of the left-right embryonic body plan

Studies in Xenopus laevis suggested that cell-extracellular matrix (ECM) interactions regulate the development of the left-right axis of asymmetry; however, the identities of ECM components and their receptors important for this process have remained unknown. We discovered that FN is required for the establishment of the asymmetric gene expression pattern in early mouse embryos by regulating morphogenesis of the node, while cellular fates of the nodal cells, canonical Wnt and Shh signaling within the node were not perturbed by the absence of FN. FN is also required for the expression of Lefty 1/2 and activation of SMADs 2 and 3 at the floor plate, while cell fate specification of the notochord and the floor plate, as well as signaling within and between these two embryonic organizing centers remained intact in FN-null mutants. Furthermore, our experiments indicate that a major cell surface receptor for FN, integrin α5β1, is also required for the development of the left-right asymmetry, and that this requirement is evolutionarily conserved in fish and mice. Taken together, our studies demonstrate the requisite role for a structural ECM protein and its integrin receptor in the development of the left-right axis of asymmetry in vertebrates.

The PAF1 complex differentially regulates cardiomyocyte specification

The specification of an appropriate number of cardiomyocytes from the lateral plate mesoderm requires a careful balance of both positive and negative regulatory signals. To identify new regulators of cardiac specification, we performed a phenotype-driven ENU mutagenesis forward genetic screen in zebrafish. In our genetic screen we identified a zebrafish ctr9 mutant with a dramatic reduction in myocardial cell number as well as later defects in primitive heart tube elongation and atrioventricular boundary patterning. Ctr9, together with Paf1, Cdc73, Rtf1 and Leo1, constitute the RNA polymerase II associated protein complex, PAF1. We demonstrate that the PAF1 complex (PAF1C) is structurally conserved among zebrafish and other metazoans and that loss of any one of the components of the PAF1C results in abnormal development of the atrioventricular boundary of the heart. However, Ctr9, Cdc73, Paf1 and Rtf1, but not Leo1, are required for the specification of an appropriate number of cardiomyocytes and elongation of the heart tube. Interestingly, loss of Rtf1 function produced the most severe defects, resulting in a nearly complete absence of cardiac precursors. Based on gene expression analyses and transplantation studies, we found that the PAF1C regulates the developmental potential of the lateral plate mesoderm and is required cell autonomously for the specification of cardiac precursors. Our findings demonstrate critical but differential requirements for PAF1C components in zebrafish cardiac specification and heart morphogenesis.

Heart chamber size in zebrafish is regulated redundantly by duplicated tbx2 genes

The Tbx2 transcription factor is implicated in growth control based on its association with human cancers. In the heart, Tbx2 represses cardiac differentiation to mediate development of the atrioventricular canal (AVC). The zebrafish genome retains two tbx2 genes, and both are required for formation of the AVC. Here, we show that both genes are also expressed earlier in the primitive heart tube, and we describe a previously unrecognized role for Tbx2 in promoting proliferation of presumptive myocardium at the heart tube stage. In contrast to single knockdowns, depletion of both gene products causes chamber defects, resulting in an expanded atrium and a smaller ventricle, associated with decreased proliferation of ventricular cardiomyocytes. The phenotype correlates with changes in the expression for known cardiac growth factors. Therefore, in zebrafish, two tbx2 genes are functionally redundant for regulating chamber development, while each gene is required independently for development of the AVC.

Zebrafish cardiac enhancer trap lines: new tools for in vivo studies of cardiovascular development and disease

Using the transposon-mediated enhancer trap (ET), we generated 18 cardiac enhancer trap (CET) transgenic zebrafish lines. They exhibit EGFP expression in defined cell types--the endocardium, myocardium, and epicardium--or in anatomical regions of the heart--the atrium, ventricle, valves, or bulbus arteriosus. Most of these expression domains are maintained into adulthood. The genomic locations of the transposon insertions were determined by thermal asymmetric interlaced polymerase chain reaction (TAIL-PCR). The expression pattern of EGFP in some CETs is unique and recapitulates expression of genes flanking the transposon insertion site. The CETs enabled us to capture the dynamics of the embryonic heart beating in vivo using fast scanning confocal microscopy coupled with image reconstruction, producing three-dimensional movies in time (4D) illustrating region-specific features of heart contraction. This collection of CET lines represents a toolbox of markers for in vivo studies of heart development, physiology, and drug screening.

Extra-embryonic syndecan 2 regulates organ primordia migration and fibrillogenesis throughout the zebrafish embryo

One of the first steps in zebrafish heart and gut organogenesis is the migration of bilateral primordia to the midline to form cardiac and gut tubes. The mechanisms that regulate this process are poorly understood. Here we show that the proteoglycan syndecan 2 (Sdc2) expressed in the extra-embryonic yolk syncytial layer (YSL) acts locally at the YSL-embryo interface to direct organ primordia migration, and is required for fibronectin and laminin matrix assembly throughout the embryo. Surprisingly, neither endogenous nor exogenous sdc2 expressed in embryonic cells can compensate for knockdown of sdc2 in the YSL, indicating that Sdc2 expressed in extra-embryonic tissues is functionally distinct from Sdc2 in embryonic cells. The effects of sdc2 knockdown in the YSL can be rescued by extra-embryonic Sdc2 lacking an extracellular proteolytic cleavage (shedding) site, but not by extra-embryonic Sdc2 lacking extracellular glycosaminoglycan (GAG) addition sites, suggesting that distinct GAG chains on extra-embryonic Sdc2 regulate extracellular matrix assembly, cell migration and epithelial morphogenesis of multiple organ systems throughout the embryo.

Zebrafish chemical screening reveals an inhibitor of Dusp6 that expands cardiac cell lineages

The dual-specificity phosphatase 6 (Dusp6) functions as a feedback regulator of fibroblast growth factor (FGF) signaling to limit the activity of extracellular signal-regulated kinases (ERKs) 1 and 2. We have identified a small-molecule inhibitor of Dusp6-(E)-2-benzylidene-3-(cyclohexylamino)-2,3-dihydro-1H-inden-1-one (BCI)-using a transgenic zebrafish chemical screen. BCI treatment blocked Dusp6 activity and enhanced FGF target gene expression in zebrafish embryos. Docking simulations predicted an allosteric binding site for BCI within the phosphatase domain. In vitro studies supported a model in which BCI inhibits Dusp6 catalytic activation by ERK2 substrate binding. We used BCI treatment at varying developmental stages to uncover a temporal role for Dusp6 in restricting cardiac progenitors and controlling heart organ size. This study highlights the power of in vivo zebrafish chemical screens to identify new compounds targeting Dusp6, a component of the FGF signaling pathway that has eluded traditional high-throughput in vitro screens.

Isolation of a ventricle-specific promoter for the zebrafish ventricular myosin heavy chain (vmhc) gene and its regulation by GATA factors during embryonic heart development

We investigated chamber-specific gene expression by isolating a 2.2-kb polymerase chain reaction product containing the 5'-flanking region of the zebrafish ventricular myosin heavy-chain gene (vmhc). Promoter analysis revealed that the fragment, consisting of nucleotides from -301 to +26, is sufficient for vmhc promoter activity. Among several putative cis-acting elements in the region, a GATA-binding site was identified to be crucial for the activity of the promoter, as evidenced by the complete abolishment of promoter activity by a single nucleotide substitution of GATA-binding site (-287, C-->T). Knockdown of GATA-binding proteins 4 and 6 (GATA4 and -6) by their antisense morpholino oligonucleotides resulted in reduced green fluorescent protein (GFP) reporter gene and endogenous vmhc expression. These findings suggest that GATA4 and -6 play a key role in the regulation of vmhc expression in the ventricle. In addition, the vmhc promoter and the transgenic zebrafish (vmhc:gfp) are useful tools to study the formation and function of the ventricle. Developmental Dynamics 238:1574-1581, 2009. (c) 2009 Wiley-Liss, Inc.

TTLL3 Is a tubulin glycine ligase that regulates the assembly of cilia

In most ciliated cell types, tubulin is modified by glycylation, a posttranslational modification of unknown function. We show that the TTLL3 proteins act as tubulin glycine ligases with chain-initiating activity. In Tetrahymena, deletion of TTLL3 shortened axonemes and increased their resistance to paclitaxel-mediated microtubule stabilization. In zebrafish, depletion of TTLL3 led to either shortening or loss of cilia in several organs, including the Kupffer's vesicle and olfactory placode. We also show that, in vivo, glutamic acid and glycine ligases oppose each other, likely by competing for shared modification sites on tubulin. We propose that tubulin glycylation regulates the assembly and dynamics of axonemal microtubules and acts either directly or indirectly by inhibiting tubulin glutamylation.

Developmental mechanisms of arsenite toxicity in zebrafish (Danio rerio) embryos

Arsenic usually accumulates in soil, water and airborne particles, from which it is taken up by various organisms. Exposure to arsenic through food and drinking water is a major public health problem affecting some countries. At present there are limited laboratory data on the effects of arsenic exposure on early embryonic development and the mechanisms behind its toxicity. In this study, we used zebrafish as a model system to investigate the effects of arsenite on early development. Zebrafish embryos were exposed to a range of sodium arsenite concentrations (0-10.0mM) between 4 and 120h post-fertilization (hpf). Survival and early development of the embryos were not obviously influenced by arsenite concentrations below 0.5mM. However, embryos exposed to higher concentrations (0.5-10.0mM) displayed reduced survival and abnormal development including delayed hatching, retarded growth and changed morphology. Alterations in neural development included weak tactile responses to light (2.0-5.0mM, 30hpf), malformation of the spinal cord and disordered motor axon projections (2.0mM, 48hpf). Abnormal cardiac function was observed as bradycardia (0.5-2.0mM, 60hpf) and altered ventricular shape (2.0mM, 48hpf). Furthermore, altered cell proliferation (2.0mM, 24hpf) and apoptosis status (2.0mM, 24 and 48hpf), as well as abnormal genomic DNA methylation patterning (2.0mM, 24 and 48hpf) were detected in the arsenite-treated embryos. All of these indicate a possible relationship between arsenic exposure and developmental failure in early embryogenesis. Our studies suggest that the negative effects of arsenic on vertebrate embryogenesis are substantial.

Effects of methotrexate on the developments of heart and vessel in zebrafish

Methotrexate (MTX), an antagonist of folic acid, can inhibit dihydrofolate reductase (DHFR) which is of great importance in the synthesis of tetrahydrofolic acid and embryonic development. In this study, we found that after being exposed to 1.5 mM MTX at 6-10 hours post-fertilization, zebrafish embryos fail to form normal cardiovascular system. In MTX-treated embryos, the morphological development of ventricle and atrium was disrupted, the cardiac twist was abnormal, the heart rate and ventricular shortening fraction were reduced, and the vascular development was disrupted. We also found that either microinjection with dhfr-gfp mRNA or treatment with folinic acid calcium salt pentahydrate (CF) could cause improved development in the heart and vessels in MTX-treated embryos, which proved that MTX induced the malformations by inhibiting DHFR. The transcript levels of genes such as hand2, mef2a, mef2c, and flk-1 were reduced in MTXtreated embryos. Compared with the MTX-treated group, the transcript levels of hand2, mef2a, mef2c, and flk-1 were increased in the MTX 1 dhfr-gfp mRNA injected group and in the MTX 1 CF group. Our results indicated that the disrupted development of the heart and vessels in MTX-treated embryos is related to the reduced transcript levels of hand2, mef2a, mef2c, and flk-1.

Hedgehog signaling plays a cell-autonomous role in maximizing cardiac developmental potential

Elucidation of the complete roster of signals required for myocardial specification is crucial to the future of cardiac regenerative medicine. Prior studies have implicated the Hedgehog (Hh) signaling pathway in the regulation of multiple aspects of heart development. However, our understanding of the contribution of Hh signaling to the initial specification of myocardial progenitor cells remains incomplete. Here, we show that Hh signaling promotes cardiomyocyte formation in zebrafish. Reduced Hh signaling creates a cardiomyocyte deficit, and increased Hh signaling creates a surplus. Through fate-mapping, we find that Hh signaling is required at early stages to ensure specification of the proper number of myocardial progenitors. Genetic inducible fate mapping in mouse indicates that myocardial progenitors respond directly to Hh signals, and transplantation experiments in zebrafish demonstrate that Hh signaling acts cell autonomously to promote the contribution of cells to the myocardium. Thus, Hh signaling plays an essential early role in defining the optimal number of cardiomyocytes, making it an attractive target for manipulation of multipotent progenitor cells.