Disruption of hemoglobin oxygen transport does not impact oxygen-dependent physiological processes in developing embryos of zebra fish (Danio rerio)

Hypoxia exacerbates the cardiotoxic effect of the polycyclic aromatic hydrocarbon, phenanthrene in Danio rerio

The Deepwater Horizon oil spill of 2010 released a mixture of polycyclic aromatic hydrocarbons (PAHs) into the Gulf of Mexico presenting a complex exposure regime for native species. Concurrently, the Gulf has experienced an increase in hypoxic events due to agricultural runoff from the Mississippi River outflow. This combination presents a unique physiological challenge to native species and a challenge for researchers. The purpose of this study was to determine how the cardiotoxic PAH, phenanthrene interacts with hypoxia to affect the cardiovascular system of larval zebrafish (Danio rerio). We exposed zebrafish larvae to 0, 1, 100, and 1000 μg/L of phenanthrene in combination with normoxia and hypoxia. At late hatching, video of hearts and vessels were used to measure heart rate (ƒH), stroke volume (S_V), cardiac output (Q), red blood cell velocity, and caudal vessel diameter. We found that the highest concentration of phenanthrene caused a 58, 80, and 84% decrease in ƒH, Q, and arterial red blood cell velocity in normoxia and an 88, 98, and 99% decrease in hypoxia, respectively. Co-exposed larvae also experienced higher rates of edema and lordosis in addition to a 33% increase in mortality rate with co-exposure to hypoxia at the 1000 μg/L concentration of phenanthrene. At 12 dpf, baseline swimming behavior was similar between treatments indicating partial recovery from embryonic exposure. This study shows that phenanthrene decreases cardiac parameters, most significantly heart rate and that this effect is exacerbated by simultaneous exposure to hypoxia.

Lmo2 (LIM-Domain-Only 2) Modulates Sphk1 (Sphingosine Kinase) and Promotes Endothelial Cell Migration

OBJECTIVE

Lmo (LIM-domain-only)2 transcription factor is involved in hematopoiesis and vascular remodeling. Sphk (sphingosine kinase)1 phosphorylates sphingosine to S1P (sphingosine-1-phosphate). We hypothesized that Lmo2 regulates Sphk1 to promote endothelial cell (EC) migration and vascular development. APPROACH AND RESULTS: Lmo2 and Sphk1 knockdown (KD) were performed in Tg(fli1:EGFP) ^y1 zebrafish and in human umbilical vein EC. Rescue of phenotypes or overexpression of these factors were achieved using mRNA encoding Lmo2 or Sphk1. EC proliferation in vivo was assessed by BrdU (bromodeoxyuridine) immunostaining and fluorescence-activated cell sorter analysis of dissociated Tg(fli1:EGFP) ^y1 embryos. Cell migration was assessed by scratch assay in human umbilical vein EC and mouse aortic rings. Lmo2 interactions with Sphk1 promoter were assessed by ChIP-PCR (chromatin immunoprecipitation-polymerase chain reaction). Lmo2 or Sphk1 KD reduced number and length of intersegmental vessels. There was no reduction in the numbers of GFP⁺ (green fluorescent protein) ECs after Lmo2 KD. However, reduced numbers of BrdU⁺GFP⁺ nuclei were observed along the dysmorphic intersegmental vessels, accumulating instead at the sprouting origin of the intersegmental vessels. This anomaly was likely because of impaired EC migration, which was confirmed in migration assays using Lmo2 KD human umbilical vein ECs and mouse aortic rings. Both in vivo and in vitro, Lmo2 KD reduced Sphk1 gene expression, associated with less Lmo2 binding to the Sphk1 promoter as assessed by ChIP-PCR. Sphk1 mRNA rescued the Lmo2 KD phenotype.

CONCLUSIONS

Our data showed that Lmo2 is necessary for Sphk1 gene expression in ECs. Lmo2 KD reduced Lmo2-Sphk1 gene interaction, impaired intersegmental vessels formation, and reduced cell migration. We identified for the first time Sphk1 as downstream effector of Lmo2.

Morphology and cardiac physiology are differentially affected by temperature in developing larvae of the marine fish mahi-mahi (Coryphaena hippurus)

Cardiovascular performance is altered by temperature in larval fishes, but how acute versus chronic temperature exposures independently affect cardiac morphology and physiology in the growing larva is poorly understood. Consequently, we investigated the influence of water temperature on cardiac plasticity in developing mahi-mahi. Morphological (e.g. standard length, heart angle) and physiological cardiac variables (e.g. heart rate f_H, stroke volume, cardiac output) were recorded under two conditions by imaging: (i) under acute temperature exposure where embryos were reared at 25°C up to 128 h post-fertilization (hpf) and then acutely exposed to 25 (rearing temperature), 27 and 30°C; and (ii) at two rearing (chronic) temperatures of 26 and 30°C and performed at 32 and 56 hpf. Chronic elevated temperature improved developmental time in mahi-mahi. Heart rates were 1.2–1.4-fold higher under exposure of elevated acute temperatures across development (Q₁₀≥2.0). Q₁₀ for heart rate in acute exposure was 1.8-fold higher compared to chronic exposure at 56 hpf. At same stage, stroke volume was temperature independent (Q₁₀∼1.0). However, larvae displayed higher stroke volume later in stage. Cardiac output in developing mahi-mahi is mainly dictated by chronotropic rather than inotropic modulation, is differentially affected by temperature during development and is not linked to metabolic changes.

Summary: Acute and chronic temperature exposures affect differentially heart rate, stroke volume and cardiac output in mahi-mahi (Coryphaena hippurus) during early development.

Concerted regulation of retinal pigment epithelium basement membrane and barrier function by angiocrine factors

The outer blood-retina barrier is established through the coordinated terminal maturation of the retinal pigment epithelium (RPE), fenestrated choroid endothelial cells (ECs) and Bruch's membrane, a highly organized basement membrane that lies between both cell types. Here we study the contribution of choroid ECs to this process by comparing their gene expression profile before (P5) and after (P30) the critical postnatal period when mice acquire mature visual function. Transcriptome analyses show that expression of extracellular matrix-related genes changes dramatically over this period. Co-culture experiments support the existence of a novel regulatory pathway: ECs secrete factors that remodel RPE basement membrane, and integrin receptors sense these changes triggering Rho GTPase signals that modulate RPE tight junctions and enhance RPE barrier function. We anticipate our results will spawn a search for additional roles of choroid ECs in RPE physiology and disease.

4D in vivo imaging of glomerular barrier function in a zebrafish podocyte injury model

AIM

Zebrafish larvae with their simplified pronephros are an ideal model to study glomerular physiology. Although several groups use zebrafish larvae to assess glomerular barrier function, temporary or slight changes are still difficult to measure. The aim of this study was to investigate the potential of in vivo two-photon microscopy (2-PM) for long-term imaging of glomerular barrier function in zebrafish larvae.

METHODS

As a proof of principle, we adapted the nitroreductase/metronidazole model of targeted podocyte ablation for 2-PM. Combination with a strain, which expresses eGFP-vitamin D-binding protein in the blood plasma, led to a strain that allowed induction of podocyte injury with parallel assessment of glomerular barrier function. We used four-dimensional (4D) 2-PM to assess eGFP fluorescence over 26 h in the vasculature and in tubules of multiple zebrafish larvae (5 days post-fertilization) simultaneously.

RESULTS

By 4D 2-PM, we observed that, under physiological conditions, eGFP fluorescence was retained in the vasculature and rarely detected in proximal tubule cells. Application of metronidazole induced podocyte injury and cell death as shown by TUNEL staining. Induction of podocyte injury resulted in a dramatic decrease of eGFP fluorescence in the vasculature over time (about 50% and 90% after 2 and 12 h respectively). Loss of vascular eGFP fluorescence was paralleled by an endocytosis-mediated accumulation of eGFP fluorescence in proximal tubule cells, indicating proteinuria.

CONCLUSION

We established a microscopy-based method to monitor the dynamics of glomerular barrier function during induction of podocyte injury in multiple zebrafish larvae simultaneously over 26 h.

Heart function and hemodynamic analysis for zebrafish embryos

The Zebrafish has emerged to become a powerful vertebrate animal model for cardiovascular research in recent years. Its advantages include easy genetic manipulation, transparency, small size, low cost, and the ability to survive without active circulation at early stages of development. Sequencing the whole genome and identifying ortholog genes with human genome made it possible to induce clinically relevant cardiovascular defects via genetic approaches. Heart function and disturbed hemodynamics need to be assessed in a reliable manner for these disease models in order to reveal the mechanobiology of induced defects. This effort requires precise determination of blood flow patterns as well as hemodynamic stress (i.e., wall shear stress and pressure) levels within the developing heart. While traditional approach involves time-lapse brightfield microscopy to track cell and tissue movements, in more recent studies fast light-sheet fluorescent microscopes are utilized for that purpose. Integration of more complicated techniques like particle image velocimetry and computational fluid dynamics modeling for hemodynamic analysis holds a great promise to the advancement of the Zebrafish studies. Here, we discuss the latest developments in heart function and hemodynamic analysis for Zebrafish embryos and conclude with our future perspective on dynamic analysis of the Zebrafish cardiovascular system. Developmental Dynamics 246:868-880, 2017. © 2017 Wiley Periodicals, Inc.

Attacking sleep from a new angle: contributions from zebrafish

Sleep consumes a third of our lifespan, but we are far from understanding how it is initiated, maintained and terminated, or what purposes it serves. To address these questions, alternative model systems have recently been recruited. The diurnal zebrafish holds the promise of bridging the gap between simple invertebrate systems, which show little neuroanatomical conservation with mammals, and well-established, but complex and nocturnal, murine systems. Zebrafish larvae can be monitored in a high-throughput fashion, pharmacologically tested by adding compounds into the water, genetically screened using transient transgenesis, and optogenetically manipulated in a non-invasive manner. Here we discuss work that has established the zebrafish as a powerful system for the study of sleep, as well as novel insights gained by exploiting its particular advantages.

The water channel aquaporin-1a1 facilitates movement of CO₂ and ammonia in zebrafish (Danio rerio) larvae

The present study tested the hypothesis that zebrafish (Danio rerio) aquaporin-1a1 (AQP1a1) serves as a multi-functional channel for the transfer of the small gaseous molecules, CO2 and ammonia, as well as water, across biological membranes. Zebrafish embryos were microinjected with a translation-blocking morpholino oligonucleotide targeted to AQP1a1. Knockdown of AQP1a1 significantly reduced rates of CO2 and ammonia excretion, as well as water fluxes, in larvae at 4 days post fertilization (dpf). Because AQP1a1 is expressed both in ionocytes present on the body surface and in red blood cells, the haemolytic agent phenylhydrazine was used to distinguish between the contributions of AQP1a1 to gas transfer in these two locations. Phenylhydrazine treatment had no effect on AQP1a1-linked excretion of CO2 or ammonia, providing evidence that AQP1a1 localized to the yolk sac epithelium, rather than red blood cell AQP1a1, is the major site of CO2 and ammonia movements. The possibility that AQP1a1 and the rhesus glycoprotein Rhcg1, which also serves as a dual CO2 and ammonia channel, act in concert to facilitate CO2 and ammonia excretion was explored. Although knockdown of each protein did not affect the abundance of mRNA and protein of the other protein under control conditions, impairment of ammonia excretion by chronic exposure to high external ammonia triggered a significant increase in the abundance of AQP1a1 mRNA and protein in 4 dpf larvae experiencing Rhcg1 knockdown. Collectively, these results suggest that AQP1a1 in zebrafish larvae facilitates the movement of CO2 and ammonia, as well as water, in a physiologically relevant fashion.

The Power of Zebrafish in Personalised Medicine

The goal of personalised medicine is to develop tailor-made therapies for patients in whom currently available therapeutics fail. This approach requires correlating individual patient genotype data to specific disease phenotype data and using these stratified data sets to identify bespoke therapeutics. Applications for personalised medicine include common complex diseases which may have multiple targets, as well as rare monogenic disorders, for which the target may be unknown. In both cases, whole genome sequence analysis (WGS) is discovering large numbers of disease associated mutations in new candidate genes and potential modifier genes. Currently, the main limiting factor is the determination of which mutated genes are important for disease progression and therefore represent potential targets for drug discovery. Zebrafish have gained popularity as a model organism for understanding developmental processes, disease mechanisms and more recently for drug discovery and toxicity testing. In this chapter, we will examine the diverse roles that zebrafish can make in the expanding field of personalised medicine, from generating humanised disease models to xenograft screening of different cancer cell lines, through to finding new drugs via in vivo phenotypic screens. We will discuss the tools available for zebrafish research and recent advances in techniques, highlighting the advantages and potential of using zebrafish for high throughput disease modeling and precision drug discovery.

Mass Transport: Circulatory System with Emphasis on Nonendothermic Species

Mass transport can be generally defined as movement of material matter. The circulatory system then is a biological example given its role in the movement in transporting gases, nutrients, wastes, and chemical signals. Comparative physiology has a long history of providing new insights and advancing our understanding of circulatory mass transport across a wide array of circulatory systems. Here we focus on circulatory function of nonmodel species. Invertebrates possess diverse convection systems; that at the most complex generate pressures and perform at a level comparable to vertebrates. Many invertebrates actively modulate cardiovascular function using neuronal, neurohormonal, and skeletal muscle activity. In vertebrates, our understanding of cardiac morphology, cardiomyocyte function, and contractile protein regulation by Ca2+ highlights a high degree of conservation, but differences between species exist and are coupled to variable environments and body temperatures. Key regulators of vertebrate cardiac function and systemic blood pressure include the autonomic nervous system, hormones, and ventricular filling. Further chemical factors regulating cardiovascular function include adenosine, natriuretic peptides, arginine vasotocin, endothelin 1, bradykinin, histamine, nitric oxide, and hydrogen sulfide, to name but a few. Diverse vascular morphologies and the regulation of blood flow in the coronary and cerebral circulations are also apparent in nonmammalian species. Dynamic adjustments of cardiovascular function are associated with exercise on land, flying at high altitude, prolonged dives by marine mammals, and unique morphology, such as the giraffe. Future studies should address limits of gas exchange and convective transport, the evolution of high arterial pressure across diverse taxa, and the importance of the cardiovascular system adaptations to extreme environments. © 2017 American Physiological Society. Compr Physiol 7:17-66, 2017.

Zebrafish Model for Functional Screening of Flow-Responsive Genes

Supplemental Digital Content is available in the text.

Objective—

Atherosclerosis is initiated at branches and bends of arteries exposed to disturbed blood flow that generates low shear stress. This mechanical environment promotes lesions by inducing endothelial cell (EC) apoptosis and dysfunction via mechanisms that are incompletely understood. Although transcriptome-based studies have identified multiple shear-responsive genes, most of them have an unknown function. To address this, we investigated whether zebrafish embryos can be used for functional screening of mechanosensitive genes that regulate EC apoptosis in mammalian arteries.

Approach and Results—

First, we demonstrated that flow regulates EC apoptosis in developing zebrafish vasculature. Specifically, suppression of blood flow in zebrafish embryos (by targeting cardiac troponin) enhanced that rate of EC apoptosis (≈10%) compared with controls exposed to flow (≈1%). A panel of candidate regulators of apoptosis were identified by transcriptome profiling of ECs from high and low shear stress regions of the porcine aorta. Genes that displayed the greatest differential expression and possessed 1 to 2 zebrafish orthologues were screened for the regulation of apoptosis in zebrafish vasculature exposed to flow or no-flow conditions using a knockdown approach. A phenotypic change was observed in 4 genes; p53-related protein (PERP) and programmed cell death 2–like protein functioned as positive regulators of apoptosis, whereas angiopoietin-like 4 and cadherin 13 were negative regulators. The regulation of perp, cdh13, angptl4, and pdcd2l by shear stress and the effects of perp and cdh13 on EC apoptosis were confirmed by studies of cultured EC exposed to flow.

Conclusions—

We conclude that a zebrafish model of flow manipulation coupled to gene knockdown can be used for functional screening of mechanosensitive genes in vascular ECs, thus providing potential therapeutic targets to prevent or treat endothelial injury at atheroprone sites.

Role of endogenous carbon monoxide in the control of breathing in zebrafish (Danio rerio)

Carbon monoxide (CO) is a gaseous signaling molecule and is produced in vivo from the intracellular breakdown of heme via the heme oxygenase (HO) family of enzymes. In this study we investigated the role of the HO-1/CO system in the control of ventilation in zebrafish, Danio rerio Immunohistochemistry revealed the presence of HO-1 in the chemoreceptive neuroepithelial cells (NECs) of larvae (4 days postfertilization) and adults, indicating the potential for endogenous CO production in the NECs. Hypoxia (20 min, water Po₂ of 30 mmHg) caused a significant increase in HO-1 activity in whole larvae and in the gills of adult fish. Zebrafish with reduced HO-1 activity (via HO-1 knockdown in larvae or zinc protoporphyrin IX treatment in adults) exhibited increased ventilation frequency (V_f) under normoxic but not hypoxic conditions. The addition of exogenous CO restored resting V_f in fish with diminished CO production, and in some cases (e.g., hypoxic sham larvae) CO modestly reduced V_f below resting levels. Larval fish were treated with phenylhydrazine (PHZ) to eliminate the potential confounding effects of CO-hemoglobin interactions that might influence ventilation. PHZ treatment did not cause changes in V_f of normoxic larvae, and the addition of CO to PHZ-exposed larvae resulted in a significant decrease in sham and HO-1-deficient fish under normoxic conditions. This study demonstrates for the first time that CO plays an inhibitory role in the control of breathing in larval and adult zebrafish.

FLNC Gene Splice Mutations Cause Dilated Cardiomyopathy

A genetic etiology has been identified in 30% to 40% of dilated cardiomyopathy (DCM) patients, yet only 50% of these cases are associated with a known causative gene variant. Thus, in order to understand the pathophysiology of DCM, it is necessary to identify and characterize additional genes. In this study, whole exome sequencing in combination with segregation analysis was used to identify mutations in a novel gene, filamin C (FLNC), resulting in a cardiac-restricted DCM pathology. Here we provide functional data via zebrafish studies and protein analysis to support a model implicating FLNC haploinsufficiency as a mechanism of DCM.

•

Deoxyribonucleic acid obtained from 2 large DCM families was studied using whole-exome sequencing and cosegregation analysis resulting in the identification of a novel disease gene, FLNC. The 2 families, from the same Italian region, harbored the same FLNC splice-site mutation (FLNC c.7251+1G>A).

•

A third U.S. family was then identified with a novel FLNC splice-site mutation (FLNC c.5669-1delG) that leads to haploinsufficiency as shown by the FLNC Western blot analysis of the heart muscle.

•

The FLNC ortholog flncb morpholino was injected into zebrafish embryos, and when flncb was knocked down caused a cardiac dysfunction phenotype.

•

On electron microscopy, the flncb morpholino knockdown zebrafish heart showed defects within the Z-discs and sarcomere disorganization.

Blood flow imaging in zebrafish by laser doppler digital holography

Microvessel blood flow imaging techniques are widely used in biomedical research and clinical diagnostics where many diseases have a vascular etiology or involvement. For testing purposes, zebrafish embryo provides an ideal animal model to achieve high-resolution imaging of superficial and deeply localized vessels. Moreover, the study of the formation of a closed circulatory system in vertebrates is a topic of recent interest in biophysics. However, most of the existing techniques are invasive due to the use of a contrast agent for imaging purposes. Recent developments in Digital Holography and Laser Doppler Holography techniques can be considered to alleviate this issue. Laser Doppler holography and transmission microscopy can be coupled to analyze blood flow in fish embryos by adapting a laser Doppler holographic setup to a standard bio-microscope: the two beams of the holographic interferometer (illumination of the object and reference), whose frequency offset is controlled, were addressed to the microscope by optical fibers. Multimodal acquisition and analysis of the data is made by acting on the frequency offset of the two beams, and on the location of the Fourier space filtered zone. In this work, we show that it is possible to select the signal of moving scatterers, and to image Red Blood Cells (RBCs) and blood vessels. Individual RBCs are imaged, and movies showing the RBC motion are obtained. Microsc. Res. Tech. 81:153-161, 2018. © 2016 Wiley Periodicals, Inc.

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.

Evidence for a role of heme oxygenase-1 in the control of cardiac function in zebrafish (Danio rerio) larvae exposed to hypoxia

Carbon monoxide (CO) is a gaseous neurotransmitter produced from the breakdown of heme via heme oxygenase-1 (HO-1; hypoxia-inducible isoform) and heme oxygenase-2 (HO-2; constitutively expressed isoform). In mammals, CO is involved in modulating cardiac function. The role of the HO-1/CO system in the control of heart function in fish, however, is unknown and investigating its physiological function in lower vertebrates will provide a better understanding of the evolution of this regulatory mechanism. We explored the role of the HO-1/CO system in larval zebrafish (Danio rerio) in vivo by investigating the impact of translational gene knockdown of HO-1 on cardiac function. Immunohistochemistry revealed the presence of HO-1 in the pacemaker cells of the heart at 4 days post-fertilization and thus the potential for CO production at these sites. Sham-treated zebrafish larvae (experiencing normal levels of HO-1) significantly increased heart rate (fH) when exposed to hypoxia (PwO2 =30 mmHg). Zebrafish larvae lacking HO-1 expression after morpholino knockdown (morphants) exhibited significantly higher fH under normoxic (but not hypoxic) conditions when compared with sham-treaded fish. The increased fH in HO-1 morphants was rescued (fH was restored to control levels) after treatment of larvae with a CO-releasing molecule (40 µmol l(-1) CORM). The HO-1-deficient larvae developed significantly larger ventricles and when exposed to hypoxia they displayed higher cardiac output ([Formula: see text]) and stroke volume (SV). These results suggest that under hypoxic conditions, HO-1 regulates [Formula: see text] and SV presumably via the production of CO. Overall, this study provides a better understanding of the role of the HO-1/CO system in controlling heart function in lower vertebrates. We demonstrate for the first time the ability for CO to be produced in presumptive pacemaker cells of the heart where it plays an inhibitory role in setting the resting cardiac frequency.

Epilepsy, Behavioral Abnormalities, and Physiological Comorbidities in Syntaxin-Binding Protein 1 (STXBP1) Mutant Zebrafish

Mutations in the synaptic machinery gene syntaxin-binding protein 1, STXBP1 (also known as MUNC18-1), are linked to childhood epilepsies and other neurodevelopmental disorders. Zebrafish STXBP1 homologs (stxbp1a and stxbp1b) have highly conserved sequence and are prominently expressed in the larval zebrafish brain. To understand the functions of stxbp1a and stxbp1b, we generated loss-of-function mutations using CRISPR/Cas9 gene editing and studied brain electrical activity, behavior, development, heart physiology, metabolism, and survival in larval zebrafish. Homozygous stxbp1a mutants exhibited a profound lack of movement, low electrical brain activity, low heart rate, decreased glucose and mitochondrial metabolism, and early fatality compared to controls. On the other hand, homozygous stxbp1b mutants had spontaneous electrographic seizures, and reduced locomotor activity response to a movement-inducing “dark-flash” visual stimulus, despite showing normal metabolism, heart rate, survival, and baseline locomotor activity. Our findings in these newly generated mutant lines of zebrafish suggest that zebrafish recapitulate clinical phenotypes associated with human syntaxin-binding protein 1 mutations.

Characterization of the zebrafish cx36.7 gene promoter: Its regulation of cardiac-specific expression and skeletal muscle-specific repression

Zebrafish connexin 36.7 (cx36.7/ecx) has been identified as a key molecule in the early stages of heart development in this species. A defect in cx36.7 causes severe heart malformation due to the downregulation of nkx2.5 expression, a result which resembles congenital heart disease in humans. It has been shown that cx36.7 is expressed specifically in early developing heart cardiomyocytes. However, the regulatory mechanism for the cardiac-restricted expression of cx36.7 remains to be elucidated. In this study we isolated the 5'-flanking promoter region of the cx36.7 gene and characterized its promoter activity in zebrafish embryos. Deletion analysis showed that a 316-bp upstream region is essential for cardiac-restricted expression. This region contains four GATA elements, the proximal two of which are responsible for promoter activation in the embryonic heart and serve as binding sites for gata4. When gata4, gata5 and gata6 were simultaneously knocked down, the promoter activity was significantly decreased. Moreover, the deletion of the region between -316 and -133bp led to EGFP expression in the embryonic trunk muscle. The distal two GATA and A/T-rich elements in this region act as repressors of promoter activity in skeletal muscle. These results suggest that cx36.7 expression is directed by cardiac promoter activation via the two proximal GATA elements as well as by skeletal muscle-specific promoter repression via the two distal GATA elements.

Vitamin D receptor signaling is required for heart development in zebrafish embryo

Vitamin D has been found to be associated with cardiovascular diseases. However, the role of vitamin D in heart development during embryonic period is largely unknown. Vitamin D induces its genomic effects through its nuclear receptor, the vitamin D receptor (VDR). The present study investigated the role of VDR on heart development by antisense-mediated knockdown approaches in zebrafish model system. In zebrafish embryos, two distinct VDR genes (vdra and vdrb) have been identified. Knockdown of vdra has little effect on heart development, whereas disrupting vdrb gene causes various cardiac phenotypes, characterized by pericardial edema, slower heart rate and laterality defects. Depletion of both vdra and vdrb (vdra/b) produce additive, but not synergistic effects. To determine whether atrioventricular (AV) cardiomyocytes are properly organized in these embryos, the expression of bmp4, which marks the developing AV boundary at 48 h post-fertilization, was examined. Notably, vdra/b-deficient embryos display ectopic expression of bmp4 towards the ventricle or throughout atrial and ventricular chambers. Taken together, these results suggest that VDR signaling plays an essential role in heart development.

Abnormal retinal development in Cloche mutant zebrafish

BACKGROUND

Functions for the early embryonic vasculature in regulating development of central nervous system tissues, such as the retina, have been suggested by in vitro studies and by in vivo manipulations that caused additional ocular vessels to develop. Here, we use an avascular zebrafish embryo, cloche-/- (clo-/-), to begin to identify necessary developmental functions of the ocular vasculature in regulating development and patterning of the neural retina, in vivo. These studies are possible in zebrafish embryos, which do not yet rely upon the vasculature for tissue oxygenation.

RESULTS

clo-/- embryos lacked early ocular vasculature and were microphthalmic, with reduced retinal cell proliferation and cell survival. Retinas of clo mutants were disorganized, with irregular synaptic layers, mispatterned expression domains of retinal transcription factors, morphologically abnormal Müller glia, reduced differentiation of specific retinal cell types, and sporadically distributed cone photoreceptors. Blockade of p53-mediated cell death did not completely rescue this phenotype and revealed ectopic cones in the inner nuclear layer. clo-/- embryos did not upregulate a molecular marker for hypoxia.

CONCLUSIONS

The disorganized retinal phenotype of clo-/- embryos is consistent with a neural and glial developmental patterning role for the early ocular vasculature that is independent of its eventual function in gas exchange.

Bisphenol A alters the cardiovascular response to hypoxia in Danio rerio embryos

The purpose of this study was to determine if the cardiovascular response to hypoxia was altered by the presence of bisphenol A (BPA) in Danio rerio embryos. It was expected that BPA exposure would affect cardiovascular parameters during hypoxia more than normoxia due to an interaction between BPA and the hypoxia-inducible factor (HIF-1α) pathway. We demonstrate that BPA exposure has a minimal effect during normoxia but can severely affect the cardiovascular system during a hypoxic event. Cardiovascular response was measured in vivo using video microscopy and digital motion analysis. RBC density increased 35% in hypoxia alone but decreased 48% with addition of 0.25mg/L BPA. Tissue vascularization (% coverage) was unaffected by hypoxia alone but decreased 37% with addition of 0.25mg/L BPA. The diameter and RBC velocity of arteries were more sensitive than veins to BPA exposure during both normoxia and hypoxia. Arterial RBC velocity decreased 42% during normoxia and 52% during hypoxia with 1mg/L BPA. This decrease in velocity may in part be due to the 86% decrease in heart rate (ƒH) observed during co-exposure to hypoxia and 5mg/L BPA. While stroke volume (SV) was unaffected by treatment, cardiac output (Q) decreased by 69% with co-exposure. ƒH and Q were not affected by BPA exposure during normoxia. Development ultimately slowed by 146% and mortality rates were 95% during hypoxia when exposed to 5mg/L BPA. Our results show for the first time that BPA exposure alters the cardiovascular system during hypoxia more so than normoxia.

Temporal cohesion of the structural, functional and molecular characteristics of the developing zebrafish heart

Heart formation is a complex, dynamic and highly coordinated process of molecular, morphogenetic and functional factors with each interacting and contributing to formation of the mature organ. Cardiac abnormalities in early life can be lethal in mammals but not in the zebrafish embryo which has been widely used to study the developing heart. While early cardiac development in the zebrafish has been well characterized, functional changes during development and how these relate to architectural, cellular and molecular aspects of development have not been well described previously. To address this we have carefully characterised cardiac structure, function, cardiomyocyte proliferation and cardiac-specific gene expression between 48 and 120 hpf in the zebrafish. We show that the zebrafish heart increases in volume and changes shape significantly between 48 and 72 hpf accompanied by a 40% increase in cardiomyocyte number. Between 96 and 120 hpf, while external heart expansion slows, there is rapid formation of a mature and extensive trabecular network within the ventricle chamber. While ejection fraction does not change during the course of development other determinants of contractile function increase significantly particularly between 72 and 96 hpf leading to an increase in cardinal vein blood flow. This study has revealed a number of novel aspects of cardiac developmental dynamics with striking temporal orchestration of structure and function within the first few days of development. These changes are associated with changes in expression of developmental and maturational genes. This study provides important insights into the complex temporal relationship between structure and function of the developing zebrafish heart.

Sensing and surviving hypoxia in vertebrates

Surviving hypoxia is one of the most critical challenges faced by vertebrates. Most species have adapted to changing levels of oxygen in their environment with specialized organs that sense hypoxia, while only few have been uniquely adapted to survive prolonged periods of anoxia. The goal of this review is to present the most recent research on oxygen sensing, adaptation to hypoxia, and mechanisms of anoxia tolerance in nonmammalian vertebrates. We discuss the respiratory structures in fish, including the skin, gills, and air-breathing organs, and recent evidence for chemosensory neuroepithelial cells (NECs) in these tissues that initiate reflex responses to hypoxia. The use of the zebrafish as a genetic and developmental model has allowed observation of the ontogenesis of respiratory and chemosensory systems, demonstration of a putative intracellular O2 sensor in chemoreceptors that may initiate transduction of the hypoxia signal, and investigation into the effects of extreme hypoxia on cardiorespiratory development. Other organisms, such as goldfish and freshwater turtles, display a high degree of anoxia tolerance, and these models are revealing important adaptations at the cellular level, such as the regulation of glutamatergic and GABAergic neurotransmission in defense of homeostasis in central neurons.

Loss of dihydrolipoyl succinyltransferase (DLST) leads to reduced resting heart rate in the zebrafish

The genetic underpinnings of heart rate regulation are only poorly understood. In search for genetic regulators of cardiac pacemaker activity, we isolated in a large-scale mutagenesis screen the embryonic lethal, recessive zebrafish mutant schneckentempo (ste). Homozygous ste mutants exhibit a severely reduced resting heart rate with normal atrio-ventricular conduction and contractile function. External electrical pacing reveals that defective excitation generation in cardiac pacemaker cells underlies bradycardia in ste^−/− mutants. By positional cloning and gene knock-down analysis we find that loss of dihydrolipoyl succinyltransferase (DLST) function causes the ste phenotype. The mitochondrial enzyme DLST is an essential player in the citric acid cycle that warrants proper adenosine-tri-phosphate (ATP) production. Accordingly, ATP levels are significantly diminished in ste^−/− mutant embryos, suggesting that limited energy supply accounts for reduced cardiac pacemaker activity in ste^−/− mutants. We demonstrate here for the first time that the mitochondrial enzyme DLST plays an essential role in the modulation of the vertebrate heart rate by controlling ATP production in the heart.

The phylogeny and ontogeny of autonomic control of the heart and cardiorespiratory interactions in vertebrates

Heart rate in vertebrates is controlled by activity in the autonomic nervous system. In spontaneously active or experimentally prepared animals, inhibitory parasympathetic control is predominant and is responsible for instantaneous changes in heart rate, such as occur at the first air breath following a period of apnoea in discontinuous breathers like inactive reptiles or species that surface to air breathe after a period of submersion. Parasympathetic control, exerted via fast-conducting, myelinated efferent fibres in the vagus nerve, is also responsible for beat-to-beat changes in heart rate such as the high frequency components observed in spectral analysis of heart rate variability. These include respiratory modulation of the heartbeat that can generate cardiorespiratory synchrony in fish and respiratory sinus arrhythmia in mammals. Both may increase the effectiveness of respiratory gas exchange. Although the central interactions generating respiratory modulation of the heartbeat seem to be highly conserved through vertebrate phylogeny, they are different in kind and location, and in most species are as yet little understood. The heart in vertebrate embryos possesses both muscarinic cholinergic and β-adrenergic receptors very early in development. Adrenergic control by circulating catecholamines seems important throughout development. However, innervation of the cardiac receptors is delayed and first evidence of a functional cholinergic tonus on the heart, exerted via the vagus nerve, is often seen shortly before or immediately after hatching or birth, suggesting that it may be coordinated with the onset of central respiratory rhythmicity and subsequent breathing.

Transcriptional response to cardiac injury in the zebrafish: systematic identification of genes with highly concordant activity across in vivo models

Background

Zebrafish is a clinically-relevant model of heart regeneration. Unlike mammals, it has a remarkable heart repair capacity after injury, and promises novel translational applications. Amputation and cryoinjury models are key research tools for understanding injury response and regeneration in vivo. An understanding of the transcriptional responses following injury is needed to identify key players of heart tissue repair, as well as potential targets for boosting this property in humans.

Results

We investigated amputation and cryoinjury in vivo models of heart damage in the zebrafish through unbiased, integrative analyses of independent molecular datasets. To detect genes with potential biological roles, we derived computational prediction models with microarray data from heart amputation experiments. We focused on a top-ranked set of genes highly activated in the early post-injury stage, whose activity was further verified in independent microarray datasets. Next, we performed independent validations of expression responses with qPCR in a cryoinjury model. Across in vivo models, the top candidates showed highly concordant responses at 1 and 3 days post-injury, which highlights the predictive power of our analysis strategies and the possible biological relevance of these genes. Top candidates are significantly involved in cell fate specification and differentiation, and include heart failure markers such as periostin, as well as potential new targets for heart regeneration. For example, ptgis and ca2 were overexpressed, while usp2a, a regulator of the p53 pathway, was down-regulated in our in vivo models. Interestingly, a high activity of ptgis and ca2 has been previously observed in failing hearts from rats and humans.

Conclusions

We identified genes with potential critical roles in the response to cardiac damage in the zebrafish. Their transcriptional activities are reproducible in different in vivo models of cardiac injury.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-852) contains supplementary material, which is available to authorized users.

The hemodynamically-regulated vascular microenvironment promotes migration of the steroidogenic tissue during its interaction with chromaffin cells in the zebrafish embryo

Background

While the endothelium-organ interaction is critical for regulating cellular behaviors during development and disease, the role of blood flow in these processes is only partially understood. The dorsal aorta performs paracrine functions for the timely migration and differentiation of the sympatho-adrenal system. However, it is unclear how the adrenal cortex and medulla achieve and maintain specific integration and whether hemodynamic forces play a role.

Methodology and Principal Findings

In this study, the possible modulation of steroidogenic and chromaffin cell integration by blood flow was investigated in the teleostean counterpart of the adrenal gland, the interrenal gland, in the zebrafish (Danio rerio). Steroidogenic tissue migration and angiogenesis were suppressed by genetic or pharmacologic inhibition of blood flow, and enhanced by acceleration of blood flow upon norepinephrine treatment. Repressed steroidogenic tissue migration and angiogenesis due to flow deficiency were recoverable following restoration of flow. The regulation of interrenal morphogenesis by blood flow was found to be mediated through the vascular microenvironment and the Fibronectin-phosphorylated Focal Adhesion Kinase (Fn-pFak) signaling. Moreover, the knockdown of krüppel-like factor 2a (klf2a) or matrix metalloproteinase 2 (mmp2), two genes regulated by the hemodynamic force, phenocopied the defects in migration, angiogenesis, the vascular microenvironment, and pFak signaling of the steroidogenic tissue observed in flow-deficient embryos, indicating a direct requirement of mechanotransduction in these processes. Interestingly, epithelial-type steroidogenic cells assumed a mesenchymal-like character and downregulated β-Catenin at cell-cell junctions during interaction with chromaffin cells, which was reversed by inhibiting blood flow or Fn-pFak signaling. Blood flow obstruction also affected the migration of chromaffin cells, but not through mechanosensitive or Fn-pFak dependent mechanisms.

Conclusions and Significance

These results demonstrate that hemodynamically regulated Fn-pFak signaling promotes the migration of steroidogenic cells, ensuring their interaction with chromaffin cells along both sides of the midline during interrenal gland development.

Zinc fingers poke zebrafish, cause thrombosis!

In this issue of Blood, Liu et al describe the creation of a null mutation for the antithrombin III gene (at3) in zebrafish by using zinc finger nuclease technology.

High-content screening in zebrafish embryos identifies butafenacil as a potent inducer of anemia

Using transgenic zebrafish (fli1:egfp) that stably express enhanced green fluorescent protein (eGFP) within vascular endothelial cells, we recently developed and optimized a 384-well high-content screening (HCS) assay that enables us to screen and identify chemicals affecting cardiovascular development and function at non-teratogenic concentrations. Within this assay, automated image acquisition procedures and custom image analysis protocols are used to quantify body length, heart rate, circulation, pericardial area, and intersegmental vessel area within individual live embryos exposed from 5 to 72 hours post-fertilization. After ranking developmental toxicity data generated from the U.S. Environmental Protection Agency's (EPA's) zebrafish teratogenesis assay, we screened 26 of the most acutely toxic chemicals within EPA's ToxCast Phase-I library in concentration-response format (0.05–50 µM) using this HCS assay. Based on this screen, we identified butafenacil as a potent inducer of anemia, as exposure from 0.39 to 3.125 µM butafenacil completely abolished arterial circulation in the absence of effects on all other endpoints evaluated. Butafenacil is an herbicide that inhibits protoporphyrinogen oxidase (PPO) – an enzyme necessary for heme production in vertebrates. Using o-dianisidine staining, we then revealed that severe butafenacil-induced anemia in zebrafish was due to a complete loss of hemoglobin following exposure during early development. Therefore, six additional PPO inhibitors within the ToxCast Phase-I library were screened to determine whether anemia represents a common adverse outcome for these herbicides. Embryonic exposure to only one of these PPO inhibitors – flumioxazin – resulted in a similar phenotype as butafenacil, albeit not as severe as butafenacil. Overall, this study highlights the potential utility of this assay for (1) screening chemicals for cardiovascular toxicity and (2) prioritizing chemicals for future hypothesis-driven and mechanism-focused investigations within zebrafish and mammalian models.

RASA1 functions in EPHB4 signaling pathway to suppress endothelial mTORC1 activity

Vascular malformations are linked to mutations in RAS p21 protein activator 1 (RASA1, also known as p120RasGAP); however, due to the global expression of this gene, it is unclear how these mutations specifically affect the vasculature. Here, we tested the hypothesis that RASA1 performs a critical effector function downstream of the endothelial receptor EPHB4. In zebrafish models, we found that either RASA1 or EPHB4 deficiency induced strikingly similar abnormalities in blood vessel formation and function. Expression of WT EPHB4 receptor or engineered receptors with altered RASA1 binding revealed that the ability of EPHB4 to recruit RASA1 is required to restore blood flow in EPHB4-deficient animals. Analysis of EPHB4-deficient zebrafish tissue lysates revealed that mTORC1 is robustly overactivated, and pharmacological inhibition of mTORC1 in these animals rescued both vessel structure and function. Furthermore, overexpression of mTORC1 in endothelial cells exacerbated vascular phenotypes in animals with reduced EPHB4 or RASA1, suggesting a functional EPHB4/RASA1/mTORC1 signaling axis in endothelial cells. Tissue samples from patients with arteriovenous malformations displayed strong endothelial phospho-S6 staining, indicating increased mTORC1 activity. These results indicate that deregulation of EPHB4/RASA1/mTORC1 signaling in endothelial cells promotes vascular malformation and suggest that mTORC1 inhibitors, many of which are approved for the treatment of certain cancers, should be further explored as a potential strategy to treat patients with vascular malformations.

ZebraBeat: a flexible platform for the analysis of the cardiac rate in zebrafish embryos

Heartbeat measurement is important in assesssing cardiac function because variations in heart rhythm can be the cause as well as an effect of hidden pathological heart conditions. Zebrafish (Danio rerio) has emerged as one of the most useful model organisms for cardiac research. Indeed, the zebrafish heart is easily accessible for optical analyses without conducting invasive procedures and shows anatomical similarity to the human heart. In this study, we present a non-invasive, simple, cost-effective process to quantify the heartbeat in embryonic zebrafish. To achieve reproducibility, high throughput and flexibility (i.e., adaptability to any existing confocal microscope system and with a user-friendly interface that can be easily used by researchers), we implemented this method within a software program. We show here that this platform, called ZebraBeat, can successfully detect heart rate variations in embryonic zebrafish at various developmental stages, and it can record cardiac rate fluctuations induced by factors such as temperature and genetic- and chemical-induced alterations. Applications of this methodology may include the screening of chemical libraries affecting heart rhythm and the identification of heart rhythm variations in mutants from large-scale forward genetic screens.

Transgenerational epigenetics: the role of maternal effects in cardiovascular development

Transgenerational epigenetics, the study of non-genetic transfer of information from one generation to the next, has gained much attention in the past few decades due to the fact that, in many instances, epigenetic processes outweigh direct genetic processes in the manifestation of aberrant phenotypes across several generations. Maternal effects, or the influences of maternal environment, phenotype, and/or genotype on offsprings' phenotypes, independently of the offsprings' genotypes, are a subcategory of transgenerational epigenetics. Due to the intimate role of the mother during early development in animals, there is much interest in investigating the means by which maternal effects can shape the individual. Maternal effects are responsible for cellular organization, determination of the body axis, initiation and maturation of organ systems, and physiological performance of a wide variety of species and biological systems. The cardiovascular system is the first to become functional and can significantly influence the development of other organ systems. Thus, it is important to elucidate the role of maternal effects in cardiovascular development, and to understand its impact on adult cardiovascular health. Topics to be addressed include: (1) how and when do maternal effects change the developmental trajectory of the cardiovascular system to permanently alter the adult's cardiovascular phenotype, (2) what molecular mechanisms have been associated with maternally induced cardiovascular phenotypes, and (3) what are the evolutionary implications of maternally mediated changes in cardiovascular phenotype?

Snail modulates the assembly of fibronectin via α5 integrin for myocardial migration in zebrafish embryos

The Snail family member snail encodes a zinc finger-containing transcriptional factor that is involved in heart formation. Yet, little is known about how Snail regulates heart development. Here, we identified that one of the duplicated snail genes, snai1b, was expressed in the heart region of zebrafish embryos. Depletion of Snai1b function dramatically reduced expression of α5 integrin, disrupted Fibronectin layer in the heart region, especially at the midline, and prevented migration of cardiac precursors, resulting in defects in cardiac morphology and function in zebrafish embryos. Injection of α5β1 protein rescued the Fibronectin layer and then the myocardial precursor migration in snai1b knockdown embryos. The results provide the molecular mechanism how Snail controls the morphogenesis of heart during embryonic development.

Metabolic and embryonic responses to terrestrial incubation of Fundulus grandis embryos across a temperature gradient

This study simulated terrestrial incubation and measured rates of embryogenesis, nitrogen elimination, heart rate, lactate production, maximum length of time a hatch could be delayed and developmental responses of terrestrially incubated Gulf killifish Fundulus grandis embryos at temperatures ranging from 20 to 30° C. Temperature had a positive relationship with rate of embryogenesis, but a negative relationship with extent of extended incubation. The 30° C treatment reached embryonic maturity 6 days before the 20° C treatment. Embryos hatched between intervals of 240 and 336, 144 and 288, 96 and 240 and 96 and 192 h after reaching developmental maturity for the 20, 23, 26 and 30° C treatments. Significantly higher concentrations of total nitrogen, in the form of ammonia and urea, were recorded in the 20 and 30° C treatments. While temperature significantly influenced lactate and ATP concentrations, no significant influence of time of incubation was detected. Terrestrial embryos displayed an ability to develop quickly during embryogenesis and prolong incubation for an extended period of time after reaching embryonic maturity. This adaptation may be a life-history trait used to minimize asynchronous hatching, cannibalism and cohort size heterogeneity.

Copper supplementation restores cytochrome c oxidase assembly defect in a mitochondrial disease model of COA6 deficiency

Mitochondrial respiratory chain biogenesis is orchestrated by hundreds of assembly factors, many of which are yet to be discovered. Using an integrative approach based on clues from evolutionary history, protein localization and human genetics, we have identified a conserved mitochondrial protein, C1orf31/COA6, and shown its requirement for respiratory complex IV biogenesis in yeast, zebrafish and human cells. A recent next-generation sequencing study reported potential pathogenic mutations within the evolutionarily conserved Cx₉CxnCx₁₀C motif of COA6, implicating it in mitochondrial disease biology. Using yeast coa6Δ cells, we show that conserved residues in the motif, including the residue mutated in a patient with mitochondrial disease, are essential for COA6 function, thus confirming the pathogenicity of the patient mutation. Furthermore, we show that zebrafish embryos with zfcoa6 knockdown display reduced heart rate and cardiac developmental defects, recapitulating the observed pathology in the human mitochondrial disease patient who died of neonatal hypertrophic cardiomyopathy. The specific requirement of Coa6 for respiratory complex IV biogenesis, its intramitochondrial localization and the presence of the Cx₉CxnCx₁₀C motif suggested a role in mitochondrial copper metabolism. In support of this, we show that exogenous copper supplementation completely rescues respiratory and complex IV assembly defects in yeast coa6Δ cells. Taken together, our results establish an evolutionarily conserved role of Coa6 in complex IV assembly and support a causal role of the COA6 mutation in the human mitochondrial disease patient.

Prolonged hypoxia increases survival even in Zebrafish (Danio rerio) showing cardiac arrhythmia

Tolerance towards hypoxia is highly pronounced in zebrafish. In this study even beneficial effects of hypoxia, specifically enhanced survival of zebrafish larvae, could be demonstrated. This effect was actually more pronounced in breakdance mutants, which phenotypically show cardiac arrhythmia. Breakdance mutants (bre) are characterized by chronically reduced cardiac output. Despite an about 50% heart rate reduction, they become adults, but survival rate significantly drops to 40%. Normoxic bre animals demonstrate increased hypoxia inducible factor 1 a (Hif-1α) expression, which indicates an activated hypoxic signaling pathway. Consequently, cardiovascular acclimation, like cardiac hypertrophy and increased erythrocyte concentration, occurs. Thus, it was hypothesized, that under hypoxic conditions survival might be even more reduced. When bre mutants were exposed to hypoxic conditions, they surprisingly showed higher survival rates than under normoxic conditions and even reached wildtype values. In hypoxic wildtype zebrafish, survival yet exceeded normoxic control values. To specify physiological acclimation, cardiovascular and metabolic parameters were measured before hypoxia started (3 dpf), when the first differences in survival rate occurred (7 dpf) and when survival rate plateaued (15 dpf). Hypoxic animals expectedly demonstrated Hif-1α accumulation and consequently enhanced convective oxygen carrying capacity. Moreover, bre animals showed a significantly enhanced heart rate under hypoxic conditions, which reached normoxic wildtype values. This improvement in convective oxygen transport ensured a sufficient oxygen and nutrient supply and was also reflected in the significantly higher mitochondrial activity. The highly optimized energy metabolism observed in hypoxic zebrafish larvae might be decisive for periods of higher energy demand due to organ development, growth and increased activity. However, hypoxia increased survival only during a short period of development and starting hypoxia before or after this phase reduced survival, particularly in bre animals. Thus, the physiological plasticity, which enables zebrafish larvae to benefit from a hypoxia, occurs only within a narrow developmental window.

The effects of hypoxia and temperature on metabolic aspects of embryonic development in the annual killifish Austrofundulus limnaeus

Embryos of Austrofundulus limnaeus are exceptional in their ability to tolerate prolonged bouts of complete anoxia. Hypoxia and anoxia are a normal part of their developmental environment. Here, we exposed embryos to a range of PO2 levels at two different temperatures (25 and 30 °C) to study the combined effects of reduced oxygen and increased temperature on developmental rate, heart rate, and metabolic enzyme capacity. Hypoxia decreased overall developmental rate and caused a stage-specific decline in heart rate. However, the rate of early development prior to the onset of organogenesis is insensitive to PO2. Increased incubation temperature caused an increase in the developmental rate at high PO2s, but hindered developmental progression under severe hypoxia. Embryonic DNA content in pre-hatching embryos was positively correlated with PO2. Citrate synthase, lactate dehydrogenase, and phosphoenolpyruvate carboxykinase capacity were all reduced in embryos developing under hypoxic conditions. Embryos of A. limnaeus are able to develop normally across a wide range of PO2s and contrary to most other vertebrates severe hypoxia is not a teratogen. Embryos of A. limnaeus do not respond to hypoxia through an increase in the capacity for enzymatic activity of the metabolic enzymes lactate dehydrogenase, citrate synthase, or phosphoenolpyruvate carboxykinase. Instead they appear to adjust whole-embryo metabolic capacity to match oxygen availability. However, decreased DNA content in hypoxia-reared embryos suggests that cellular enzymatic capacity may remain unchanged in response to hypoxia, and the reduced capacity may rather indicate reduced cell number in hypoxic embryos.

Autophagy is essential for cardiac morphogenesis during vertebrate development

Genetic analyses indicate that autophagy, an evolutionarily conserved lysosomal degradation pathway, is essential for eukaryotic differentiation and development. However, little is known about whether autophagy contributes to morphogenesis during embryogenesis. To address this question, we examined the role of autophagy in the early development of zebrafish, a model organism for studying vertebrate tissue and organ morphogenesis. Using zebrafish that transgenically express the fluorescent autophagy reporter protein, GFP-LC3, we found that autophagy is active in multiple tissues, including the heart, during the embryonic period. Inhibition of autophagy by morpholino knockdown of essential autophagy genes (including atg5, atg7, and becn1) resulted in defects in morphogenesis, increased numbers of dead cells, abnormal heart structure, and reduced organismal survival. Further analyses of cardiac development in autophagy-deficient zebrafish revealed defects in cardiac looping, abnormal chamber morphology, aberrant valve development, and ectopic expression of critical transcription factors including foxn4, tbx5, and tbx2. Consistent with these results, Atg5-deficient mice displayed abnormal Tbx2 expression and defects in valve development and chamber septation. Thus, autophagy plays an essential, conserved role in cardiac morphogenesis during vertebrate development.

Zebrafish as a model of cardiac disease

The zebrafish has been rapidly adopted as a model for cardiac development and disease. The transparency of the embryo, its limited requirement for active oxygen delivery, and ease of use in genetic manipulations and chemical exposure have made it a powerful alternative to rodents. Novel technologies like TALEN/CRISPR-mediated genome engineering and advanced imaging methods will only accelerate its use. Here, we give an overview of heart development and function in the fish and highlight a number of areas where it is most actively contributing to the understanding of cardiac development and disease. We also review the current state of research on a feature that we only could wish to be conserved between fish and human; cardiac regeneration.

A multi-endpoint in vivo larval zebrafish (Danio rerio) model for the assessment of integrated cardiovascular function

INTRODUCTION

Despite effective in vitro preclinical strategies to identify cardiovascular (CV) liabilities, there remains a need for early functional assessment prior to complex in vivo mammalian models. The larval zebrafish (Danio rerio, Zf) has been suggested for this role: previous data suggest that cardiac electrophysiology and vascular ultrastructure are comparable with mammals, and also indicate responsiveness of individual Zf CV system endpoints to some functional modulators. Little information is, however, available regarding integrated functional CV responses to drug treatment. Consequently, we developed a novel larval Zf model capable of simultaneous quantification of chronotropic, inotropic and arrhythmic effects, alongside measures of blood flow and vessel diameter.

METHODS

Non-invasive video analysis of the heart and dorsal aorta of anaesthetized and agarose-embedded larval ZF was used to measure multiple cardiovascular endpoints, simultaneously, following treatment with a range of functional modulators of CV physiology.

RESULTS

Changes in atrial and ventricular beat frequencies were detected in response to acute treatment with cardio-stimulants (adrenaline and theophylline), and negative chrono/inotropes (cisapride, haloperidol, terfenadine and verapamil). Arrhythmias were also observed including terfenadine-induced 2:1 atrial-ventricular (A-V) block, a previously proposed hERG surrogate measure. Significant increases in blood flow were detected in response to adrenaline and theophylline exposure; and decreases after cisapride, haloperidol, terfenadine, and verapamil treatment. Using dorsal aorta (DA) blood flow and ventricular beat rate, surrogate stoke volumes were also calculated for all compounds.

DISCUSSION

These data support the use of this approach for CV function studies. Moreover the throughput and compound requirements (approximately 3 compounds/person effort/week and <10 mg) make our approach potentially suitable for higher throughput drug safety and efficacy applications, pending further assessment of ZF-mammalian pharmacological comparability.

Blood flow suppresses vascular Notch signalling via dll4 and is required for angiogenesis in response to hypoxic signalling

Aims

The contribution of blood flow to angiogenesis is incompletely understood. We examined the effect of blood flow on Notch signalling in the vasculature of zebrafish embryos, and whether blood flow regulates angiogenesis in zebrafish with constitutively up-regulated hypoxic signalling.

Methods and results

Developing zebrafish (Danio rerio) embryos survive via diffusion in the absence of circulation induced by knockdown of cardiac troponin T2 or chemical cardiac cessation. The absence of blood flow increased vascular Notch signalling in 48 h post-fertilization old embryos via up-regulation of the Notch ligand dll4. Despite this, patterning of the intersegmental vessels is not affected by absent blood flow. We therefore examined homozygous vhl mutant zebrafish that have constitutively up-regulated hypoxic signalling. These display excessive and aberrant angiogenesis from 72 h post-fertilization, with significantly increased endothelial number, vessel diameter, and length. The absence of blood flow abolished these effects, though normal vessel patterning was preserved.

Conclusion

We show that blood flow suppresses vascular Notch signalling via down-regulation of dll4. We have also shown that blood flow is required for angiogenesis in response to hypoxic signalling but is not required for normal vessel patterning. These data indicate important differences in hypoxia-driven vs. developmental angiogenesis.