Nitroreductase-mediated cell/tissue ablation in zebrafish: a spatially and temporally controlled ablation method with applications in developmental and regeneration studies

Chemogenetic Tools for Causal Cellular and Neuronal Biology

Chemogenetic technologies enable selective pharmacological control of specific cell populations. An increasing number of approaches have been developed that modulate different signaling pathways. Selective pharmacological control over G protein-coupled receptor signaling, ion channel conductances, protein association, protein stability, and small molecule targeting allows modulation of cellular processes in distinct cell types. Here, we review these chemogenetic technologies and instances of their applications in complex tissues in vivo and ex vivo.

Stat3 Regulates Liver Progenitor Cell-Driven Liver Regeneration in Zebrafish

After liver injury, regeneration manifests as either (1) hepatocytes proliferating to restore the lost hepatocyte mass or (2) if hepatocyte proliferation is compromised, biliary epithelial cells (BECs) dedifferentiating into liver progenitor cells (LPCs), which subsequently differentiate into hepatocytes. Following pharmacogenetic ablation of hepatocytes in Tg(fabp10a:CFP-NTR) zebrafish, resulting in severe liver injury, signal transducer and activator of transcription 3 (Stat3) and its target gene and negative regulator, socs3a, were upregulated in regenerating livers. Using either Stat3 inhibitors, JSI-124 and S3I-201, or stat3 zebrafish mutants, we investigated the role of Stat3 in LPC-driven liver regeneration. Although Stat3 suppression reduced the size of regenerating livers, BEC dedifferentiation into LPCs was unaffected. However, regenerating livers displayed a delay in LPC-to-hepatocyte differentiation and a significant reduction in the number of BECs. While no difference in cell death was detected, Stat3 inhibition significantly reduced LPC proliferation. Notably, stat3 mutants phenocopied the effects of Stat3 chemical inhibitors, although the mutant phenotype was incompletely penetrant. Intriguingly, a subset of socs3a mutants also displayed a lower number of BECs in regenerating livers. We conclude that the Stat3/Socs3a pathway is necessary for the proper timing of LPC-to-hepatocyte differentiation and establishing the proper number of BECs during LPC-driven liver regeneration.

Zebrafish as a Model for Obesity and Diabetes

Obesity and diabetes now considered global epidemics. The prevalence rates of diabetes are increasing in parallel with the rates of obesity and the strong connection between these two diseases has been coined as “diabesity.” The health risks of overweight or obesity include Type 2 diabetes mellitus (T2DM), coronary heart disease and cancer of numerous organs. Both obesity and diabetes are complex diseases that involve the interaction of genetics and environmental factors. The underlying pathogenesis of obesity and diabetes are not well understood and further research is needed for pharmacological and surgical management. Consequently, the use of animal models of obesity and/or diabetes is important for both improving the understanding of these diseases and to identify and develop effective treatments. Zebrafish is an attractive model system for studying metabolic diseases because of the functional conservation in lipid metabolism, adipose biology, pancreas structure, and glucose homeostasis. It is also suited for identification of novel targets associated with the risk and treatment of obesity and diabetes in humans. In this review, we highlight studies using zebrafish to model metabolic diseases, and discuss the advantages and disadvantages of studying pathologies associated with obesity and diabetes in zebrafish.

Fundamental parameters of the developing thymic epithelium in the mouse

The numbers of thymic epithelial cells (TECs) and thymocytes steadily increase during embryogenesis. To examine this dynamic, we generated several TEC-specific transgenic mouse lines, which express fluorescent proteins in the nucleus, the cytosol and in the membranes under the control of the Foxn1 promoter. These tools enabled us to determine TEC numbers in tissue sections by confocal fluorescent microscopy, and in the intact organ by light-sheet microscopy. Compared to histological procedures, flow cytometric analysis of thymic cellularity is shown to underestimate the numbers of TECs by one order of magnitude; using enzymatic digestion of thymic tissue, the loss of cortical TECs (cTECs) is several fold greater than that of medullary TECs (mTECs), although different cTEC subsets appear to be still present in the final preparation. Novel reporter lines driven by Psmb11 and Prss16 promoters revealed the trajectory of differentiation of cTEC-like cells, and, owing to the additional facility of conditional cell ablation, allowed us to follow the recovery of such cells after their depletion during embryogenesis. Multiparametric histological analyses indicate that the new transgenic reporter lines not only reveal the unique morphologies of different TEC subsets, but are also conducive to the analysis of the complex cellular interactions in the thymus.

Triptolide attenuates proteinuria and podocyte apoptosis via inhibition of NF-κB/GADD45B

Podocyte injury is a primary contributor to proteinuria. Triptolide is a major active component of Tripterygium wilfordii Hook F that exhibits potent antiproteinuric effects. We used our previously developed in vivo zebrafish model of inducible podocyte-target injury and found that triptolide treatment effectively alleviated oedema, proteinuria and foot process effacement. Triptolide also inhibited podocyte apoptosis in our zebrafish model and in vitro. We also examined the mechanism of triptolide protection of podocyte. Whole-genome expression profiles of cultured podocytes demonstrated that triptolide treatment downregulated apoptosis pathway-related GADD45B expression. Specific overexpression of gadd45b in zebrafish podocytes abolished the protective effects of triptolide. GADD45B is a mediator of podocyte apoptosis that contains typical NF-κB binding sites in the promoter region, and NF-κB p65 primarily transactivates this gene. Triptolide inhibited NF-κB signalling activation and binding of NF-κB to the GADD45B promoter. Taken together, our findings demonstrated that triptolide attenuated proteinuria and podocyte apoptosis via inhibition of NF-κB/GADD45B signalling, which provides a new understanding of the antiproteinuric effects of triptolide in glomerular diseases.

Analysis of Beta-cell Function Using Single-cell Resolution Calcium Imaging in Zebrafish Islets

Pancreatic beta-cells respond to increasing blood glucose concentrations by secreting the hormone insulin. The dysfunction of beta-cells leads to hyperglycemia and severe, life-threatening consequences. Understanding how the beta-cells operate under physiological conditions and what genetic and environmental factors might cause their dysfunction could lead to better treatment options for diabetic patients. The ability to measure calcium levels in beta-cells serves as an important indicator of beta-cell function, as the influx of calcium ions triggers insulin release. Here we describe a protocol for monitoring the glucose-stimulated calcium influx in zebrafish beta-cells by using GCaMP6s, a genetically encoded sensor of calcium. The method allows monitoring the intracellular calcium dynamics with single-cell resolution in ex vivo mounted islets. The glucose-responsiveness of beta-cells within the same islet can be captured simultaneously under different glucose concentrations, which suggests the presence of functional heterogeneity among zebrafish beta-cells. Furthermore, the technique provides high temporal and spatial resolution, which reveals the oscillatory nature of the calcium influx upon glucose stimulation. Our approach opens the doors to use the zebrafish as a model to investigate the contribution of genetic and environmental factors to beta-cell function and dysfunction.

Chemical Amputation and Regeneration of the Pharynx in the Planarian Schmidtea mediterranea

Planarians are flatworms that are extremely efficient at regeneration. They owe this ability to a large number of stem cells that can rapidly respond to any type of injury. Common injury models in these animals remove large amounts of tissue, which damages multiple organs. To overcome this broad tissue damage, we describe here a method to selectively remove a single organ, the pharynx, in the planarian Schmidtea mediterranea. We achieve this by soaking animals in a solution containing the cytochrome oxidase inhibitor sodium azide. Brief exposure to sodium azide causes extrusion of the pharynx from the animal, which we call "chemical amputation." Chemical amputation removes the entire pharynx, and generates a small wound where the pharynx attaches to the intestine. After extensive rinsing, all amputated animals regenerate a fully functional pharynx in approximately one week. Stem cells in the rest of the body drive regeneration of the new pharynx. Here, we provide a detailed protocol for chemical amputation, and describe both histological and behavioral methods to assess successful amputation and regeneration.

Activatable Near-Infrared Fluorescent Probe for Dipeptidyl Peptidase IV and Its Bioimaging Applications in Living Cells and Animals

Visualization of endogenous disease-associated enzymes is of great clinical significance, as it could allow earlier clinical diagnosis and timely intervention. Herein, we first synthesized and characterized an enzyme-activatable near-infrared fluorescent probe, GP-DM, for determining the activity of dipeptidyl peptidase IV (DPP IV), which is associated with various pathological processes, especially in diabetes and malignant tumors. GP-DM emitted significant turn-on NIR fluorescent signals simultaneously in response to DPP IV, making it favorable for accurately and dynamically monitoring DPP IV activity in vitro and in vivo. GP-DM exhibited excellent specificity and sensitivity in DPP IV imaging, as indicated by its higher catalytic activity than other human serine hydrolases and by its strong anti-interference ability to a complex biological matrix, which was fully characterized in a series of phenotyping reactions and inhibition assays. Encouraged by the advantages mentioned above, we successfully used GP-DM to evaluate endogenous DPP IV activity in various biological samples (plasma and tissue preparations) and living tumor cells and performed real-time in vivo bioimaging of DPP IV in zebrafish and tumor-bearing nude mice. All of the results reflected and highlighted the potential application value of GP-DM in the early detection of pathologies, individual tailoring of drug therapy, and image-guided tumor resection. Furthermore, our results revealed that DPP IV, a key target enzyme, is closely associated with the migration and proliferation of cancer cells and regulating the biological activity of DPP IV may be a useful approach for cancer therapy.

Down-regulation of interleukin 7 receptor (IL-7R) contributes to central nervous system demyelination

Interleukin 7 receptor (IL-7R) has been associated with the pathogenesis of multiple sclerosis (MS), though the mechanisms are not clear. Because myelin expression is highly conserved between zebrafish and mammals, zebrafish have become an ideal model for studying demyelination. We used a transgenic (Tg; mbp:nfsB-egfp) zebrafish line in which oligodendrocytes expressed green fluorescent protein (GFP) from the larval stage to adulthood. Exposing adult transgenic zebrafish to metronidazole induced demyelination that resembled the morphological changes associated with the early stages of MS. The metronidazole-induced demyelination was confirmed by magnetic resonance imaging (MRI) for the first time. Microarray analysis revealed down-regulation of IL-7R during demyelination. Targeted knockdown of IL-7R demonstrated that IL-7R is essential for myelination in embryonic and larval zebrafish. Moreover, IL-7R down-regulation induced signaling via the JAK/STAT pathway leading to apoptosis in oligodendrocytes. These findings contribute to our understanding of the role of IL-7R in demyelination, and provide a rationale for the development of IL-7R-based therapies for MS and other demyelinating diseases.

Oxidative stress-dependent MMP-13 activity underlies glucose neurotoxicity

BACKGROUND

A complication of diabetes is neuropathy, a condition of sensory axon degeneration that originates in the epidermis. The mechanisms remain unknown but reactive oxygen species (ROS) have been implicated in this condition. In this study, we assessed the role of ROS and a candidate downstream target, MMP-13 in glucose-induced sensory axon degeneration in zebrafish and mice.

METHODS

The effects of glucose on metabolism and sensory axon degeneration were assessed using qPCR and live imaging. ROS were analyzed using pentafluorobenzene-sulfonyl fluorescein and activation of the NF-κB stress response was determined using Tg(NF-κB:GFP) zebrafish. The role of MMP-13 and ROS in glucose-dependent axon degeneration was determined in zebrafish following treatment with the antioxidant, N-acetylcysteine and the MMP-13 inhibitor, DB04760. Neuropathic mice fed on a high-fat/high-sugar diet were treated with the MMP-13 inhibitor, CL-82198 to assess sensory recovery.

RESULTS

Glucose treatment of zebrafish induced metabolic changes that resemble diabetes. Sensory axon degeneration was mediated by ROS-induced MMP-13 and prevented upon antioxidant treatment or MMP-13 inhibition. MMP-13 inhibition also reversed neuropathy in diabetic mice.

CONCLUSION

We demonstrate that zebrafish are suitable to study glucose-induced neurotoxicity. Given the effects in zebrafish and mice, MMP-13 inhibition may be beneficial in the treatment of human diabetic neuropathy.

The Efficacy of Puromycin and Adriamycin for Induction of Glomerular Failure in Larval Zebrafish Validated by an Assay of Glomerular Permeability Dynamics

Defects in the glomerular filtration barrier (GFB) play a major role in the onset of human renal diseases. Highly ramified glomerular cells named podocytes are a critical component of the GFB. Injury to podocytes results in abnormal excretion of plasma proteins, which can lead to chronic kidney disease. The conserved paired nephron of larval zebrafish is an excellent model for assessing glomerular function and injury. The efficacy of two known podocyte toxins was tested to refine models of acute podocyte injury in larval zebrafish. The validated compound was then used to test a novel assay of the dynamics of abnormal protein excretion. Injected adriamycin was found to be unsuitable for induction of glomerular injury due to off-target cardiovascular toxicity. In contrast, puromycin treatment resulted in a loss of discriminative filtration, measured by excretion of 70 kDa dextran, and podocyte effacement confirmed by electron microscopy. The dynamics of dextran excretion during puromycin injury modeled the onset of glomerular damage within 24 hours postinjection. These data validate puromycin for induction of acute podocyte injury in zebrafish larvae and describe a semihigh-throughput assay for quantifying the dynamics of abnormal protein excretion.

Effects of fin fold mesenchyme ablation on fin development in zebrafish

The evolution of the tetrapod limb involved an expansion and elaboration of the endoskeletal elements, while the fish fin rays were lost. Loss of fin-specific genes, and regulatory changes in key appendicular patterning genes have been identified as mechanisms of limb evolution, however their contributions to cellular organization and tissue differences between fins and limbs remains poorly understood. During early larval fin development, hoxa13a/hoxd13a-expressing fin fold mesenchyme migrate through the median and pectoral fin along actinotrichia fibrils, non-calcified skeletal elements crucial for supporting the fin fold. Fin fold mesenchyme migration defects have previously been proposed as a mechanism of fin dermal bone loss during tetrapod evolution as it has been shown they contribute directly to the fin ray osteoblast population. Using the nitroreductase/metronidazole system, we genetically ablated a subset of hoxa13a/hoxd13a-expressing fin fold mesenchyme to assess its contributions to fin development. Following the ablation of fin fold mesenchyme in larvae, the actinotrichia are unable to remain rigid and the median and pectoral fin folds collapse, resulting in a reduced fin fold size. The remaining cells following ablation are unable to migrate and show decreased actinodin1 mesenchymal reporter activity. Actinodin proteins are crucial structural component of the actinotrichia. Additionally, we show a decrease in hoxa13a, hoxd13a, fgf10a and altered shha, and ptch2 expression during larval fin development. A continuous treatment of metronidazole leads to fin ray defects at 30dpf. Fewer rays are present compared to stage-matched control larvae, and these rays are shorter and less defined. These results suggest the targeted hoxa13a/hoxd13a-expressing mesenchyme contribute to their own successful migration through their contributions to actinotrichia. Furthermore, due to their fate as fin ray osteoblasts, we propose their initial ablation, and subsequent disorganization produces truncated fin dermal bone elements during late larval stages.

Generation of all-male-like sterile zebrafish by eliminating primordial germ cells at early development

Production of all-male and sterile fish may not only substantially improve yield but also be crucial for the application of genome modified species in aquaculture. Previously, it was reported that the fish lacking primordial germ cells (PGCs) becomes infertile, and nitroreductase, an enzyme converting non-toxic metronidazole (MTZ) into toxic metabolites, induces targeted toxicity to kill the cells expressing it. In this study, we generated a transgenic zebrafish line of Tg(nanos3:nfsB-mCherry-nanos3 3'UTR) in which the NfsB nitroreductase is solely expressed in PGCs. Treating the embryos derived from the female transgenic zebrafish with MTZ from 0 through 2 dpf (days post fertilization), we found that the germ cells were completely eliminated in the ones older than 2.5 dpf. At 20 dpf, the MTZ-treated juvenile had no germ cells in their gonads. At 100 dpf, the MTZ-treated adult exhibited male-like morphology and showed normal mating behaviors although they had no germ cells but only supporting cells in their gonads. Taken together, our results demonstrated that conditional elimination of PGCs during early development make the zebrafish male-like and infertile. It may provide an alternative strategy to make sterile and all-male farmed fish that is good for increasing aquaculture yield and preventing the genome modified species from potential ecological risks.

Tbx5a lineage tracing shows cardiomyocyte plasticity during zebrafish heart regeneration

During development, mesodermal progenitors from the first heart field (FHF) form a primitive cardiac tube, to which progenitors from the second heart field (SHF) are added. The contribution of FHF and SHF progenitors to the adult zebrafish heart has not been studied to date. Here we find, using genetic tbx5a lineage tracing tools, that the ventricular myocardium in the adult zebrafish is mainly derived from tbx5a+ cells, with a small contribution from tbx5a- SHF progenitors. Notably, ablation of ventricular tbx5a+-derived cardiomyocytes in the embryo is compensated by expansion of SHF-derived cells. In the adult, tbx5a expression is restricted to the trabeculae and excluded from the outer cortical layer. tbx5a-lineage tracing revealed that trabecular cardiomyocytes can switch their fate and differentiate into cortical myocardium during adult heart regeneration. We conclude that a high degree of cardiomyocyte cell fate plasticity contributes to efficient regeneration.

Conditional Chemogenetic Ablation of Photoreceptor Cells in Xenopus Retina

Xenopus is an attractive model system for regeneration studies, as it exhibits an extraordinary regenerative capacity compared to mammals. It is commonly used to study body part regeneration following amputation, for instance of the limb, the tail, or the retina. Models with more subtle injuries are also needed for human degenerative disease modeling, allowing for the study of stem cell recruitment for the regeneration of a given cellular subtype. We present here a model to ablate photoreceptor cells in the Xenopus retina. This method is based on the nitroreductase/metronidazole (NTR/MTZ) system, a combination of chemical and genetic tools, allowing for the conditional ablation of targeted cells. This type of approach establishes Xenopus as a powerful model to study cellular regeneration and stem cell regulation.

Development and growth of organs in living whole embryo and larval grafts in zebrafish

Age-related systemic environments influence neurogenesis and organ regeneration of heterochronic parabiotic partners; however, the difficulty of manipulating small embryos prevents the effects of aged systemic environments on primitive organs at the developmental stage from being analysed. Here, we describe a novel transplantation system to support whole living embryos/larvae as grafts in immunodeficient zebrafish by the intrusion of host blood vessels into the grafts, allowing bodies similar to those of heterochronic parabiosis to be generated by subcutaneous grafting. Although grafted embryos/larvae formed most organs, not all organogenesis was supported equally; although the brain, eyes and the intestine usually developed, the liver, testes and heart developed insufficiently or even occasionally disappeared. Removal of host germ cells stimulated testis development in grafted embryos. These results indicate that primitive testes are susceptible to the systemic environments that originated from the germ cells of aged hosts and imply that the primitive liver and heart are similar. Upon applying this method to embryonic lethal mutants, various types of organs, including testes that developed in germ-cell-removed recipients, and viable offspring were obtained from the mutants. This unique transplantation system will lead to new insights into the age-related systemic environments that are crucial for organogenesis in vertebrates.

Regeneration Genetics

Understanding how and why animals regenerate complex tissues has the potential to transform regenerative medicine. Here we present an overview of genetic approaches that have recently been applied to dissect mechanisms of regeneration. We describe new advances that relate to central objectives of regeneration biologists researching different tissues and species, focusing mainly on vertebrates. These objectives include defining the cellular sources and key cell behaviors in regenerating tissue, elucidating molecular triggers and brakes for regeneration, and defining the earliest events that control the presence of these molecular factors.

Re-evaluating the functional landscape of the cardiovascular system during development

The cardiovascular system facilitates body-wide distribution of oxygen, a vital process for the development and survival of virtually all vertebrates. However, the zebrafish, a vertebrate model organism, appears to form organs and survive mid-larval periods without a functional cardiovascular system. Despite such dispensability, it is the first organ to develop. Such enigma prompted us to hypothesize other cardiovascular functions that are important for developmental and/or physiological processes. Hence, systematic cellular ablations and functional perturbations were performed on the zebrafish cardiovascular system to gain comprehensive and body-wide understanding of such functions and to elucidate the underlying mechanisms. This approach identifies a set of organ-specific genes, each implicated for important functions. The study also unveils distinct cardiovascular mechanisms, each differentially regulating their expressions in organ-specific and oxygen-independent manners. Such mechanisms are mediated by organ-vessel interactions, circulation-dependent signals, and circulation-independent beating-heart-derived signals. A comprehensive and body-wide functional landscape of the cardiovascular system reported herein may provide clues as to why it is the first organ to develop. Furthermore, these data could serve as a resource for the study of organ development and function.

Summary: The body-wide landscape of the cardiovascular functions during development is reported. Such landscape may provide clues as to why the cardiovascular system is the first organ to develop.

A turn-on endoplasmic reticulum-targeted two-photon fluorescent probe for hydrogen sulfide and bio-imaging applications in living cells, tissues, and zebrafish

As one of the important gas signal molecules, hydrogen sulfide (H₂S) is associated with many important physiological processes in living organisms. Organelles, especially endoplasmic reticulum (ER), play a crucial role in the cell metabolism. Accordingly, the detection of H₂S in the ER is of high interest. Toward this goal, we have described the development of the first ER-targeted fluorescent H₂S probe (Na-H ₂ S-ER). The new probe exhibited favorable features, such as a large turn-on fluorescence signal (45-fold fluorescence enhancement), high sensitivity and selectivity. The probe was successfully employed for imaging exogenous and endogenous H₂S in the living HeLa cells. Significantly, the new probe Na-H ₂ S-ER was employed to visualize H₂S in the ER of living cells for the first time. In addition, the probe was also successfully used for imaging H₂S in the living tissues up to a depth of 100 μm and in the living zebrafish.

Role of branchiomotor neurons in controlling food intake of zebrafish larvae

The physical act of eating or feeding involves the coordinated action of several organs like eyes and jaws, and associated neural networks. Moreover, the activity of the neural networks controlling jaw movements (branchiomotor circuits) is regulated by the visual, olfactory, gustatory and hypothalamic systems, which are largely well characterized at the physiological level. By contrast, the behavioral output of the branchiomotor circuits and the functional consequences of disruption of these circuits by abnormal neural development are poorly understood. To begin to address these questions, we sought to evaluate the feeding ability of zebrafish larvae, a direct output of the branchiomotor circuits, and developed a qualitative assay for measuring food intake in zebrafish larvae at 7 days post-fertilization. We validated the assay by examining the effects of ablating the branchiomotor neurons. Metronidazole-mediated ablation of nitroreductase-expressing branchiomotor neurons resulted in a predictable reduction in food intake without significantly affecting swimming ability, indicating that the assay is robust. Laser-mediated ablation of trigeminal motor neurons resulted in a significant decrease in food intake, indicating that the assay is sensitive. Importantly, in larvae of a genetic mutant with severe loss of branchiomotor neurons, food intake was abolished. These studies establish a foundation for dissecting the neural circuits driving a motor behavior essential for survival.

Tcf7l2 plays pleiotropic roles in the control of glucose homeostasis, pancreas morphology, vascularization and regeneration

Type 2 diabetes (T2D) is a disease characterized by impaired insulin secretion. The Wnt signaling transcription factor Tcf7l2 is to date the T2D-associated gene with the largest effect on disease susceptibility. However, the mechanisms by which TCF7L2 variants affect insulin release from β-cells are not yet fully understood. By taking advantage of a tcf7l2 zebrafish mutant line, we first show that these animals are characterized by hyperglycemia and impaired islet development. Moreover, we demonstrate that the zebrafish tcf7l2 gene is highly expressed in the exocrine pancreas, suggesting potential bystander effects on β-cell growth, differentiation and regeneration. Finally, we describe a peculiar vascular phenotype in tcf7l2 mutant larvae, characterized by significant reduction in the average number and diameter of pancreatic islet capillaries. Overall, the zebrafish Tcf7l2 mutant, characterized by hyperglycemia, pancreatic and vascular defects, and reduced regeneration proves to be a suitable model to study the mechanism of action and the pleiotropic effects of Tcf7l2, the most relevant T2D GWAS hit in human populations.

Investigating the application of a nitroreductase-expressing transgenic zebrafish line for high-throughput toxicity testing

Nitroreductase enzymes are responsible for the reduction of nitro functional groups to amino functional groups, and are found in a range of animal models, zebrafish (Danio rerio) excluded. Transgenic zebrafish models have been developed for tissue-specific cell ablation, which use nitroreductase to ablate specific tissues or cell types following exposure to the non-toxic pro-drug metronidazole (MTZ). When metabolized by nitroreductase, MTZ produces a potent cytotoxin, which specifically ablates the tissue in which metabolism occurs. Uses, beyond tissue-specific cell ablation, are possible for the hepatocyte-specific Tg(l-fabp:CFP-NTR)^s891 zebrafish line, including investigations of the role of nitroreductase in the toxicity of nitrated compounds. The hepatic ablation characteristics of this transgenic line were explored, in order to expand its potential uses. Embryos were exposed at 48, 72, or 96 hours post fertilization (hpf) to a range of MTZ concentrations, and the ablation profiles were compared. Ablation occurred at a 10-fold lower concentration than previously reported. Embryos were exposed to a selection of other compounds, with and without MTZ, in order to investigate alternative uses for this transgenic line. Test compounds were selected based on: their ability to undergo nitroreduction, known importance of hepatic metabolism to toxicity, and known pharmaceutical hepatotoxins. Selected compounds included nitrated polycyclic aromatic hydrocarbons (nitro-PAHs), the PAHs retene and benzo[a]pyrene, and the pharmaceuticals acetaminophen and flutamide. The results suggest a range of potential roles of the liver in the toxicity of these compounds, and highlight the additional uses of this transgenic model in toxicity testing.

Fate bias during neural regeneration adjusts dynamically without recapitulating developmental fate progression

BACKGROUND

Regeneration of neurons in the central nervous system is poor in humans. In other vertebrates neural regeneration does occur efficiently and involves reactivation of developmental processes. Within the neural retina of zebrafish, Müller glia are the main stem cell source and are capable of generating progenitors to replace lost neurons after injury. However, it remains largely unknown to what extent Müller glia and neuron differentiation mirror development.

METHODS

Following neural ablation in the zebrafish retina, dividing cells were tracked using a prolonged labelling technique. We investigated to what extent extrinsic feedback influences fate choices in two injury models, and whether fate specification follows the histogenic order observed in development.

RESULTS

By comparing two injury paradigms that affect different subpopulations of neurons, we found a dynamic adaptability of fate choices during regeneration. Both injuries followed a similar time course of cell death, and activated Müller glia proliferation. However, these newly generated cells were initially biased towards replacing specifically the ablated cell types, and subsequently generating all cell types as the appropriate neuron proportions became re-established. This dynamic behaviour has implications for shaping regenerative processes and ensuring restoration of appropriate proportions of neuron types regardless of injury or cell type lost.

CONCLUSIONS

Our findings suggest that regenerative fate processes are more flexible than development processes. Compared to development fate specification we observed a disruption in stereotypical birth order of neurons during regeneration Understanding such feedback systems can allow us to direct regenerative fate specification in injury and diseases to regenerate specific neuron types in vivo.

tomm22 Knockdown-Mediated Hepatocyte Damages Elicit Both the Formation of Hybrid Hepatocytes and Biliary Conversion to Hepatocytes in Zebrafish Larvae

The liver has a highly regenerative capacity. In the normal liver, hepatocytes proliferate to restore lost liver mass. However, when hepatocyte proliferation is impaired, biliary epithelial cells (BECs) activate and contribute to hepatocytes. We previously reported in zebrafish that upon severe hepatocyte ablation, BECs extensively contribute to regenerated hepatocytes. It was also speculated that BEC-driven liver regeneration might occur in another zebrafish liver injury model in which temporary knockdown of the mitochondrial import gene tomm22 by morpholino antisense oligonucleotides (MO) induces hepatocyte death. Given the importance of multiple BEC-driven liver regeneration models for better elucidating the mechanisms underlying innate liver regeneration in the diseased liver, we hypothesized that BECs would contribute to hepatocytes in tomm22 MO-injected larvae. In this MO-based liver injury model, by tracing the lineage of BECs, we found that BECs significantly contributed to hepatocytes. Moreover, we found that surviving, preexisting hepatocytes become BEC-hepatocyte hybrid cells in tomm22 MO-injected larvae. Intriguingly, both the inhibition of Wnt/β-catenin signaling and macrophage ablation suppressed the formation of the hybrid hepatocytes. This new liver injury model in which both hepatocytes and BECs contribute to regenerated hepatocytes will aid in better understanding the mechanisms of innate liver regeneration in the diseased liver.

Heritable natural variation of an anxiety-like behavior in larval zebrafish

Complex behaviors are often observed at a spectrum in the population, and psychiatric disorders represent extremes of such behavioral spectra. While grasping the underlying cellular and molecular basis of these disorders represents a major challenge, it is believed that studies of complex behaviors in model organisms, where genotyping and phenotyping can be more conveniently carried out and cause-effect relationships can be further discerned, will help address this challenge. Here we report the characterization of a natural dark aversion behavior in larval zebrafish, which is previously shown to be fear or anxiety-associated. Phenotyping ∼200 individuals using a light/dark choice assay uncovered that, while a majority of individuals displayed medium level of dark aversion (mda), a small number of individuals exhibited strong dark aversion (sda), and a third small cohort showed variable dark aversion (vda). Through selective breeding and phenotyping of the next generation, we demonstrated that both the sda and vda traits are heritable, with sda being invariable while vda being highly variable across multiple trials. Additionally, sda appears to be recessive and vda appears to be dominant over the common allele(s) in the population. Moreover, compared to vda, sda showed increased thigmotaxis (preference for the walls in an open field), another measure of anxiety. Together, these findings reveal a naturally heritable variation of anxiety-like behavior in a tractable model organism, thereby laying foundation for future dissection of the underlying molecular and cellular mechanisms.

Macrophages, but not neutrophils, are critical for proliferation of Burkholderia cenocepacia and ensuing host-damaging inflammation

Bacteria of the Burkholderia cepacia complex (Bcc) can cause devastating pulmonary infections in cystic fibrosis (CF) patients, yet the precise mechanisms underlying inflammation, recurrent exacerbations and transition from chronic stages to acute infection and septicemia are not known. Bcc bacteria are generally believed to have a predominant extracellular biofilm life style in infected CF lungs, similar to Pseudomonas aeruginosa, but this has been challenged by clinical observations which show Bcc bacteria predominantly in macrophages. More recently, Bcc bacteria have emerged in nosocomial infections of patients hospitalized for reasons unrelated to CF. Research has abundantly shown that Bcc bacteria can survive and replicate in mammalian cells in vitro, yet the importance of an intracellular life style during infection in humans is unknown. Here we studied the contribution of innate immune cell types to fatal pro-inflammatory infection caused by B. cenocepacia using zebrafish larvae. In strong contrast to the usual protective role for macrophages against microbes, our results show that these phagocytes significantly worsen disease outcome. We provide new insight that macrophages are critical for multiplication of B. cenocepacia in the host and for development of a fatal, pro-inflammatory response that partially depends on Il1-signalling. In contrast, neutrophils did not significantly contribute to disease outcome. In subcutaneous infections that are dominated by neutrophil-driven phagocytosis, the absence of a functional NADPH oxidase complex resulted in a small but measurably higher increase in bacterial growth suggesting the oxidative burst helps limit bacterial multiplication; however, neutrophils were unable to clear the bacteria. We suggest that paradigm-changing approaches are needed for development of novel antimicrobials to efficiently disarm intracellular bacteria of this group of highly persistent, opportunistic pathogens.

Sheath Cell Invasion and Trans-differentiation Repair Mechanical Damage Caused by Loss of Caveolae in the Zebrafish Notochord

The notochord, a conserved axial structure required for embryonic axis elongation and spine development, consists of giant vacuolated cells surrounded by an epithelial sheath [1-3]. During morphogenesis, vacuolated cells maintain their structural integrity despite being under constant mechanical stress [4]. We hypothesized that the high density of caveolae present in vacuolated cells [5, 6] could buffer mechanical tension. Caveolae are 50- to 80-nm membrane invaginations lined by cage-like polygonal structures [7, 8] formed by caveolin 1 (Cav1) or Cav3 and one of the cavin proteins [6, 9-11]. Recent in vitro work has shown that plasma membrane caveolae constitute a membrane reservoir that can buffer mechanical stresses such as stretching or osmotic swelling [12]. Moreover, mechanical integrity of vascular and muscle cells is partly dependent on caveolae [13-15]. However, the in vivo mechano-protective roles of caveolae have only begun to be explored. Using zebrafish mutants for cav1, cav3, and cavin1b, we show that caveolae are essential for notochord integrity. Upon loss of caveola function, vacuolated cells collapse at discrete positions under the mechanical strain of locomotion. Then, sheath cells invade the inner notochord and differentiate into vacuolated cells, thereby restoring notochord function and allowing normal spine development. Our data further indicate that nucleotides released by dying vacuolated cells promote sheath cell vacuolization and trans-differentiation. This work reveals a novel structural role for caveolae in vertebrates and provides unique insights into the mechanisms that safeguard notochord and spine development.

Hif-1α regulates macrophage-endothelial interactions during blood vessel development in zebrafish

Macrophages are known to interact with endothelial cells during developmental and pathological angiogenesis but the molecular mechanisms modulating these interactions remain unclear. Here, we show a role for the Hif-1α transcription factor in this cellular communication. We generated hif-1aa;hif-1ab double mutants in zebrafish, hereafter referred to as hif-1α mutants, and find that they exhibit impaired macrophage mobilization from the aorta-gonad-mesonephros (AGM) region as well as angiogenic defects and defective vascular repair. Importantly, macrophage ablation is sufficient to recapitulate the vascular phenotypes observed in hif-1α mutants, revealing for the first time a macrophage-dependent angiogenic process during development. Further substantiating our observations of vascular repair, we find that most macrophages closely associated with ruptured blood vessels are Tnfα-positive, a key feature of classically activated macrophages. Altogether, our data provide genetic evidence that Hif-1α regulates interactions between macrophages and endothelial cells starting with the mobilization of macrophages from the AGM.

The molecular mechanism regulating macrophage interaction with endothelial cells during development is unclear. Here, the authors show that in zebrafish mutation of hypoxia-inducible factor-1α impairs macrophage mobilization from the aorta-gonad-mesonephros, causing defects in angiogenesis and vessel repair.

Distinct Roles for Matrix Metalloproteinases 2 and 9 in Embryonic Hematopoietic Stem Cell Emergence, Migration, and Niche Colonization

Hematopoietic stem/progenitor cells (HSPCs) are formed during ontogeny from hemogenic endothelium in the ventral wall of the dorsal aorta (VDA). Critically, the cellular mechanism(s) allowing HSPC egress and migration to secondary niches are incompletely understood. Matrix metalloproteinases (MMPs) are inflammation-responsive proteins that regulate extracellular matrix (ECM) remodeling, cellular interactions, and signaling. Here, inhibition of vascular-associated Mmp2 function caused accumulation of fibronectin-rich ECM, retention of runx1/cmyb⁺ HSPCs in the VDA, and delayed caudal hematopoietic tissue (CHT) colonization; these defects were absent in fibronectin mutants, indicating that Mmp2 facilitates endothelial-to-hematopoietic transition via ECM remodeling. In contrast, Mmp9 was dispensable for HSPC budding, being instead required for proper colonization of secondary niches. Significantly, these migration defects were mimicked by overexpression and blocked by knockdown of C-X-C motif chemokine-12 (cxcl12), suggesting that Mmp9 controls CHT homeostasis through chemokine regulation. Our findings indicate Mmp2 and Mmp9 play distinct but complementary roles in developmental HSPC production and migration.

Subcellular Redox Targeting: Bridging in Vitro and in Vivo Chemical Biology

Networks of redox sensor proteins within discrete microdomains regulate the flow of redox signaling. Yet, the inherent reactivity of redox signals complicates the study of specific redox events and pathways by traditional methods. Herein, we review designer chemistries capable of measuring flux and/or mimicking subcellular redox signaling at the cellular and organismal level. Such efforts have begun to decipher the logic underlying organelle-, site-, and target-specific redox signaling in vitro and in vivo. These data highlight chemical biology as a perfect gateway to interrogate how nature choreographs subcellular redox chemistry to drive precision redox biology.

A macrophage relay for long-distance signaling during postembryonic tissue remodeling

Macrophages have diverse functions in immunity as well as in development and homeostasis. We identified a function for these cells in long-distance communication during postembryonic tissue remodeling. Ablation of macrophages in zebrafish prevented melanophores from coalescing into adult pigment stripes. Melanophore organization depends on signals provided by cells of the yellow xanthophore lineage via airinemes, long filamentous projections with vesicles at their tips. We show that airineme extension from originating cells, as well as vesicle deposition on target cells, depend on interactions with macrophages. These findings identify a role for macrophages in relaying long-range signals between nonimmune cells. This signaling modality may function in the remodeling and homeostasis of other tissues during normal development and disease.

ptf1a+ , ela3l- cells are developmentally maintained progenitors for exocrine regeneration following extreme loss of acinar cells in zebrafish larvae

The exocrine pancreas displays a significant capacity for regeneration and renewal. In humans and mammalian model systems, the partial loss of exocrine tissue, such as after acute pancreatitis or partial pancreatectomy induces rapid recovery via expansion of surviving acinar cells. In mouse it was further found that an almost complete removal of acinar cells initiates regeneration from a currently not well-defined progenitor pool. Here, we used the zebrafish as an alternative model to study cellular mechanisms of exocrine regeneration following an almost complete removal of acinar cells. We introduced and validated two novel transgenic approaches for genetically encoded conditional cell ablation in the zebrafish, either by caspase-8-induced apoptosis or by rendering cells sensitive to diphtheria toxin. By using the ela3l promoter for exocrine-specific expression, we show that both approaches allowed cell-type-specific removal of >95% of acinar tissue in larval and adult zebrafish without causing any signs of unspecific side effects. We find that zebrafish larvae are able to recover from a virtually complete acinar tissue ablation within 2 weeks. Using short-term lineage-tracing experiments and EdU incorporation assays, we exclude duct-associated Notch-responsive cells as the source of regeneration. Rather, a rare population of slowly dividing ela3l-negative cells expressing ptf1a and CPA was identified as the origin of the newly forming exocrine cells. Cells are actively maintained, as revealed by a constant number of these cells at different larval stages and after repeated cell ablation. These cells establish ela3l expression about 4-6 days after ablation without signs of increased proliferation in between. With onset of ela3l expression, cells initiate rapid proliferation, leading to fast expansion of the ela3l-positive population. Finally, we show that this proliferation is blocked by overexpression of the Wnt-signaling antagonist dkk1b In conclusion, we show a conserved requirement for Wnt signaling in exocrine tissue expansion and reveal a potential novel progenitor or stem cell population as a source for exocrine neogenesis after complete loss of acinar cells.

Retinal Degeneration and Regeneration-Lessons From Fishes and Amphibians

PURPOSE OF REVIEW

Retinal degenerative diseases have immense socio-economic impact. Studying animal models that recapitulate human eye pathologies aids in understanding the pathogenesis of diseases and allows for the discovery of novel therapeutic strategies. Some non-mammalian species are known to have remarkable regenerative abilities and may provide the basis to develop strategies to stimulate self-repair in patients suffering from these retinal diseases.

RECENT FINDINGS

Non-mammalian organisms, such as zebrafish and Xenopus, have become attractive model systems to study retinal diseases. Additionally, many fish and amphibian models of retinal cell ablation and cell lineage analysis have been developed to study regeneration. These investigations highlighted several cellular sources for retinal repair in different fish and amphibian species. Moreover, major differences in repair mechanisms have been reported in these animal models.

SUMMARY

This review aims to emphasize first on the importance of zebrafish and Xenopus models in studying the pathogenesis of retinal diseases and, second, on the different modes of regeneration processes in these model organisms.

Precision Optogenetic Tool for Selective Single- and Multiple-Cell Ablation in a Live Animal Model System

Cell ablation is a strategy to study cell lineage and function during development. Optogenetic methods are an important cell-ablation approach, and we have previously developed a mini singlet oxygen generator (miniSOG) tool that works in the living Caenorhabditis elegans. Here, we use directed evolution to generate miniSOG2, an improved tool for cell ablation via photogenerated reactive oxygen species. We apply miniSOG2 to a far more complex model animal system, Drosophila melanogaster, and demonstrate that it can be used to kill a single neuron in a Drosophila larva. In addition, miniSOG2 is able to photoablate a small group of cells in one of the larval wing imaginal discs, resulting in an adult with one incomplete and one normal wing. We expect miniSOG2 to be a useful optogenetic tool for precision cell ablation at a desired developmental time point in live animals, thus opening a new window into cell origin, fate and function, tissue regeneration, and developmental biology.

Phloroglucinol accelerates the regeneration of liver damaged by H2O2or MNZ treatment in zebrafish

ROSs can cause oxidative damage to biological macromolecules. Particularly, liver is a vital organ in vertebrates and easily attacked by ROS. PG attenuates H₂O₂-induced oxidative stress, even in liver.

Zebrafish Pancreas Development and Regeneration: Fishing for Diabetes Therapies

The zebrafish pancreas shares its basic organization and cell types with the mammalian pancreas. In addition, the developmental pathways that lead to the establishment of the pancreatic islets of Langherhans are generally conserved from fish to mammals. Zebrafish provides a powerful tool to probe the mechanisms controlling establishment of the pancreatic endocrine cell types from early embryonic progenitor cells, as well as the regeneration of endocrine cells after damage. This knowledge is, in turn, applicable to refining protocols to generate renewable sources of human pancreatic islet cells that are critical for regulation of blood sugar levels. Here, we review how previous and ongoing studies in zebrafish and beyond are influencing the understanding of molecular mechanisms underlying various forms of diabetes and efforts to develop cell-based approaches to cure this increasingly widespread disease.

Rapid Recovery of Visual Function Associated with Blue Cone Ablation in Zebrafish

Hurdles in the treatment of retinal degeneration include managing the functional rewiring of surviving photoreceptors and integration of any newly added cells into the remaining second-order retinal neurons. Zebrafish are the premier genetic model for such questions, and we present two new transgenic lines allowing us to contrast vision loss and recovery following conditional ablation of specific cone types: UV or blue cones. The ablation of each cone type proved to be thorough (killing 80% of cells in each intended cone class), specific, and cell-autonomous. We assessed the loss and recovery of vision in larvae via the optomotor behavioural response (OMR). This visually mediated behaviour decreased to about 5% or 20% of control levels following ablation of UV or blue cones, respectively (P<0.05). We further assessed ocular photoreception by measuring the effects of UV light on body pigmentation, and observed that photoreceptor deficits and recovery occurred (p<0.01) with a timeline coincident to the OMR results. This corroborated and extended previous conclusions that UV cones are required photoreceptors for modulating body pigmentation, addressing assumptions that were unavoidable in previous experiments. Functional vision recovery following UV cone ablation was robust, as measured by both assays, returning to control levels within four days. In contrast, robust functional recovery following blue cone ablation was unexpectedly rapid, returning to normal levels within 24 hours after ablation. Ablation of cones led to increased proliferation in the retina, though the rapid recovery of vision following blue cone ablation was demonstrated to not be mediated by blue cone regeneration. Thus rapid visual recovery occurs following ablation of some, but not all, cone subtypes, suggesting an opportunity to contrast and dissect the sources and mechanisms of outer retinal recovery during cone photoreceptor death and regeneration.

Radial glia regulate vascular patterning around the developing spinal cord

Vascular networks surrounding individual organs are important for their development, maintenance, and function; however, how these networks are assembled remains poorly understood. Here we show that CNS progenitors, referred to as radial glia, modulate vascular patterning around the spinal cord by acting as negative regulators. We found that radial glia ablation in zebrafish embryos leads to excessive sprouting of the trunk vessels around the spinal cord, and exclusively those of venous identity. Mechanistically, we determined that radial glia control this process via the Vegf decoy receptor sFlt1: sflt1 mutants exhibit the venous over-sprouting observed in radial glia-ablated larvae, and sFlt1 overexpression rescues it. Genetic mosaic analyses show that sFlt1 function in trunk endothelial cells can limit their over-sprouting. Together, our findings identify CNS-resident progenitors as critical angiogenic regulators that determine the precise patterning of the vasculature around the spinal cord, providing novel insights into vascular network formation around developing organs.

DOI:http://dx.doi.org/10.7554/eLife.20253.001

Senescent intimal foam cells are deleterious at all stages of atherosclerosis

Advanced atherosclerotic lesions contain senescent cells, but the role of these cells in atherogenesis remains unclear. Using transgenic and pharmacological approaches to eliminate senescent cells in atherosclerosis-prone low-density lipoprotein receptor-deficient (Ldlr^-/-) mice, we show that these cells are detrimental throughout disease pathogenesis. We find that foamy macrophages with senescence markers accumulate in the subendothelial space at the onset of atherosclerosis, where they drive pathology by increasing expression of key atherogenic and inflammatory cytokines and chemokines. In advanced lesions, senescent cells promote features of plaque instability, including elastic fiber degradation and fibrous cap thinning, by heightening metalloprotease production. Together, these results demonstrate that senescent cells are key drivers of atheroma formation and maturation and suggest that selective clearance of these cells by senolytic agents holds promise for the treatment of atherosclerosis.

A High-Content Larval Zebrafish Brain Imaging Method for Small Molecule Drug Discovery

Drug discovery in whole-organisms such as zebrafish is a promising approach for identifying biologically-relevant lead compounds. However, high content imaging of zebrafish at cellular resolution is challenging due to the difficulty in orienting larvae en masse such that the cell type of interest is in clear view. We report the development of the multi-pose imaging method, which uses 96-well round bottom plates combined with a standard liquid handler to repose the larvae within each well multiple times, such that an image in a specific orientation can be acquired. We have validated this method in a chemo-genetic zebrafish model of dopaminergic neuron degeneration. For this purpose, we have developed an analysis pipeline that identifies the larval brain in each image and then quantifies neuronal health in CellProfiler. Our method achieves a SSMD* score of 6.96 (robust Z’-factor of 0.56) and is suitable for screening libraries up to 10⁵ compounds in size.

Analysis of myelinated axon formation in zebrafish

Myelin is a lipid-rich sheath formed by the spiral wrapping of specialized glial cells around axon segments. Myelinating glia allow for rapid transmission of nerve impulses and metabolic support of axons, and the absence of or disruption to myelin results in debilitating motor, cognitive, and emotional deficits in humans. Because myelin is a jawed vertebrate innovation, zebrafish are one of the simplest vertebrate model systems to study the genetics and development of myelinating glia. The morphogenetic cellular movements and genetic program that drive myelination are conserved between zebrafish and mammals, and myelin develops rapidly in zebrafish larvae, within 3-5days postfertilization. Myelin ultrastructure can be visualized in the zebrafish from larval to adult stages via transmission electron microscopy, and the dynamic development of myelinating glial cells may be observed in vivo via transgenic reporter lines in zebrafish larvae. Zebrafish are amenable to genetic and pharmacological screens, and screens for myelinating glial phenotypes have revealed both genes and drugs that promote myelin development, many of which are conserved in mammalian glia. Recently, zebrafish have been employed as a model to understand the complex dynamics of myelinating glia during development and regeneration. In this chapter, we describe these key methodologies and recent insights into mechanisms that regulate myelination using the zebrafish model.

Optimization of a Neurotoxin to Investigate the Contribution of Excitatory Interneurons to Speed Modulation In Vivo

Precise control of speed during locomotion is essential for adaptation of behavior in different environmental contexts [1-4]. A central question in locomotion lies in understanding which neural populations set locomotor frequency during slow and fast regimes. Tackling this question in vivo requires additional non-invasive tools to silence large populations of neurons during active locomotion. Here we generated a stable transgenic line encoding a zebrafish-optimized botulinum neurotoxin light chain fused to GFP (BoTxBLC-GFP) to silence synaptic output over large populations of motor neurons or interneurons while monitoring active locomotion. By combining calcium imaging, electrophysiology, optogenetics, and behavior, we show that expression of BoTxBLC-GFP abolished synaptic release while maintaining characterized activity patterns and without triggering off-target effects. As chx10(+) V2a interneurons (V2as) are well characterized as the main population driving the frequency-dependent recruitment of motor neurons during fictive locomotion [5-14], we validated our silencing method by testing the effect of silencing chx10(+) V2as during active and fictive locomotion. Silencing of V2as selectively abolished fast locomotor frequencies during escape responses. In addition, spontaneous slow locomotion occurred less often and at frequencies lower than in controls. Overall, this silencing approach confirms that V2a excitation is critical for the production of fast stimulus-evoked swimming and also reveals a role for V2a excitation in the production of slower spontaneous locomotor behavior. Altogether, these results establish BoTxBLC-GFP as an ideal tool for in vivo silencing for probing the development and function of neural circuits from the synaptic to the behavioral level.

Liver regeneration - mechanisms and models to clinical application

Liver regeneration has been studied for many decades and the mechanisms underlying regeneration of the normal liver following resection or moderate damage are well described. A large number of factors extrinsic (such as bile acids and circulating growth factors) and intrinsic to the liver interact to initiate and regulate liver regeneration. Less well understood, and more clinically relevant, are the factors at play when the abnormal liver is required to regenerate. Fatty liver disease, chronic scarring, prior chemotherapy and massive liver injury can all inhibit the normal programme of regeneration and can lead to liver failure. Understanding these mechanisms could enable the rational targeting of specific therapies to either reduce the factors inhibiting regeneration or directly stimulate liver regeneration. Although animal models of liver regeneration have been highly instructive, the clinical relevance of some models could be improved to bridge the gap between our in vivo model systems and the clinical situation. Likewise, modern imaging techniques such as spectroscopy will probably improve our understanding of whole-organ metabolism and how this predicts the liver's regenerative capacity. This Review describes briefly the mechanisms underpinning liver regeneration, the models used to study this process, and discusses areas in which failed or compromised liver regeneration is clinically relevant.

Cell migration during heart regeneration in zebrafish

Zebrafish possess the remarkable ability to regenerate injured hearts as adults, which contrasts the very limited ability in mammals. Although very limited, mammalian hearts do in fact have measurable levels of cardiomyocyte regeneration. Therefore, elucidating mechanisms of zebrafish heart regeneration would provide information of naturally occurring regeneration to potentially apply to mammalian studies, in addition to addressing this biologically interesting phenomenon in itself. Studies over the past 13 years have identified processes and mechanisms of heart regeneration in zebrafish. After heart injury, pre-existing cardiomyocytes dedifferentiate, enter the cell cycle, and repair the injured myocardium. This process requires interaction with epicardial cells, endocardial cells, and vascular endothelial cells. Epicardial cells envelope the heart, while endocardial cells make up the inner lining of the heart. They provide paracrine signals to cardiomyocytes to regenerate the injured myocardium, which is vascularized during heart regeneration. In addition, accumulating results suggest that local migration of these major cardiac cell types have roles in heart regeneration. In this review, we summarize the characteristics of various heart injury methods used in the research community and regeneration of the major cardiac cell types. Then, we discuss local migration of these cardiac cell types and immune cells during heart regeneration. Developmental Dynamics 245:774-787, 2016. © 2016 Wiley Periodicals, Inc.

Gfap-positive radial glial cells are an essential progenitor population for later-born neurons and glia in the zebrafish spinal cord

Radial glial cells are presumptive neural stem cells (NSCs) in the developing nervous system. The direct requirement of radial glia for the generation of a diverse array of neuronal and glial subtypes, however, has not been tested. We employed two novel transgenic zebrafish lines and endogenous markers of NSCs and radial glia to show for the first time that radial glia are essential for neurogenesis during development. By using the gfap promoter to drive expression of nuclear localized mCherry we discerned two distinct radial glial-derived cell types: a major nestin+/Sox2+ subtype with strong gfap promoter activity and a minor Sox2+ subtype lacking this activity. Fate mapping studies in this line indicate that gfap+ radial glia generate later-born CoSA interneurons, secondary motorneurons, and oligodendroglia. In another transgenic line using the gfap promoter-driven expression of the nitroreductase enzyme, we induced cell autonomous ablation of gfap+ radial glia and observed a reduction in their specific derived lineages, but not Blbp+ and Sox2+/gfap-negative NSCs, which were retained and expanded at later larval stages. Moreover, we provide evidence supporting classical roles of radial glial in axon patterning, blood-brain barrier formation, and locomotion. Our results suggest that gfap+ radial glia represent the major NSC during late neurogenesis for specific lineages, and possess diverse roles to sustain the structure and function of the spinal cord. These new tools will both corroborate the predicted roles of astroglia and reveal novel roles related to development, physiology, and regeneration in the vertebrate nervous system. GLIA 2016;64:1170-1189.

Endoderm specification and liver development

The endoderm is the innermost embryonic germ layer, and in zebrafish, it gives rise to the lining of the gut, the gills, liver, pancreas, gallbladder, and derivatives of the pharyngeal pouch. These organs form the gastrointestinal tract and are involved with the absorption, delivery, and metabolism of nutrients. The liver has a central role in regulating these processes because it controls carbohydrate and lipid metabolism, protein synthesis, and breakdown of endogenous and xenobiotic products. Liver dysfunction frequently leads to significant morbidity and mortality; however, in most settings of organ injury, the liver exhibits remarkable regenerative capacity. In this chapter, we review the principal mechanisms of endoderm and liver formation and provide protocols to assess liver formation and liver regeneration.

Development of an Ethanol-induced Fibrotic Liver Model in Zebrafish to Study Progenitor Cell-mediated Hepatocyte Regeneration

Sustained liver fibrosis with continuation of extracellular matrix (ECM) protein build-up results in the loss of cellular competency of the liver, leading to cirrhosis with hepatocellular dysfunction. Among multiple hepatic insults, alcohol abuse can lead to significant health problems including liver failure and hepatocellular carcinoma. Nonetheless, the identity of endogenous cellular sources that regenerate hepatocytes in response to alcohol has not been properly investigated. Moreover, few studies have effectively modeled hepatocyte regeneration upon alcohol-induced injury. We recently reported on establishing an ethanol (EtOH)-induced fibrotic liver model in zebrafish in which hepatic progenitor cells (HPCs) gave rise to hepatocytes upon near-complete hepatocyte loss in the presence of fibrogenic stimulus. Furthermore, through chemical screens using this model, we identified multiple small molecules that enhance hepatocyte regeneration. Here we describe in detail the procedures to develop an EtOH-induced fibrotic liver model and to perform chemical screens using this model in zebrafish. This protocol will be a critical tool to delineate the molecular and cellular mechanisms of how hepatocyte regenerates in the fibrotic liver. Furthermore, these methods will facilitate potential discovery of novel therapeutic strategies for chronic liver disease in vivo.

A toolbox to study epidermal cell types in zebrafish

Epithelia provide a crucial protective barrier for our organs and are also the sites where the majority of carcinomas form. Most studies on epithelia and carcinomas use cell culture or organisms where high-resolution live imaging is inaccessible without invasive techniques. Here, we introduce the developing zebrafish epidermis as an excellent in vivo model system for studying a living epithelium. We developed tools to fluorescently tag specific epithelial cell types and express genes in a mosaic fashion using five Gal4 lines identified from an enhancer trap screen. When crossed to a variety of UAS effector lines, we can now track, ablate or monitor single cells at sub-cellular resolution. Using photo-cleavable morpholino oligonucleotides that target gal4, we can also express genes in a mosaic fashion at specific times during development. Together, this system provides an excellent in vivo alternative to tissue culture cells, without the intrinsic concerns of culture conditions or transformation, and enables the investigation of distinct cell types within living epithelial tissues.

Radial glia inhibit peripheral glial infiltration into the spinal cord at motor exit point transition zones

In the mature vertebrate nervous system, central and peripheral nervous system (CNS and PNS, respectively) GLIA myelinate distinct motor axon domains at the motor exit point transition zone (MEP TZ). How these cells preferentially associate with and myelinate discrete, non-overlapping CNS versus PNS axonal segments, is unknown. Using in vivo imaging and genetic cell ablation in zebrafish, we demonstrate that radial glia restrict migration of PNS glia into the spinal cord during development. Prior to development of radial glial endfeet, peripheral cells freely migrate back and forth across the MEP TZ. However, upon maturation, peripherally located cells never enter the CNS. When we ablate radial glia, peripheral glia ectopically migrate into the spinal cord during developmental stages when they would normally be restricted. These findings demonstrate that radial glia contribute to both CNS and PNS development and control the unidirectional movement of glial cell types across the MEP TZ early in development. GLIA 2016. GLIA 2016;64:1138-1153.