Genetic inducible fate mapping in larval zebrafish reveals origins of adult insulin-producing β-cells

Mechanistic insights and approaches for beta cell regeneration

Diabetes is characterized by variable loss of insulin-producing beta cells, and new regenerative approaches to increasing the functional beta cell mass of patients hold promise for reversing disease progression. In this Review, we summarize recent chemical biology breakthroughs advancing our knowledge of beta cell regeneration. We present current chemical-based tools, sensors and mechanistic insights into pathways that can be targeted to enhance beta cell regeneration in model organisms. We group the pathways according to the cellular processes they affect, that is, proliferation, conversion of other mature cell types to beta cells and beta cell differentiation from progenitor-like populations. We also suggest assays for assessing the functionality of the regenerated beta cells. Although regeneration processes differ between animal models, such as zebrafish, mice and pigs, regenerative mechanisms identified in any one animal model may be translatable to humans. Overall, chemical biology-based approaches in beta cell regeneration give hope that specific molecular pathways can be targeted to enhance beta cell regeneration.

Evaluation of aspartame effects at environmental concentration on early development of zebrafish: Morphology and transcriptome1

The use of aspartame as an artificial sweetener is prevalent in a wide range of everyday food products, potentially leading to health complications such as obesity, diabetes mellitus, autism spectrum disorders, and neurodegeneration. Aspartame has also been detected in natural water bodies at a concentration of 0.49 μg/L, yet research on its ecotoxicological effects on aquatic life remains scarce. This study aimed to investigate the potential negative effects of environmentally relevant concentrations of aspartame on the development of various tissues and organs in zebrafish embryos. We used a zebrafish model to treat embryos with aspartame at environmental concentration and those higher than in the environment-up to 1000 times. We observed that after exposure to aspartame body length increased, pigmentation was delayed, and neutrophil production inhibited in zebrafish. Furthermore, transcriptome analysis revealed that early exposure of zebrafish embryos to aspartame affected the transcriptomics of various systems, primarily by downregulating genes related to immune cell production, eye and optic nerve development, nervous system development, and growth hormone-related transcription. Most of the genes associated with ferroptosis were upregulated. This study provides new insights into the ecotoxicological effects of aspartame on aquatic environments.

Non-apoptotic caspase events and Atf3 expression underlie direct neuronal differentiation of adult neural stem cells

Neural stem cells (NSCs) generate neurons over a lifetime in adult vertebrate brains. In the adult zebrafish pallium, NSCs persist long term through balanced fate decisions. These decisions include direct neuronal conversions, i.e. delamination and neurogenesis without a division. To characterize this process, we reanalyze intravital imaging data of adult pallial NSCs, and observe shared delamination dynamics between NSCs and committed neuronal progenitors. Searching for mechanisms predicting direct NSC conversions, we build an NSC-specific genetic tracer of Caspase3/7 activation (Cas3*/Cas7*) in vivo. We show that non-apoptotic Cas3*/7* events occur in adult NSCs and are biased towards lineage termination under physiological conditions, with a predominant generation of single neurons. We further identify the transcription factor Atf3 as necessary for this bias. Finally, we show that the Cas3*/7* pathway is engaged by NSCs upon parenchymal lesion and correlates with NSCs more prone to lineage termination and neuron formation. These results provide evidence for non-apoptotic caspase events occurring in vertebrate adult NSCs and link these events with the NSC fate decision of direct conversion, which is important for long-term NSC population homeostasis.

Leveraging zebrafish to investigate pancreatic development, regeneration, and diabetes

The zebrafish has become an outstanding model for studying organ development and tissue regeneration, which is prominently leveraged for studies of pancreatic development, insulin-producing β-cells, and diabetes. Although studied for more than two decades, many aspects remain elusive and it has only recently been possible to investigate these due to technical advances in transcriptomics, chemical-genetics, genome editing, drug screening, and in vivo imaging. Here, we review recent findings on zebrafish pancreas development, β-cell regeneration, and how zebrafish can be used to provide novel insights into gene functions, disease mechanisms, and therapeutic targets in diabetes, inspiring further use of zebrafish for the development of novel therapies for diabetes.

Neural plate progenitors give rise to both anterior and posterior pituitary cells

The pituitary is the master neuroendocrine gland, which regulates body homeostasis. It consists of the anterior pituitary/adenohypophysis harboring hormones producing cells and the posterior pituitary/neurohypophysis, which relays the passage of hormones from the brain to the periphery. It is accepted that the adenohypophysis originates from the oral ectoderm (Rathke's pouch), whereas the neural ectoderm contributes to the neurohypophysis. Single-cell transcriptomics of the zebrafish pituitary showed that cyp26b1-positive astroglial pituicytes of the neurohypophysis and prop1-positive adenohypophyseal progenitors expressed common markers implying lineage relatedness. Genetic tracing identifies that, in contrast to the prevailing dogma, neural plate precursors of zebrafish (her4.3+) and mouse (Sox1+) contribute to both neurohypophyseal and a subset of adenohypophyseal cells. Pituicyte-derived retinoic-acid-degrading enzyme Cyp26b1 fine-tunes differentiation of prop1+ progenitors into hormone-producing cells. These results challenge the notion that adenohypophyseal cells are exclusively derived from non-neural ectoderm and demonstrate that crosstalk between neuro- and adeno-hypophyseal cells affects differentiation of pituitary cells.

Zebrafish pancreatic β cell clusters undergo stepwise regeneration using Neurod1-expressing cells from different cell lineages

Pancreatic β cell clusters produce insulin and play a central role in glucose homeostasis. The regenerative capacity of mammalian β cells is limited and the loss of β cells causes diabetes. In contrast, zebrafish β cell clusters have a high regenerative capacity, making them an attractive model to study β cell cluster regeneration. How zebrafish β cell clusters regenerate, when the regeneration process is complete, and the identification of the cellular source of regeneration are fundamental questions that require investigation. Here, using larval and adult zebrafish, we demonstrate that pancreatic β cell clusters undergo a two-step regeneration process, regenerating functionality and then β cell numbers. Additionally, we found that all regenerating pancreatic β cells arose from Neurod1-expressing cells and that cells from different lineages contribute to both functional and β cell number recovery throughout their life. Furthermore, we found that during development and neogenesis, as well as regeneration, all β cells undergo Neurod1expression in zebrafish. Together, these results shed light on the fundamental cellular mechanisms underlying β cell cluster development, neogenesis, and regeneration.

Decoding pancreatic endocrine cell differentiation and β cell regeneration in zebrafish

In contrast to mice, zebrafish have an exceptional yet elusive ability to replenish lost β cells in adulthood. Understanding this framework would provide mechanistic insights for β cell regeneration, which may be extrapolated to humans. Here, we characterize a krt4-expressing ductal cell type, which is distinct from the putative Notch-responsive cells, showing neogenic competence and giving rise to the majority of endocrine cells during postembryonic development. Furthermore, we demonstrate a marked ductal remodeling process featuring a Notch-responsive to krt4+ luminal duct transformation during late development, indicating several origins of krt4+ ductal cells displaying similar transcriptional patterns. Single-cell transcriptomics upon a series of time points during β cell regeneration unveil a previously unrecognized dlb+ transitional endocrine precursor cell, distinct regulons, and a differentiation trajectory involving cellular shuffling through differentiation and dedifferentiation dynamics. These results establish a model of zebrafish pancreatic endocrinogenesis and highlight key values of zebrafish for translational studies of β cell regeneration.

Dietary exposure to the food preservative tert-Butylhydroquinone (tBHQ) impairs zebrafish (Danio rerio) survival, growth, organ development, and gene expression in Nrf2a-dependent and independent ways

Tert-Butylhydroquinone (tBHQ), a preservative used to prevent oxidative deterioration of oil, fat, and meat products, has been linked to both chemoprotective and adverse effects. This study investigates the impact of dietary tBHQ consumption on survival, growth parameters, organ development, and gene expression in zebrafish (Danio rerio). As tBHQ activates the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2a), a zebrafish line with a mutation in the DNA-binding domain of Nrf2a was used to identify Nrf2a-dependent vs independent effects. Homozygous Nrf2a wildtype (wt) and mutant (m) larvae were fed a diet containing 5% tBHQ or a control diet. Survival and growth parameters were assessed at 15 days and at 5 months, and samples were collected for RNA sequencing at 5 months. Dietary exposure to tBHQ throughout the larval and juvenile periods negatively impacted growth and survival. RNA-seq analysis found differentially expressed genes related to growth and development and upregulation of several immune system-related pathways. The findings herein demonstrate that dietary tBHQ exposure may impair growth and survival in both Nrf2a dependent and independent manners.

Ptf1a function and transcriptional cis-regulation, a cornerstone in vertebrate pancreas development

Vertebrate pancreas organogenesis is a stepwise process regulated by a complex network of signaling and transcriptional events, progressively steering the early endoderm toward pancreatic fate. Many crucial players of this process have been identified, including signaling pathways, cis-regulatory elements, and transcription factors (TFs). Pancreas-associated transcription factor 1a (PTF1A) is one such TF, crucial for pancreas development. PTF1A mutations result in dramatic pancreatic phenotypes associated with severe complications, such as neonatal diabetes and impaired food digestion due to exocrine pancreatic insufficiency. Here, we present a brief overview of vertebrate pancreas development, centered on Ptf1a function and transcriptional regulation, covering similarities and divergences in three broadly studied organisms: human, mouse and zebrafish.

Molecular profiling of enteric nervous system cell lineages

The enteric nervous system (ENS) is an extensive network of enteric neurons and glial cells that is intrinsic to the gut wall and regulates almost all aspects of intestinal physiology. While considerable advancement has been made in understanding the genetic programs regulating ENS development, there is limited understanding of the molecular pathways that control ENS function in adult stages. One of the limitations in advancing the molecular characterization of the adult ENS relates to technical difficulties in purifying healthy neurons and glia from adult intestinal tissues. To overcome this, we developed novel methods for performing transcriptomic analysis of enteric neurons and glia, which are based on the isolation of fluorescently labeled nuclei. Here we provide a step-by-step protocol for the labeling of adult mouse enteric neuronal nuclei using adeno-associated-virus-mediated gene transfer, isolation of the labeled nuclei by fluorimetric analysis, RNA purification and nuclear RNA sequencing. This protocol has also been adapted for the isolation of enteric neuron and glia nuclei from myenteric plexus preparations from adult zebrafish intestine. Finally, we describe a method for visualization and quantification of RNA in myenteric ganglia: Spatial Integration of Granular Nuclear Signals (SIGNS). By following this protocol, it takes ~3 d to generate RNA and create cDNA libraries for nuclear RNA sequencing and 4 d to carry out high-resolution RNA expression analysis on ENS tissues.

Tel2 regulates redifferentiation of bipotential progenitor cells via Hhex during zebrafish liver regeneration

Upon extensive hepatocyte loss or impaired hepatocyte proliferation, liver regeneration occurs via biliary epithelial cell (BEC) transdifferentiation, which includes dedifferentiation of BECs into bipotential progenitor cells (BP-PCs) and then redifferentiation of BP-PCs to nascent hepatocytes and BECs. This BEC-driven liver regeneration involves reactivation of hepatoblast markers, but the underpinning mechanisms and their effects on liver regeneration remain largely unknown. Using a zebrafish extensive hepatocyte ablation model, we perform an N-ethyl-N-nitrosourea (ENU) forward genetic screen and identify a liver regeneration mutant, liver logan (lvl), in which the telomere maintenance 2 (tel2) gene is mutated. During liver regeneration, the tel2 mutation specifically inhibits transcriptional activation of a hepatoblast marker, hematopoietically expressed homeobox (hhex), in BEC-derived cells, which blocks BP-PC redifferentiation. Mechanistic studies show that Tel2 associates with the hhex promoter region and promotes hhex transcription. Our results reveal roles of Tel2 in the BP-PC redifferentiation process of liver regeneration by activating hhex.

A δ-cell subpopulation with a pro-β-cell identity contributes to efficient age-independent recovery in a zebrafish model of diabetes

Restoring damaged β-cells in diabetic patients by harnessing the plasticity of other pancreatic cells raises the questions of the efficiency of the process and of the functionality of the new Insulin-expressing cells. To overcome the weak regenerative capacity of mammals, we used regeneration-prone zebrafish to study β-cells arising following destruction. We show that most new insulin cells differ from the original β-cells as they coexpress Somatostatin and Insulin. These bihormonal cells are abundant, functional and able to normalize glycemia. Their formation in response to β-cell destruction is fast, efficient, and age-independent. Bihormonal cells are transcriptionally close to a subset of δ-cells that we identified in control islets and that are characterized by the expression of somatostatin 1.1 (sst1.1) and by genes essential for glucose-induced Insulin secretion in β-cells such as pdx1, slc2a2 and gck. We observed in vivo the conversion of monohormonal sst1.1-expressing cells to sst1.1+ ins + bihormonal cells following β-cell destruction. Our findings support the conclusion that sst1.1 δ-cells possess a pro-β identity enabling them to contribute to the neogenesis of Insulin-producing cells during regeneration. This work unveils that abundant and functional bihormonal cells benefit to diabetes recovery in zebrafish.

Broadening the phenotypic spectrum and physiological insights related to EIF2S3 variants

Mental deficiency, epilepsy, hypogonadism, microcephaly, and obesity syndrome is a severe X-linked syndrome caused by pathogenic variants in EIF2S3. The gene encodes the γ subunit of the eukaryotic translation initiation factor-2, eIF2, essential for protein translation. A recurrent frameshift variant is described in severely affected patients while missense variants usually cause a moderate phenotype. We identified a novel missense variant (c.433A>G, p.(Met145Val)) in EIF2S3 in a mildly affected patient. Studies on zebrafish confirm the pathogenicity of this novel variant and three previously published missense variants. CRISPR/Cas9 knockout of eif2s3 in zebrafish embryos recapitulate the human microcephaly and show increased neuronal cell death. Abnormal high glucose levels were identified in mutant embryos, caused by beta cell and pancreatic progenitor deficiency, not related to apoptosis. Additional studies in patient-derived fibroblasts did not reveal apoptosis. Our results provide new insights into disease physiopathology, suggesting tissue-dependent mechanisms.

Contributions of biliary epithelial cells to hepatocyte homeostasis and regeneration in zebrafish

Whether transdifferentiation of the biliary epithelial cells (BECs) to hepatocytes occurs under physiological conditions and contributes to liver homeostasis remains under long-term debate. Similar questions have been raised under pathological circumstances if a fibrotic liver is suffered from severe injuries. To address these questions in zebrafish, we established a sensitive lineage tracing system specific for the detection of BEC-derived hepatocytes. The BEC-to-hepatocyte transdifferentiation occurred and became minor contributors to hepatocyte homeostasis in a portion of adult individuals. The BEC-derived hepatocytes distributed in clusters in the liver. When a fibrotic liver underwent extreme hepatocyte damages, BEC-to-hepatocyte transdifferentiation acted as the major origin of regenerating hepatocytes. In contrast, partial hepatectomy failed to induce the BEC-to-hepatocyte conversion. In conclusion, based on a sensitive lineage tracing system, our results suggest that BECs are able to transdifferentiate into hepatocytes and contribute to both physiological hepatocyte homeostasis and pathological regeneration.

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Developed sensitivity system to trace BECs derived hepatocytes in liver homeostasis

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BECs convert to hepatocytes in liver homeostasis but are individually heterogeneous

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BECs are the primary regeneration sources in the extreme injury of the fibrotic liver

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BECs fail to contribute to new hepatocytes after partial hepatectomy

Inducible Mosaic Cell Labeling Provides Insights Into Pancreatic Islet Morphogenesis

Pancreatic islets, discrete microorgans embedded within the exocrine pancreas, contain beta cells which are critical for glucose homeostasis. Loss or dysfunction of beta cells leads to diabetes, a disease with expanding global prevalence, and for which regenerative therapies are actively being pursued. Recent efforts have focused on producing mature beta cells in vitro, but it is increasingly recognized that achieving a faithful three-dimensional islet structure is crucial for generating fully functional beta cells. Our current understanding of islet morphogenesis is far from complete, due to the deep internal location of the pancreas in mammalian models, which hampers direct visualization. Zebrafish is a model system well suited for studies of pancreas morphogenesis due to its transparency and the accessible location of the larval pancreas. In order to further clarify the cellular mechanisms of islet formation, we have developed new tools for in vivo visualization of single-cell dynamics. Our results show that clustering islet cells make contact and interconnect through dynamic actin-rich processes, move together while remaining in close proximity to the duct, and maintain high protrusive motility after forming clusters. Quantitative analyses of cell morphology and motility in 3-dimensions lays the groundwork to define therapeutically applicable factors responsible for orchestrating the morphogenic behaviors of coalescing endocrine cells.

Non-thalamic origin of zebrafish sensory nuclei implies convergent evolution of visual pathways in amniotes and teleosts

Ascending visual projections similar to the mammalian thalamocortical pathway are found in a wide range of vertebrate species, but their homology is debated. To get better insights into their evolutionary origin, we examined the developmental origin of a thalamic-like sensory structure of teleosts, the preglomerular complex (PG), focusing on the visual projection neurons. Similarly to the tectofugal thalamic nuclei in amniotes, the lateral nucleus of PG receives tectal information and projects to the pallium. However, our cell lineage study in zebrafish reveals that the majority of PG cells are derived from the midbrain, unlike the amniote thalamus. We also demonstrate that the PG projection neurons develop gradually until late juvenile stages. Our data suggest that teleost PG, as a whole, is not homologous to the amniote thalamus. Thus, the thalamocortical-like projections evolved from a non-forebrain cell population, which indicates a surprising degree of variation in the vertebrate sensory systems.

Enteric glia as a source of neural progenitors in adult zebrafish

The presence and identity of neural progenitors in the enteric nervous system (ENS) of vertebrates is a matter of intense debate. Here, we demonstrate that the non-neuronal ENS cell compartment of teleosts shares molecular and morphological characteristics with mammalian enteric glia but cannot be identified by the expression of canonical glial markers. However, unlike their mammalian counterparts, which are generally quiescent and do not undergo neuronal differentiation during homeostasis, we show that a relatively high proportion of zebrafish enteric glia proliferate under physiological conditions giving rise to progeny that differentiate into enteric neurons. We also provide evidence that, similar to brain neural stem cells, the activation and neuronal differentiation of enteric glia are regulated by Notch signalling. Our experiments reveal remarkable similarities between enteric glia and brain neural stem cells in teleosts and open new possibilities for use of mammalian enteric glia as a potential source of neurons to restore the activity of intestinal neural circuits compromised by injury or disease.

Endothelial Cell Dynamics in Vascular Development: Insights From Live-Imaging in Zebrafish

The formation of the vertebrate vasculature involves the acquisition of endothelial cell identities, sprouting, migration, remodeling and maturation of functional vessel networks. To understand the cellular and molecular processes that drive vascular development, live-imaging of dynamic cellular events in the zebrafish embryo have proven highly informative. This review focusses on recent advances, new tools and new insights from imaging studies in vascular cell biology using zebrafish as a model system.

In vivo generation and regeneration of β cells in zebrafish

The pathological feature of diabetes, hyperglycemia, is a result of an inadequate number and/or function of insulin producing β cells. Replenishing functional β cells is a strategy to cure the disease. Although β-cell regeneration occurs in animal models under certain conditions, human β cells are refractory to proliferation. A better understanding of both the positive and the negative regulatory mechanisms of β-cell regeneration in animal models is essential to develop novel strategies capable of inducing functional β cells in patients. Zebrafish are an attractive model system for studying β-cell regeneration due to the ease to which genetic and chemical-genetic approaches can be used as well as their high regenerative capacity. Here, we highlight the current state of β-cell regeneration studies in zebrafish with an emphasis on cell signaling mechanisms.

Key Developmental Regulators Suggest Multiple Origins of Pancreatic Beta Cell Regeneration

Extensive efforts have been done to try to restore the lost β cell mass for the cure of diabetes. Animal models have been established to provide evidences of cellular origins and contextual regulators of β cell regeneration. Here, we used a zebrafish β cell ablation and regeneration model to investigate β cell neogenesis in the first few days after a near-total β cell loss. Regeneration of β cells first occurred within 7 h post-treatment. Developmental regulators such as neurod, pdx1, mnx1, and nkx2.2a were active in the regenerating β cells, while at the same time suggesting different subpopulations of regenerative cellular origins. Using Cre/loxP-based lineage tracing, we showed that intrapancreatic ductal cells resisted to give rise to regenerating β cells. Given that transdifferentiation of α cell and δ cell can regenerate β cell, here we have provided further molecular evidence highly suggesting that the regenerating β cells originate from multiple cellular origins.

A novel group of secretory cells regulates development of the immature intestinal stem cell niche through repression of the main signaling pathways driving proliferation

The intestinal epithelium has constant turnover throughout the life of the organ, with apoptosis of cells at the tips of folds or villi releasing cells into the lumen. Due to constant turnover, epithelial cells need to be constantly replaced. Epithelial cells are supplied by stem cell niches that form at the base of the interfold space (zebrafish) and crypts (birds and mammals). Within the adult stem cell niche of mammals, secretory cells such as Paneth and goblet cells play a role in modulation of proliferation and stem cell activity, producing asymmetric divisions. Progeny of asymmetric divisions move up the fold or villi, giving rise to all of the epithelial cell types. Although much is known about function and organization of the adult intestinal stem cell niche, less is understood about regulation within the immature stem cell compartment. Following smooth muscle formation, the intestinal epithelium folds and proliferation becomes restricted to the interfold base. Symmetric divisions continue in the developing interfold niche until stem cell progeny begin asymmetric divisions, producing progeny that migrate up the developing folds. Proliferative progeny from the developing stem cell niche begin migrating out of the niche during the third week post-embryogenesis (zebrafish) or during the postnatal period (mammals). Regulation and organization of epithelial proliferation in the immature stem cell niche may be regulated by signals comparable to the adult niche. Here we identify a novel subset of secretory cells associated with the developing stem cell niche that receive Notch signaling (referred to as NRSCs). Inhibition of the embryonic NRSCs between 74 hpf to 120 hpf increases epithelial proliferation as well as EGF and IGF signaling. Inhibition of post-embryonic NRSCs (6 hpf to 12 dpf) also increases epithelial proliferation and expression level of Wnt target genes. We conclude that NRSCs play a role in modulation of epithelial proliferation through repression of signaling pathways that drive proliferation during both embryogenesis and the post embryonic period.

Nano-Sampling and Reporter Tools to Study Metabolic Regulation in Zebrafish

In the past years, evidence has emerged that hallmarks of human metabolic disorders can be recapitulated in zebrafish using genetic, pharmacological or dietary interventions. An advantage of modeling metabolic diseases in zebrafish compared to other "lower organisms" is the presence of a vertebrate body plan providing the possibility to study the tissue-intrinsic processes preceding the loss of metabolic homeostasis. While the small size of zebrafish is advantageous in many aspects, it also has shortcomings such as the difficulty to obtain sufficient amounts for biochemical analyses in response to metabolic challenges. A workshop at the European Zebrafish Principal Investigator meeting in Trento, Italy, was dedicated to discuss the advantages and disadvantages of zebrafish to study metabolic disorders. This perspective article by the participants highlights strategies to achieve improved tissue-resolution for read-outs using "nano-sampling" approaches for metabolomics as well as live imaging of zebrafish expressing fluorescent reporter tools that inform on cellular or subcellular metabolic processes. We provide several examples, including the use of reporter tools to study the heterogeneity of pancreatic beta-cells within their tissue environment. While limitations exist, we believe that with the advent of new technologies and more labs developing methods that can be applied to minimal amounts of tissue or single cells, zebrafish will further increase their utility to study energy metabolism.

Animal models of obesity and diabetes mellitus

More than one-third of the worldwide population is overweight or obese and therefore at risk of developing type 2 diabetes mellitus. In order to mitigate this pandemic, safer and more potent therapeutics are urgently required. This necessitates the continued use of animal models to discover, validate and optimize novel therapeutics for their safe use in humans. In order to improve the transition from bench to bedside, researchers must not only carefully select the appropriate model but also draw the right conclusions. In this Review, we consolidate the key information on the currently available animal models of obesity and diabetes and highlight the advantages, limitations and important caveats of each of these models.

Switch and Trace: Recombinase Genetics in Zebrafish

Transgenic approaches are instrumental for labeling and manipulating cells and cellular machineries in vivo. Transgenes have traditionally been static entities that remained unaltered following genome integration, limiting their versatility. The development of DNA recombinase-based methods to modify, excise, or rearrange transgene cassettes has introduced versatile control of transgene activity and function. In particular, recombinase-controlled transgenes enable regulation of exogenous gene expression, conditional mutagenesis, and genetic lineage tracing. In zebrafish, transgenesis-based recombinase genetics using Cre/lox, Flp/FRT, and ΦC31 are increasingly applied to study development and homeostasis, and to generate disease models. Intersected with the versatile imaging capacity of the zebrafish model and recent breakthroughs in genome editing, we review and discuss past, current, and potential future approaches and resources for recombinase-based techniques in zebrafish.

In vivo imaging of emerging endocrine cells reveals a requirement for PI3K-regulated motility in pancreatic islet morphogenesis

The three-dimensional architecture of the pancreatic islet is integral to beta cell function, but the process of islet formation remains poorly understood due to the difficulties of imaging internal organs with cellular resolution. Within transparent zebrafish larvae, the developing pancreas is relatively superficial and thus amenable to live imaging approaches. We performed in vivo time-lapse and longitudinal imaging studies to follow islet development, visualizing both naturally occurring islet cells and cells arising with an accelerated timecourse following an induction approach. These studies revealed previously unappreciated fine dynamic protrusions projecting between neighboring and distant endocrine cells. Using pharmacological compound and toxin interference approaches, and single-cell analysis of morphology and cell dynamics, we determined that endocrine cell motility is regulated by phosphoinositide 3-kinase (PI3K) and G-protein-coupled receptor (GPCR) signaling. Linking cell dynamics to islet formation, perturbation of protrusion formation disrupted endocrine cell coalescence, and correlated with decreased islet cell differentiation. These studies identified novel cell behaviors contributing to islet morphogenesis, and suggest a model in which dynamic exploratory filopodia establish cell-cell contacts that subsequently promote cell clustering.

Life-Long Neurogenic Activity of Individual Neural Stem Cells and Continuous Growth Establish an Outside-In Architecture in the Teleost Pallium

Spatiotemporal variations of neurogenesis are thought to account for the evolution of brain shape. In the dorsal telencephalon (pallium) of vertebrates, it remains unresolved which ancestral neurogenesis mode prefigures the highly divergent cytoarchitectures that are seen in extant species. To gain insight into this question, we developed genetic tools to generate here the first 4-dimensional (3D + birthdating time) map of pallium construction in the adult teleost zebrafish. Using a Tet-On-based genetic birthdating strategy, we identify a “sequential stacking” construction mode where neurons derived from the zebrafish pallial germinal zone arrange in outside-in, age-related layers from a central core generated during embryogenesis. We obtained no evidence for overt radial or tangential neuronal migrations. Cre-lox-mediated tracing, which included following Brainbow clones, further demonstrates that this process is sustained by the persistent neurogenic activity of individual pallial neural stem cells (NSCs) from embryo to adult. Together, these data demonstrate that the spatiotemporal control of NSC activity is an important driver of the macroarchitecture of the zebrafish adult pallium. This simple mode of pallium construction shares distinct traits with pallial genesis in mammals and non-mammalian amniotes such as birds or reptiles, suggesting that it may exemplify the basal layout from which vertebrate pallial architectures were elaborated.

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Neurons of the teleost pallium are arranged in concentric age-dependent layers

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Neurons of the central pallial domain, Dc, are born during embryogenesis

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Most pallial neurons are generated from ventricular her4-positive radial glia

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The majority of individual pallial radial glia are neurogenic throughout life

Endoderm Jagged induces liver and pancreas duct lineage in zebrafish

Liver duct paucity is characteristic of children born with Alagille Syndrome (ALGS), a disease associated with JAGGED1 mutations. Here, we report that zebrafish embryos with compound homozygous mutations in two Notch ligand genes, jagged1b (jag1b) and jagged2b (jag2b) exhibit a complete loss of canonical Notch activity and duct cells within the liver and exocrine pancreas, whereas hepatocyte and acinar pancreas development is not affected. Further, animal chimera studies demonstrate that wild-type endoderm cells within the liver and pancreas can rescue Notch activity and duct lineage specification in adjacent cells lacking jag1b and jag2b expression. We conclude that these two Notch ligands are directly and solely responsible for all duct lineage specification in these organs in zebrafish. Our study uncovers genes required for lineage specification of the intrahepatopancreatic duct cells, challenges the role of duct cells as progenitors, and suggests a genetic mechanism for ALGS ductal paucity.The hepatopancreatic duct cells connect liver hepatocytes and pancreatic acinar cells to the intestine, but the mechanism for their lineage specification is unclear. Here, the authors reveal that Notch ligands Jagged1b and Jagged2b induce duct cell lineage in the liver and pancreas of the zebrafish.

Different developmental histories of beta-cells generate functional and proliferative heterogeneity during islet growth

The proliferative and functional heterogeneity among seemingly uniform cells is a universal phenomenon. Identifying the underlying factors requires single-cell analysis of function and proliferation. Here we show that the pancreatic beta-cells in zebrafish exhibit different growth-promoting and functional properties, which in part reflect differences in the time elapsed since birth of the cells. Calcium imaging shows that the beta-cells in the embryonic islet become functional during early zebrafish development. At later stages, younger beta-cells join the islet following differentiation from post-embryonic progenitors. Notably, the older and younger beta-cells occupy different regions within the islet, which generates topological asymmetries in glucose responsiveness and proliferation. Specifically, the older beta-cells exhibit robust glucose responsiveness, whereas younger beta-cells are more proliferative but less functional. As the islet approaches its mature state, heterogeneity diminishes and beta-cells synchronize function and proliferation. Our work illustrates a dynamic model of heterogeneity based on evolving proliferative and functional beta-cell states.

Βeta-cells have recently been shown to be heterogeneous with regard to morphology and function. Here, the authors show that β-cells in zebrafish switch from proliferative to functional states with increasing time since β-cell birth, leading to functional and proliferative heterogeneity.

Distinct Levels of Reactive Oxygen Species Coordinate Metabolic Activity with Beta-cell Mass Plasticity

The pancreatic beta-cells control glucose homeostasis by secreting insulin in response to nutrient intake. The number of beta-cells is under tight metabolic control, as this number increases with higher nutrient intake. However, the signaling pathways matching nutrition with beta-cell mass plasticity remain poorly defined. By applying pharmacological and genetic manipulations, we show that reactive oxygen species (ROS) regulate dose-dependently beta-cell proliferation in vivo and in vitro. In particular, reducing ROS levels in beta-cells blocks their proliferation in response to nutrients. Using a non-invasive genetic sensor of intracellular hydrogen peroxide (H₂O₂), we reveal that glucose can directly increase the levels of H₂O₂. Furthermore, a moderate increase in H₂O₂ levels can stimulate beta-cell proliferation. Interestingly, while high H₂O₂ levels are inhibitory to beta-cell proliferation, they expand beta-cell mass in vivo by inducing rapid beta-cell neogenesis. Our study thus reveals a ROS-level-dependent mechanism linking nutrients with beta-cell mass plasticity. Hence, given the requirement of ROS for beta-cell mass expansion, antioxidant therapies should be applied with caution in diabetes.

Systematic comparison of 2A peptides for cloning multi-genes in a polycistronic vector

Cloning of multiple genes in a single vector has greatly facilitated both basic and translational studies that require co-expression of multiple factors or multi-units of complex protein. Many strategies have been adopted, among which 2A “self-cleaving” peptides have garnered increased interest for their polycistronic nature, small size and high “cleavage” efficiency. However, broad application of 2 A peptides is limited by the lack of systematic comparison of different 2As alone or in combination. Here we characterized the effect of varying gene position and 2As on the expression of proteins encoded in bi-, tri-, or quad-cistronic constructs. Using direct cardiac reprogramming as an example, we further determined the effect of varied 2As on the efficiency of fluorescent cell labeling and cell fate conversion. We found that the expression of fluorophores decreased as it was moved towards the end of the construct while reprogramming was most efficient with the fluorophore at the second position. Moreover, quad-cistronic TPE2A constructs resulted in more efficient reprogramming than 3P2A or PTE2A constructs. We expect that the bi-, tri-, and quad-cistronic vectors constructed here and our results on protein expression ratios from different 2A constructs could serve to guide future utilization of 2A peptides in basic research and clinical applications.

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.

Modeling Pancreatic Endocrine Cell Adaptation and Diabetes in the Zebrafish

Glucose homeostasis is an important element of energy balance and is conserved in organisms from fruit fly to mammals. Central to the control of circulating glucose levels in vertebrates are the endocrine cells of the pancreas, particularly the insulin-producing β-cells and the glucagon producing α-cells. A feature of α- and β-cells is their plasticity, an ability to adapt, in function and number as a response to physiological and pathophysiological conditions of increased hormone demand. The molecular mechanisms underlying these adaptive responses that maintain glucose homeostasis are incompletely defined. The zebrafish is an attractive model due to the low cost, high fecundity, and amenability to genetic and compound screens, and mechanisms governing the development of the pancreatic endocrine cells are conserved between zebrafish and mammals. Post development, both β- and α-cells of zebrafish display plasticity as in mammals. Here, we summarize the studies of pancreatic endocrine cell adaptation in zebrafish. We further explore the utility of the zebrafish as a model for diabetes, a relevant topic considering the increase in diabetes in the human population.

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.

A conserved bacterial protein induces pancreatic beta cell expansion during zebrafish development

Resident microbes play important roles in the development of the gastrointestinal tract, but their influence on other digestive organs is less well explored. Using the gnotobiotic zebrafish, we discovered that the normal expansion of the pancreatic β cell population during early larval development requires the intestinal microbiota and that specific bacterial members can restore normal β cell numbers. These bacteria share a gene that encodes a previously undescribed protein, named herein BefA (β Cell Expansion Factor A), which is sufficient to induce β cell proliferation in developing zebrafish larvae. Homologs of BefA are present in several human-associated bacterial species, and we show that they have conserved capacity to stimulate β cell proliferation in larval zebrafish. Our findings highlight a role for the microbiota in early pancreatic β cell development and suggest a possible basis for the association between low diversity childhood fecal microbiota and increased diabetes risk.

Embryonic origin and lineage hierarchies of the neural progenitor subtypes building the zebrafish adult midbrain

Neurogenesis in the post-embryonic vertebrate brain varies in extent and efficiency between species and brain territories. Distinct neurogenesis modes may account for this diversity, and several neural progenitor subtypes, radial glial cells (RG) and neuroepithelial progenitors (NE), have been identified in the adult zebrafish brain. The neurogenic sequences issued from these progenitors, and their contribution to brain construction, remain incompletely understood. Here we use genetic tracing techniques based on conditional Cre recombination and Tet-On neuronal birthdating to unravel the neurogenic sequence operating from NE progenitors in the zebrafish post-embryonic optic tectum. We reveal that a subpopulation of her5-positive NE cells of the posterior midbrain layer stands at the top of a neurogenic hierarchy involving, in order, the amplification pool of the tectal proliferation zone (TPZ), followed by her4-positive RG cells with transient neurogenic activity. We further demonstrate that the adult her5-positive NE pool is issued in lineage from an identically located NE pool expressing the same gene in the embryonic neural tube. Finally, we show that these features are reminiscent of the neurogenic sequence and embryonic origin of the her9-positive progenitor NE pool involved in the construction of the lateral pallium at post-embryonic stages. Together, our results highlight the shared recruitment of an identical neurogenic strategy by two remote brain territories, where long-lasting NE pools serve both as a growth zone and as the life-long source of young neurogenic RG cells.

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Zebrafish post-embryonic tectal neurogenesis is driven by neuroepithelial progenitors.

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The neuroepithelial progenitor pool is long-lasting and expresses Her5 life long.

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Tectal radial glia originate from the her5-positive pool and are transiently neurogenic.

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The post-embryonic neurogenic sequences of the tectum and lateral pallium are similar.

Assessment of Toxicological Perturbations and Variants of Pancreatic Islet Development in the Zebrafish Model

The pancreatic islets, largely comprised of insulin-producing beta cells, play a critical role in endocrine signaling and glucose homeostasis. Because they have low levels of antioxidant defenses and a high perfusion rate, the endocrine islets may be a highly susceptible target tissue of chemical exposures. However, this endpoint, as well as the integrity of the surrounding exocrine pancreas, is often overlooked in studies of developmental toxicology. Disruption of development by toxicants can alter cell fate and migration, resulting in structural alterations that are difficult to detect in mammalian embryo systems, but that are easily observed in the zebrafish embryo model (Danio rerio). Using endogenously expressed fluorescent protein markers for developing zebrafish beta cells and exocrine pancreas tissue, we documented differences in islet area and incidence rates of islet morphological variants in zebrafish embryos between 48 and 96 h post fertilization (hpf), raised under control conditions commonly used in embryotoxicity assays. We identified critical windows for chemical exposures during which increased incidences of endocrine pancreas abnormalities were observed following exposure to cyclopamine (2–12 hpf), Mono-2-ethylhexyl phthalate (MEHP) (3–48 hpf), and Perfluorooctanesulfonic acid (PFOS) (3–48 hpf). Both islet area and length of the exocrine pancreas were sensitive to oxidative stress from exposure to the oxidant tert-butyl hydroperoxide during a highly proliferative critical window (72 hpf). Finally, pancreatic dysmorphogenesis following developmental exposures is discussed with respect to human disease.

Sox9b is a mediator of retinoic acid signaling restricting endocrine progenitor differentiation

Centroacinar cells (CACs) are ductal Notch-responsive progenitors that in the larval zebrafish pancreas differentiate to form new islets and ultimately contribute to the majority of the adult endocrine mass. Uncovering the mechanisms regulating CAC differentiation will facilitate understanding how insulin-producing β cells are formed. Previously we reported retinoic acid (RA) signaling and Notch signaling both regulate larval CAC differentiation, suggesting a shared downstream intermediate. Sox9b is a transcription factor important for islet formation whose expression is upregulated by Notch signaling in larval CACs. Here we report that sox9b expression in larval CACs is also regulated by RA signaling. Therefore, we hypothesized that Sox9b is an intermediate between both RA- and Notch-signaling pathways. In order to study the role of Sox9b in larval CACs, we generated two cre/lox based transgenic tools, which allowed us to express full-length or truncated Sox9b in larval CACs. In this way we were able to perform spatiotemporal-controlled Sox9b gain- and loss-of-function studies and observe the subsequent effect on progenitor differentiation. Our results are consistent with Sox9b regulating CAC differentiation by being a downstream intermediate of both RA- and Notch-signaling pathways. We also demonstrate that adult zebrafish with only one functional allele of sox9b undergo accelerated β-cell regeneration, an observation consistent with sox9b regulating CAC differentiation in adults.

A GAL4-inducible transgenic tool kit for the in vivo modulation of Rho GTPase activity in zebrafish

BACKGROUND

Rho GTPases are small monomeric G-proteins that play key roles in many cellular processes. Due to Rho GTPases' widespread expression and broad functions, analyses of their function during late development require tissue-specific modulation of activity. The GAL4/UAS system provides an excellent tool for investigating the function of Rho GTPases in vivo. With this in mind, we created a transgenic tool kit enabling spatial and temporal modulation of Rho GTPase activity in zebrafish.

RESULTS

Transgenic constructs were assembled driving dominant-negative, constitutively active, and wild-type versions of Cdc42, RhoA, and Rac1 under 10XUAS control. The self-cleaving viral peptide F2A was utilized to allow bicistronic expression of a fluorescent reporter and Rho GTPase. Global heat shock of hsp70l:gal4(+) transgenic embryos confirmed GAL4-specific construct expression. Western blot analysis indicated myc-tagged Rho GTPases were expressed only in the presence of GAL4. Construct expression was confined to proper cells when combined with pou4f3:gal4 or ptf1a:gal4. Finally, transgene expression resulted in reproducible defects in lens formation, indicating that the transgenes are functional in vivo.

CONCLUSIONS

We generated and validated 10 transgenic lines, creating a versatile tool kit for the temporal-spatial modulation of Cdc42, RhoA, and Rac1 activity in vivo. These lines will enable systematic analysis of Rho GTPase function in any tissue of interest. Developmental Dynamics 245:844-853, 2016. © 2016 Wiley Periodicals, Inc.

sept7b is required for the differentiation of pancreatic endocrine progenitors

Protection or restoration of pancreatic β-cell mass as a therapeutic treatment for type 1 diabetes requires understanding of the mechanisms that drive the specification and development of pancreatic endocrine cells. Septins are filamentous small GTPases that function in the regulation of cell division, cytoskeletal organization and membrane remodeling, and are involved in various tissue-specific developmental processes. However, their role in pancreatic endocrine cell differentiation remains unknown. Here we show by functional manipulation techniques in transgenic zebrafish lines that suppression of sept7b, the zebrafish ortholog of human SEPT7, profoundly increases the number of endocrine progenitors but limits their differentiation, leading to reduction in β- and α-cell mass. Furthermore, we discovered that shh (sonic hedgehog) expression in the endoderm, essential for the development of pancreatic progenitors of the dorsal pancreatic bud, is absent in larvae depleted of sept7b. We also discovered that sept7b is important for the differentiation of ventral pancreatic bud-derived cells: sept7b-depleted larvae exhibit downregulation of Notch receptors notch1a and notch1b and show precocious differentiation of NeuroD-positive endocrine cells in the intrapancreatic duct and gut epithelium. Collectively, this study provides a novel insight into the development of pancreatic endocrine progenitors, revealing an essential role for sept7b in endocrine progenitor differentiation.

Advancements in zebrafish applications for 21st century toxicology

The zebrafish model is the only available high-throughput vertebrate assessment system, and it is uniquely suited for studies of in vivo cell biology. A sequenced and annotated genome has revealed a large degree of evolutionary conservation in comparison to the human genome. Due to our shared evolutionary history, the anatomical and physiological features of fish are highly homologous to humans, which facilitates studies relevant to human health. In addition, zebrafish provide a very unique vertebrate data stream that allows researchers to anchor hypotheses at the biochemical, genetic, and cellular levels to observations at the structural, functional, and behavioral level in a high-throughput format. In this review, we will draw heavily from toxicological studies to highlight advances in zebrafish high-throughput systems. Breakthroughs in transgenic/reporter lines and methods for genetic manipulation, such as the CRISPR-Cas9 system, will be comprised of reports across diverse disciplines.

Centroacinar cells: At the center of pancreas regeneration

The process of regeneration serves to heal injury by replacing missing cells. Understanding regeneration can help us replace cell populations lost during disease, such as the insulin-producing β cells lost in diabetic patients. Centroacinar cells (CACs) are a specialized ductal pancreatic cell type that act as progenitors to replace β cells in the zebrafish. However, whether CACs contribute to β-cell regeneration in adult mammals remains controversial. Here we review the current understanding of the role of CACs as endocrine progenitors during regeneration in zebrafish and mammals.

FGF1 Mediates Overnutrition-Induced Compensatory β-Cell Differentiation

Increased insulin demand resulting from insulin resistance and/or overnutrition induces a compensatory increase in β-cell mass. The physiological factors responsible for the compensation have not been fully characterized. In zebrafish, overnutrition rapidly induces compensatory β-cell differentiation through triggering the release of a paracrine signal from persistently activated β-cells. We identified Fgf1 signaling as a key component of the overnutrition-induced β-cell differentiation signal in a small molecule screen. Fgf1 was confirmed as the overnutrition-induced β-cell differentiation signal, as inactivation of fgf1 abolished the compensatory β-cell differentiation. Furthermore, expression of human FGF1 solely in β-cells in fgf1(-/-) animals rescued the compensatory response, indicating that β-cells can be the source of FGF1. Additionally, constitutive secretion of FGF1 with an exogenous signal peptide increased β-cell number in the absence of overnutrition. These results demonstrate that fgf1 is necessary and FGF1 expression in β-cells is sufficient for the compensatory β-cell differentiation. We further show that FGF1 is secreted during prolonged activation of cultured mammalian β-cells and that endoplasmic reticulum stress acts upstream of FGF1 release. Thus, the recently discovered antidiabetes function of FGF1 may act partially through increasing β-cell differentiation.

Adipocyte-specific blockade of gamma-secretase, but not inhibition of Notch activity, reduces adipose insulin sensitivity

Objective

As the obesity pandemic continues to expand, novel molecular targets to reduce obesity-related insulin resistance and Type 2 Diabetes (T2D) continue to be needed. We have recently shown that obesity is associated with reactivated liver Notch signaling, which, in turn, increases hepatic insulin resistance, opening up therapeutic avenues for Notch inhibitors to be repurposed for T2D. Herein, we tested the systemic effects of γ-secretase inhibitors (GSIs), which prevent endogenous Notch activation, and confirmed these effects through creation and characterization of two different adipocyte-specific Notch loss-of-function mouse models through genetic ablation of the Notch transcriptional effector Rbp-Jk (A-Rbpj) and the obligate γ-secretase component Nicastrin (A-Nicastrin).

Methods

Glucose homeostasis and both local adipose and systemic insulin sensitivity were examined in GSI-treated, A-Rbpj and A-Nicastrin mice, as well as vehicle-treated or control littermates, with complementary in vitro studies in primary hepatocytes and 3T3-L1 adipocytes.

Results

GSI-treatment increases hepatic insulin sensitivity in obese mice but leads to reciprocal lowering of adipose glucose disposal. While A-Rbpj mice show normal body weight, adipose development and mass and unchanged adipose insulin sensitivity as control littermates, A-Nicastrin mice are relatively insulin-resistant, mirroring the GSI effect on adipose insulin action.

Conclusions

Notch signaling is dispensable for normal adipocyte function, but adipocyte-specific γ-secretase blockade reduces adipose insulin sensitivity, suggesting that specific Notch inhibitors would be preferable to GSIs for application in T2D.

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γ-secretase inhibitors (GSIs) are non-specific inhibitors of Notch signaling.

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GSI-treatment of obese mice increases hepatic, but lowers adipose insulin sensitivity.

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Adipocyte-specific Notch inhibition does not affect adipose mass or glucose homeostasis.

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Adipocyte-specific γ-secretase blockade reduces adipose insulin sensitivity.

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Specific Notch inhibitors may be preferable to GSIs for treatment of Type 2 Diabetes.

Deviant development of pancreatic beta cells from embryonic exposure to PCB-126 in zebrafish

Exposures to co-planar PCBs and dioxins have been associated with diabetes in epidemiologic studies. Individuals may be predisposed to diseases such as diabetes as a result of exposure to environmental contaminants during early life, resulting in dysmorphic pancreatic islets or metabolically fragile β-cells. We tested the hypothesis that embryonic exposure to a model Ahr-ligand, PCB-126 would cause structural and/or functional alterations to the developing primary pancreatic islet in the zebrafish (Danio rerio). To assess β-cell development, transgenic zebrafish embryos (Tg(ins:GFP) and Tg(ins:mcherry) were exposed to nominal concentrations of 2 or 5nM PCB-126 or DMSO from 24-48h post fertilization (hpf), and imaged via time-lapse microscopy from 80-102hpf. We identified defects including hypomorphic islets, altered islet migration, islet fragmentation, and formation of ectopic β-cells. As we recently showed the transcription factor Nrf2a is protective in PCB-126 embryotoxicity, we then assessed the transcriptional function of the islets in wildtype and nrf2a(fh318/fh318) mutant embryos. We measured gene expression of preproinsulin a, somatostatin2, pdx1, ghrelin, and glucagon. Expression of preproinsulin a increased with PCB treatment in wildtype embryos, while expression of all measured pancreas genes was altered by the nrf2a mutant genotype, suggesting misregulation of the glucose homeostasis axis in those embryos, independent of PCB treatment. This study shows that embryonic exposure to PCB-126 can result in deviant development of the pancreatic islet and suggests that Nrf2a plays a role in regulating glucose homeostasis during development.

The Ribosome Biogenesis Protein Nol9 Is Essential for Definitive Hematopoiesis and Pancreas Morphogenesis in Zebrafish

Ribosome biogenesis is a ubiquitous and essential process in cells. Defects in ribosome biogenesis and function result in a group of human disorders, collectively known as ribosomopathies. In this study, we describe a zebrafish mutant with a loss-of-function mutation in nol9, a gene that encodes a non-ribosomal protein involved in rRNA processing. nol9sa1022/sa1022 mutants have a defect in 28S rRNA processing. The nol9sa1022/sa1022 larvae display hypoplastic pancreas, liver and intestine and have decreased numbers of hematopoietic stem and progenitor cells (HSPCs), as well as definitive erythrocytes and lymphocytes. In addition, ultrastructural analysis revealed signs of pathological processes occurring in endothelial cells of the caudal vein, emphasizing the complexity of the phenotype observed in nol9sa1022/sa1022 larvae. We further show that both the pancreatic and hematopoietic deficiencies in nol9sa1022/sa1022 embryos were due to impaired cell proliferation of respective progenitor cells. Interestingly, genetic loss of Tp53 rescued the HSPCs but not the pancreatic defects. In contrast, activation of mRNA translation via the mTOR pathway by L-Leucine treatment did not revert the erythroid or pancreatic defects. Together, we present the nol9sa1022/sa1022 mutant, a novel zebrafish ribosomopathy model, which recapitulates key human disease characteristics. The use of this genetically tractable model will enhance our understanding of the tissue-specific mechanisms following impaired ribosome biogenesis in the context of an intact vertebrate.

Centroacinar Cells Are Progenitors That Contribute to Endocrine Pancreas Regeneration

Diabetes is associated with a paucity of insulin-producing β-cells. With the goal of finding therapeutic routes to treat diabetes, we aim to find molecular and cellular mechanisms involved in β-cell neogenesis and regeneration. To facilitate discovery of such mechanisms, we use a vertebrate organism where pancreatic cells readily regenerate. The larval zebrafish pancreas contains Notch-responsive progenitors that during development give rise to adult ductal, endocrine, and centroacinar cells (CACs). Adult CACs are also Notch responsive and are morphologically similar to their larval predecessors. To test our hypothesis that adult CACs are also progenitors, we took two complementary approaches: 1) We established the transcriptome for adult CACs. Using gene ontology, transgenic lines, and in situ hybridization, we found that the CAC transcriptome is enriched for progenitor markers. 2) Using lineage tracing, we demonstrated that CACs do form new endocrine cells after β-cell ablation or partial pancreatectomy. We concluded that CACs and their larval predecessors are the same cell type and represent an opportune model to study both β-cell neogenesis and β-cell regeneration. Furthermore, we show that in cftr loss-of-function mutants, there is a deficiency of larval CACs, providing a possible explanation for pancreatic complications associated with cystic fibrosis.