Targeted ablation of beta cells in the embryonic zebrafish pancreas using E. coli nitroreductase

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.

Unraveling Verapamil's Multidimensional Role in Diabetes Therapy: From β-Cell Regeneration to Cholecystokinin Induction in Zebrafish and MIN6 Cell-Line Models

This study unveils verapamil's compelling cytoprotective and proliferative effects on pancreatic β-cells amidst diabetic stressors, spotlighting its unforeseen role in augmenting cholecystokinin (CCK) expression. Through rigorous investigations employing MIN6 β-cells and zebrafish models under type 1 and type 2 diabetic conditions, we demonstrate verapamil's capacity to significantly boost β-cell proliferation, enhance glucose-stimulated insulin secretion, and fortify cellular resilience. A pivotal revelation of our research is verapamil's induction of CCK, a peptide hormone known for its role in nutrient digestion and insulin secretion, which signifies a novel pathway through which verapamil exerts its therapeutic effects. Furthermore, our mechanistic insights reveal that verapamil orchestrates a broad spectrum of gene and protein expressions pivotal for β-cell survival and adaptation to immune-metabolic challenges. In vivo validation in a zebrafish larvae model confirms verapamil's efficacy in fostering β-cell recovery post-metronidazole infliction. Collectively, our findings advocate for verapamil's reevaluation as a multifaceted agent in diabetes therapy, highlighting its novel function in CCK upregulation alongside enhancing β-cell proliferation, glucose sensing, and oxidative respiration. This research enriches the therapeutic landscape, proposing verapamil not only as a cytoprotector but also as a promoter of β-cell regeneration, thereby offering fresh avenues for diabetes management strategies aimed at preserving and augmenting β-cell functionality.

PERK inhibition in zebrafish mimics human Wolcott-Rallison syndrome phenotypes

Wolcott-Rallison Syndrome (WRS) is the most common cause of permanent neonatal diabetes mellitus among consanguineous families. The diabetes associated with WRS is non-autoimmune, insulin-requiring and associated with skeletal dysplasia and growth retardation. The therapeutic options for WRS patients rely on permanent insulin pumping or on invasive transplants of liver and pancreas. WRS has a well identified genetic cause: loss-of-function mutations in the gene coding for an endoplasmic reticulum kinase named PERK (protein kinase R-like ER kinase). Currently, WRS research is facilitated by cellular and rodent models with PERK ablation. While these models have unique strengths, cellular models incompletely replicate the organ/system-level complexity of WRS, and rodents have limited scalability for efficiently screening potential therapeutics. To address these challenges, we developed a new in vivo model of WRS by pharmacologically inhibiting PERK in zebrafish. This small vertebrate displays high fecundity, rapid development of organ systems and is amenable to highly efficient in vivo drug testing. PERK inhibition in zebrafish produced typical WRS phenotypes such as glucose dysregulation, skeletal defects, and impaired development. PERK inhibition in zebrafish also produced broad-spectrum WRS phenotypes such as impaired neuromuscular function, compromised cardiac function and muscular integrity. These results show that zebrafish holds potential as a versatile model to study WRS mechanisms and contribute to the identification of promising therapeutic options for WRS.

col1a2+ fibroblasts/muscle progenitors finetune xanthophore countershading by differentially expressing csf1a/1b in embryonic zebrafish

Animals evolve diverse pigment patterns to adapt to the natural environment. Countershading, characterized by a dark-colored dorsum and a light-colored ventrum, is one of the most prevalent pigment patterns observed in vertebrates. In this study, we reveal a mechanism regulating xanthophore countershading in zebrafish embryos. We found that csf1a and csf1b mutants altered xanthophore countershading differently: csf1a mutants lack ventral xanthophores, while csf1b mutants have reduced dorsal xanthophores. Further study revealed that csf1a is expressed throughout the trunk, whereas csf1b is expressed dorsally. Ectopic expression of csf1a or csf1b in neurons attracted xanthophores into the spinal cord. Blocking csf1 signaling by csf1ra mutants disrupts spinal cord distribution and normal xanthophores countershading. Single-cell RNA sequencing identified two col1a2+ populations: csf1ahighcsf1bhigh muscle progenitors and csf1ahighcsf1blow fibroblast progenitors. Ablation of col1a2+ fibroblast and muscle progenitors abolished xanthophore patterns. Our study suggests that fibroblast and muscle progenitors differentially express csf1a and csf1b to modulate xanthophore patterning, providing insights into the mechanism of countershading.

Foxp and Skor family proteins control differentiation of Purkinje cells from Ptf1a- and Neurog1-expressing progenitors in zebrafish

Cerebellar neurons, such as GABAergic Purkinje cells (PCs), interneurons (INs) and glutamatergic granule cells (GCs) are differentiated from neural progenitors expressing proneural genes, including ptf1a, neurog1 and atoh1a/b/c. Studies in mammals previously suggested that these genes determine cerebellar neuron cell fate. However, our studies on ptf1a;neurog1 zebrafish mutants and lineage tracing of ptf1a-expressing progenitors have revealed that the ptf1a/neurog1-expressing progenitors can generate diverse cerebellar neurons, including PCs, INs and a subset of GCs in zebrafish. The precise mechanisms of how each cerebellar neuron type is specified remains elusive. We found that genes encoding the transcriptional regulators Foxp1b, Foxp4, Skor1b and Skor2, which are reportedly expressed in PCs, were absent in ptf1a;neurog1 mutants. foxp1b;foxp4 mutants showed a strong reduction in PCs, whereas skor1b;skor2 mutants completely lacked PCs, and displayed an increase in immature GCs. Misexpression of skor2 in GC progenitors expressing atoh1c suppressed GC fate. These data indicate that Foxp1b/4 and Skor1b/2 function as key transcriptional regulators in the initial step of PC differentiation from ptf1a/neurog1-expressing neural progenitors, and that Skor1b and Skor2 control PC differentiation by suppressing their differentiation into GCs.

Selective Cell Ablation Using an Improved Prodrug-Converting Nitroreductase

Selective cell ablation is an invaluable tool to investigate the function of cell types, the regeneration of cells, and the modeling of diseases associated with cell loss. The nitroreductase (NTR)-mediated cell ablation system is a simple method enabling the elimination of targeted cells through the expression of a nitroreductase enzyme and the application of a prodrug (such as metronidazole). The prodrug is reduced to a cytotoxic product by nitroreductase, thereby leading to DNA damage-induced cell death. In species with elevated regenerative capacity such as zebrafish, removing the prodrug allows endogenous tissue to replace the lost cells. Herein, we describe a method for the use of a markedly improved nitroreductase enzyme for spatially and temporally controlled targeted cell ablation in the zebrafish. Recently, we identified an NTR variant (NTR 2.0) that achieves effective targeted cell ablation at concentrations of metronidazole well below those causing toxic side effects. NTR 2.0 thereby enables the ablation of "resistant" cell types and novel cell ablation paradigms. These advances simplify investigations of cell function, enable interrogations of the effects of chronic inflammation on regenerative processes and facilitate modeling of degenerative diseases associated with chronic cell loss. Techniques for transgenic nitroreductase expression and prodrug application are discussed.

Pancreatic islet cell plasticity: Pathogenic or therapeutically exploitable?

The development of pancreatic islet endocrine cells is a tightly regulated process leading to the generation of distinct cell types harbouring different hormones in response to small changes in environmental stimuli. Cell differentiation is driven by transcription factors that are also critical for the maintenance of the mature islet cell phenotype. Alteration of the insulin-secreting β-cell transcription factor set by prolonged metabolic stress, associated with the pathogenesis of diabetes, obesity or pregnancy, results in the loss of β-cell identity through de- or transdifferentiation. Importantly, the glucose-lowering effects of approved and experimental antidiabetic agents, including glucagon-like peptide-1 mimetics, novel peptides and small molecules, have been associated with preventing or reversing β-cell dedifferentiation or promoting the transdifferentiation of non-β-cells towards an insulin-positive β-cell-like phenotype. Therefore, we review the manifestations of islet cell plasticity in various experimental settings and discuss the physiological and therapeutic sides of this phenomenon, focusing on strategies for preventing β-cell loss or generating new β-cells in diabetes. A better understanding of the molecular mechanisms underpinning islet cell plasticity is a prerequisite for more targeted therapies to help prevent β-cell decline in diabetes.

A metagenomic library cloning strategy that promotes high-level expression of captured genes to enable efficient functional screening

Functional screening of environmental DNA (eDNA) libraries is a potentially powerful approach to discover enzymatic "unknown unknowns", but is usually heavily biased toward the tiny subset of genes preferentially transcribed and translated by the screening strain. We have overcome this by preparing an eDNA library via partial digest with restriction enzyme FatI (cuts CATG), causing a substantial proportion of ATG start codons to be precisely aligned with strong plasmid-encoded promoter and ribosome-binding sequences. Whereas we were unable to select nitroreductases from standard metagenome libraries, our FatI strategy yielded 21 nitroreductases spanning eight different enzyme families, each conferring resistance to the nitro-antibiotic niclosamide and sensitivity to the nitro-prodrug metronidazole. We showed expression could be improved by co-expressing rare tRNAs and encoded proteins purified directly using an embedded His6-tag. In a transgenic zebrafish model of metronidazole-mediated targeted cell ablation, our lead MhqN-family nitroreductase proved ∼5-fold more effective than the canonical nitroreductase NfsB.

Transcriptomic comparison of two selective retinal cell ablation paradigms in zebrafish reveals shared and cell-specific regenerative responses

Retinal Müller glia (MG) can act as stem-like cells to generate new neurons in both zebrafish and mice. In zebrafish, retinal regeneration is innate and robust, resulting in the replacement of lost neurons and restoration of visual function. In mice, exogenous stimulation of MG is required to reveal a dormant and, to date, limited regenerative capacity. Zebrafish studies have been key in revealing factors that promote regenerative responses in the mammalian eye. Increased understanding of how the regenerative potential of MG is regulated in zebrafish may therefore aid efforts to promote retinal repair therapeutically. Developmental signaling pathways are known to coordinate regeneration following widespread retinal cell loss. In contrast, less is known about how regeneration is regulated in the context of retinal degenerative disease, i.e., following the loss of specific retinal cell types. To address this knowledge gap, we compared transcriptomic responses underlying regeneration following targeted loss of rod photoreceptors or bipolar cells. In total, 2,531 differentially expressed genes (DEGs) were identified, with the majority being paradigm specific, including during early MG activation phases, suggesting the nature of the injury/cell loss informs the regenerative process from initiation onward. For example, early modulation of Notch signaling was implicated in the rod but not bipolar cell ablation paradigm and components of JAK/STAT signaling were implicated in both paradigms. To examine candidate gene roles in rod cell regeneration, including several immune-related factors, CRISPR/Cas9 was used to create G0 mutant larvae (i.e., "crispants"). Rod cell regeneration was inhibited in stat3 crispants, while mutating stat5a/b, c7b and txn accelerated rod regeneration kinetics. These data support emerging evidence that discrete responses follow from selective retinal cell loss and that the immune system plays a key role in regulating "fate-biased" regenerative processes.

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.

An inducible model of chronic hyperglycemia

Transgene driven expression of Escherichia coli nitroreductase (NTR1.0) renders animal cells susceptible to the antibiotic metronidazole (MTZ). Many NTR1.0/MTZ ablation tools have been reported in zebrafish, which have significantly impacted regeneration studies. However, NTR1.0-based tools are not appropriate for modeling chronic cell loss as prolonged application of the required MTZ dose (10 mM) is deleterious to zebrafish health. We established that this dose corresponds to the median lethal dose (LD50) of MTZ in larval and adult zebrafish and that it induced intestinal pathology. NTR2.0 is a more active nitroreductase engineered from Vibrio vulnificus NfsB that requires substantially less MTZ to induce cell ablation. Here, we report on the generation of two new NTR2.0-based zebrafish lines in which acute β-cell ablation can be achieved without MTZ-associated intestinal pathology. For the first time, we were able to sustain β-cell loss and maintain elevated glucose levels (chronic hyperglycemia) in larvae and adults. Adult fish showed significant weight loss, consistent with the induction of a diabetic state, indicating that this paradigm will allow the modeling of diabetes and associated pathologies.

Excess pancreatic elastase alters acinar-β cell communication by impairing the mechano-signaling and the PAR2 pathways

Type 1 (T1D) or type 2 diabetes (T2D) are caused by a deficit of functional insulin-producing β cells. Thus, the identification of β cell trophic agents could allow the development of therapeutic strategies to counteract diabetes. The discovery of SerpinB1, an elastase inhibitor that promotes human β cell growth, prompted us to hypothesize that pancreatic elastase (PE) regulates β cell viability. Here, we report that PE is up-regulated in acinar cells and in islets from T2D patients, and negatively impacts β cell viability. Using high-throughput screening assays, we identified telaprevir as a potent PE inhibitor that can increase human and rodent β cell viability in vitro and in vivo and improve glucose tolerance in insulin-resistant mice. Phospho-antibody microarrays and single-cell RNA sequencing analysis identified PAR2 and mechano-signaling pathways as potential mediators of PE. Taken together, our work highlights PE as a potential regulator of acinar-β cell crosstalk that acts to limit β cell viability, leading to T2D.

IFT46 gene promoter-driven ciliopathy disease model in zebrafish

Ciliopathies are human genetic disorders caused by abnormal formation and dysfunction of cellular cilia. Cilia are microtubule-based organelles that project into the extracellular space and transduce molecular and chemical signals from the extracellular environment or neighboring cells. Intraflagellar transport (IFT) proteins are required for the assembly and maintenance of cilia by transporting proteins along the axoneme which consists of complexes A and B. IFT46, a core IFT-B protein complex, is required for cilium formation and maintenance during vertebrate embryonic development. Here, we introduce transgenic zebrafish lines under the control of ciliated cell-specific IFT46 promoter to recapitulate human ciliopathy-like phenotypes. We generated a Tg(IFT46:GAL4-VP16) line to temporo-spatially control the expression of effectors including fluorescent reporters or nitroreductase based on the GAL4/UAS system, which expresses GAL4-VP16 chimeric transcription factors in most ciliated tissues during embryonic development. To analyze the function of IFT46-expressing ciliated cells during zebrafish development, we generated the Tg(IFT46:GAL4-VP16;UAS;nfsb-mCherry) line, a ciliated cell-specific injury model induced by nitroreductase (NTR)/metrodinazole (MTZ). Conditionally, controlled ablation of ciliated cells in transgenic animals exhibited ciliopathy-like phenotypes including cystic kidneys and pericardial and periorbital edema. Altogether, we established a zebrafish NTR/MTZ-mediated ciliated cell injury model that recapitulates ciliopathy-like phenotypes and may be a vertebrate animal model to further investigate the etiology and therapeutic approaches to human ciliopathies.

The developing epicardium regulates cardiac chamber morphogenesis by promoting cardiomyocyte growth

The epicardium, the outermost layer of the heart, is an important regulator of cardiac regeneration. However, a detailed understanding of the crosstalk between the epicardium and myocardium during development requires further investigation. Here, we generated three models of epicardial impairment in zebrafish by mutating the transcription factor genes tcf21 and wt1a, and ablating tcf21+ epicardial cells. Notably, all three epicardial impairment models exhibited smaller ventricles. We identified the initial cause of this phenotype as defective cardiomyocyte growth, resulting in reduced cell surface and volume. This failure of cardiomyocyte growth was followed by decreased proliferation and increased abluminal extrusion. By temporally manipulating its ablation, we show that the epicardium is required to support cardiomyocyte growth mainly during early cardiac morphogenesis. By transcriptomic profiling of sorted epicardial cells, we identified reduced expression of FGF and VEGF ligand genes in tcf21-/- hearts, and pharmacological inhibition of these signaling pathways in wild type partially recapitulated the ventricular growth defects. Taken together, these data reveal distinct roles of the epicardium during cardiac morphogenesis and signaling pathways underlying epicardial-myocardial crosstalk.

Non-Invasive Monitoring of Cutaneous Wound Healing in Non-Diabetic and Diabetic Model of Adult Zebrafish Using OCT Angiography

A diabetic wound presents a severe risk of infections and other complications because of its slow healing. Evaluating the pathophysiology during wound healing is imperative for wound care, necessitating a proper diabetic wound model and assay for monitoring. The adult zebrafish is a rapid and robust model for studying human cutaneous wound healing because of its fecundity and high similarities to human wound repair. OCTA as an assay can provide three-dimensional (3D) imaging of the tissue structure and vasculature in the epidermis, enabling monitoring of the pathophysiologic alterations in the zebrafish skin wound. We present a longitudinal study for assessing the cutaneous wound healing of the diabetic adult zebrafish model using OCTA, which is of importance for the diabetes research using the alternative animal models. We used non-diabetic (n = 9) and type 1 diabetes mellitus (DM) adult zebrafish models (n = 9). The full-thickness wound was generated on the fish skin, and the wound healing was monitored with OCTA for 15 days. The OCTA results demonstrated significant differences between diabetic and non-diabetic wound healing, involving delayed tissue remodeling and impaired angiogenesis for the diabetic wound, leading to slow wound recovery. The adult zebrafish model and OCTA technique may benefit long-term metabolic disease studies using zebrafish for drug development.

Next-generation plasmids for transgenesis in zebrafish and beyond

Transgenesis is an essential technique for any genetic model. Tol2-based transgenesis paired with Gateway-compatible vector collections has transformed zebrafish transgenesis with an accessible modular system. Here, we establish several next-generation transgenesis tools for zebrafish and other species to expand and enhance transgenic applications. To facilitate gene regulatory element testing, we generated Gateway middle entry vectors harboring the small mouse beta-globin minimal promoter coupled to several fluorophores, CreERT2 and Gal4. To extend the color spectrum for transgenic applications, we established middle entry vectors encoding the bright, blue-fluorescent protein mCerulean and mApple as an alternative red fluorophore. We present a series of p2A peptide-based 3' vectors with different fluorophores and subcellular localizations to co-label cells expressing proteins of interest. Finally, we established Tol2 destination vectors carrying the zebrafish exorh promoter driving different fluorophores as a pineal gland-specific transgenesis marker that is active before hatching and through adulthood. exorh-based reporters and transgenesis markers also drive specific pineal gland expression in the eye-less cavefish (Astyanax). Together, our vectors provide versatile reagents for transgenesis applications in zebrafish, cavefish and other models.

Type I Diabetes in Zebrafish Reduces Sperm Quality and Increases Insulin and Glucose Transporter Transcripts

Type I diabetes is a prominent human pathology with increasing incidence in the population; however, its cause is still unknown. This disease promotes detrimental effects on reproduction, such as lower sperm motility and DNA integrity. Hence, the investigation of the underlying mechanisms of this metabolic disturbance in reproduction and its transgenerational consequences is of the utmost importance. The zebrafish is a useful model for this research considering its high homology with human genes as well as its fast generation and regeneration abilities. Therefore, we aimed to investigate sperm quality and genes relevant to diabetes in the spermatozoa of Tg(ins:nfsb-mCherry) zebrafish, a model for type I diabetes. Diabetic Tg(ins:nfsb-mCherry) males showed significantly higher expression of transcripts for insulin a (insa) and glucose transporter (slc2a2) compared to controls. Sperm obtained from the same treatment group showed significantly lower sperm motility, plasma membrane viability, and DNA integrity compared to that from the control group. Upon sperm cryopreservation, sperm freezability was reduced, which could be a consequence of poor initial sperm quality. Altogether, the data showed similar detrimental effects related to type I diabetes in zebrafish spermatozoa at the cellular and molecular levels. Therefore, our study validates the zebrafish model for type I diabetes research in germ cells.

Transsynaptic labeling and transcriptional control of zebrafish neural circuits

Deciphering the connectome, the ensemble of synaptic connections that underlie brain function is a central goal of neuroscience research. The trans-Tango genetic approach, initially developed for anterograde transsynaptic tracing in Drosophila, can be used to map connections between presynaptic and postsynaptic partners and to drive gene expression in target neurons. Here, we describe the successful adaptation of trans-Tango to visualize neural connections in a living vertebrate nervous system, that of the zebrafish. Connections were validated between synaptic partners in the larval retina and brain. Results were corroborated by functional experiments in which optogenetic activation of retinal ganglion cells elicited responses in neurons of the optic tectum, as measured by trans-Tango-dependent expression of a genetically encoded calcium indicator. Transsynaptic signaling through trans-Tango reveals predicted as well as previously undescribed synaptic connections, providing a valuable in vivo tool to monitor and interrogate neural circuits over time.

A metagenomic library cloning strategy that promotes high-level expression of captured genes to enable efficient functional screening

Functional screening of environmental DNA (eDNA) libraries is a potentially powerful approach to discover enzymatic "unknown unknowns", but is usually heavily biased toward the tiny subset of genes preferentially transcribed and translated by the screening strain. We have overcome this by preparing an eDNA library via partial digest with restriction enzyme FatI (cuts CATG), causing a substantial proportion of ATG start codons to be precisely aligned with strong plasmid-encoded promoter and ribosome-binding sequences. Whereas we were unable to select nitroreductases from standard metagenome libraries, our FatI strategy yielded 21 nitroreductases spanning eight different enzyme families, each conferring resistance to the nitro-antibiotic niclosamide and sensitivity to the nitro-prodrug metronidazole. We showed expression could be improved by co-expressing rare tRNAs and encoded proteins purified directly using an embedded His6-tag. In a transgenic zebrafish model of metronidazole-mediated targeted cell ablation, our lead MhqN-family nitroreductase proved ~5-fold more effective than the canonical nitroreductase NfsB.

Research Progress on the Construction and Application of a Diabetic Zebrafish Model

Diabetes is a metabolic disease characterized by high blood glucose levels. With economic development and lifestyle changes, the prevalence of diabetes is increasing yearly. Thus, it has become an increasingly serious public health problem in countries around the world. The etiology of diabetes is complex, and its pathogenic mechanisms are not completely clear. The use of diabetic animal models is helpful in the study of the pathogenesis of diabetes and the development of drugs. The emerging vertebrate model of zebrafish has many advantages, such as its small size, large number of eggs, short growth cycle, simple cultivation of adult fish, and effective improvement of experimental efficiency. Thus, this model is highly suitable for research as an animal model of diabetes. This review not only summarizes the advantages of zebrafish as a diabetes model, but also summarizes the construction methods and challenges of zebrafish models of type 1 diabetes, type 2 diabetes, and diabetes complications. This study provides valuable reference information for further study of the pathological mechanisms of diabetes and the research and development of new related therapeutic drugs.

Regenerative potential and limitations in a zebrafish model of hyperglycemia-induced nerve degeneration

BACKGROUND

Previous work from our lab has described a model of motor nerve degeneration in hyperglycemic zebrafish larvae which resembles mammalian models of diabetic peripheral neuropathy (DPN). Here, we optimized the hyperglycemic-induction protocol, characterized deficits in nerve structure and behavioral function, and then examined the regenerative potential following recovery from the hyperglycemic state.

RESULTS

In agreement with our previous work, hyperglycemia induced motor nerve degeneration and behavioral deficits. However, the optimized protocol initiated disruption of tight junctions within the blood-nerve barrier, a phenotype apparent in mammalian models of DPN. Following a 10-day recovery period, regeneration of motor nerve components was apparent, but behavioral deficits persisted. We next examined the effect of hyperglycemia on the musculoskeletal system and found subtle deficits in muscle that resolved following recovery, and robust deficits in the skeletal system which persisted following recovery.

CONCLUSION

Here we optimized our previous model of hyperglycemia-induced motor nerve degeneration to more closely align with that observed in mammalian models and then characterized the regenerative potential following recovery from hyperglycemia. Notably, we observed striking impairments to skeletal development, which underscores the global impact hyperglycemia has across systems, and provides a framework for elucidating molecular mechanisms responsible for regenerative events moving forward.

BefA, a microbiota-secreted membrane disrupter, disseminates to the pancreas and increases β cell mass

Microbiome dysbiosis is a feature of diabetes, but how microbial products influence insulin production is poorly understood. We report the mechanism of BefA, a microbiome-derived protein that increases proliferation of insulin-producing β cells during development in gnotobiotic zebrafish and mice. BefA disseminates systemically by multiple anatomic routes to act directly on pancreatic islets. We detail BefA's atomic structure, containing a lipid-binding SYLF domain, and demonstrate that it permeabilizes synthetic liposomes and bacterial membranes. A BefA mutant impaired in membrane disruption fails to expand β cells, whereas the pore-forming host defense protein, Reg3, stimulates β cell proliferation. Our work demonstrates that membrane permeabilization by microbiome-derived and host defense proteins is necessary and sufficient for β cell expansion during pancreas development, potentially connecting microbiome composition with diabetes risk.

Generation and characterization of a novel gne Knockout Model in Zebrafish

GNE Myopathy is a rare, recessively inherited neuromuscular worldwide disorder, caused by a spectrum of bi-allelic mutations in the human GNE gene. GNE encodes a bi-functional enzyme responsible for the rate-limiting step of sialic acid biosynthesis pathway. However, the process in which GNE mutations lead to the development of a muscle pathology is not clear yet. Cellular and mouse models for GNE Myopathy established to date have not been informative. Further, additional GNE functions in muscle have been hypothesized. In these studies, we aimed to investigate gne functions using zebrafish genetic and transgenic models, and characterized them using macroscopic, microscopic, and molecular approaches. We first established transgenic zebrafish lineages expressing the human GNE cDNA carrying the M743T mutation, driven by the zebrafish gne promoter. These fish developed entirely normally. Then, we generated a gne knocked-out (KO) fish using the CRISPR/Cas9 methodology. These fish died 8–10 days post-fertilization (dpf), but a phenotype appeared less than 24 h before death and included progressive body axis curving, deflation of the swim bladder and decreasing movement and heart rate. However, muscle histology uncovered severe defects, already at 5 dpf, with compromised fiber organization. Sialic acid supplementation did not rescue the larvae from this phenotype nor prolonged their lifespan. To have deeper insights into the potential functions of gne in zebrafish, RNA sequencing was performed at 3 time points (3, 5, and 7 dpf). Genotype clustering was progressive, with only 5 genes differentially expressed in gne KO compared to gne WT siblings at 3 dpf. Enrichment analyses of the primary processes affected by the lack of gne also at 5 and 7 dpf point to the involvement of cell cycle and DNA damage/repair processes in the gne KO zebrafish. Thus, we have established a gne KO zebrafish lineage and obtained new insights into gne functions. This is the only model where GNE can be related to clear muscle defects, thus the only animal model relevant to GNE Myopathy to date. Further elucidation of gne precise mechanism-of-action in these processes could be relevant to GNE Myopathy and allow the identification of novel therapeutic targets.

Homeostatic plasticity in the retina

Vision begins in the retina, whose intricate neural circuits extract salient features of the environment from the light entering our eyes. Neurodegenerative diseases of the retina (e.g., inherited retinal degenerations, age-related macular degeneration, and glaucoma) impair vision and cause blindness in a growing number of people worldwide. Increasing evidence indicates that homeostatic plasticity (i.e., the drive of a neural system to stabilize its function) can, in principle, preserve retinal function in the face of major perturbations, including neurodegeneration. Here, we review the circumstances and events that trigger homeostatic plasticity in the retina during development, sensory experience, and disease. We discuss the diverse mechanisms that cooperate to compensate and the set points and outcomes that homeostatic retinal plasticity stabilizes. Finally, we summarize the opportunities and challenges for unlocking the therapeutic potential of homeostatic plasticity. Homeostatic plasticity is fundamental to understanding retinal development and function and could be an important tool in the fight to preserve and restore vision.

Deacetylated nimbin analog N2 fortifies alloxan-induced pancreatic β-cell damage in insulin-resistant zebrafish larvae by upregulating phosphoenolpyruvate carboxykinase (PEPCK) and insulin levels

This study aims to evaluate the protective behaviour of N2, a semi-natural analog of nimbin, for its anti-diabetic efficacy against alloxan-induced oxidative damage and β-cell dysfunction in in-vivo zebrafish larvae. A 500 μM of alloxan was exposed to zebrafish larvae for 24 h to induce oxidative stress in the pancreatic β-cells and co-exposed with N2 to study the protection of N2 by inhibiting ROS by DCFH-DA, DHE and NDA staining along with Cellular damage, apoptosis and lipid peroxidation. The zebrafish was further exposed to 500 μM alloxan for 72 h to induce β-cell destruction along with depleted glucose uptake and co-exposed to N2 to study the protective mechanism. Glucose levels were estimated, and PCR was used to verify the mRNA expression of phosphoenolpyruvate carboxykinase (PEPCK) and insulin. Alloxan induced (24 h) oxidative stress in the pancreatic β-cells in which N2's co-exposure inhibited ROS by eliminating O^-₂ radicals and restoring the glutathione levels, thus preventing cellular damage and lipid peroxidation. The zebrafish exposed to 500 μM alloxan for 72 h was observed with β-cell destruction along with depleted glucose uptake when stained with 2NBDG, wherein N2 was able to protect the pancreatic β-cells from oxidative damage, promoted high glucose uptake and reduced glucose levels. N2 stimulated insulin production and downregulated PEPCK by inhibiting gluconeogenesis, attenuating post-prandial hyperglycemia. N2 may contribute to anti-oxidant protection against alloxan-induced β-cell damage and anti-hyperglycemic activity, restoring insulin function and suppressing PEPCK expression.

Macrophages and neutrophils are necessary for ER stress-induced β cell loss

Persistent endoplasmic reticulum (ER) stress induces islet inflammation and β cell loss. How islet inflammation contributes to β cell loss remains uncertain. We have reported previously that chronic overnutrition-induced ER stress in β cells causes Ripk3-mediated islet inflammation, macrophage recruitment, and a reduction of β cell numbers in a zebrafish model. We show here that β cell loss results from the intricate communications among β cells, macrophages, and neutrophils. Macrophage-derived Tnfa induces cxcl8a in β cells. Cxcl8a, in turn, attracts neutrophils to macrophage-contacted “hotspots” where β cell loss occurs. We also show potentiation of chemokine expression in stressed mammalian β cells by macrophage-derived TNFA. In Akita and db/db mice, there is an increase in CXCL15-positive β cells and intra-islet neutrophils. Blocking neutrophil recruitment in Akita mice preserves β cell mass and slows diabetes progression. These results reveal an important role of neutrophils in persistent ER stress-induced β cell loss.

Yang et al. show a pivotal role of communications among β cells, macrophages, and neutrophils in chronic overnutrition-induced loss of pancreatic β cells in a diabetes-prone zebrafish model.

Macrophages trigger cardiomyocyte proliferation by increasing epicardial vegfaa expression during larval zebrafish heart regeneration

Cardiac injury leads to the loss of cardiomyocytes, which are rapidly replaced by the proliferation of the surviving cells in zebrafish, but not in mammals. In both the regenerative zebrafish and non-regenerative mammals, cardiac injury induces a sustained macrophage response. Macrophages are required for cardiomyocyte proliferation during zebrafish cardiac regeneration, but the mechanisms whereby macrophages facilitate this crucial process are fundamentally unknown. Using heartbeat-synchronized live imaging, RNA sequencing, and macrophage-null genotypes in the larval zebrafish cardiac injury model, we characterize macrophage function and reveal that these cells activate the epicardium, inducing cardiomyocyte proliferation. Mechanistically, macrophages are specifically recruited to the epicardial-myocardial niche, triggering the expansion of the epicardium, which upregulates vegfaa expression to induce cardiomyocyte proliferation. Our data suggest that epicardial Vegfaa augments a developmental cardiac growth pathway via increased endocardial notch signaling. The identification of this macrophage-dependent mechanism of cardiac regeneration highlights immunomodulation as a potential strategy for enhancing mammalian cardiac repair.

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Heart regeneration in larval zebrafish is characterized in detail

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Macrophage ablation blocks cardiomyocyte proliferation after cardiac injury

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Macrophages synapse with epicardial cells and promote their proliferation

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Epicardial Vegfaa drives cardiomyocyte proliferation during cardiac regeneration

A New Transgenic Line for Rapid and Complete Neutrophil Ablation

Zebrafish lines expressing nitroreductase (NTR) in specific cell compartments, which sensitizes those cells to metronidazole (MTZ)-mediated ablation, have proven extremely useful for studying tissue regeneration and investigating cell function. In contrast to many cells, neutrophils are comparatively resistant to the NTR/MTZ targeted ablation strategy. Recently, a rationally engineered variant of NTR (NTR 2.0) has been described that exhibits greatly improved MTZ-mediated ablation efficacy in zebrafish. We show that a transgenic line with neutrophil-restricted expression of NTR 2.0 demonstrates complete neutrophil ablation, with an MTZ dose 100-fold less than current treatment regimens, and with treatment durations as short as 5 h.

Hypothalamic Galanin-producing neurons regulate stress in zebrafish through a peptidergic, self-inhibitory loop

Animals possess neuronal circuits inducing stress to avoid or cope with threats present in their surroundings, for instance, by promoting behaviors, such as avoidance and escape. However, mechanisms must exist to tightly control responses to stressors, since overactivation of stress circuits is deleterious for the wellbeing of an organism. The underlying neuronal dynamics responsible for controlling behavioral responses to stress have remained unclear. Here, we describe a neuronal circuit in the hypothalamus of zebrafish larvae that inhibits stress-related behaviors and prevents excessive activation of the neuroendocrine pathway hypothalamic-pituitary-interrenal axis. Central components of this circuit are neurons secreting the neuropeptide Galanin, as ablation of these neurons led to abnormally high levels of stress. Surprisingly, we found that Galanin has a self-inhibitory action on Galanin-producing neurons. Our results suggest that hypothalamic Galanin-producing neurons play an important role in fine-tuning stress responses by preventing potentially harmful overactivation of stress-regulating circuits.

Follistatin-controlled activin-HNF4α-coagulation factor axis in liver progenitor cells determines outcome of acute liver failure

BACKGROUND AND AIMS

In patients with acute liver failure (ALF) who suffer from massive hepatocyte loss, liver progenitor cells (LPCs) take over key hepatocyte functions, which ultimately determines survival. This study investigated how the expression of hepatocyte nuclear factor 4α (HNF4α), its regulators, and targets in LPCs determines clinical outcome of patients with ALF.

APPROACH AND RESULTS

Clinicopathological associations were scrutinized in 19 patients with ALF (9 recovered and 10 receiving liver transplantation). Regulatory mechanisms between follistatin, activin, HNF4α, and coagulation factor expression in LPC were investigated in vitro and in metronidazole-treated zebrafish. A prospective clinical study followed up 186 patients with cirrhosis for 80 months to observe the relevance of follistatin levels in prevalence and mortality of acute-on-chronic liver failure. Recovered patients with ALF robustly express HNF4α in either LPCs or remaining hepatocytes. As in hepatocytes, HNF4α controls the expression of coagulation factors by binding to their promoters in LPC. HNF4α expression in LPCs requires the forkhead box protein H1-Sma and Mad homolog 2/3/4 transcription factor complex, which is promoted by the TGF-β superfamily member activin. Activin signaling in LPCs is negatively regulated by follistatin, a hepatocyte-derived hormone controlled by insulin and glucagon. In contrast to patients requiring liver transplantation, recovered patients demonstrate a normal activin/follistatin ratio, robust abundance of the activin effectors phosphorylated Sma and Mad homolog 2 and HNF4α in LPCs, leading to significantly improved coagulation function. A follow-up study indicated that serum follistatin levels could predict the incidence and mortality of acute-on-chronic liver failure.

CONCLUSIONS

These results highlight a crucial role of the follistatin-controlled activin-HNF4α-coagulation axis in determining the clinical outcome of massive hepatocyte loss-induced ALF. The effects of insulin and glucagon on follistatin suggest a key role of the systemic metabolic state in ALF.

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

NTR 2.0: a rationally engineered prodrug-converting enzyme with substantially enhanced efficacy for targeted cell ablation

Transgenic expression of bacterial nitroreductase (NTR) enzymes sensitizes eukaryotic cells to prodrugs such as metronidazole (MTZ), enabling selective cell-ablation paradigms that have expanded studies of cell function and regeneration in vertebrates. However, first-generation NTRs required confoundingly toxic prodrug treatments to achieve effective cell ablation, and some cell types have proven resistant. Here we used rational engineering and cross-species screening to develop an NTR variant, NTR 2.0, which exhibits ~100-fold improvement in MTZ-mediated cell-specific ablation efficacy, eliminating the need for near-toxic prodrug treatment regimens. NTR 2.0 therefore enables sustained cell-loss paradigms and ablation of previously resistant cell types. These properties permit enhanced interrogations of cell function, extended challenges to the regenerative capacities of discrete stem cell niches, and novel modeling of chronic degenerative diseases. Accordingly, we have created a series of bipartite transgenic reporter/effector resources to facilitate dissemination of NTR 2.0 to the research community.

A single-cell atlas of de novo β-cell regeneration reveals the contribution of hybrid β/δ-cells to diabetes recovery in zebrafish

Regeneration-competent species possess the ability to reverse the progression of severe diseases by restoring the function of the damaged tissue. However, the cellular dynamics underlying this capability remain unexplored. Here, we have used single-cell transcriptomics to map de novo β-cell regeneration during induction and recovery from diabetes in zebrafish. We show that the zebrafish has evolved two distinct types of somatostatin-producing δ-cells, which we term δ1- and δ2-cells. Moreover, we characterize a small population of glucose-responsive islet cells, which share the hormones and fate-determinants of both β- and δ1-cells. The transcriptomic analysis of β-cell regeneration reveals that β/δ hybrid cells provide a prominent source of insulin expression during diabetes recovery. Using in vivo calcium imaging and cell tracking, we further show that the hybrid cells form de novo and acquire glucose-responsiveness in the course of regeneration. The overexpression of dkk3, a gene enriched in hybrid cells, increases their formation in the absence of β-cell injury. Finally, interspecies comparison shows that plastic δ1-cells are partially related to PP cells in the human pancreas. Our work provides an atlas of β-cell regeneration and indicates that the rapid formation of glucose-responsive hybrid cells contributes to the resolution of diabetes in zebrafish.

MNK2 deficiency potentiates β-cell regeneration via translational regulation

Regenerating pancreatic β-cells is a potential curative approach for diabetes. We previously identified the small molecule CID661578 as a potent inducer of β-cell regeneration, but its target and mechanism of action have remained unknown. We now screened 257 million yeast clones and determined that CID661578 targets MAP kinase-interacting serine/threonine kinase 2 (MNK2), an interaction we genetically validated in vivo. CID661578 increased β-cell neogenesis from ductal cells in zebrafish, neonatal pig islet aggregates and human pancreatic ductal organoids. Mechanistically, we found that CID661578 boosts protein synthesis and regeneration by blocking MNK2 from binding eIF4G in the translation initiation complex at the mRNA cap. Unexpectedly, this blocking activity augmented eIF4E phosphorylation depending on MNK1 and bolstered the interaction between eIF4E and eIF4G, which is necessary for both hypertranslation and β-cell regeneration. Taken together, our findings demonstrate a targetable role of MNK2-controlled translation in β-cell regeneration, a role that warrants further investigation in diabetes.

Shifting the focus of zebrafish toward a model of the tumor microenvironment

Cancer cells exist in a complex ecosystem with numerous other cell types in the tumor microenvironment (TME). The composition of this tumor/TME ecosystem will vary at each anatomic site and affects phenotypes such as initiation, metastasis, and drug resistance. A mechanistic understanding of the large number of cell-cell interactions between tumor and TME requires models that allow us to both characterize as well as genetically perturb this complexity. Zebrafish are a model system optimized for this problem, because of the large number of existing cell-type-specific drivers that can label nearly any cell in the TME. These include stromal cells, immune cells, and tissue resident normal cells. These cell-type-specific promoters/enhancers can be used to drive fluorophores to facilitate imaging and also CRISPR cassettes to facilitate perturbations. A major advantage of the zebrafish is the ease by which large numbers of TME cell types can be studied at once, within the same animal. While these features make the zebrafish well suited to investigate the TME, the model has important limitations, which we also discuss. In this review, we describe the existing toolset for studying the TME using zebrafish models of cancer and highlight unique biological insights that can be gained by leveraging this powerful resource.

Differential Responses of Neural Retina Progenitor Populations to Chronic Hyperglycemia

Diabetic retinopathy is a frequent complication of longstanding diabetes, which comprises a complex interplay of microvascular abnormalities and neurodegeneration. Zebrafish harboring a homozygous mutation in the pancreatic transcription factor pdx1 display a diabetic phenotype with survival into adulthood, and are therefore uniquely suitable among zebrafish models for studying pathologies associated with persistent diabetic conditions. We have previously shown that, starting at three months of age, pdx1 mutants exhibit not only vascular but also neuro-retinal pathologies manifesting as photoreceptor dysfunction and loss, similar to human diabetic retinopathy. Here, we further characterize injury and regenerative responses and examine the effects on progenitor cell populations. Consistent with a negative impact of hyperglycemia on neurogenesis, stem cells of the ciliary marginal zone show an exacerbation of aging-related proliferative decline. In contrast to the robust Müller glial cell proliferation seen following acute retinal injury, the pdx1 mutant shows replenishment of both rod and cone photoreceptors from slow-cycling, neurod-expressing progenitors which first accumulate in the inner nuclear layer. Overall, we demonstrate a diabetic retinopathy model which shows pathological features of the human disease evolving alongside an ongoing restorative process that replaces lost photoreceptors, at the same time suggesting an unappreciated phenotypic continuum between multipotent and photoreceptor-committed progenitors.

A cerebellar internal model calibrates a feedback controller involved in sensorimotor control

Animals must adapt their behavior to survive in a changing environment. Behavioral adaptations can be evoked by two mechanisms: feedback control and internal-model-based control. Feedback controllers can maintain the sensory state of the animal at a desired level under different environmental conditions. In contrast, internal models learn the relationship between the motor output and its sensory consequences and can be used to recalibrate behaviors. Here, we present multiple unpredictable perturbations in visual feedback to larval zebrafish performing the optomotor response and show that they react to these perturbations through a feedback control mechanism. In contrast, if a perturbation is long-lasting, fish adapt their behavior by updating a cerebellum-dependent internal model. We use modelling and functional imaging to show that the neuronal requirements for these mechanisms are met in the larval zebrafish brain. Our results illustrate the role of the cerebellum in encoding internal models and how these can calibrate neuronal circuits involved in reactive behaviors depending on the interactions between animal and environment.

REST is a major negative regulator of endocrine differentiation during pancreas organogenesis

In this study, Rovira et al. report that inactivation of the transcriptional repressor REST causes a drastic increase in pancreatic endocrine progenitors and endocrine cells, and establish that REST is a major negative regulator of embryonic pancreas endocrine differentiation in mice and zebrafish. Their findings show that REST-dependent inhibition ensures a balanced production of endocrine cells from embryonic pancreatic progenitors.

Multiple transcription factors have been shown to promote pancreatic β-cell differentiation, yet much less is known about negative regulators. Earlier epigenomic studies suggested that the transcriptional repressor REST could be a suppressor of endocrinogenesis in the embryonic pancreas. However, pancreatic Rest knockout mice failed to show abnormal numbers of endocrine cells, suggesting that REST is not a major regulator of endocrine differentiation. Using a different conditional allele that enables profound REST inactivation, we observed a marked increase in pancreatic endocrine cell formation. REST inhibition also promoted endocrinogenesis in zebrafish and mouse early postnatal ducts and induced β-cell-specific genes in human adult duct-derived organoids. We also defined genomic sites that are bound and repressed by REST in the embryonic pancreas. Our findings show that REST-dependent inhibition ensures a balanced production of endocrine cells from embryonic pancreatic progenitors.

12-Lipoxygenase governs the innate immune pathogenesis of islet inflammation and autoimmune diabetes

Macrophages and related myeloid cells are innate immune cells that participate in the early islet inflammation of type 1 diabetes (T1D). The enzyme 12-lipoxygenase (12-LOX) catalyzes the formation of proinflammatory eicosanoids, but its role and mechanisms in myeloid cells in the pathogenesis of islet inflammation have not been elucidated. Leveraging a model of islet inflammation in zebrafish, we show here that macrophages contribute significantly to the loss of β cells and the subsequent development of hyperglycemia. The depletion or inhibition of 12-LOX in this model resulted in reduced macrophage infiltration into islets and the preservation of β cell mass. In NOD mice, the deletion of the gene encoding 12-LOX in the myeloid lineage resulted in reduced insulitis with reductions in proinflammatory macrophages, a suppressed T cell response, preserved β cell mass, and almost complete protection from the development of T1D. 12-LOX depletion caused a defect in myeloid cell migration, a function required for immune surveillance and tissue injury responses. This effect on migration resulted from the loss of the chemokine receptor CXCR3. Transgenic expression of the gene encoding CXCR3 rescued the migratory defect in zebrafish 12-LOX morphants. Taken together, our results reveal a formative role for innate immune cells in the early pathogenesis of T1D and identify 12-LOX as an enzyme required to promote their prodiabetogenic phenotype in the context of autoimmunity.

Protective effects of alpha-lipoic acid on hair cell damage in diabetic zebrafish model

Hearing impairment is one of the complications in diabetes mellitus; however, there are very few therapeutic studies on it. In this study, we investigated the protective effect of alpha-lipoic acid (ALA) on hearing loss in diabetic transgenic zebrafish and confirmed that ALA protects the loss of hair cells (HCs) caused by hyperglycemia. The data indicated that ALA has a protective effect on the damage to HCs in diabetic zebrafish.

Characteristics of the New Insulin-Resistant Zebrafish Model

Insulin resistance, which occurs when insulin levels are sufficiently high over a prolonged period, causing the cells to fail to respond normally to the hormone. As a system for insulin resistance and diabetes drug development, insulin-resistant rodent models have been clearly established, but there is a limitation to high-throughput drug screening. Recently, zebrafish have been identified as an excellent system for drug discovery and identification of therapeutic targets, but studies on insulin resistance models have not been extensively performed. Therefore, we aimed to make a rapid insulin-resistant zebrafish model that complements the existing rodent models. To establish this model, zebrafish were treated with 10 μM insulin for 48 h. This model showed characteristics of insulin-resistant disease such as damaged pancreatic islets. Then we confirmed the recovery of the pancreatic islets after pioglitazone treatment. In addition, it was found that insulin-resistant drugs have as significant an effect in zebrafish as in humans, and these results proved the value of the zebrafish insulin resistance model for drug selection. In addition, RNA sequencing was performed to elucidate the mechanism involved. KEGG pathway enrichment analysis of differentially expressed genes showed that insulin resistance altered gene expression due to the MAPK signaling and calcium signaling pathways. This model demonstrates the utility of the zebrafish model for drug testing and drug discovery in insulin resistance and diabetes.

Large-scale phenotypic drug screen identifies neuroprotectants in zebrafish and mouse models of retinitis pigmentosa

Retinitis pigmentosa (RP) and associated inherited retinal diseases (IRDs) are caused by rod photoreceptor degeneration, necessitating therapeutics promoting rod photoreceptor survival. To address this, we tested compounds for neuroprotective effects in multiple zebrafish and mouse RP models, reasoning drugs effective across species and/or independent of disease mutation may translate better clinically. We first performed a large-scale phenotypic drug screen for compounds promoting rod cell survival in a larval zebrafish model of inducible RP. We tested 2934 compounds, mostly human-approved drugs, across six concentrations, resulting in 113 compounds being identified as hits. Secondary tests of 42 high-priority hits confirmed eleven lead candidates. Leads were then evaluated in a series of mouse RP models in an effort to identify compounds effective across species and RP models, that is, potential pan-disease therapeutics. Nine of 11 leads exhibited neuroprotective effects in mouse primary photoreceptor cultures, and three promoted photoreceptor survival in mouse rd1 retinal explants. Both shared and complementary mechanisms of action were implicated across leads. Shared target tests implicated parp1-dependent cell death in our zebrafish RP model. Complementation tests revealed enhanced and additive/synergistic neuroprotective effects of paired drug combinations in mouse photoreceptor cultures and zebrafish, respectively. These results highlight the value of cross-species/multi-model phenotypic drug discovery and suggest combinatorial drug therapies may provide enhanced therapeutic benefits for RP patients.

Exocrine pancreas proteases regulate β-cell proliferation in zebrafish ciliopathy models and in murine systems

Pancreatic β-cells are a critical cell type in the pathology of diabetes. Models of genetic syndromes featuring diabetes can provide novel mechanistic insights into regulation of β-cells in the context of disease. We previously examined β-cell mass in models of two ciliopathies, Alström Syndrome (AS) and Bardet-Biedl Syndrome (BBS), which are similar in the presence of metabolic phenotypes, including obesity, but exhibit strikingly different rates of diabetes. Zebrafish models of these disorders show deficient β-cells with diabetes in AS models and an increased β-cells absent diabetes in BBS models, indicating β-cell generation or maintenance that correlates with disease prevalence. Using transcriptome analyses, differential expression of several exocrine pancreas proteases with directionality that was consistent with β-cell numbers were identified. Based on these lines of evidence, we hypothesized that pancreatic proteases directly impact β-cells. In the present study, we examined this possibility and found that pancreatic protease genes contribute to proper maintenance of normal β-cell numbers, proliferation in larval zebrafish, and regulation of AS and BBS β-cell phenotypes. Our data suggest that these proteins can be taken up directly by cultured β-cells and ex vivo murine islets, inducing proliferation in both. Endogenous uptake of pancreatic proteases by β-cells was confirmed in vivo using transgenic zebrafish and in intact murine pancreata. Taken together, these findings support a novel proliferative signaling role for exocrine pancreas proteases through interaction with endocrine β-cells.

Summary: In this paper, we examine the role of exocrine pancreas protease genes in β-cell number using zebrafish and murine models, and identify a novel relationship between the two.

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.

Reinforcing one-carbon metabolism via folic acid/Folr1 promotes β-cell differentiation

Diabetes can be caused by an insufficiency in β-cell mass. Here, we performed a genetic screen in a zebrafish model of β-cell loss to identify pathways promoting β-cell regeneration. We found that both folate receptor 1 (folr1) overexpression and treatment with folinic acid, stimulated β-cell differentiation in zebrafish. Treatment with folinic acid also stimulated β-cell differentiation in cultures of neonatal pig islets, showing that the effect could be translated to a mammalian system. In both zebrafish and neonatal pig islets, the increased β-cell differentiation originated from ductal cells. Mechanistically, comparative metabolomic analysis of zebrafish with/without β-cell ablation and with/without folinic acid treatment indicated β-cell regeneration could be attributed to changes in the pyrimidine, carnitine, and serine pathways. Overall, our results suggest evolutionarily conserved and previously unknown roles for folic acid and one-carbon metabolism in the generation of β-cells.

Involvement of Cerebellar Neural Circuits in Active Avoidance Conditioning in Zebrafish

When animals repeatedly receive a combination of neutral conditional stimulus (CS) and aversive unconditional stimulus (US), they learn the relationship between CS and US, and show conditioned fear responses after CS. They show passive responses such as freezing or panic movements (classical or Pavlovian fear conditioning), or active behavioral responses to avoid aversive stimuli (active avoidance). Previous studies suggested the roles of the cerebellum in classical fear conditioning but it remains elusive whether the cerebellum is involved in active avoidance conditioning. In this study, we analyzed the roles of cerebellar neural circuits during active avoidance in adult zebrafish. When pairs of CS (light) and US (electric shock) were administered to wild-type zebrafish, about half of them displayed active avoidance. The expression of botulinum toxin, which inhibits the release of neurotransmitters, in cerebellar granule cells (GCs) or Purkinje cells (PCs) did not affect conditioning-independent swimming behaviors, but did inhibit active avoidance conditioning. Nitroreductase (NTR)-mediated ablation of PCs in adult zebrafish also impaired active avoidance. Furthermore, the inhibited transmission of GCs or PCs resulted in reduced fear-conditioned Pavlovian fear responses. Our findings suggest that the zebrafish cerebellum plays an active role in active avoidance conditioning.

A Novel Lysolecithin Model for Visualizing Damage in vivo in the Larval Zebrafish Spinal Cord

Background: Lysolecithin is commonly used to induce demyelinating lesions in the spinal cord and corpus callosum of mammalian models. Although these models and clinical patient samples are used to study neurodegenerative diseases, such as multiple sclerosis (MS), they do not allow for direct visualization of disease-related damage in vivo. To overcome this limitation, we created and characterized a focal lysolecithin injection model in zebrafish that allows us to investigate the temporal dynamics underlying lysolecithin-induced damage in vivo. Results: We injected lysolecithin into 4-6 days post-fertilization (dpf) zebrafish larval spinal cords and, coupled with in vivo, time-lapse imaging, observed hallmarks consistent with mammalian models of lysolecithin-induced demyelination, including myelinating glial cell loss, myelin perturbations, axonal sparing, and debris clearance. Conclusion: We have developed and characterized a lysolecithin injection model in zebrafish that allows us to investigate myelin damage in a living, vertebrate organism. This model may be a useful pre-clinical screening tool for investigating the safety and efficacy of novel therapeutic compounds that reduce damage and/or promote repair in neurodegenerative disorders, such as MS.

State-of-the-art microscopy to understand islets of Langerhans: what to expect next?

The discovery of Langerhans and microscopic description of islets in the pancreas were crucial steps in the discovery of insulin. Over the past 150 years, many discoveries in islet biology and type 1 diabetes have been made using powerful microscopic techniques. In the past decade, combination of new probes, animal and tissue models, application of new biosensors and automation of light and electron microscopic methods and other (sub)cellular imaging modalities have proven their potential in understanding the beta cell under (patho)physiological conditions. The imaging evolution, from fluorescent jellyfish to real‐time intravital functional imaging, the revolution in automation and data handling and the increased resolving power of analytical imaging techniques are now converging. Here, we review innovative approaches that address islet biology from new angles by studying cells and molecules at high spatiotemporal resolution and in live models. Broad implementation of these cellular imaging techniques will shed new light on cause/consequence of (mal)function in islets of Langerhans in the years to come.

Thyroid Hormones Regulate Goblet Cell Differentiation and Fgf19-Fgfr4 Signaling

Hypothyroidism is a common pathological condition characterized by insufficient activity of the thyroid hormones (THs), thyroxine (T4), and 3,5,3'-triiodothyronine (T3), in the whole body or in specific tissues. Hypothyroidism is associated with inadequate development of the intestine as well as gastrointestinal diseases. We used a zebrafish model of hypothyroidism to identify and characterize TH-modulated genes and cellular pathways controlling intestine development. In the intestine of hypothyroid juveniles and adults, the number of mucus-secreting goblet cells was reduced, and this phenotype could be rescued by T3 treatment. Transcriptome profiling revealed dozens of differentially expressed genes in the intestine of hypothyroid adults compared to controls. Notably, the expression of genes encoding to Fgf19 and its receptor Fgfr4 was markedly increased in the intestine of hypothyroid adults, and treatment with T3 normalized it. Blocking fibroblast growth factor (FGF) signaling, using an inducible dominant-negative Fgfr transgenic line, rescued the number of goblet cells in hypothyroid adults. These results show that THs inhibit the Fgf19-Fgfr4 signaling pathway, which is associated with inhibition of goblet cell differentiation in hypothyroidism. Both the TH and Fgf19-Fgfr4 signaling pathways can be pharmaceutical targets for the treatment of TH-related gastrointestinal diseases.

In vivo screen identifies a SIK inhibitor that induces β cell proliferation through a transient UPR

It is known that β cell proliferation expands the β cell mass during development and under certain hyperglycemic conditions in the adult, a process that may be used for β cell regeneration in diabetes. Here, through a new high-throughput screen using a luminescence ubiquitination-based cell cycle indicator (LUCCI) in zebrafish, we identify HG-9-91-01 as a driver of proliferation and confirm this effect in mouse and human β cells. HG-9-91-01 is an inhibitor of salt-inducible kinases (SIKs), and overexpression of Sik1 specifically in β cells blocks the effect of HG-9-91-01 on β cell proliferation. Single-cell transcriptomic analyses of mouse β cells demonstrate that HG-9-91-01 induces a wave of activating transcription factor (ATF)6-dependent unfolded protein response (UPR) before cell cycle entry. Importantly, the UPR wave is not associated with an increase in insulin expression. Additional mechanistic studies indicate that HG-9-91-01 induces multiple signalling effectors downstream of SIK inhibition, including CRTC1, CRTC2, ATF6, IRE1 and mTOR, which integrate to collectively drive β cell proliferation.