Defining hepatic dysfunction parameters in two models of fatty liver disease in zebrafish larvae

A high-cholesterol zebrafish diet promotes hypercholesterolemia and fasting-associated liver steatosis

Zebrafish are an ideal model organism to study lipid metabolism and to elucidate the molecular underpinnings of human lipid-associated disorders. Unlike murine models, to which various standardized high lipid diets such as a high-cholesterol diet (HCD) are available, there has yet to be a uniformly adopted zebrasfish HCD protocol. In this study, we have developed an improved HCD protocol and thoroughly tested its impact on zebrafish lipid deposition and lipoprotein regulation in a dose- and time- dependent manner. The diet stability, reproducibility, and fish palatability were also validated. Fish fed HCD developed hypercholesterolemia as indicated by significantly elevated ApoB-containing lipoproteins (ApoB-LP) and increased plasma levels of cholesterol and cholesterol esters. Feeding of the HCD to larvae for 8 days produced hepatic steatosis that become more stable and severer after 1 day of fasting and was associated with an opaque liver phenotype (dark under transmitted light). Unlike larvae, adult fish fed HCD for 14 days followed by a 3 day fast did not develop a stable fatty liver phenotype, though the fish had higher ApoB-LP levels in plasma and an up-regulated lipogenesis gene fasn in adipose tissue. In conclusion, our HCD zebrafish protocol represents an effective and reliable approach for studying the temporal characteristics of the physiological and biochemical responses to high levels of dietary cholesterol and provides insights into the mechanisms that may underlie fatty liver disease.

Embryonic Amoxicillin Exposure Has Limited Impact on Liver Development but Increases Susceptibility to NAFLD in Zebrafish Larvae

Amoxicillin is commonly used in clinical settings to target bacterial infection and is frequently prescribed during pregnancy. Investigations into its developmental toxicity and effects on disease susceptibility are not comprehensive. Our present study examined the effects of embryonic amoxicillin exposure on liver development and function, especially the effects on susceptibility to non-alcoholic fatty liver disease (NAFLD) using zebrafish as an animal model. We discovered that embryonic amoxicillin exposure did not compromise liver development, nor did it induce liver toxicity. However, co-treatment of amoxicillin and clavulanic acid diminished BESP expression, caused bile stasis and induced liver toxicity. Embryonic amoxicillin exposure resulted in elevated expression of lipid synthesis genes and exacerbated hepatic steatosis in a fructose-induced NAFLD model, indicating embryonic amoxicillin exposure increased susceptibility to NAFLD in zebrafish larvae. In summary, this research broadens our understanding of the risks of amoxicillin usage during pregnancy and provides evidence for the impact of embryonic amoxicillin exposure on disease susceptibility in offspring.

Genome-wide detection of positive and balancing signatures of selection shared by four domesticated rainbow trout populations (Oncorhynchus mykiss)

BACKGROUND

Evolutionary processes leave footprints along the genome over time. Highly homozygous regions may correspond to positive selection of favorable alleles, while maintenance of heterozygous regions may be due to balancing selection phenomena. We analyzed data from 176 fish from four disconnected domestic rainbow trout populations that were genotyped using a high-density Axiom Trout genotyping 665K single nucleotide polymorphism array, including 20 from the US and 156 from three French lines. Using methods based on runs of homozygosity and extended haplotype homozygosity, we detected signatures of selection in these four populations.

RESULTS

Nine genomic regions that included 253 genes were identified as being under positive selection in all four populations Most were located on chromosome 2 but also on chromosomes 12, 15, 16, and 20. In addition, four heterozygous regions that contain 29 genes that are putatively under balancing selection were also shared by the four populations. These were located on chromosomes 10, 13, and 19. Regardless of the homozygous or heterozygous nature of the regions, in each region, we detected several genes that are highly conserved among vertebrates due to their critical roles in cellular and nuclear organization, embryonic development, or immunity. We identified new candidate genes involved in rainbow trout fitness, as well as 17 genes that were previously identified to be under positive selection, 10 of which in other fishes (auts2, atp1b3, zp4, znf135, igf-1α, brd2, col9a2, mrap2, pbx1, and emilin-3).

CONCLUSIONS

Using material from disconnected populations of different origins allowed us to draw a genome-wide map of signatures of positive selection that are shared between these rainbow trout populations, and to identify several regions that are putatively under balancing selection. These results provide a valuable resource for future investigations of the dynamics of genetic diversity and genome evolution during domestication.

Non-Genetic-Induced Zebrafish Model for Type 2 Diabetes with Emphasis on Tools in Model Validation

The unrelenting increase in the incidence of type 2 diabetes (T2D) necessitates the urgent need for effective animal models to mimic its pathophysiology. Zebrafish possess human-like metabolic traits and share significant genetic similarities, making them valuable candidates for studying metabolic disorders, including T2D. This review emphasizes the critical role of animal models in diabetes research, especially focusing on zebrafish as an alternative model organism. Different approaches to a non-genetic model of T2D in zebrafish, such as the glucose solution, diet-induced, chemical-induced, and combined diet-induced and glucose solution methods, with an emphasis on model validation using indicators of T2D, were highlighted. However, a significant drawback lies in the validation of these models. Some of these models have not extensively demonstrated persistent hyperglycemia or response to insulin resistance and glucose tolerance tests, depicted the morphology of the pancreatic β-cell, or showed their response to antidiabetic drugs. These tools are crucial in T2D pathology. Future research on non-genetic models of T2D in zebrafish must extensively focus on validating the metabolic deficits existing in the model with the same metabolic defects in humans and improve on the existing models for a better understanding of the molecular mechanisms underlying T2D and exploring potential therapeutic interventions.

A high-cholesterol zebrafish diet promotes hypercholesterolemia and fasting-associated liver triglycerides accumulation

Zebrafish are an ideal model organism to study lipid metabolism and to elucidate the molecular underpinnings of human lipid-associated disorders. In this study, we provide an improved protocol to assay the impact of a high-cholesterol diet (HCD) on zebrafish lipid deposition and lipoprotein regulation. Fish fed HCD developed hypercholesterolemia as indicated by significantly elevated ApoB-containing lipoproteins (ApoB-LP) and increased plasma levels of cholesterol and cholesterol esters. Feeding of the HCD to larvae (8 days followed by a 1 day fast) and adult female fish (2 weeks, followed by 3 days of fasting) was also associated with a fatty liver phenotype that presented as severe hepatic steatosis. The HCD feeding paradigm doubled the levels of liver triacylglycerol (TG), which was striking because our HCD was only supplemented with cholesterol. The accumulated liver TG was unlikely due to increased de novo lipogenesis or inhibited β-oxidation since no differentially expressed genes in these pathways were found between the livers of fish fed the HCD versus control diets. However, fasted HCD fish had significantly increased lipogenesis gene fasn in adipose tissue and higher free fatty acids (FFA) in plasma. This suggested that elevated dietary cholesterol resulted in lipid accumulation in adipocytes, which supplied more FFA during fasting, promoting hepatic steatosis. In conclusion, our HCD zebrafish protocol represents an effective and reliable approach for studying the temporal characteristics of the physiological and biochemical responses to high levels of dietary cholesterol and provides insights into the mechanisms that may underlie fatty liver disease.

Zebrafish as a Useful Model System for Human Liver Disease

Liver diseases represent a significant global health challenge, thereby necessitating extensive research to understand their intricate complexities and to develop effective treatments. In this context, zebrafish (Danio rerio) have emerged as a valuable model organism for studying various aspects of liver disease. The zebrafish liver has striking similarities to the human liver in terms of structure, function, and regenerative capacity. Researchers have successfully induced liver damage in zebrafish using chemical toxins, genetic manipulation, and other methods, thereby allowing the study of disease mechanisms and the progression of liver disease. Zebrafish embryos or larvae, with their transparency and rapid development, provide a unique opportunity for high-throughput drug screening and the identification of potential therapeutics. This review highlights how research on zebrafish has provided valuable insights into the pathological mechanisms of human liver disease.

Zebrafish facilitate non-alcoholic fatty liver disease research: Tools, models and applications

Non-alcoholic fatty liver disease (NAFLD) has become an increasingly epidemic metabolic disease worldwide. NAFLD can gradually deteriorate from simple liver steatosis, inflammation and fibrosis to liver cirrhosis and/or hepatocellular carcinoma. Zebrafish are vertebrate animal models that are genetically and metabolically conserved with mammals and have unique advantages such as high fecundity, rapid development ex utero and optical transparency. These features have rendered zebrafish an emerging model system for liver diseases and metabolic diseases favoured by many researchers in recent years. In the present review, we summarize a series of tools for zebrafish NAFLD research and the models established through different dietary feeding, hepatotoxic chemical treatments and genetic manipulations via transgenic or genome editing technologies. We also discuss how zebrafish models facilitate NAFLD studies by providing novel insights into NAFLD pathogenesis, toxicology research, and drug evaluation and discovery.

The immune-related circRNA-miRNA-mRNA ceRNA regulatory network in the liver of turbot (Scophthalmus maximus L.) induced by Vibrio anguillarum

Recently, Vibrio anguillarum, a Gram-negative pathogenic bacterium, has been becoming a major constraint on the development of the turbot aquaculture industry because of its characteristics of worldwide distribution, broad host range and potentially devastating impacts. Although the functions of protein-coding mRNAs in the immune response against bacterial infection have been reported, as well as several non-coding RNAs (ncRNAs), such as circular RNAs (circRNAs) and microRNAs (miRNAs), the relationships between mRNAs and ncRNAs in the immune system of turbot liver are still limited during bacterial infection. In present study, the comprehensive analyses of whole-transcriptome sequencing were conducted in turbot liver infected by V. anguillarum. The differential expression was analyzed in the data of circRNAs, miRNAs, and mRNAs. The interactions of miRNA-circRNA pairs and miRNA-mRNA pairs were predicted basing on the negative regulatory relationships between miRNAs and their target circRNAs\mRNAs. The circRNA-related ceRNA regulatory networks were constructed for the analyses of regulated mechanism in turbot immune system. Subsequently, the RT-qPCR was carried out to verify the results of sequencing. Finally, we identified 31 circRNAs, 53 miRNAs and 948 mRNAs with differential expression. Gene set enrichment analyses using Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways showed that innate immunity was principally activated at the early stages of infection, while adaptive immunity was activated after 24 h. Finally, 65 circRNA-miRNA-mRNA pathways were constructed, based on the hypothesis of ceRNA regulatory networks. In conclusion, our findings provide new insights on the underlying immune response to bacterial infection and identify novel target genes for the prevention and control of disease in turbot.

Evaluation of Metal-Organic Framework MIL-89 nanoparticles toxicity on embryonic zebrafish development

Metal-Organic Framework MIL-89 nanoparticles garnered remarkable attention for their widespread use in technological applications. However, the impact of these nanomaterials on human and environmental health is still limited, and concerns regarding the potential risk of exposure during manipulation is constantly rising. Therefore, the extensive use of nanomaterials in the medical field necessitates a comprehensive assessment of their safety and interaction with different tissues of the body system. In this study, we evaluated the systemic toxicity of nanoMIL-89 using Zebrafish embryos as a model system to determine the acute developmental effect. Zebrafish embryos were exposed to a range of nanoMIL-89 concentrations (1 - 300 µM) at 4 h post-fertilization (hpf) for up to 120 hpf. The viability and hatching rate were evaluated at 24-72 hpf, whereas the cardiac function was assessed at 72 and 96 hpf, and the neurodevelopment and hepatic steatosis at 120 hpf. Our study shows that nanoMIL-89 exerted no developmental toxicity on zebrafish embryos at low concentrations (1-10 µM). However, the hatching time and heart development were affected at high concentrations of nanoMIL-89 (> 30 µM). Our findings add novel information into the available data about the in vivo toxicity of nanoMIL-89 and demonstrate its innocuity and safe use in biological, environmental, and medical applications.

Zebrafish Embryos and Larvae as Alternative Animal Models for Toxicity Testing

Prerequisite to any biological laboratory assay employing living animals is consideration about its necessity, feasibility, ethics and the potential harm caused during an experiment. The imperative of these thoughts has led to the formulation of the 3R-principle, which today is a pivotal scientific standard of animal experimentation worldwide. The rising amount of laboratory investigations utilizing living animals throughout the last decades, either for regulatory concerns or for basic science, demands the development of alternative methods in accordance with 3R to help reduce experiments in mammals. This demand has resulted in investigation of additional vertebrate species displaying favourable biological properties. One prominent species among these is the zebrafish (Danio rerio), as these small laboratory ray-finned fish are well established in science today and feature outstanding biological characteristics. In this review, we highlight the advantages and general prerequisites of zebrafish embryos and larvae before free-feeding stages for toxicological testing, with a particular focus on cardio-, neuro, hepato- and nephrotoxicity. Furthermore, we discuss toxicokinetics, current advances in utilizing zebrafish for organ toxicity testing and highlight how advanced laboratory methods (such as automation, advanced imaging and genetic techniques) can refine future toxicological studies in this species.

Loss of mpv17 affected early embryonic development via mitochondria dysfunction in zebrafish

MVP17 encodes a mitochondrial inner-membrane protein, and mutation of human MVP17 can cause mitochondria DNA depletion syndrome (MDDS). However, the underlying function of mpv17 is still elusive. Here, we developed a new mutant with mpv17 knockout by using the CRISPR/Cas9 system. The mpv17^-/- zebrafish showed developmental defects in muscles, liver, and energy supply. The mpv17^-/- larvae hardly survived beyond a month, and they showed abnormal growth during the development stage. Abnormal swimming ability was also found in the mpv17^-/- zebrafish. The transmission electron microscope (TEM) observation indicated that the mpv17^-/- zebrafish underwent severe mitochondria dysfunction and the disorder of mitochondrial cristae. As an energy producer, the defects of mitochondria significantly reduced ATP content in mpv17^-/- zebrafish, compared to wild-type zebrafish. We hypothesized that the disorder of mitochondria cristae was contributed to the dysfunction of muscle and liver in the mpv17^-/- zebrafish. Moreover, the content of major energy depot triglycerides (TAG) was decreased dramatically. Interestingly, after rescued with normal exogenous mitochondria by microinjection, the genes involved in the TAG metabolism pathway were recovered to a normal level. Taken together, this is the first report of developmental defects in muscles, liver, and energy supply via mitochondria dysfunction, and reveals the functional mechanism of mpv17 in zebrafish.

Transgenic fluorescent zebrafish lines that have revolutionized biomedical research

Since its debut in the biomedical research fields in 1981, zebrafish have been used as a vertebrate model organism in more than 40,000 biomedical research studies. Especially useful are zebrafish lines expressing fluorescent proteins in a molecule, intracellular organelle, cell or tissue specific manner because they allow the visualization and tracking of molecules, intracellular organelles, cells or tissues of interest in real time and in vivo. In this review, we summarize representative transgenic fluorescent zebrafish lines that have revolutionized biomedical research on signal transduction, the craniofacial skeletal system, the hematopoietic system, the nervous system, the urogenital system, the digestive system and intracellular organelles.

Imaging cytoplasmic lipid droplets in vivo with fluorescent perilipin 2 and perilipin 3 knock-in zebrafish

Cytoplasmic lipid droplets are highly dynamic storage organelles that are critical for cellular lipid homeostasis. While the molecular details of lipid droplet dynamics are a very active area of investigation, this work has been primarily performed in cultured cells. Taking advantage of the powerful transgenic and in vivo imaging opportunities available in zebrafish, we built a suite of tools to study lipid droplets in real time from the subcellular to the whole organism level. Fluorescently tagging the lipid droplet-associated proteins, perilipin 2 and perilipin 3, in the endogenous loci permits visualization of lipid droplets in the intestine, liver, and adipose tissue. Using these tools, we found that perilipin 3 is rapidly loaded on intestinal lipid droplets following a high-fat meal and later replaced by perilipin 2. These powerful new tools will facilitate studies on the role of lipid droplets in different tissues, under different genetic and physiological manipulations, and in a variety of human disease models.

Chronic exposure to ethanol alters the expression of miR-155, miR-122 and miR-217 in alcoholic liver disease in an adult zebrafish model

AIM

The aim of this study was to evaluate the hepatic and circulating expression of miR-155, miR-122 and miR-217 in a model of chronic exposure to ethanol in adult zebrafish.

METHODS

Wild-type adult zebrafish were divided into two groups (n = 281): an EG (exposed to 0.5% v/v Ethanol in aquarium water) and a CG (without ethanol). After 28 days the animals were euthanized, followed by histopathological analysis, quantification of lipids, triglycerides and inflammatory cytokines in liver tissue. miR-155, miR-122 and miR-217 gene expression was quantified in liver tissue and serum.

RESULTS

We observed hepatic lesions and increased accumulation of hepatic lipids in the EG. The expression of il-1β was higher in the EG, but there were no differences in il-10 and tnf-α between groups. In the liver, expression of miR-122 and miR-155 was higher in the EG. The circulating expression of miR-155 and miR-217 was significantly higher in the EG.

CONCLUSION

Chronic exposure to ethanol in zebrafish leads to altered hepatic and circulating expression of miR-155, miR-122 and miR-217. This confirms its potential as a biomarker and therapeutic target.

Arsenic induced redox imbalance triggers the unfolded protein response in the liver of zebrafish

Inorganic arsenic (iAs) is one of the most endemic toxicants worldwide and oxidative stress is a key cellular pathway underlying iAs toxicity. Other cellular stress response pathways, such as the unfolded protein response (UPR), are also impacted by iAs exposure, however it is not known how these pathways intersect to cause disease. We optimized the use of zebrafish larvae to identify the relationship between these cellular stress response pathways and arsenic toxicity. We found that the window of iAs susceptibility during zebrafish development corresponds with the development of the liver, and that even a 24-h exposure can cause lethality if administered to mature larvae, but not to early embryos. Acute exposure of larvae to iAs generates reactive oxygen species (ROS), an antioxidant response, endoplasmic reticulum (ER) stress and UPR activation in the liver. An in vivo assay using transgenic larvae expressing a GFP-tagged secreted glycoprotein in hepatocytes (Tg(fabp10a:Gc-EGFP)) revealed acute iAs exposure selectively decreased expression of Gc-EGFP, indicating that iAs impairs secretory protein folding in the liver. The transcriptional output of UPR activation is preceded by ROS production and activation of genes involved in the oxidative stress response. These studies implicate redox imbalance as the mechanism of iAs-induced ER stress and suggest that crosstalk between these pathways underlie iAs-induced hepatic toxicity.

A homozygous MRPL24 mutation causes a complex movement disorder and affects the mitoribosome assembly

Mitochondrial ribosomal protein large 24 (MRPL24) is 1 of the 82 protein components of mitochondrial ribosomes, playing an essential role in the mitochondrial translation process. We report here on a baby girl with cerebellar atrophy, choreoathetosis of limbs and face, intellectual disability and a combined defect of complexes I and IV in muscle biopsy, caused by a homozygous missense mutation identified in MRPL24. The variant predicts a Leu91Pro substitution at an evolutionarily conserved site. Using human mutant cells and the zebrafish model, we demonstrated the pathological role of the identified variant. In fact, in fibroblasts we observed a significant reduction of MRPL24 protein and of mitochondrial respiratory chain complex I and IV subunits, as well a markedly reduced synthesis of the mtDNA-encoded peptides. In zebrafish we demonstrated that the orthologue gene is expressed in metabolically active tissues, and that gene knockdown induced locomotion impairment, structural defects and low ATP production. The motor phenotype was complemented by human WT but not mutant cRNA. Moreover, sucrose density gradient fractionation showed perturbed assembly of large subunit mitoribosomal proteins, suggesting that the mutation leads to a conformational change in MRPL24, which is expected to cause an aberrant interaction of the protein with other components of the 39S mitoribosomal subunit.

Zebrafish miR-462-731 is required for digestive organ development

MicroRNAs (miRNAs), as important regulators of post-transcriptional gene expression, play important roles in the occurrence and function of organs. In this study, morpholino (MO) knockdown of miR-462/miR-731 was used to investigate the potential mechanisms of the miR-462-731 cluster during zebrafish liver development. The results showed significant reduction of digestive organs, especially liver and exocrine pancreas after the miR-462/miR-731 knockdown, and those phenotypes could be partially rescued by corresponding miRNA duplex. Acinar cells of the exocrine pancreas were also severely affected with pancreatic insufficiency. In particular, knockdown of miR-462 caused pancreas morphogenesis abnormity with specific bilateral exocrine pancreas. Additionally, it was found that miR-731 played a role in liver and exocrine pancreas development by directly targeting dkk3b, instead of the down-regulation of Wnt/β-catenin signaling. These findings contribute significantly to our understanding of molecular mechanisms of miR-462-731 cluster in development of digestive organs.

Tamoxifen-induced hepatotoxicity via lipid accumulation and inflammation in zebrafish

Tamoxifen is a clinical drug for estrogen receptor (ER)-positive breast cancer. Recently, it has been detected in aquatic environment. The residual drugs will produce certain biological activity and create a risk to aquatic organism when they enter the water environment. Therefore, it has great significance to study the ecotoxicity of tamoxifen. In the study, we used zebrafish as a model of aquatic to investigate the ecotoxic mechanism of tamoxifen to aquatic. We found that tamoxifen induced liver lipid accumulation in zebrafish, which showed a significant hepatotoxicity with smaller liver area and bigger yolk area. Though biochemical and pathologic measurement, tamoxifen treated group showed higher transaminase and lipid content. The elevated liver lipid synthesis might due to the increase of lipid metabolism related gene Srebf1, Srebf2 and Fasn. Moreover, inflammatory cytokine Tnf-α, Il-1β And Il-6 were increased. This result confirmed the toxicity of tamoxifen to aquatic, suggested liver injury was the main characteristic of its ecotoxicity. This study indicated it is important to avoid tamoxifen discharging into the aquatic ecology and provided a theoretical basis of prevention tamoxifen-induced ecotoxicity to aquatic.

Impaired Liver Size and Compromised Neurobehavioral Activity are Elicited by Chitosan Nanoparticles in the Zebrafish Embryo Model

The use of chitosan nanoparticles (ChNPs) in various biological and environmental applications is attracting great interest. However, potential side effects related to ChNP toxicity remain the major limitation hampering their wide application. For the first time, we investigate the potential organ-specific (cardiac, hepatic, and neuromuscular) toxicity of ChNPs (size 100–150 nm) using the zebrafish embryo model. Our data highlight the absence of both acute and teratogenic toxic effects of ChNPs (~100% survival rate) even at the higher concentration employed (200 mg/L). Although no single sign of cardiotoxicity was observed upon exposure to 200 mg/L of ChNPs, as judged by heartbeat rate, the corrected QT interval (QTc, which measures the time between the start of the Q wave and the end of the T wave in the heart's electrical cycle), maximum cardiac arrest, and ejection fraction assays, the same dosage elicited the impairment of both liver size (decreased liver size, but without steatosis and lipid yolk retention) and neurobehavioral activity (increased movement under different light conditions). Although the observed toxic effect failed to affect embryo survival, whether a prolonged ChNP treatment may induce other potentially harmful effects remains to be elucidated. By reporting new insights on their organ-specific toxicity, our results add novel and useful information into the available data concerning the in vivo effect of ChNPs.

Construction of a two-photon fluorescent probe for ratiometric imaging of hypochlorous acid in alcohol-induced liver injury

A ratiometric two-photon fluorescent probe for HClO was deliberately constructed to reveal the generation of HClO in alcohol-induced liver injury.

Mesenchymal stromal cells prevent progression of liver fibrosis in a novel zebrafish embryo model

Chronic liver damage leads to the onset of fibrogenesis. Rodent models for liver fibrosis have been widely used, but are less suitable for screening purposes. Therefore the aim of our study was to design a novel model for liver fibrosis in zebrafish embryos, suitable for high throughput screening. Furthermore, we evaluated the efficacy of mesenchymal stromal cells (MSCs) to inhibit the fibrotic process and thereby the applicability of this model to evaluate therapeutic responses. Zebrafish embryos were exposed to TAA or CCL4 and mRNA levels of fibrosis-related genes (Collagen-1α1, Hand-2, and Acta-2) and tissue damage-related genes (TGF-β and SDF-1a, SDF-1b) were determined, while Sirius-red staining was used to estimate collagen deposition. Three days after start of TAA exposure, MSCs were injected after which the fibrotic response was determined. In contrast to CCL4, TAA resulted in an upregulation of the fibrosis-related genes, increased extracellular matrix deposition and decreased liver sizes suggesting the onset of fibrosis. The applicability of this model to evaluate therapeutic responses was shown by local treatment with MSCs which resulted in decreased expression of the fibrosis-related RNA markers. In conclusion, TAA induces liver fibrosis in zebrafish embryos, thereby providing a promising model for future mechanistic and therapeutic studies.

Liver-directed microRNA-7a depletion induces nonalcoholic fatty liver disease by stabilizing YY1-mediated lipogenic pathways in zebrafish

Nonalcoholic fatty liver disease (NAFLD) has been associated with the function and changes in expression levels of microRNAs (miRs). MiR-7 has been proven to play an important role in many cellular processes; however, its functions in the context of liver lipogenesis remain unknown. We applied the microRNA-sponge (miR-SP) technology and generated transgenic miR-7a-SP models (hC7aSP and bC7aSP), which disrupted the activities of hepatic miR-7a and induced the early onset of NAFLD and nonalcoholic steatohepatitis (NASH) in zebrafish. We identified a novel miR-7a target, YY1, and demonstrated novel miR-7a functions to regulate zebrafish hepatic lipid metabolism by controlling YY1 stabilization through the regulation of the expression of lipogenic signaling pathways. Correspondingly, liver specific miR-7a depletion functionally promoted lipid accumulation in hC7ASP livers. NASH hC7aSP increased the expression of inflammatory genes (il-1b, il-6, tnf-α, ifn-γ, nfkb2, and NF-kB) and endoplasmic reticulum stress markers (atf6, ern2, ire1, perk, hspa5 and ddit3). Molecular analysis revealed that miR-7a-SP can stabilize YY1 expression and contribute to the accumulation of hepatic triglycerides by reducing the CHOP-10 expression in the hC7aSP and then inducing the transactivation of C/EBP-α and PPAR-γ expression. PPAR-γ antagonists and miR-7a mimic treatment ameliorate hC7aSP NASH phenotypes.

CONCLUSION

Our results suggest that miR-7a-SP acts as a lipid enhancer by directly increasing YY1 stability to disrupt CHOP-10-dependent suppression of lipogenic pathways, resulting in increased lipid accumulation. MiR-7a expression improves liver steatosis and steatohepatitis in hC7aSPs, which suggests a novel strategy for the prevention and early treatment of NASH in humans.

Inorganic arsenic causes fatty liver and interacts with ethanol to cause alcoholic liver disease in zebrafish

The rapid increase in fatty liver disease (FLD) incidence is attributed largely to genetic and lifestyle factors; however, environmental toxicants are a frequently overlooked factor that can modify the effects of more common causes of FLD. Chronic exposure to inorganic arsenic (iAs) is associated with liver disease in humans and animal models, but neither the mechanism of action nor the combinatorial interaction with other disease-causing factors has been fully investigated. Here, we examined the contribution of iAs to FLD using zebrafish and tested the interaction with ethanol to cause alcoholic liver disease (ALD). We report that zebrafish exposed to iAs throughout development developed specific phenotypes beginning at 4 days post-fertilization (dpf), including the development of FLD in over 50% of larvae by 5 dpf. Comparative transcriptomic analysis of livers from larvae exposed to either iAs or ethanol revealed the oxidative stress response and the unfolded protein response (UPR) caused by endoplasmic reticulum (ER) stress as common pathways in both these models of FLD, suggesting that they target similar cellular processes. This was confirmed by our finding that arsenic is synthetically lethal with both ethanol and a well-characterized ER-stress-inducing agent (tunicamycin), suggesting that these exposures work together through UPR activation to cause iAs toxicity. Most significantly, combined exposure to sub-toxic concentrations of iAs and ethanol potentiated the expression of UPR-associated genes, cooperated to induce FLD, reduced the expression of as3mt, which encodes an arsenic-metabolizing enzyme, and significantly increased the concentration of iAs in the liver. This demonstrates that iAs exposure is sufficient to cause FLD and that low doses of iAs can potentiate the effects of ethanol to cause liver disease.

This article has an associated First Person interview with the first author of the paper.

Summary: Using zebrafish, the authors show that exposure to a common environmental contaminant, inorganic arsenic, increases the risk of alcoholic liver disease.

Dexamethasone-induced hepatomegaly and steatosis in larval zebrafish

Fish hepatobiliary syndrome, characterized by hepatomegaly and fatty liver, has been frequently reported in many cultured fish species and has caused a dramatic economic loss in China. Glucocorticoids are thought to be important non-nutritional factors for hepatomegaly and fatty liver development. In the present study, a dexamethasone-induced zebrafish model of fatty liver and hepatomegaly was established, and the role of glucocorticoid receptor (GR) in the development of hepatomegaly and fatty liver was investigated using developing zebrafish. Exposure of larval zebrafish at 5 days post fertilization (dpf) to dexamethasone for 24 hr caused significant increases of liver size and number of fish with hepatic steatosis at 6 dpf. The increase of liver size caused by dexamethasone was significantly reversed by treatment with RU486, a GR antagonist, and by gene knock-down with a morpholino against the GR. The dexamethasone-induced hepatic steatosis was also inhibited by treatment with RU486. Overall, the results highlight larval zebrafish as a useful model for stress-induced liver failure.

Effects of chronic high stocking density on liver proteome of rainbow trout (Oncorhynchus mykiss)

The main aim of the present study was to assess the effects of chronic high stocking density on liver proteome of rainbow trout. Rainbow trout juveniles (42.6 ± 2.3 g average body weight) were randomly distributed into six tanks at two stocking densities (low stocking density (LD) = 20 kg m^-3 and high stocking density (HD) = 80 kg m^-3). Both treatments were performed in triplicate tanks for a period of 60 days. High stocking density caused a reduction in the growth performance compared with LD fish. Lysozyme activity increased with stocking density, while serum complement activity presented the opposite pattern. Serum cortisol and total protein levels did not show significant differences (P > 0.05) between experimental groups. The fish reared at high stocking density showed significantly lower osmolality and globulin values but higher albumin level. The HD group had significantly higher activities of catalase, glutathione peroxidase and superoxide dismutase, and malondialdehyde content in the liver when compared to the LD group. Comparative proteomics was used to determine the proteomic responses in livers of rainbow trout reared at high stocking density for 60 days. Out of nine protein spots showing altered abundance (>1.5-folds, P < 0.05), eight spots were successfully identified. Two proteins including apolipoprotein A-I-2 precursor and mitochondrial stress-70 protein were found to increase in HD group. The spots found to decrease in the HD group were identified as follows: 2-peptidylprolyl isomerase A, two isoforms of glyceraldehydes-3-phosphate dehydrogenase, an unnamed protein product similar to fructose-bisphosphate aldolase, 78 kDa glucose-regulated protein, and serum albumin 1 protein.

Naringenin inhibits alcoholic injury by improving lipid metabolism and reducing apoptosis in zebrafish larvae

Alcoholic liver disease (ALD) includes a spectrum of hepatic abnormalities that range from isolated alcoholic steatosis to steatohepatitis and cirrhosis. Naringenin, a predominant flavanone in grapefruit, increases resistance to oxidative stress and inflammation and protects against multiple organ injury in various animal models. However, the specific mechanisms responsible for protection against alcoholic injury are poorly understood. In the present study, we aimed to investigate the effect of naringenin on alcoholic events and the molecular regulatory mechanisms of naringenin in the liver and whole body of zebrafish larvae following exposure to 350 mmol/l ethanol for 32 h. Zebrafish larvae {4 days post‑fertilization (dpf); wild-type (WT) and a transgenic line with liver-specific eGFP expression [Tg(lfabp10α-eGFP)]} were used to establish an alcoholic fatty liver model in order to evaluate the effects of naringenin treatment on anti-alcoholic injury. Naringenin significantly reduced alcoholic liver morphological phenotypes and the expression of alcohol and lipid metabolism-related genes, including cyp2y3, cyp3a65, hmgcra, hmgcrb, fasn, fabp10α, fads2 and echs1, in zebrafish larvae. Naringenin also attenuated hepatic apoptosis in larvae as detected by TUNEL staining, consistent with the expression of critical biomarkers of endoplasmic reticulum stress and of DNA damage genes (chop, gadd45αa and edem1). The present study showed that naringenin inhibited alcohol-induced liver steatosis and injury in zebrafish larvae by reducing apoptosis and DNA damage and by harmonizing alcohol and lipid metabolism.

MPI depletion enhances O-GlcNAcylation of p53 and suppresses the Warburg effect

Rapid cellular proliferation in early development and cancer depends on glucose metabolism to fuel macromolecule biosynthesis. Metabolic enzymes are presumed regulators of this glycolysis-driven metabolic program, known as the Warburg effect; however, few have been identified. We uncover a previously unappreciated role for Mannose phosphate isomerase (MPI) as a metabolic enzyme required to maintain Warburg metabolism in zebrafish embryos and in both primary and malignant mammalian cells. The functional consequences of MPI loss are striking: glycolysis is blocked and cells die. These phenotypes are caused by induction of p53 and accumulation of the glycolytic intermediate fructose 6-phosphate, leading to engagement of the hexosamine biosynthetic pathway (HBP), increased O-GlcNAcylation, and p53 stabilization. Inhibiting the HBP through genetic and chemical methods reverses p53 stabilization and rescues the Mpi-deficient phenotype. This work provides mechanistic evidence by which MPI loss induces p53, and identifies MPI as a novel regulator of p53 and Warburg metabolism.

DOI:http://dx.doi.org/10.7554/eLife.22477.001

Histological Analyses of Acute Alcoholic Liver Injury in Zebrafish

Alcoholic Liver Disease (ALD) refers to damage to the liver due to acute or chronic alcohol abuse. It is among the leading causes of alcohol-related morbidity and mortality and affects more than 2 million people in the United States. A better understanding of the cellular and molecular mechanisms underlying alcohol-induced liver injury is crucial for developing effective treatment for ALD. Zebrafish larvae exhibit hepatic steatosis and fibrogenesis after just 24 h of exposure to 2% ethanol, making them useful for the study of acute alcoholic liver injury. This work describes the procedure for acute ethanol treatment in zebrafish larvae and shows that it causes steatosis and swelling of the hepatic blood vessels. A detailed protocol for Hematoxylin and Eosin (H&E) staining that is optimized for the histological analysis of the zebrafish larval liver, is also described. H&E staining has several unique advantages over immunofluorescence, as it marks all liver cells and extracellular components simultaneously and can readily detect hepatic injury, such as steatosis and fibrosis. Given the increasing usage of zebrafish in modeling toxin and virus-induced liver injury, as well as inherited liver diseases, this protocol serves as a reference for the histological analyses performed in all these studies.

Hesperidin Protects against Acute Alcoholic Injury through Improving Lipid Metabolism and Cell Damage in Zebrafish Larvae

Alcoholic liver disease (ALD) is a series of abnormalities of liver function, including alcoholic steatosis, steatohepatitis, and cirrhosis. Hesperidin, the major constituent of flavanone in grapefruit, is proved to play a role in antioxidation, anti-inflammation, and reducing multiple organs damage in various animal experiments. However, the underlying mechanism of resistance to alcoholic liver injury is still unclear. Thus, we aimed to investigate the protective effects of hesperidin against ALD and its molecular mechanism in this study. We established an ALD zebrafish larvae model induced by 350 mM ethanol for 32 hours, using wild-type and transgenic line with liver-specific eGFP expression Tg (lfabp10α:eGFP) zebrafish larvae (4 dpf). The results revealed that hesperidin dramatically reduced the hepatic morphological damage and the expressions of alcohol and lipid metabolism related genes, including cyp2y3, cyp3a65, hmgcra, hmgcrb, fasn, and fads2 compared with ALD model. Moreover, the findings demonstrated that hesperidin alleviated hepatic damage as well, which is reflected by the expressions of endoplasmic reticulum stress and DNA damage related genes (chop, gadd45αa, and edem1). In conclusion, this study revealed that hesperidin can inhibit alcoholic damage to liver of zebrafish larvae by reducing endoplasmic reticulum stress and DNA damage, regulating alcohol and lipid metabolism.

Using zebrafish to model liver diseases-Where do we stand?

Purpose of Review

The liver is the largest internal organ and performs both exocrine and endocrine function that is necessary for survival. Liver failure is among the leading causes of death and represents a major global health burden. Liver transplantation is the only effective treatment for end-stage liver diseases. Animal models advance our understanding of liver disease etiology and hold promise for the development of alternative therapies. Zebrafish has become an increasingly popular system for modeling liver diseases and complements the rodent models.

Recent Findings

The zebrafish liver contains main cell types that are found in mammalian liver and exhibits similar pathogenic responses to environmental insults and genetic mutations. Zebrafish have been used to model neonatal cholestasis, cholangiopathies, such as polycystic liver disease, alcoholic liver disease, and non-alcoholic fatty liver disease. It also provides a unique opportunity to study the plasticity of liver parenchymal cells during regeneration.

Summary

In this review, we summarize the recent work of building zebrafish models of liver diseases. We highlight how these studies have brought new knowledge of disease mechanisms. We also discuss the advantages and challenges of using zebrafish to model liver diseases.

ZeGlobalTox: An Innovative Approach to Address Organ Drug Toxicity Using Zebrafish

Toxicity is one of the major attrition causes during the drug development process. In that line, cardio-, neuro-, and hepatotoxicities are among the main reasons behind the retirement of drugs in clinical phases and post market withdrawal. Zebrafish exploitation in high-throughput drug screening is becoming an important tool to assess the toxicity and efficacy of novel drugs. This animal model has, from early developmental stages, fully functional organs from a physiological point of view. Thus, drug-induced organ-toxicity can be detected in larval stages, allowing a high predictive power on possible human drug-induced liabilities. Hence, zebrafish can bridge the gap between preclinical in vitro safety assays and rodent models in a fast and cost-effective manner. ZeGlobalTox is an innovative assay that sequentially integrates in vivo cardio-, neuro-, and hepatotoxicity assessment in the same animal, thus impacting strongly in the 3Rs principles. It Reduces, by up to a third, the number of animals required to assess toxicity in those organs. It Refines the drug toxicity evaluation through novel physiological parameters. Finally, it might allow the Replacement of classical species, such as rodents and larger mammals, thanks to its high predictivity (Specificity: 89%, Sensitivity: 68% and Accuracy: 78%).

Making It New Again: Insight Into Liver Development, Regeneration, and Disease From Zebrafish Research

The adult liver of most vertebrates is predominantly comprised of hepatocytes. However, these cells must work in concert with biliary, stellate, vascular, and immune cells to accomplish the vast array of hepatic functions required for physiological homeostasis. Our understanding of liver development was accelerated as zebrafish emerged as an ideal vertebrate system to study embryogenesis. Through work in zebrafish and other models, it is now clear that the cells in the liver develop in a coordinated fashion during embryogenesis through a complex yet incompletely understood set of molecular guidelines. Zebrafish research has uncovered many key players that govern the acquisition of hepatic potential, cell fate, and plasticity. Although rare, some hepatobiliary diseases-especially biliary atresia-are caused by developmental defects; we discuss how research using zebrafish to study liver development has informed our understanding of and approaches to liver disease. The liver can be injured in response to an array of stressors including viral, mechanical/surgical, toxin-induced, immune-mediated, or inborn defects in metabolism. The liver has thus evolved the capacity to efficiently repair and regenerate. We discuss the emerging field of using zebrafish to study liver regeneration and highlight recent advances where zebrafish genetics and imaging approaches have provided novel insights into how cell plasticity contributes to liver regeneration.

Inhibition of vascular endothelial growth factor signaling facilitates liver repair from acute ethanol-induced injury in zebrafish

Alcoholic liver disease (ALD) results from alcohol overconsumption and is among the leading causes of liver-related morbidity and mortality worldwide. Elevated expression of vascular endothelial growth factor (VEGF) and its receptors has been observed in ALD, but how it contributes to ALD pathophysiology is unclear. Here, we investigated the impact of VEGF signaling inhibition on an established zebrafish model of acute alcoholic liver injury. Kdrl activity was blocked by chemical inhibitor treatment or by genetic mutation. Exposing 4-day-old zebrafish larvae to 2% ethanol for 24 h induced hepatic steatosis, angiogenesis and fibrogenesis. The liver started self-repair once ethanol was removed. Although inhibiting Kdrl did not block the initial activation of hepatic stellate cells during ethanol treatment, it suppressed their proliferation, extracellular matrix protein deposition and fibrogenic gene expression after ethanol exposure, thus enhancing the liver repair. It also ameliorated hepatic steatosis and attenuated hepatic angiogenesis that accelerated after the ethanol treatment. qPCR showed that hepatic stellate cells are the first liver cell type to increase the expression of VEGF ligand and receptor genes in response to ethanol exposure. Both hepatic stellate cells and endothelial cells, but not hepatic parenchymal cells, expressed kdrl upon ethanol exposure and were likely the direct targets of Kdrl inhibition. Ethanol-induced steatosis and fibrogenesis still occurred in cloche mutants that have hepatic stellate cells but lack hepatic endothelial cells, and Kdrl inhibition suppressed both phenotypes in the mutants. These results suggest that VEGF signaling mediates interactions between activated hepatic stellate cells and hepatocytes that lead to steatosis. Our study demonstrates the involvement of VEGF signaling in regulating sustained liver injuries after acute alcohol exposure. It also provides a proof of principle of using the zebrafish model to identify molecular targets for developing ALD therapies.

Endoderm specification and liver development

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

trappc11 is required for protein glycosylation in zebrafish and humans

Activation of the unfolded protein response (UPR) can be either adaptive or pathological. We term the pathological UPR that causes fatty liver disease a "stressed UPR." Here we investigate the mechanism of stressed UPR activation in zebrafish bearing a mutation in thetrappc11gene, which encodes a component of the transport protein particle (TRAPP) complex.trappc11mutants are characterized by secretory pathway defects, reflecting disruption of the TRAPP complex. In addition, we uncover a defect in protein glycosylation intrappc11mutants that is associated with reduced levels of lipid-linked oligosaccharides (LLOs) and compensatory up-regulation of genes in the terpenoid biosynthetic pathway that produces the LLO anchor dolichol. Treating wild-type larvae with terpenoid or LLO synthesis inhibitors phenocopies the stressed UPR seen intrappc11mutants and is synthetically lethal withtrappc11mutation. We propose that reduced LLO level causing hypoglycosylation is a mechanism of stressed UPR induction intrappc11mutants. Of importance, in human cells, depletion of TRAPPC11, but not other TRAPP components, causes protein hypoglycosylation, and lipid droplets accumulate in fibroblasts from patients with theTRAPPC11mutation. These data point to a previously unanticipated and conserved role for TRAPPC11 in LLO biosynthesis and protein glycosylation in addition to its established function in vesicle trafficking.

High fat plus high cholesterol diet lead to hepatic steatosis in zebrafish larvae: a novel model for screening anti-hepatic steatosis drugs

Background

Non-alcoholic fatty liver disease (NAFLD), characterized as excessive lipid accumulation within hepatocytes, is growing in prevalence. The exploitation of effective drugs for NAFLD has been proven challenging. Herein, we aimed to establish a dietary model of hepatic steatosis using transparent zebrafish larvae in which high-throughput chemical screens could be conducted.

Methods

Zebrafish larvae fed with high fat (HF) diet and high fat plus high cholesterol (HFC) diet were compared to the control. We analyzed intrahepatic lipid accumulation, biological indexes and various pathways including lipid metabolism, ER stress and inflammation. In addition, the effects of ezetimibe and simvastatin on HFC diet-induced steatosis were evaluated.

Results

Zebrafish larvae fed with HF and HFC diets developed steatosis for 7 and 10 days. The incidence and degree of steatosis were more severe in HFC diet-fed larvae compared with the control and HF diet-fed larvae, suggesting that adding cholesterol to the HF diet promotes the hepatic lipid accumulation. These data were confirmed by the pathological observation. Biological indexes, free cholesterol (FC), total cholesterol (TC) and triacylglycerol (TG) were elevated in the liver of HFC diet-fed larvae compared with the control and HF diet-fed larvae. Additionally, the expression levels of endoplasmic reticulum (ER) stress and lipolytic molecules (atf6, hspa5, hsp90b1, pparab, cpt1a and acox3) were significantly up-regulated in the liver of HF and HFC diets-fed larvae compared to the control, whereas the expression of lipogenic molecules (acaca, fasn, srebf2, hmgcs1 and hmgcra) were decreased in the liver of HF and HFC diets-fed larvae compared to the control. To validate the reliability of the HFC model and utility value for screening potential anti-steaotsis drugs, HFC-fed larvae were treated with two accepted lipid-lowing drugs (ezetimibe and simvastatin). The results showed that these drugs significantly ameliorated HFC-induced steatosis.

Conclusion

Our results demonstrate that the zebrafish larvae steatosis model established and validated in this study could be used for in vivo steatosis studies and drug screening.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-015-0036-z) contains supplementary material, which is available to authorized users.

Zebrafish: an important tool for liver disease research

As the incidence of hepatobiliary diseases increases, we must improve our understanding of the molecular, cellular, and physiological factors that contribute to the pathogenesis of liver disease. Animal models help us identify disease mechanisms that might be targeted therapeutically. Zebrafish (Danio rerio) have traditionally been used to study embryonic development but are also important to the study of liver disease. Zebrafish embryos develop rapidly; all of their digestive organs are mature in larvae by 5 days of age. At this stage, they can develop hepatobiliary diseases caused by developmental defects or toxin- or ethanol-induced injury and manifest premalignant changes within weeks. Zebrafish are similar to humans in hepatic cellular composition, function, signaling, and response to injury as well as the cellular processes that mediate liver diseases. Genes are highly conserved between humans and zebrafish, making them a useful system to study the basic mechanisms of liver disease. We can perform genetic screens to identify novel genes involved in specific disease processes and chemical screens to identify pathways and compounds that act on specific processes. We review how studies of zebrafish have advanced our understanding of inherited and acquired liver diseases as well as liver cancer and regeneration.

Strategies, models and biomarkers in experimental non-alcoholic fatty liver disease research

Non-alcoholic fatty liver disease encompasses a spectrum of liver diseases, including simple steatosis, steatohepatitis, liver fibrosis and cirrhosis and hepatocellular carcinoma. Non-alcoholic fatty liver disease is currently the most dominant chronic liver disease in Western countries due to the fact that hepatic steatosis is associated with insulin resistance, type 2 diabetes mellitus, obesity, metabolic syndrome and drug-induced injury. A variety of chemicals, mainly drugs, and diets is known to cause hepatic steatosis in humans and rodents. Experimental non-alcoholic fatty liver disease models rely on the application of a diet or the administration of drugs to laboratory animals or the exposure of hepatic cell lines to these drugs. More recently, genetically modified rodents or zebrafish have been introduced as non-alcoholic fatty liver disease models. Considerable interest now lies in the discovery and development of novel non-invasive biomarkers of non-alcoholic fatty liver disease, with specific focus on hepatic steatosis. Experimental diagnostic biomarkers of non-alcoholic fatty liver disease, such as (epi)genetic parameters and '-omics'-based read-outs are still in their infancy, but show great promise. In this paper, the array of tools and models for the study of liver steatosis is discussed. Furthermore, the current state-of-art regarding experimental biomarkers such as epigenetic, genetic, transcriptomic, proteomic and metabonomic biomarkers will be reviewed.

Development of an Animal Model for Alcoholic Liver Disease in Zebrafish

Alcoholic liver disease (ALD) continues to be a major cause of liver-related morbidity and mortality worldwide. To date, no zebrafish animal model has demonstrated the characteristic manifestations of ALD in the setting of chronic alcohol exposure. The aim of this study was to develop a zebrafish animal model for ALD. Male adult zebrafish were housed in a 1% (v/v) ethanol solution up to 3 months. A histopathological study showed the characteristic features of alcoholic liver steatosis and steatohepatitis in the early stages of alcohol exposure, including fat droplet accumulation, ballooning degeneration of the hepatocytes, and Mallory body formation. As the exposure time increased, collagen deposition in the extracellular matrix was observed by Sirius red staining and immunofluorescence staining. Finally, anaplastic hepatocytes with pleomorphic nuclei were arranged in trabecular patterns and formed nodules in the zebrafish liver. Over the time course of 1% ethanol exposure, upregulations of lipogenesis, fibrosis, and tumor-related genes were also revealed by semiquantitative and quantitative real-time reverse transcription-polymerase chain reaction. As these data reflect characteristic liver damage by alcohol in humans, this zebrafish animal model may serve as a powerful tool to study the pathogenesis and treatment of ALD and its related disorders in humans.

The lure of zebrafish in liver research: regulation of hepatic growth in development and regeneration

The liver is an essential organ that plays a pivotal role in metabolism, digestion and nutrient storage. Major efforts have been made to develop zebrafish (Danio rerio) as a model system to study the pathways regulating hepatic growth during liver development and regeneration. Zebrafish offer unique advantages over other vertebrates including in vivo imaging at cellular resolution and the capacity for large-scale chemical and genetic screens. Here, we review the cellular and molecular mechanisms that regulate hepatic growth during liver development in zebrafish. We also highlight emerging evidence that developmental pathways are reactivated following liver injury to facilitate regeneration. Finally, we discuss how zebrafish have transformed drug discovery efforts and enabled the identification of drugs that stimulate hepatic growth and provide hepatoprotection in pre-clinical models of liver injury, with the ultimate goal of identifying novel therapeutic approaches to treat liver disease.

Molecularly defined unfolded protein response subclasses have distinct correlations with fatty liver disease in zebrafish

The unfolded protein response (UPR) is a complex network of sensors and target genes that ensure efficient folding of secretory proteins in the endoplasmic reticulum (ER). UPR activation is mediated by three main sensors, which regulate the expression of hundreds of targets. UPR activation can result in outcomes ranging from enhanced cellular function to cell dysfunction and cell death. How this pathway causes such different outcomes is unknown. Fatty liver disease (steatosis) is associated with markers of UPR activation and robust UPR induction can cause steatosis; however, in other cases, UPR activation can protect against this disease. By assessing the magnitude of activation of UPR sensors and target genes in the liver of zebrafish larvae exposed to three commonly used ER stressors (tunicamycin, thapsigargin and Brefeldin A), we have identified distinct combinations of UPR sensors and targets (i.e. subclasses) activated by each stressor. We found that only the UPR subclass characterized by maximal induction of UPR target genes, which we term a stressed-UPR, induced steatosis. Principal component analysis demonstrated a significant positive association between UPR target gene induction and steatosis. The same principal component analysis showed significant correlation with steatosis in samples from patients with fatty liver disease. We demonstrate that an adaptive UPR induced by a short exposure to thapsigargin prior to challenging with tunicamycin reduced both the induction of a stressed UPR and steatosis incidence. We conclude that a stressed UPR causes steatosis and an adaptive UPR prevents it, demonstrating that this pathway plays dichotomous roles in fatty liver disease.

Activating transcription factor 6 is necessary and sufficient for alcoholic fatty liver disease in zebrafish

Fatty liver disease (FLD) is characterized by lipid accumulation in hepatocytes and is accompanied by secretory pathway dysfunction, resulting in induction of the unfolded protein response (UPR). Activating transcription factor 6 (ATF6), one of three main UPR sensors, functions to both promote FLD during acute stress and reduce FLD during chronic stress. There is little mechanistic understanding of how ATF6, or any other UPR factor, regulates hepatic lipid metabolism to cause disease. We addressed this using zebrafish genetics and biochemical analyses and demonstrate that Atf6 is necessary and sufficient for FLD. atf6 transcription is significantly upregulated in the liver of zebrafish with alcoholic FLD and morpholino-mediated atf6 depletion significantly reduced steatosis incidence caused by alcohol. Moreover, overexpression of active, nuclear Atf6 (nAtf6) in hepatocytes caused FLD in the absence of stress. mRNA-Seq and qPCR analyses of livers from five day old nAtf6 transgenic larvae revealed upregulation of genes promoting glyceroneogenesis and fatty acid elongation, including fatty acid synthase (fasn), and nAtf6 overexpression in both zebrafish larvae and human hepatoma cells increased the incorporation of ¹⁴C-acetate into lipids. Srebp transcription factors are key regulators of lipogenic enzymes, but reducing Srebp activation by scap morpholino injection neither prevented FLD in nAtf6 transgenics nor synergized with atf6 knockdown to reduce alcohol-induced FLD. In contrast, fasn morpholino injection reduced FLD in nAtf6 transgenic larvae and synergistically interacted with atf6 to reduce alcoholic FLD. Thus, our data demonstrate that Atf6 is required for alcoholic FLD and epistatically interacts with fasn to cause this disease, suggesting triglyceride biogenesis as the mechanism of UPR induced FLD.

Fatty liver disease (steatosis) is the most common liver disease worldwide and is commonly caused by obesity, type 2 diabetes, or alcohol abuse. All of these conditions are associated with impaired hepatocyte protein secretion, resulting in hypoproteinemia that contributes to the systemic complications of these diseases. The unfolded protein response (UPR) is activated in response to stress in the protein secretory pathway and a wealth of data indicates that UPR activation can contribute to steatosis, but the mechanistic basis for this relationship is poorly understood. We identify activating transcription factor 6 (Atf6), one of three UPR sensors, as necessary and sufficient for steatosis and show that Atf6 activation can promote lipogenesis, providing a direct connection between the stress response and lipid metabolism. Blocking Atf6 in zebrafish larvae prevents alcohol-induced steatosis and Atf6 overexpression in zebrafish hepatocytes induces genes that drive lipogenesis, increases lipid production and causes steatosis. Fatty acid synthase (fasn) is a key lipogenic enzyme and we show that fasn is required for fatty liver in response to both ethanol and Atf6 overexpression. Our findings point to Atf6 as a potential therapeutic target for fatty liver disease.

In vivo cell biology in zebrafish - providing insights into vertebrate development and disease

Over the past decades, studies using zebrafish have significantly advanced our understanding of the cellular basis for development and human diseases. Zebrafish have rapidly developing transparent embryos that allow comprehensive imaging of embryogenesis combined with powerful genetic approaches. However, forward genetic screens in zebrafish have generated unanticipated findings that are mirrored by human genetic studies: disruption of genes implicated in basic cellular processes, such as protein secretion or cytoskeletal dynamics, causes discrete developmental or disease phenotypes. This is surprising because many processes that were assumed to be fundamental to the function and survival of all cell types appear instead to be regulated by cell-specific mechanisms. Such discoveries are facilitated by experiments in whole animals, where zebrafish provides an ideal model for visualization and manipulation of organelles and cellular processes in a live vertebrate. Here, we review well-characterized mutants and newly developed tools that underscore this notion. We focus on the secretory pathway and microtubule-based trafficking as illustrative examples of how studying cell biology in vivo using zebrafish has broadened our understanding of the role fundamental cellular processes play in embryogenesis and disease.

Ethanol metabolism and oxidative stress are required for unfolded protein response activation and steatosis in zebrafish with alcoholic liver disease

Secretory pathway dysfunction and lipid accumulation (steatosis) are the two most common responses of hepatocytes to ethanol exposure and are major factors in the pathophysiology of alcoholic liver disease (ALD). However, the mechanisms by which ethanol elicits these cellular responses are not fully understood. Recent data indicates that activation of the unfolded protein response (UPR) in response to secretory pathway dysfunction can cause steatosis. Here, we examined the relationship between alcohol metabolism, oxidative stress, secretory pathway stress and steatosis using zebrafish larvae. We found that ethanol was immediately internalized and metabolized by larvae, such that the internal ethanol concentration in 4-day-old larvae equilibrated to 160 mM after 1 hour of exposure to 350 mM ethanol, with an average ethanol metabolism rate of 56 μmol/larva/hour over 32 hours. Blocking alcohol dehydrogenase 1 (Adh1) and cytochrome P450 2E1 (Cyp2e1), the major enzymes that metabolize ethanol, prevented alcohol-induced steatosis and reduced induction of the UPR in the liver. Thus, we conclude that ethanol metabolism causes ALD in zebrafish. Oxidative stress generated by Cyp2e1-mediated ethanol metabolism is proposed to be a major culprit in ALD pathology. We found that production of reactive oxygen species (ROS) increased in larvae exposed to ethanol, whereas inhibition of the zebrafish CYP2E1 homolog or administration of antioxidants reduced ROS levels. Importantly, these treatments also blocked ethanol-induced steatosis and reduced UPR activation, whereas hydrogen peroxide (H₂O₂) acted as a pro-oxidant that synergized with low doses of ethanol to induce the UPR. Collectively, these data demonstrate that ethanol metabolism and oxidative stress are conserved mechanisms required for the development of steatosis and hepatic dysfunction in ALD, and that these processes contribute to ethanol-induced UPR activation and secretory pathway stress in hepatocytes.