Zebrafish models for cancer

Hooked! Modeling human disease in zebrafish

Zebrafish have been widely used as a model system for studying developmental processes, but in the last decade, they have also emerged as a valuable system for modeling human disease. The development and function of zebrafish organs are strikingly similar to those of humans, and the ease of creating mutant or transgenic fish has facilitated the generation of disease models. Here, we highlight the use of zebrafish for defining disease pathways and for discovering new therapies.

Vasa-Like DEAD-Box RNA Helicases of Schistosoma mansoni

Genome sequences are available for the human blood flukes, Schistosoma japonicum, S. mansoni and S. haematobium. Functional genomic approaches could aid in identifying the role and importance of these newly described schistosome genes. Transgenesis is established for functional genomics in model species, which can lead to gain- or loss-of-functions, facilitate vector-based RNA interference, and represents an effective forward genetics tool for insertional mutagenesis screens. Progress toward routine transgenesis in schistosomes might be expedited if germ cells could be reliably localized in cultured schistosomes. Vasa, a member of the ATP-dependent DEAD-box RNA helicase family, is a prototypic marker of primordial germ cells and the germ line in the Metazoa. Using bioinformatics, 33 putative DEAD-box RNA helicases exhibiting conserved motifs that characterize helicases of this family were identified in the S. mansoni genome. Moreover, three of the helicases exhibited vasa-like sequences; phylogenetic analysis confirmed the three vasa-like genes-termed Smvlg1, Smvlg2, and Smvlg3-were members of the Vasa/PL10 DEAD-box subfamily. Transcripts encoding Smvlg1, Smvlg2, and Smvlg3 were cloned from cDNAs from mixed sex adult worms, and quantitative real time PCR revealed their presence in developmental stages of S. mansoni with elevated expression in sporocysts, adult females, eggs, and miracidia, with strikingly high expression in the undeveloped egg. Whole mount in situ hybridization (WISH) analysis revealed that Smvlg1, Smvlg2 and Smvlg3 were transcribed in the posterior ovary where the oocytes mature. Germ cell specific expression of schistosome vasa-like genes should provide an informative landmark for germ line transgenesis of schistosomes, etiologic agents of major neglected tropical diseases.

A novel zebrafish human tumor xenograft model validated for anti-cancer drug screening

The development of a relatively simple, reliant and cost-effective animal test will greatly facilitate drug development. In this study, our goal was the establishment of a rapid, simple, sensitive and reproducible zebrafish xenograft model for anti-cancer drug screening. We optimized the conditions for the cancer cell xenograft in terms of injected cell numbers, incubation temperature and time. A range of human carcinoma cell types were stained with a fluorescent dye prior to injection into the fish larvae. Subsequent cancer cell dissemination was observed under fluorescent microscopy. Differences in injected cell numbers were reflected in the rate of dissemination from the xenograft site. Paclitaxel, known as a microtubule stabilizer, dose-dependently inhibited cancer cell dissemination in our zebrafish xenograft model. An anti-migratory drug, LY294002 (phosphatidylinositol 3-kinase inhibitor) also decreased the cancer cell dissemination. Chemical modifications to increase cancer drug pharmacokinetics, such as increased solubility (17-DMAG compared to geldanamycin) could also be assessed in our xenograft model. In addition to testing our new model using known anti-cancer drugs, we carried out further validation by screening a tagged triazine library. Two novel anti-cancer drug candidates were discovered. Therefore, our zebrafish xenograft model provides a vertebrate animal system for the rapid screening and pre-clinical testing of novel anti-cancer agents, prior to the requirement for testing in mammals. Our model system should greatly facilitate drug development for cancer therapy because of its speed, simplicity and reproducibility.

Generation of transgenic zebrafish with liver-specific expression of EGFP-Lc3: a new in vivo model for investigation of liver autophagy

Transgenic expression of GFP-Lc3 is a useful tool for an in vivo model to monitor the formation of autophagosomes during the autophagy process. So far, two transgenic animals (mice and zebrafish) with expression of GFP-Lc3 have been reported. Liver is one of the most important organs for autophagy research. Here, we generated a transgenic zebrafish line with liver-specific EGFP-Lc3 expression. By exposing transgenic larvae to the autophagy inducer, Torin1, we observed a substantial increase in the number of EGFP-Lc3 puncta in the liver as well as the increase of Lc3-II protein. Notably, addition of a chloroquine (CQ) led to further increase of EGFP-Lc3 puncta in liver cells due to the blockage of lysosomal function and degradation stage of autophagy. Thus, the newly established transgenic line will be a useful in vivo model to investigate liver autophagy, and, in particular, the involvement of autophagy in basic biology and diseases in the liver.

Screening of anti-cancer agent using zebrafish: comparison with the MTT assay

The MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide) assay is a classical method for screening cytotoxic anti-cancer agents. Candidate drugs from the MTT assay need in vivo models to test their efficiency and to assess the absorption, distribution, metabolism, excretion, and toxicity of the drugs. An in vivo screening model could increase the rate of development of anti-cancer drugs. Here, we used zebrafish to screen a library of 502 natural compounds and compared the results with those from an MTT assay of the MCF7 breast cancer cell line. We identified 59 toxic compounds in the zebrafish screen, 21 of which were also identified by the MTT assay, and 28 of which were already known for their anti-cancer and apoptosis-inducing effects. These compounds induced apoptosis and activated the p53 pathway in zebrafish within 3h treatment. Our results indicate that zebrafish is a simple, reliable and highly efficient in vivo tool for cancer drug screening, and could complement the MTT assay.

EuFishBioMed (COST Action BM0804): a European network to promote the use of small fishes in biomedical research

Small fresh water fishes such as the zebrafish (Danio rerio) have become important model organisms for biomedical research. They currently represent the best vertebrate embryo models in which it is possible to derive quantitative data on gene expression, signaling events, and cell behavior in real time in the living animal. Relevant phenotypes in fish mutants are similar to those of other vertebrate models and human diseases. They can be analyzed in great detail and much faster than in mammals. In recent years, approximately 2500 genetically distinct fish lines have been generated by European research groups alone. Their potential, including their possible use by industry, is far from being exploited. To promote zebrafish research in Europe, EuFishBioMed was founded and won support by the EU COST programme ( http://www.cost.esf.org/ ). The main objective of EuFishBioMed is to establish a platform of knowledge exchange for research on small fish models with a strong focus on widening its biomedical applications and an integration of European research efforts and resources. EuFishBioMed currently lists more than 300 member laboratories in Europe, offers funding for short-term laboratory visits, organizes and co-sponsors meetings and workshops, and has successfully lobbied for the establishment of a European Zebrafish Resource Centre. To maintain this network in the future, beyond the funding period of the COST Action, we are currently establishing the European Society for Fish Models in Biology and Medicine.

Small molecule screening in zebrafish: swimming in potential drug therapies

Phenotype-driven chemical genetic screens in zebrafish have become a proven approach for both dissection of developmental mechanisms and discovery of potential therapeutics. A library of small molecules can be arrayed into multiwell plates containing zebrafish embryos. The embryo becomes a whole organism in vivo bioassay that can produce a phenotype upon treatment. Screens have been performed that are based simply on the morphology of the embryo. Other screens have scored complex phenotypes using whole mount in situ hybridization, fluorescent transgenic reporters, and even tracking of embryo movement. The availability of many well-characterized zebrafish mutants has also enabled the discovery of chemical suppressors of genetic phenotypes. Importantly, the application of chemical libraries that already contain FDA-approved drugs has allowed the rapid translation of hits from zebrafish chemical screens to clinical trials.

Documented and potential research impacts of subclinical diseases in zebrafish

The zebrafish (Danio rerio) has become a very important animal model in biomedical research. In contrast with other models, such as mice, there has been relatively little documentation or control of subclinical disease in zebrafish research facilities. Several infectious and noninfectious conditions are consistently detected by histopathology in apparently healthy D. rerio. The most commonly observed infectious agent in zebrafish is Pseudoloma neurophilia, which is a microsporidian organism that targets the central nervous system, peripheral nerves, and occasionally other tissues. Mycobacteriosis, caused by Mycobacterium chelonae and other species, is also a frequent finding. Less commonly encountered agents include Pseudocapillaria tomentosa, which can cause extensive proliferative enteritis, and a myxozoan (Myxidium sp.) that inhabits the urinary tract but appears to cause few if any pathological changes. Noninfectious diseases that are often clinically unapparent in zebrafish include hepatic megalocytosis, bile and pancreatic ductal proliferation, and neoplasms of the ultimobranchial gland, gastrointestinal tract, and testis. To date, there is little information on the degree to which these conditions may impact research in subclinically affected fish, but there is reason to believe that they should be considered as potentially significant causes of nonprotocol variation in experiments. Therefore, it is imperative that research facilities monitor their stocks for the presence of these occult diseases and be aware of their existence when interpreting study results. Furthermore, for underlying disease conditions that cannot be readily eradicated, it is essential to determine the physiological and immunological changes that they elicit in zebrafish. Understanding the cause, modes of transmission, and distribution of the pathogens would provide useful information for the development of control and prevention strategies.

Calcium imaging in the zebrafish

The zebrafish (Danio rerio) has emerged as a new model system during the last three decades. The fact that the zebrafish larva is transparent enables sophisticated in vivo imaging. While being the vertebrate, the reduced complexity of its nervous system and small size make it possible to follow large-scale activity in the whole brain. Its genome is sequenced and many genetic and molecular tools have been developed that simplify the study of gene function. Since the mid 1990s, the embryonic development and neuronal function of the larval, and later, adult zebrafish have been studied using calcium imaging methods. The choice of calcium indicator depends on the desired number of cells to study and cell accessibility. Dextran indicators have been used to label cells in the developing embryo from dye injection into the one-cell stage. Dextrans have also been useful for retrograde labeling of spinal cord neurons and cells in the olfactory system. Acetoxymethyl (AM) esters permit labeling of larger areas of tissue such as the tectum, a region responsible for visual processing. Genetically encoded calcium indicators have been expressed in various tissues by the use of cell-specific promoters. These studies have contributed greatly to our understanding of basic biological principles during development and adulthood, and of the function of disease-related genes in a vertebrate system.

An inducible kras(V12) transgenic zebrafish model for liver tumorigenesis and chemical drug screening

Because Ras signaling is frequently activated by major hepatocellular carcinoma etiological factors, a transgenic zebrafish constitutively expressing the kras^V12 oncogene in the liver was previously generated by our laboratory. Although this model depicted and uncovered the conservation between zebrafish and human liver tumorigenesis, the low tumor incidence and early mortality limit its use for further studies of tumor progression and inhibition. Here, we employed a mifepristone-inducible transgenic system to achieve inducible kras^V12 expression in the liver. The system consisted of two transgenic lines: the liver-driver line had a liver-specific fabp10 promoter to produce the LexPR chimeric transactivator, and the Ras-effector line contained a LexA-binding site to control EGFP-kras^V12 expression. In double-transgenic zebrafish (driver-effector) embryos and adults, we demonstrated mifepristone-inducible EGFP-kras^V12 expression in the liver. Robust and homogeneous liver tumors developed in 100% of double-transgenic fish after 1 month of induction and the tumors progressed from hyperplasia by 1 week post-treatment (wpt) to carcinoma by 4 wpt. Strikingly, liver tumorigenesis was found to be ‘addicted’ to Ras signaling for tumor maintenance, because mifepristone withdrawal led to tumor regression via cell death in transgenic fish. We further demonstrated the potential use of the transparent EGFP-kras^V12 larvae in inhibitor treatments to suppress Ras-driven liver tumorigenesis by targeting its downstream effectors, including the Raf-MEK-ERK and PI3K-AKT-mTOR pathways. Collectively, this mifepristone-inducible and reversible kras^V12 transgenic system offers a novel model for understanding hepatocarcinogenesis and a high-throughput screening platform for anti-cancer drugs.