The use of in vivo zebrafish assays in drug toxicity screening.

Anecdotal evidence has long suggested that zebrafish may be a good model to predict toxicity of human drugs. As summarized in this review, several groups have recently conducted systematic evaluations of zebrafish toxicity end points using large numbers of pharmacologically relevant compounds. Assays of particular interest include those for cardiotoxicity, ototoxicity, seizure liability, developmental toxicity and gastrointestinal motility. Results suggest that zebrafish assays can attain an acceptable level of predictivity, ranging from "sufficient" (65 - 75% predictivity) to "good" (75 - 85% predictivity) based on guidelines established for novel in vitro tests by the European Centre for the Validation of Alternative Methods. Further validation will probably be required to definitely establish zebrafish as a standard model for toxicity testing.

In vivo imaging of zebrafish digestive organ function using multiple quenched fluorescent reporters.

Optical clarity of larvae makes the zebrafish ideal for real-time analyses of vertebrate organ function through the use of fluorescent reporters of enzymatic activities. A key function of digestive organs is to couple the generation of enzymes with mechanical processes that enable nutrient availability and absorption. However, it has been extremely difficult, and in many cases not possible, to directly observe digestive processes in a live vertebrate. Here we describe a new method to visualize intestinal protein and lipid processing simultaneously in live zebrafish larvae using a quenched fluorescent protein (EnzChek) and phospholipid (PED6). By employing these reagents, we found that wild-type larvae exhibit significant variation in intestinal phospholipase and protease activities within a group but display a strong correlation between the activities within individuals. Furthermore, we found that pancreas function is essential for larval digestive protease activity but not for larval intestinal phospholipase activity. Although fat-free (ffr) mutant larvae were previously described to exhibit impaired lipid processes, we found they also had significantly reduced protease activity. Finally, we selected and evaluated compounds that were previously suggested to have altered phospholipase activity and are known or suspected to have inflammatory effects in the intestinal tract including nonsteroidal anti-inflammatory drugs, and identified a compound that significantly increases intestinal phospholipid processing. Thus the multiple fluorescent reporter-based methodology facilitates the rapid analysis of digestive organ function in live zebrafish larvae.

Automated, quantitative screening assay for antiangiogenic compounds using transgenic zebrafish.

Pathologic angiogenesis has emerged as an important therapeutic target in several major diseases. Zebrafish offer the potential for high-throughput drug discovery in a whole vertebrate system. We developed the first quantitative, automated assay for antiangiogenic compound identification using zebrafish embryos. This assay uses transgenic zebrafish with fluorescent blood vessels to facilitate image analysis. We developed methods for automated drugging and imaging of zebrafish in 384-well plates and developed a custom algorithm to quantify the number of angiogenic blood vessels in zebrafish. The assay was used to screen the LOPAC1280 compound library for antiangiogenic compounds. Two known antiangiogenic compounds, SU4312 and AG1478, were identified as hits. Additionally, one compound with no previously known antiangiogenic activity, indirubin-3'-monoxime (IRO), was identified. We showed that each of the hit compounds had dose-dependent antiangiogenic activity in zebrafish. The IC(50) of SU4312, AG1478, and IRO in the zebrafish angiogenesis assay was 1.8, 8.5, and 0.31 micromol/L, respectively. IRO had the highest potency of the hit compounds. Moreover, IRO inhibited human umbilical vein endothelial cell tube formation and proliferation (IC(50) of 6.5 and 0.36 micromol/L, respectively). It is therefore the first antiangiogenic compound discovered initially in a zebrafish assay that also has demonstrable activity in human endothelial cell-based angiogenesis assays.

Zebrafish assays for drug toxicity screening.

Zebrafish are vertebrate organisms that are of growing interest for preclinical drug discovery applications. Zebrafish embryos develop most of the major organ systems present in mammals, including the cardiovascular, nervous and digestive systems, in < 1 week. Additional characteristics that make them advantageous for compound screening are their small size, transparency and ability to absorb compounds through the water. Furthermore, gene function analysis with antisense technology is now routine procedure. Thus, it is relatively simple to assess whether compounds or gene knockdowns cause toxic effects in zebrafish. Assays are being developed to exploit the unique characteristics of zebrafish for pharmacological toxicology. This review discusses assays that may be used to assess in vivo toxicity and provides examples of compounds known to be toxic to humans that have been demonstrated to function similarly in zebrafish.

Neuroprotection of MPTP-induced toxicity in zebrafish dopaminergic neurons.

Parkinson's disease is characterized by a severe loss of dopaminergic neurons resulting in a range of motor deficits. The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is known to cause a similar loss of dopaminergic neurons in the human midbrain with corresponding Parkinsonian symptoms. Several animal species have also shown sensitivity to MPTP, including primates, mice, goldfish, and, most recently, zebrafish. This study demonstrates that the effect of MPTP on dopaminergic neurons in zebrafish larvae is mediated by the same pathways that have been demonstrated in mammalian species. MPTP-induced neurodegeneration was prevented by co-incubation with either the monoamine oxidase-B (MAO-B) inhibitor l-deprenyl or the dopamine transporter (DAT) inhibitor nomifensine. Furthermore, targeted inactivation of the DAT gene by antisense morpholinos also protected neurons from MPTP damage. Thus, the mechanism for MPTP-induced dopaminergic neuron toxicity in mammals is conserved in zebrafish larvae. Effects on swimming behavior and touch response that result from MPTP damage are partially ameliorated by both l-deprenyl and DAT knockdown.

Downregulation of gene expression with negatively charged peptide nucleic acids (PNAs) in zebrafish embryos.

We found that negatively charged, highly soluble PNA analogs with alternating phosphonates (HypNA-pPNAs) are effective and specific antisense agents in zebrafish embryos, showing comparable potency and greater specificity against chordin, ntl and uroD. In addition, we successfully phenocopied a dharma mutant that had not been found susceptible to MO knockdown. Both MO and HypNA-pPNAs against a tumor suppressor gene induced comparable upregulation of p53, illustrating similar effects on transcription profiles. HypNA-pPNAs are therefore a valuable alternative for reverse genetic studies, enabling the targeting of previously inaccessible genes in zebrafish or validating newly identified orthologs, and perhaps for reverse genetic studies in other organisms.

Zebrafish: from disease modeling to drug discovery.

The study of zebrafish, a leading model organism for developmental biology, is rapidly expanding to include human disease. Zebrafish models based on known disease mechanisms have been developed in several therapeutic areas, including blood diseases, diabetes, muscular dystrophy, neurodegenerative disease, angiogenesis and lipid metabolism. This review summarizes recent progress in disease model development, and outlines the potential of zebrafish to contribute to drug discovery through the identification of novel drug targets, validation of those targets and screening for new therapeutic compounds.