Miriam Martínez - 21 July 2021
Zebrafish as a Translatable Model System
Are zebrafish suitable for human drug discovery?
In previous articles, we have discussed the advantages of phenotype-based screening in zebrafish for drug discovery. Although being able to perform high-throughput phenotypic screens in an in vivo context is really attractive, you may be wondering: how relevant is the output from zebrafish screens for human biology? In other words, what is the translatability of zebrafish models to humans?
From translatable to translational research
The term “translational research” is increasingly being used by scientific and medical institutions to convey the message that the research activities ultimately serve the public. It describes the transfer of basic preclinical discoveries into human applications, such as new treatments that improve the health of the population. Translational research includes all stages of clinical development, from phase-I to phase-IV clinical trials.
It must be distinguished from “translatable research”, which describes the basic or preclinical research that generates outcomes with likely human applicability, and therefore, that potentially could lead to translation. In this sense, animal models are key tools in translatable research, since they allow to show efficacy and understand the mechanism of action of candidate therapeutic drugs in living whole-organism systems. So that the results can be extrapolated to humans in later phases, it is important to work with animal models that better resemble the human situation we want to study.
In order to answer the question we are concerned with today, it is essential to understand how zebrafish compare to humans in terms of targets, physiology, pharmacology, and drug metabolism, particularly during the first few days of life when it is possible to house them in multi-well plates suited for screening .
Zebrafish: a translatable model system for drug discovery
Zebrafish entered the laboratory settings as a model organism for the study of developmental biology. They are ideal for this purpose because they are fertilized externally, can generate hundreds of embryos from a breeding pair, and are transparent. More recently, zebrafish has emerged as a powerful preclinical model for human disease, as their disease characteristics, etiology and progression, and molecular mechanisms are clinically relevant and highly conserved .
Conserved drug targets
Zebrafish have become a popular organism for the study of gene function as well as for understanding the biology underlying human disease. The reason for that is the significant similarity that the zebrafish genome yields to the human genome: the vast majority of human genes (70%) have counterparts (known as orthologs) in the zebrafish genome. Among those, genes related to human disease rise up to 84% conservation. Zebrafish ortholog genes show resemblances with humans in DNA sequence, expression pattern (where and when the gene is active in the body), and encoded proteins; in particular within their functional domains. Indeed, evolutionary conservation in protein active sites may explain why zebrafish display homologous pharmacological effects to humans for tested molecules .
Additionally, gene location studies have revealed large conservation of chromosome segments between zebrafish and human genes (syntenic correspondence), whereby two or more genes that are found on the same chromosome in zebrafish are also found on the same chromosome in humans .
This overall genetic similarity makes zebrafish an important model for understanding how genes work in health and disease. Indeed, knowing the relationship between zebrafish and human genomes allows for identifying roles for human genes from zebrafish mutations. Likewise, modeling human-identified disease genes in zebrafish may help find new targets for drug development.
Among model systems that are amenable to screening, zebrafish stand out for their highly conserved integrative physiology. Most zebrafish organs perform the same functions as their human counterparts, consist of the same cell types, and exhibit well-conserved physiology.
Cardiovascular physiology, for instance, is highly conserved between humans and zebrafish at anatomical, cellular, and membrane-biology levels. Although the zebrafish heart is different from the human heart with only systemic circulation, human heart rate and cardiac electrophysiology are strikingly closer to zebrafish than they are to rodents. Action potential (AP) parameters, including AP amplitude, resting membrane potential, and repolarization phase (QT interval) are highly similar between zebrafish and humans .
Zebrafish’ central nervous system shares neuroarchitecture and cellular morphology with humans. It also has a very similar neurotransmitter system, which seems to play similar roles to those observed in mammals. Zebrafish brain presents a highly conserved organization, having all typical sensory functions (vision, olfaction, taste, hearing, and tactile), and all the sensory pathways share significant homology with humans. The blood–brain barrier (BBB) is also present in zebrafish, which develops 3 days post-fertilization, and controls small molecules’ permeability with functional similarities to humans .
The same analogies apply to other organ systems, including the liver, kidneys, pancreas, and so on, which also show similar physiological features to those of humans .
As previously discussed, disease processes and related proteins are well-conserved between zebrafish and humans, which means that active drugs for humans often have the same target in zebrafish, especially molecules that interact with the active region of the target protein. There are some examples of drugs that work in zebrafish but not in humans, and vice versa, but evidence from more than two decades of drug screening in zebrafish suggests that conservation of pharmacological effect is high for the majority of drugs where the phenotypic correlation is rigorous .
Zebrafish drug screenings revealed that not only were the pharmacological effects conserved in humans and zebrafish but so too were the majority of drug-drug interactions. These findings suggested that the distribution, metabolism, and excretion of drugs are similar in zebrafish. Actually, there is strong evidence of the existence in zebrafish of active regulation of drug distribution across physiological boundaries such as the BBB, and by conserved tissue-specific transporters with similar pharmacokinetic properties.
There is also evidence of substantial functional parallels across diverse mechanisms of drug metabolism, including cytochrome P450 (CYP) and other multiple non-CYP enzymes. However, great care must be taken in simple extrapolation, as drug metabolism may vary widely across different developmental stages, and the presence of a part of any pathway does not infer the conservation of the remaining components.
Finally, it is clear that drug excretion is regulated in zebrafish, but few studies of drug filtration, reabsorption, or excretion have been undertaken to date.
In summary, genetic, physiological, metabolic, and pharmacologic conservation configures zebrafish as a valuable tool to model a wide range of human disorders, perform target validation studies, and discover new drugs to treat multiple diseases. Indeed, zebrafish have already contributed to several examples of successful phenotype-based drug discovery. To date, numerous drugs discovered through zebrafish screens have entered clinical trials, which supports further zebrafish as a translatable model system. Some have discovered novel compound classes with therapeutic potential, whereas others have identified repurposing opportunities for existing drugs. Luckily, the understanding of zebrafish biology runs in parallel with the improvement of the technologies for manipulating the zebrafish genome. Thus, we predict an ever-increasing role of zebrafish in accelerating the emergence of precision medicine and for validating the target genes that will come from the enormous well of knowledge provided by the combination of artificial intelligence and systems biology .
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By Miriam Martínez
Miriam is a Human Biologist expert in neuropharmacology. After a master’s degree in Pharmaceutical and Biotech Industry, she obtained her PhD in Biomedicine from Pompeu Fabra University (Barcelona). During her doctorate, she focused her research on the behavioral analysis of animal models for neurophenotypical characterization. Since then, she has been working in the healthcare marketing and publicity sector, where she has contributed to developing marketing campaigns for several pharmaceutical brands. In 2021, she joined ZeClinics with a branding and marketing strategy focus.