By Emaan Moshin, Fall 2019.
In most research facilities, the use of mice has become second nature in order to study the human body. Whether it is isolating mouse organs to observe the effects of a mutated gene or analyzing the impact a new medicine can have on curing a disease, mice are critical for scientific researchers. However, in the past few years, researchers have begun using a species that does not seem to be the obvious choice for experiments––zebrafish.
It may come as a surprise that the slender, diminutive zebrafish with its vibrant stripes is an emerging research organism. However, they have actually been in use for many decades. George Streisinger, an American biologist at the University of Oregon, led the widespread effort around 1944 to adopt the zebrafish as a scientific model. He used gamma-rays to spontaneously mutate zebrafish DNA and look for offspring that had peculiar phenotypes, such as pigmentation defects.[1] After his death, his colleagues at the University of Oregon continued to lead efforts in understanding how the zebrafish could be used as a model organism.
Although the organism has been used since the 1940s [2], researchers are just beginning to have the necessary tools to understand the full potential that zebrafish hold. Due to their miniscule size, it is easier for labs to maintain and buy zebrafish compared to mice. Additionally, one of the most important aspects of the fish involves its transparent embryo. This transparent embryo allows scientists to observe cells growing and dividing and the development of organs without killing the organisms. The ease in handling zebrafish is not the only intriguing aspect of the fish. Zebrafish are not that different from mice in respect to protein-coding genes shared with humans. 70 percent of protein-coding human genes are related to genes found in zebrafish.[3] In comparison, 85 percent of protein-coding human genes are related to genes found in mice.[4]
One field in which zebrafish models are proving very useful is cardiovascular diseases. Unlike mice, zebrafish do not need a functional cardiovascular system to survive in the embryonic stage. Therefore, researchers are able to observe phenotypic differences in zebrafish with severe cardiovascular deformities that could not be studied in mice.[5] The 2012 paper, “Zebrafish Mef2ca and Mef2cb are essential for both first and second heart field cardiomyocyte differentiation,” used the ability of zebrafish to oxygenate through diffusion alone, to explore the function of Mef2ca proteins to control gene expression important for cardiac cell differentiation. Mef2ca and Mef2cb are a part of the family of transcription factors, which are important regulators of cellular differentiation. In previous experiments, Mef2ca proteins were discovered to be linked with early heart development. In this experiment, researchers examined what happens when a zebrafish embryo has a dual loss of both the Mef2ca and Mef2cb proteins. It was found that embryos that lack both Mef2ca and Mef2cb proteins have defects in muscle cell differentiation in the heart. However, surprisingly, the loss of either the Mef2ca or Mef2cb gene does not seem to affect the function of the heart. The use of zebrafish in this experiment was incredibly beneficial to the researchers since “studies in mice have so far failed to determine whether Mef2 activity is essential for all CM differentiation.”[6]
Another area in which zebrafish outperform mice is in toxicology models. In one study, researchers placed zebrafish in different samples of water from the Danube river and observed the development of embryo abnormalities. In fact, “The results indicated that all 22 extracts were capable of causing mortality to some extent.”[7] Since researchers could observe the development of the zebrafish embryos, they saw eye defects, rare pigments, and little to no blood circulation in these embryos. After testing the water quality using zebrafish, the researchers were able to confirm their results by attempting to identify the potent mixtures in the water. These chemicals were confirmed to have a high-level toxic potency by comparing them to an artificial mixture created in the lab. The purpose of this experiment was not necessarily to identify the chemical toxins in water but rather to explore if zebrafish could be used in assessing the overall quality of water.
Even with the success of using zebrafish in various scientific fields, one of the obvious disadvantages of using zebrafish is that they are not mammals. Zebrafish are poikilotherms, organisms whose internal temperature varies considerably, unlike humans and mice who are homeotherms and able to maintain internal temperature. Due to these differences, drugs can be metabolized differently in zebrafish compared to humans, making it difficult to use zebrafish for clinical research. Additionally, zebrafish, and fish in general, unlike rodents, do not tolerate inbreeding, and rapidly lose fertility with inbreeding.[9] The aquatic environment makes it difficult for applications of 3D analysis of zebrafish embryos as well as the administration of water-insoluble drugs.
Although zebrafish are making significant contributions in numerous scientific fields, there are still advances that need to be made in order to use zebrafish to their full capability. In the past few years, high-throughput technology has increased the feasibility of using zebrafish embryos in toxicity assays in screening for chemicals and drugs.[10] Researchers in China are also discovering ways in which zebrafish embryo development can be used to further test water quality. Zebrafish in laboratories can fill research niches that mice models have proven unsuccessful in. Perhaps in the next few years, the next big discovery in science may be attributed to zebrafish.
[1] Meyers, Jason. 2018,“Zebrafish: Development of Vertebrate Model Organism,” Current Protocols, Volume 16, Issue, 1-5.
[2] Your Genome. “Why use zebrafish in research.” Accessed November 1. https://www.yourgenome.org/facts/why-use-the-zebrafish-in-research
[3] Welcome Trust. “Zebrafish genome yields significant similarity to human genome.” https://wellcome.ac.uk/news/zebrafish-genome-yields-significant-similarity-human-genome
[4] National Human Genome Research Institution. “Why Mouse Matters.” Accessed November 10, 2019. https://www.genome.gov/10001345/importance-of-mouse-genome
[5] National Human Genome Research Institution. “Knockout Mice Fact Sheet.” Accessed November 1, 2019. https://www.genome.gov/about-genomics/fact-sheets/Knockout-Mice-Fact-Sheet
[6] Yaniv Hinits, Luyuan Pan et al. 2012, “Zebrafish Mef2ca and Mef2cb are essential for both first and second heart field cardiomyocyte differentiation,” Developmental Biology, Volume 369, Issue 2,199-210
[7] Poon, K. L., & Brand, T. 2013. “The zebrafish model system in cardiovascular research: A tiny fish with mighty prospects.” Global cardiology science & practice, 9–28.
[8] Ying Shao, Hongxia Xiao, et al. 2019, “Integrated zebrafish-based tests as an investigation strategy for water quality assessment.” Water Research, Volume 150, 252-260.
[9] Kalueff AV, Stewart AM, Gerlai R. 2014, “Zebrafish as a new emerging model for studying complex brain disorders.” Trends Pharmacol Sci.
[10] Horzmann, K. A., & Freeman, J. L. 2018, “Making Waves: New Developments in Toxicology with the Zebrafish. Toxicological sciences.” Society of Toxicology, 5–12.