This blog post comes to us from a rising 3rd year in the NGP, Riley St. Clair. Riley writes:
Neurodevelopment. Stem cells. Cancer. Regeneration.
These are some of the biggest scientific research topics of today. But a small animal, the zebrafish, has been helping scientists uncover the mechanisms underlying these processes to advance human knowledge and make important discoveries to one day find therapies to treat human disease.
Zebrafish have made an entrance into the fields of biology and neuroscience. The zebrafish is a teleost fish, meaning that both the lower and upper jaws are able to move. As teleosts, the zebrafish belong to the family that also includes goldfish, catfish, and eels1. In the 1980’s, Dr. George Streisinger, a professor at the University of Oregon, realized the potential of zebrafish as a model organism to study development2. To this day, the zebrafish continues to be a great model for development and many other biological processes for many reasons:
- There is genetic similarity: It may not seem obvious, but many of the genes in the zebrafish are homologous to those of humans. In fact, about 70% of the proteins found in humans are also present in zebrafish3. These proteins are involved in fundamental processes such as cell division, proliferation and migration. Because of this, scientists can use zebrafish as a model organism that allows us to learn more about human health and disease.
- The embryos are externally fertilized and transparent: Studying embryonic development in mammalian model organisms, such as mice, can be difficult due to in utero development – occurring in the uterus of the pregnant female. External fertilization, in addition to the near-transparency of the embryos, allows for easy visualization of development. Although the embryos are small, low magnification light microscopes can be used to watch development occur – from the very first cell divisions to the emergence of active swimming and food seeking behaviors!
- Development is rapid: Embryonic development is completed in only three days4. This advantage gives scientists the ability to study developmental processes quickly and in real time.
- They produce large clutches of eggs: Zebrafish lay and fertilize large numbers of eggs each time they mate. This gives us statistical power – so we can conduct experiments and make accurate conclusions with confidence.
- They can be manipulated genetically: Even in Dr. Streisinger’s seminal 1981 paper, he showed how the genome of zebrafish could be slightly altered for use in gene identification and function studies2. This is an extremely powerful tool for scientists – by studying the cellular or behavioral effects after mutating or removing a specific gene, we can make strong inferences as to the function of the proteins that are encoded by that gene.
The 1980’s and 1990’s were also the decades in which the amazing protein Green Fluorescent Protein, or GFP, was being isolated, sequenced, and first utilized in model organisms. It was Dr. Martin Chalfie and his lab at Columbia University that pioneered the technique of adding the GFP gene to specific parts of the genome called promoters. Promoters are what “turn on” genes and help the encoded information ultimately turn into proteins in cells. They are important regulators of cellular activity and are a major force in dictating the fate of different cell types. Knowing this, Dr. Chalfie attached GFP to unique promoters and thus could make specific cell types fluoresce! Today, there are numerous GFP transgenic lines of zebrafish. For example, the image below is the isl2b:GFP zebrafish, which expresses GFP in retinal ganglion cells. These cells are found in the retina and their projections connect to the visual processing area of the brain and help give the ability of sight. These projections enter the brain behind the eye in this lateral view. This transgenic zebrafish line also expresses GFP in motor neurons of the spinal cord – which can be seen down the body of the fish.
With all of these tools available in the research world of zebrafish, scientists have been able to study many different processes and diseases in the zebrafish, including cancer research, stem cells, and regeneration.
Zebrafish, like other animals, develop tumors either unexpectedly or from known environmental causes. Therefore, cancer researchers have recently been using zebrafish to model how tumors originate, grow, and metastasize. Furthermore, transgenic zebrafish can model certain cancers by activating a specific oncogene. An oncogene is a gene that can cause cancer. Transgenic zebrafish can also model cancer by expressing a malfunctioning tumor suppressor gene, one that helps prevent cancer5. Additionally, zebrafish can absorb many drugs through their skin, making identifying and studying novel chemotherapy compounds easier5.
Collaborators at Harvard Medical School and several Boston research hospitals, including the Dana-Farber Cancer Institute, Boston Children’s Hospital, and Brighan and Women’s Hospital, are investigating how neuroblastomas develop. A neuroblastoma is a type of cancer that affects children and generally has a poor prognosis. MYCN is an oncogene implicated in the onset of neuroblastomas. If the tumor cells have high levels of MYCN, this puts the patients at high risk, with a 5-year survival rate of less than fifty percent6. Dr. Thomas Look and collaborators are studying how MYCN contributes to neuroblastoma pathogenesis in the hopes of finding a therapeutic target. To do this, they used a transgenic zebrafish line that was modified to have elevated levels of MYCN to model the high-risk patient population. This causes neuroblastoma-like tumors to arise in the zebrafish, which can be studied. The scientists found that an increased amount of MYCN in the tumor cells caused them to resist apoptosis. This is a normal process of cellular death that occurs when cells are no longer healthy. However, the results of Dr. Look and colleague’s investigation show that neuroblastoma cells have an increased amount of MYCN and resist cell death. This could be one mechanism in which the cells continue to grow and thus promote the formation of the tumor. Future investigation could uncover the potential of MYCN to be a drug target to help fight neuroblastomas.
Regeneration and Stem Cell Research
Zebrafish and other teleost fish have the amazing ability to regenerate. Not only can they regrow amputated fins, but they can also regenerate organs including the brain and heart7. This makes them great candidates to study the process of regeneration and the cellular response to injury. Because they can regenerate, stem cell scientists have suggested that there may be a population of cells in many parts of the brain that are responsible for replenishing injured cells.
A research group in Germany studies how the brain repairs itself after injury. A normal response to brain injury in humans is a process called reactive gliosis. This is when microglia, a type of immune cell found in the central nervous system, is recruited to remove the damaged cells. However, this process can be detrimental and cause scar tissue that can cause further brain damage. Furthermore, a type of proliferative cell called radial glial cells are present during mammalian development but turn into astrocytes in adults – further prohibiting regeneration in the adult human brain. However, adult zebrafish have radial glial cells and this may be one reason as to why they can regenerate their brain after injury. Dr. Uwe Strahle’s lab wanted to determine the differences between zebrafish and human injury response. To do this, they caused a small lesion in the brain of zebrafish expressing GFP in astrocytes – a subset of which are the cells they believed to be radial glial cells. After injury, Dr. Strahle’s lab observed an increase in cell proliferation and discovered that scar tissue formed only temporarily. They also found another population of cells, normally found in the human brain and contribute to scar formation, were not as active in the zebrafish. Identifying the differences between zebrafish and human injury response may help future studies discover therapeutic ways to treat brain injury.
These are only two of the numerous examples of what scientists are doing all over the world using zebrafish as their model system. This is due to the myriad of characteristics that make zebrafish a great model organism for scientific research, including rapid embryonic development, transparency, genetic similarity to humans, and the ability to regenerate. Fish around on the web, and you’ll be sure to find many more articles in all sorts of interesting topics.
Zhu, S., et al. (2012). Activated ALK collaborates with MYCN in neuroblastoma pathogenesis. Cancer Cell 21(3):362-273.
Marz, M., Schmidt, R., Rastegar, S., & Strahle, U. (2011). Regenerative response following stab injury in the adult zebrafish telencephalon. Developmental Dynamics 240:2221-2231.
- Nusslein-Volhard, C., Gilmour, D. T., & Dahm, R. (2002). Zebrafish as a system to study development and organogenesis. In C. Nusslein-Volhard & R. Dahm (Eds.), Zebrafish (pp. 1-5). New York: Oxford University Press.
- Streisinger, G., Walker, C., Dower, N., Knauber, D., & Singer, F. (1981). Production of clones of homozygous diploid zebra fish (Brachydanio rerio). Nature 291:293-296.
- Howe, K., et al. (2013). The zebrafish reference genome sequence and its relationship to the human genome. Nature 496:498-503.
- Kimmel, C. B., Ballard, W. W., Kimmel, S. R., Ullmann, B., & Schilling, T. F. (1995). Stages of embryonic development of the zebrafish. Developmental Dynamics 203:253-310.
- Stoletov, K., & Klemke, R. (2008). Catch of the day: zebrafish as a human cancer model. Oncogene 27:4509-4520.
- American Cancer Society. (2015). Survival rates for neuroblastoma based on risk groups. Retrieved from http://www.cancer.org/cancer/neuroblastoma/detailedguide/neuroblastoma-survival-rates
- Gemberling M., Bailey, T. J., Hyde, D. R., & Poss, K. D. (2013). The zebrafish as a model for complex tissue regeneration. Trends in Genetics 29(11): 611-620.