Written by Dr. Marija Cvetanovic Edited by Larissa Nitschke
Purkinje cells (a type of neuron in the cerebellum) are the most vulnerable cells in many Spinocerebellar Ataxias (SCAs). While animal models of SCA have been very fruitful in understanding the mechanisms of Purkinje cell neurodegeneration, none of these models have allowed for visualization of neurodegenerative processes in live animals as the disease progresses – until now. In the laboratory of Dr. Reinhard Köster, researchers have developed a zebrafish model of SCA that allows for the expression of SCA-causing mutant protein in Purkinje cells and proteins that can be used to monitor Purkinje cell changes. As zebrafish larvae are almost transparent, researchers can now study pathogenic changes in neurons in a live animal during disease progression.
Since the 1993 discovery of the mutation that causes Spinocerebellar Ataxia Type 1 (SCA1), we have significantly increased our understanding of disease pathogenesis using animal models. While there are advantages and disadvantages of using any model, most researchers would agree that the similarity between humans and the animal used, plus the cost of creating and caring for the animals, are critical determinants of which model to choose. Mouse models, for instance, are useful to study pathogenesis at the molecular, cellular, tissue and behavioral level, but are costly to house and maintain. Fruit fly models, on the other hand, allow high-throughput studies (that is, studies that can produce a lot of relevant data quickly) of disease modifying properties but are much farther from human beings evolutionarily. Unfortunately, neither of these animal models allow us to follow up changes in neurons in the same animal throughout disease progression – to study the neurons, the animal must be euthanized and the brain must be dissected. Understanding how neurons are affected during disease progression, however, is very important. Observing the same neurons over time could increase our understanding of disease processes and inform us about the optimal timing for therapies. For example, if we were to identify changes in neurons that occur just prior to the onset of motor symptoms, this might mean that these changes are a contributing factor to behavioral pathology. This could also tell us the stage at which neurons start dying and disease thus becomes irreversible.
In an effort to examine how cells behave over time, many researchers use zebrafish. The fact that zebrafish embryos (larvae) are mostly transparent means that we can follow changes in neurons throughout disease progression. Moreover, in most SCAs, Purkinje cells in the cerebellum are the neurons that are most affected by the disease-causing mutant protein, and the zebrafish cerebellum has an anatomy and function that is quite similar to the human cerebellum. Zebrafish are also inexpensive and produce hundreds of offspring weekly, providing researchers with a large number of animals to study.
Using state-of-the-art genetic approaches, Dr. Reinhard Köster’s laboratory at the Technical University of Braunschweig in Germany created a zebrafish model of SCA that expresses two types of protein in their Purkinje cells: a disease-causing SCA mutant protein, and a fluorescent reporter protein to monitor degenerative changes and cell death.