Transparent fish models such as the zebrafish and Japanese rice fish, or medaka, make it possible to observe processes within the living body without interference from tissue pigmentation. These models are frequently used for medical research, such as cancer research, but are less applicable to the study of aging due to their relatively long life spans – about five years for zebrafish and two years for the medaka. A new transparent fish model developed at the Leibniz Institute on Aging—Fritz Lipmann Institute (FLI) opens up new opportunities to study aging processes due to its short lifespan and genetic similarity to humans.
The new model, named “klara,” was created through CRISPR/Cas9 gene editing of the African turquoise killifish (Nothobranchius furzeri), which normally has vibrant coloration in the wild. As the shortest-living vertebrate kept in laboratory vivaria, killifish have already been used for aging research in the past. The researchers aimed to improve N. furzeri’s use as a model organism by rendering it transparent for in vivo microscopy. This task required the simultaneous deactivation of three loci responsible for pigmentation in the killifish. To perform the gene editing, the team leveraged the complete N. furzeri genome sequence previously produced at FLI.
The resulting organism is the world’s first transparent killifish, giving researchers a clear look at the animal’s internal organs. The team produced a total of about 200 “klara” fish, including both male and female organisms. In addition to its short lifespan, the killifish model also has the advantage of exhibiting signs of aging similar to those seen in mammals, including telomere shortening, cellular senescence and cognitive decline. The researchers established an additional klara line incorporating an additional gene at a locus associated with cell senescence. When the genes at this locus are expressed, the modified cells produce a green fluorescent protein, allowing the progression of aging to be visualized as senescent cells accumulate. This research was published in eLife.
“With the klara line, we now can investigate the role of senescent cells in the living organisms on a molecular level. By labeling them with fluorophores and then examining them under a fluorescence microscope, we can learn where they appear in the body and whether they are possibly clustered at certain locations, and what effects their removal has on the surrounding cells and tissues,” said first author Johannes Krug.