Zebrafish and Medaka: new model organisms for modern biomedical research

Abstract

Although they are primitive vertebrates, zebrafish (Danio rerio) and medaka (Oryzias latipes) have surpassed other animals as the most used model organisms based on their many advantages. Studies on gene expression patterns, regulatory cis-elements identification, and gene functions can be facilitated by using zebrafish embryos via a number of techniques, including transgenesis, in vivo transient assay, overexpression by injection of mRNAs, knockdown by injection of morpholino oligonucleotides, knockout and gene editing by CRISPR/Cas9 system and mutagenesis. In addition, transgenic lines of model fish harboring a tissue-specific reporter have become a powerful tool for the study of biological sciences, since it is possible to visualize the dynamic expression of a specific gene in the transparent embryos. In particular, some transgenic fish lines and mutants display defective phenotypes similar to those of human diseases. Therefore, a wide variety of fish model not only sheds light on the molecular mechanisms underlying disease pathogenesis in vivo but also provides a living platform for high-throughput screening of drug candidates. Interestingly, transgenic model fish lines can also be applied as biosensors to detect environmental pollutants, and even as pet fish to display beautiful fluorescent colors. Therefore, transgenic model fish possess a broad spectrum of applications in modern biomedical research, as exampled in the following review.

Fig. 1

miR-3906 silences different target genes at different muscle developmental stages. a At the early muscle development, Dkk3a interacts with its receptor Itgα6b, resulting in the phosphorylation of p38a and the formation of the Smad2/3a/4 complex, which in turn, activates the myf5 promoter activity. When myf5 is highly transcribed, the intronic miR-3906 suppresses the transcription of myf5 through silencing the Dkk3a [, –43]. b At the late muscle development, miR-3906 starts transcription at its own promoter and switches to silence homer1b to control the homeostasis of intracellular calcium concentration ([Ca²⁺]_i) in fast muscle cells [40]. Either miR-3906-knockdown or homer-1b-overexpression causes the increase of Homer-1b protein, resulting in an enhanced level of [Ca²⁺]_i, which in turn, disrupts sarcomeric actin filament organization. In contrast, either miR-3906-overexpression or homer-1b-knockdown causes the decrease of Homer-1b, resulting in a reduced [Ca²⁺]_i and thus a defective muscle phenotype [40]

Fig. 2

miR-1 and miR-206 silence different target genes and play opposing roles in zebrafish angiogenesis. Both miR-1 and miR-206 are muscle-specific microRNAs and share identical seed sequences. However, they silence different target genes to affect the secreted VegfAa level through different pathways [62]. The miR-1/SARS/VegfAa pathway plays a positive role in angiogenesis since SARS, a negative factor for VegfAa promoter transcription, is silenced by miR-1, resulting in the increase of VegfAa. However, the miR-206/VegfAa pathway plays a negative role since VegfAa is silenced directly by miR-206. Dynamic changes of miR-1 and miR-206 levels are also observed [62]. The miR-1 level gradually increases between 12 and 20 hpf and significantly increases further between 20 and 32 hpf, while the miR-206 level is only slightly changed during this same period. Consequently, VegfAa increases greatly from 24 to 30 hpf, which might be responsible for the continuous increase of miR-1/SARS/VegfAa pathway, but not miR-206/VegfAa pathway. Therefore, temporal regulation of the expression of miR-1 and miR-206 with different target genes occur during embryonic angiogenesis in somitic cells of zebrafish