Recent advances in bioorthogonal catalysis are increasing the capacity of researchers to manipulate the fate of molecules in complex biological systems. A bioorthogonal uncaging strategy is presented, which is triggered by heterogeneous gold catalysis and facilitates the activation of a structurally diverse range of therapeutics in cancer cell culture. Furthermore, this solid‐supported catalytic system enabled locally controlled release of a fluorescent dye into the brain of a zebrafish for the first time, offering a novel way to modulate the activity of bioorthogonal reagents in the most fragile and complex organs.

The zebrafish is increasingly recognized as a model organism for translational research into human neuropathology. The zebrafish brain exhibits fundamental resemblance with human neuroanatomical and neurochemical pathways, and hallmarks of human brain pathology such as protein aggregation, neuronal degeneration and activation of glial cells, for example, can be modeled and recapitulated in the fish central nervous system. Genetic manipulation, imaging, and drug screening are areas where zebrafish excel with the ease of introducing mutations and transgenes, the expression of fluorescent markers that can be detected in vivo in the transparent larval stages overtime, and simple treatment of large numbers of fish larvae at once followed by automated screening and imaging. In this review, we summarize how zebrafish have successfully been employed to model human neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, amyotrophic lateral sclerosis, and Huntington’s disease. We discuss advantages and disadvantages of choosing zebrafish as a model for these neurodegenerative conditions.

In adult zebrafish, relatively quiescent progenitor cells show lesion-induced generation of motor neurons. Developmental motor neuron generation from the spinal motor neuron progenitor domain (pMN) sharply declines already at 48 hours post-fertilisation (hpf). After that, mostly oligodendrocytes are generated from the same domain. We demonstrate here that within 48 hours after a spinal lesion or specific genetic ablation of motor neurons at 72 hpf, the pMN domain reverts to motor neuron generation at the expense of oligodendrogenesis. In contrast, generation of dorsal Pax2-positive interneurons was not altered. Larval motor neuron regeneration can be boosted by dopaminergic drugs, similar to adult regeneration. We use larval lesions to show that pharmacological suppression of the cellular response of the innate immune system inhibits motor neuron regeneration. Hence, we have established a rapid larval regeneration paradigm. Both, mechanical lesion or motor neuron ablation are sufficient to reveal a high degree of developmental flexibility of pMN progenitor cells. In addition, we show an important influence of the immune system on motor neuron regeneration from these progenitor cells.

The zebrafish genome contains ten genes that encode class II cytokine-like peptides, of which the two that are related most closely to mammalian interferon gamma (IFN-γ) were named IFN-γ1 and IFN-γ2. Although the zebrafish has become a popular model system to study immune mechanisms, and although interferons are central regulators of immunity, which zebrafish cytokines correspond functionally to mammalian IFN-γ has not been established. We used zebrafish embryos to assay the functions of IFN-γ1 and IFN-γ2, and have identified a subset of zebrafish homologs of the mammalian IFN-responsive genes as IFN-γ targets in the zebrafish embryo: these genes are upregulated in response to raised levels of either IFN-γ1 or IFN-γ2. Infection studies using two different pathogens show that IFN-γ signalling is required for resistance against bacterial infections in the young embryo and that the levels of IFN-γ need to be regulated tightly: raising IFN-γ levels sensitizes fish embryos against bacterial infection. Embryos injected with high doses of Escherichia coli are able to clear the bacteria within a day, and the γ-interferons are necessary for this defence reaction. The protective response to Yersinia ruckeri, a natural fish pathogen that is lethal at low doses, also requires IFN-γ. As in the induction of target genes, the two interferons act at least partly redundantly. Together with the previously demonstrated type III interferon response, these results show that the counterparts of the mammalian viral and bacterial interferon-dependent defence functions are in place in zebrafish embryos, and suggest that zebrafish IFN-γ1 and IFN-γ2 are functionally equivalent to mammalian IFN-γ.

Metastasis, the dispersal of cancer cells from a primary tumor to secondary sites within the body, is the leading cause of cancer-related death. Animal models have been an indispensable tool to investigate the complex interactions between the cancer cells and the tumor microenvironment during the metastatic cascade. The zebrafish (Danio rerio) has emerged as a powerful vertebrate model for studying metastatic events in vivo. The zebrafish has many attributes including ex-utero development, which facilitates embryonic manipulation, as well as optically transparent tissues, which enables in vivo imaging of fluorescently labeled cells in real time. Here, we summarize the techniques which have been used to study cancer biology and metastasis in the zebrafish model organism, including genetic manipulation and transgenesis, cell transplantation, live imaging, and high-throughput compound screening. Finally, we discuss studies using the zebrafish, which have complemented and benefited metastasis research.